Case Study : Robot Assembly of Pneumatic Valves (1/3)

This is a quick feasibility study that I was asked to carry out for a Swedish world leading manufacturer of compressors, generators, construction and mining equipment, industrial tools and assembly systems.  They wanted a swift appraisal of the economics for the robot assembly of their pneumatic valves.

INTRODUCTION

The client company has committed to capital investment in a two-armed robot for the assembly of its range of pneumatic cylinders at one of its Swedish manufacturing plants.  The robot will not be fully utilised and another product is required to economically justify the installation.

The manufacturing plant currently assembles two product families :

1) Pneumatic cylinders
2) Pneumatic valves

The feasibility of assembling a model of pneumatic valve is investigated for the client company.

VOLUME REQUIREMENTS OF THE SPECIFIC PNEUMATIC VALVE MODEL

The pneumatic valve annual production volume is 30 000 units.  The valve is currently assembled manually and the client company assumes that demand for the valve will increase to 40 000 within 2 years.  Three workers are currently required for product assembly.  The product has a total of 63 separate parts, of which, 31 are unique parts.

MANUAL ASSEMBLY


The manual assembly of the valve has been studied to create individual times for the 97 operations.  The manual assembly worksheet, shown below, gives the sequence of operations and their corresponding operation times.  The worksheet shows that the cycle time for the complete assembly is 389 seconds.  One worker can assemble 11 786 valves in one year, with single shift working at a labour efficiency of 70 percent :

225 working shifts per annum (single shift working)
= 5 940 000 working seconds per annum (440 minutes / shift)
= 4 158 000 working seconds per year at 70 percent labour efficiency
= 10 689 units assembled per annum.    .

ROBOT ASSEMBLY

Certain assembly operations can be executed by the robot without a re-design of the pneumatic valve.  The assembly sequence for the robot assembly is given at the end of this article and an estimate for the robot capital expenditure is :

(a) Cost of the robot and controller = 50 000 euros.

(b) Turret and eight grippers = 5 000 euros

(c) Fixture number 32 = 3 000 euros, fixture number 33 = 2 000 euros, fixture number 34 = 2 000 euros, fixture number 35 = 3000 euros, fixture number 36 = 2 000 euros, fixture number 37 = 500 euros, fixture number 38 = 500 euros, fixture number 39 = 2 000 euros

(d) Arm-2 0-ring tools (6 off), including tool holder = 1 800 euros.  Arm-2 screwdriver bit and friction screwdriver bit = 200 euros.

(e) Greasing station = 2 000 euros

( f) Labelling station = 5 000 euros

(g) Cleaning station = 2 000 euros

(h) Eight vibratory linear feeders at 3 000 euros each = 24 000 euros

Total = 105 000 euros

CYCLE TIME

It is estimated that the cycle time for the robot and manual assembly of the valve would be 456 seconds.  Using this estimate, one robot can assemble 5210 valves in one year, with single shift working at a robot efficiency of 80 percent :

225 working shifts per annum (single shift)
= 5 940 000 working seconds per annum (440 minutes/shift)
= 4 752 000 working seconds per year at 80% robot efficiency
= 10 421 units assembled per year (single shift)

The robot can assemble approximately the same number of products per year as one worker, considering single shift working.  However, certain operations (using the existing product design) must be executed manually.

ANNUAL COST SAVINGS

The annual cost saving of using the robot is one worker per year.  If the annual cost of an operator is 50 000 euros per year (including taxes, social charges, pension contributions, overhead contribution, etc.) then the cost saving would be approximately 50 000 euros per year.

PAYBACK PERIOD

The payback period for using the robot is 2 years, for the assembly of 10 421 units per year.

VALVE SPECIFICATIONS

If the valve is to be re-designed then it must have the following performance characteristics :

(a) It must achieve a flow rate of 2.2 litres per second for 10 000 000 cycles of operation, without leakage from port to port.

(b) The upper sealing gasket must not drop off when the body sub-assembly is transported between operations.

(c) The inner sleeve 0-rings must be stable during assembly of the inner sleeve sub-assembly to the valve body.

(d) The activation time of the unit must be better than 0.02 seconds.

(e) The operating air pressure for the double acting valve should be lessthan 1.2 kg/cm2 and less than 2.5 kg/cm2 for the spring return valve.

(f) The customer should have the option of achieving flow rates between 0 and 50 percent of the maximum and between 50 and 100 percent of the maximum, using a convenient design feature.

Case Study : Robot Assembly of Pneumatic Valves (2/3)

RECOMMENDATIONS FOR A RADICAL RE-DESIGN OF THE PNEUMATIC VALVE TO REDUCE THE NUMBER OF PARTS IN THE ASSEMBLY

The following design changes are recommended to reduce the cost of the assembly of the pneumatic valve :

(1) Eliminate the choke screw housing (6) by providing an internal thread in the valve body, where the choke screw housing sub-assembly is currently situated.  This would involve the use of a choke screw and O-ring only, thus eliminating two parts.    .

(2) Eliminate the gasket (2) and top cover (23) by moulding the airway into the integral body and top cover. This would eliminate two parts but would require the bodies to be stocked in two styles to accommodate single acting and double acting valves.

(3) Eliminate the piston sleeve (9) by reducing the bore of the body at this point so that the piston is guided by the body, instead of the sleeve.

(4) Eliminate the three sleeves (9), (12), (14) and integrally mould the three sleeves as one part.

(5) Eliminate the piston by integrally moulding it with the spool piece, for single acting valves.

(6) Eliminate the spool piece O-rings / sealing rings and locate them on the spool piece sleeves.

(7) Eliminate one end piece and integrate it with the valve body.

(8) Eliminate the indicator and integrate it with the spool piece.

(9) Eliminate the label and print it directly onto the valve.

(10) Eliminate the cover screws and incorporate (a) a bayonet fitting or, (b) a screw thread between the body and end cover.

(11) Eliminate the short spring by changing the following design features of the long springs :
(a) spring wire gauge
(b) number of turns per inch
(c) spring material
(d) external diameter of spring

CONSIDERATIONS FOR THE RE-DESIGN OF THE PNEUMATIC VALVE

There are many factors that must be considered when re-designing the pneumatic valve for assembly.  The effect of changing one design feature of a part may have an effect on the design of the other parts.  The performance of the valve can be reduced by adverse design changes, or there may be an increase in the manufacturing costs of the product parts, due to new tooling costs.  A number of factors must be considered when the re-designed valve is being evaluated :

(1) A capital investment has been made by the client company in mould tooling for the valve body.  If other part features are to be integrated within the body, or if the body is to be split into more than one component, then there will be an investment required for the new mould tooling.

(2) If the choke screw housing feature is to be integrated within the valve body then the body tooling modification cost, and the scrapped choke screw housing tooling cost, must be considered.

(3) The assembly of the O-ring seals to the spool piece presents the problem of expanding the O-rings over the part and then allowing them to contract into the o-ring groove.  The task can be simplified by re-designing the joints between the spool piece sleeves.  Unfortunately, the current design of joints has been carefully chosen to avoid the possibility of an O-ring passing over a joint between two sleeves.  It would be very difficult to achieve this same performance, so that the valve would operate for more than 3 000 000 cycles without a loss in performance.

(4) The spool piece is surrounded by sleeves having a multitude of holes in them.  It would be logical to eliminate the sleeves and to direct the flow of air from one port directly to another port.  However, these sleeves are required to provide an even flow path around the spool piece to maintain the required flow rate.  Larger ports could be moulded into the valve body, but this could cause the  O-rings to be damaged as they passed over the ports.

(5) The piston is guided by the piston sleeve.  This piston guide design feature could be integrated within the valve body.  The inside diameter of the piston guide must be such that the air pressure required to operate the valve is no greater than that already required.  Additionally, it must still be possible to insert internal parts to the valve body.  If the piston sleeve is integrated into one half of the body only, for spring return valves, then all of the parts associated with the spool piece can be inserted from one end of the of the valve.  This option would, of course, require there to be two valve bodies for the product range.

(6) The thrust from a new single spring must be such that it can overcome the action of the fluid pressure and the friction between the spool piece O-rings and the sleeves.

(7) The sleeves are moulded as separate components because, as an integral part, it would be difficult to get the correct distribution of plastic in the mould.  If the sleeve must be split for this reason then it would be advantageous to situate spool piece O-ring seals between the sleeves.

(8) It must be impossible to inadvertently unscrew the choke screw out of the body.  If the valve was to be re-designed so that the choke screw could be inserted into the body after assembly of the cover, or into a body with an integral top cover, difficulties may arise.  If the choke screw can be inserted after the cover, or cover feature, then it could also be removed by screwing.  The addition of a retaining part would be counter-productive and, therefore, a stamping operation would be more efficient.  The tops of the choke screw holes would be deformed after insertion of the choke screw, thus retaining it.

(9) During manual assembly of the spool piece sub-assembly to the valve body, special tools are required to assist the operator.  The outside diameter of the sealing ring is much larger than the inside diameter of the spool piece sleeve.  A special tool is required to contract the rings before insertion into the valve body.  The operation is so complex that it may not be efficient to carry it out by a robot, in its current state of design.

(10) The operation of inserting the spool piece sub-assembly into the valve body is so complex that it is not feasible for it to be done by the robot.

(11) The piston and lip seal can only be inserted into the sleeve in one direction.  This is because the lip of the seal has a larger diameter than the inside of the sleeve.  The piston could be inserted in both directions if the seal was an O-ring.

(12) If one of the end pieces were to be integrally moulded with the body then all parts could only be inserted into the body from one direction.  The sealing of the indicator would create special problems.  If the seal is inserted by the robot then it cannot be sufficiently located.  Otherwise, the robot would not be able to assemble the seal.  During movement of the indicator, the seal may be removed from its housing.

(13) Integration of the top cover would make it impossible to change the routing of the signal air because the gasket would no longer be present.

(14) The current pneumatic valve design currently has two springs to generate the required thrust.

(15) The pin is required for the stability of the long spring, during operation.

(16) For aesthetics, the top cover and end covers must be of aluminium, to give the impression of robustness to the product.

(17) The dimensions of the inlet / outlet ports must be kept the same for compatibility with complimentary and substituted products.

Case Study : Robot Assembly of Pneumatic Valves (3/3)

MANUAL ASSEMBLY WORKSHEET

1 = Part identification number
2 = Number of times that the operation is carried out consecutively
3 = Two digit manual handling code
4 = Manual handling time per part
5 = Two-digit manual insertion code
6 = Manual insertion time per part
7 = Operation time in seconds (2) x [(4)+(6)]
8 = Operation cost, centimes 0.4 x (7)
9 = Figures for the estimation of the theoretical minimum number of parts

_1    2    _3    ___4    _5    ___6    _____7    ___8    9
23    1    30    01.95    00    01.5    003.45    01.38    1    PICK UP TOP COVER
                                   99    12.0    012.00    03.00        CLEAN THE TOP COVER
30    1    88    06.35    33    05.0    011.35    04.54        PEEL OFF LABEL
06    2    11    01.80                       003.60    01.44        PICK CHOKE S. HOUSING
05    2    03    01.69    30    02.0    007.38    02.95        PICK UP O-RING
04    2    10    01.50    00    01.5    006.00    02.40        PICK UP CHOKE SCREW
                                                                                      AUTO. SCREW CHOKE
07    2    03    01.69    30    02.0    007.38    02.95        PICK UP O-RING
01    1    30    01.95    00    01.5    003.45    01.38    1    PICK UP VALVE BODY
29    2    33    02.51    40    04.5    014.02    05.60        PICK UP BLANKING PC.
01    1    98    09.00                       009.00    03.60        TURN BODY UP/DOWN
        2    30    01.95    31    05.0    013.90    05.56        PICK UP CH. SCREW S/A
02    1    38    03.34    43    07.5    010.84    04.34        INSERT GASKET
        1                         02    02.5    002.50    01.00        INSERT COVER TO BODY
19    1    10    01.50                       001.50    00.60        PICK UP PISTON
18    1    10    01.50    30    02.0    003.50    01.40        INS. LIP SEAL TO PISTON
                                                                                      PICK UP O-RING TOOL
16    1    03    01.69    00    01.5    003.19    01.28        INSERT O-RING TO TOOL
17    1    00    01.13    30    02.0    003.13    01.25        INS. SPOOL TO O-RING
                                                                                      PICK UP MIDDLE O-RING
16    2    03    01.69    00    01.50    006.38    02.55        INS. O-RING TO TOOL
17    2    00    01.13    30    02.00    006.26    02.50        INS. O-RING TO SPOOL
15    3    03    01.69    44    08.50    030.57    12.23        SEAL RING TO SPOOL
24    2    10    01.50                         003.00    01.20        PICK UP INDICATOR
20    2    03    01.69    30    02.00    007.38    02.95        INSERT O-RING TO IND.
22    2    30    01.95    30    02.00    007.90    03.16        INSERT ENDPIECE
09    2    10    01.50                         003.00    01.20        PICK UP PISTON SLEEVE
08    4    03    01.69    30    02.00    014.76    05.90        INS. 0-RING TO SLEEVE
10    2    10    01.50    30    02.00    007.00    02.80        INS. SEAL TO SLEEVE
12    2    10    01.50    30    02.00    007.00    02.80        INS. SEAL TO PISTON
                                                                                        PICK UP O-RING TOOL
11    2    03    01.64    30    02.00    007.38    02.95        INS. O-RING TO SLEEVE
        1    10    01.50    00    01.50    003.00    01.20        SLEEVES TO FIXTURE
14    1    00    01.13                         001.13    00.45        PICK UP HALF SLEEVES
13    1    03    01.69    30    02.00    003.69                   INS. 0-RING TO HALF-SLV
                                   00    01.50    001.50                    INSERT HALF PISTON
        1    00    01.13                         001.13                    PICK UP VALVE BODY
                                                                                      INS. TOOL TO VLV. BODY
                                    30    02.00    002.00                  INSERT VALVE BODY
                                                                                      PICK UP O-RING TOOL
13    1    03    01.69    30    02.00    003.69                   INS. O-RING TO TOOL
        1                         12    05.00    005.00                   INS. O-RING TO VALVE
        1    10    01.50    30    02.00    003.50                   INS. SLEEVE TO BODY
        1    10    01.50    30    02.00    003.50                   INS. PISTON TO SLEEVE
        2    00    01.13                         002.26                   PICK UP END PIECE
21    2    23    02.36    43    07.50    019.72                  GASKET TO ENDPIECE
        2                         02    02.50    005.00                  ENDPIECE TO BODY
25    8    11    01.80    00    01.50    026.40                  INS. COVER SCREW
        8                         92    05.00    040.00    16.00     FASTEN COV. SCREWS
        1    98    09.00                         009.00    03.60     TURN VALVE BODY
        1    99    12.00                         012.00                  LUBRICATE VLV. BODY
                                                                                     PICK UP SPOOL TOOL
        1    00    01.13    31    05.00    006.13            PICK UP SPOOL PIECE
                                                                               PUT TOOL DOWN
26    1    00    01.13    02    02.50    003.63            INS. SPRING TO SPOOL
27    1    00    01.13    02    02.50    003.63            INS. SPRING TO SPOOL
28    1    10    01.50    02    02.50    004.00            INS. PIN TO SPRING
        1    00    01.13    00    01.50    002.63            INS. VALVE TO FIXTURE
02    1    23    02.36    43    07.50    009.86            INS. GASKET TO BODY
                                                        389.22

ASSEMBLY SEQUENCE FOR THE ROBOT ASSEMBLY OF THE PNEUMATIC VALVE, WITH MANUAL ASSISTANCE

Note - Operations marked with an asterisk (*) are carried out manually.

 (1) Pick up the cover (23) from the pallet and insert into fixture (32).
 (2) Automatically clean the cover.
 (3) Automatically feed and insert the label (30) to top cover.
*(4) Pick up the choke screw housing (6).
*(5) Pick up the O-ring seal (5) and insert onto choke screw housing.
*(6) Pick up the choke screw (4) and insert into the choke screw housing.
*(7) Fasten choke screw in choke screw housing by friction screwdriver.
*(8) Pick up O-ring seal (7) and insert into choke screw housing.
*(9) Pick up valve body (1).
*(10) Pick up gasket and assemble to valve body.
 (11) Pick up valve body and insert into fixture (33).
 (12) Automatically feed and insert the blanking piece (29) into the valve body.
 (13) Rotate the body in the fixture by 180 degrees.
 (14) Pick up the choke screw sub-assembly and insert into the valve body.
 (15) Pick up the top cover and insert into the valve body.
 (16) Automatically feed the piston (19) and insert into fixture (34).
 (17) Automatically feed the lip-seal and insert into the piston.
*(18) Pick up O-ring tool.
*(19) Pick up O-ring (16) and insert onto O-ring tool.
*(20) Pick up spool piece (17) and insert into tool.
*(21) Insert O-ring into spool piece.
*(22) Pick up O-ring tool.
*(23) Pick up O-ring and insert into O-ring tool.
*(24) Pick up spool piece and insert into tool.
*(25) Insert O-ring into spool piece.
*(26) Pick up O-ring tool.
*(27) Pick up O-ring and insert into tool.
*(28) Insert O-ring into spool piece.
*(29) Pick up sealing ring (15) and insert onto spool piece.
*(30) Pick up end piece.
*(31) Pick up end piece gasket (21) and insert into end piece.
*(32) Insert end piece sub-assembly into magazine.
 (33) Pick up end piece and insert into fixture (35).
 (34) Automatically feed and pick up end piece O-ring (20) and insert end piece.
 (35) Automatically feed and pick up indicator (24) and insert into end piece.
 (36) Automatically feed the piston sleeve (9) and insert into fixture (36).
 (37) Automatically feed the O-ring (8) and insert into piston sleeve.
 (38) Automatically feed the lip-seal (10) and insert into piston sleeve.
 (39) Automatically feed the sleeve (12) and insert into piston sleeve.
 (40) Automatically feed O-ring (11) and insert into sleeve.
 (41) Pick up piston sleeve and sleeve and insert into fixture (37).
 (42) Automatically feed middle sleeve O-ring (13).
 (43) Automatically feed middle sleeve (12) and insert into O-ring.
 (44) Insert middle sleeve and O-ring into sleeve.
 (45) Pick up the body and insert onto sleeves.
*(46) Insert O-ring into valve body.
 (47) Pick up sleeve and piston sleeve and insert into valve body.
 (48) Pick up piston and insert into piston sleeve.
 (49) Pick up end piece and insert into valve body.
 (50) Automatically feed end piece screw and insert into valve body.
 (51) Fasten end piece screw into valve body.
 (52) Rotate the valve body by 180 degrees.
 (53) Lubricate the valve.
 (54) Insert spool piece tool into body.
 (55) Insert spool piece into body.
 (56) Remove spool piece tool.
 (57) Automatically feed and insert long spring (26) into spool piece.
 (58) Automatically feed and insert short spring (27) into spool piece.
 (59) Automatically feed and insert pin (28) into spring.
 (60) Repeat operations 49 to 51.
 (61) Pick up the valve and place onto the test station.
 (62) Pick up the gasket (3) and insert into the valve body.

Robot Parts (1/4)

I originally published this article under the title, “The Presentation of Parts for Robot Assembly” in the book “Advances in Manufacturing Technology”, Kogan Page, London, ISBN 1.85091.3951 ...

The presentation of parts for robot assembly involves the selection of the correct parts handling devices and it influences the robot degrees of freedom required. The design of appropriate feeders is discussed, with an emphasis on their flexibility.  A classification system is described that allows parts to be categorised by their design features and physical properties. The performance of an automatic parts feeder is shown to depend upon the design of the part that is being handled. A selection procedure is described that enables the correct handling device and robot configuration to be chosen for a particular application. An expert system is shown to be the best method of acquiring design information about the handle-ability of a part.  A software package that simplifies the selection of parts feeders and robot configurations is described. The importance of knowledge transfer between industrialists and researchers, in defining relevant handling devices, is discussed. The development of an enhanced CAD system is the subject of a further publication.

INTRODUCTION

The presentation of parts to a robot presents some of the most difficult problems in robot assembly. Single cell robot assembly systems may assemble a complete product consisting of several parts. These parts have to be presented to the robot at the correct rate and in a known orientation, or a limited number of known orientations. The rate of supply of parts to the robot cell is seldom a problem because cycle times are usually long. The orientation of the part, at the exit of the parts feeding device, is critical because this influences many other factors.  The orientation of a particular design of part at the feeder exit can be predicted using knowledge of handling device design. Parts are classified according to size, geometry, etc. so that feeding device performance can be qualified. Using a standard parts coding system, feeder performance can be matched with that required for a particular design of part. The orientation of the part, at the exit of the automatic feeder, can be predicted and the need for extra robot degrees of freedom can be determined. The presentation of parts for robot assembly is a complex problem and it’s best carried out using a software application.

PARTS PRESENTATION TECHNIQUES

A multitude of automatic feeders are available to handle a wide variety of parts. However, only a small proportion of these automatic feeders are economically viable for robot assembly.  For robot assembly, an automatic parts feeder must have a high general-purpose content and a low special-purpose content, so that the flexibility of the robot is not compromised by the inflexibility of its feeders. The vibratory linear feeder has a low cost special-purpose feed track that is mounted on a general-purpose drive unit and frame.  The device is very flexible because changeover is effected by removing the current feed track and replacing it with a feed track for the next part. The vibratory bowl feeder consists of medium cost special-purpose tooling that is mounted around the periphery of a general-purpose bowl. The feeder is generally inflexible and the time associated with part changeover makes it unsuitable for many applications with small batch sizes. The horizontal pallet transfer system has low cost special-purpose pallets that move into, and out of, the work zone by a general-purpose transfer system. Flexibility is achieved by using different pallet configurations or by simply changing the pallet contents. The 'Hitachi' type feeder works on a similar principle to the vibratory bowl feeder, with the special-purpose tooling being replaced by a vision system. Within certain geometrical and size limitations, this device is highly flexible; using a vision system to identify part orientations. The programmable belt feeder uses special-purpose pushers and gates, activated by a vision system or sensors, mounted above a general-purpose belt.  Product changeover is achieved by using a different vision system computer program or by replacing the pushers and gates.

Robot Parts (2/4)

PARTS CLASSIFICATION FOR FEEDING

It’s important to be able to classify or describe the features of a part so that particular part shapes can be identified.  Firstly, a part can be classified according to it's basic shape, i.e. rotational or non-rotational. Each rotational or non-rotational part has a certain aspect ratio that allows it to be classified as being a disc, short cylinder, long cylinder, flat, long or cubic. Secondly, the amount of symmetry that a part possesses can be quantified. The amount of symmetry is determined by defining how often an orientation is repeated when the part is rotated through three mutually perpendicular axes. Thirdly, the amount of symmetry that a part possesses can be identified. The asymmetrical feature or features are those that cause the part not to have symmetry about an axis or axes.  Fourthly, the bulk properties of a part can be identified to estimate the loss in performance of those feeders which deliver parts from bulk random orientation.  Properties such as overlapping, tangling, nesting or stickiness reduce the feed rate and may even prevent feeding, depending upon the magnitude of the adverse property.  Lastly, the physical properties of a part can preclude it from being handled by certain automatic feeders.  Other properties, such as abrasiveness or a delicate surface finish, may cause problems with different feeder designs.

PERFORMANCE OF FEEDING DEVICES

Each robot assembly handling device has its own performance characteristics. A given device is able to handle a limited number of parts within a certain size range and geometry class.  The orientation efficiency of a feeder, for parts with no adverse physical properties, is unimportant for robot assembly because the relatively long cycle time means that the demand rate for parts is low. The orientation efficiency for automatic feeders which sort out parts with adverse physical properties from bulk random orientation can be extremely low or zero if the adverse physical property is severe. Parts with severe adverse physical properties cannot be sorted from bulk random orientation and other methods of handling must be chosen. A typical solution to this problem is to present the part on a horizontal pallet transfer system. These handling devices are loaded manually or, preferably at the point of manufacture, using pick and place devices.

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ROBOT ASSEMBLY HANDLING CHARTS

The required attitude of a part, on insertion, influences the choice of handling device and it also affects the number of robot degrees of freedom required.  A particular feeding device, if it can handle the part under consideration, may be able to present a part in only one unique orientation or it may be able to present the part in a number of unique orientations. The orientation(s) of the part at the feeder exit are determined by considering the design of orientation tooling that is required.  For vision system controlled feeders, knowledge is required of whether or not the part's orientation can be deduced by the vision system. If the attitude of the part at the feeder exit is the same as that required for insertion then a minimum number of degrees of freedom are required from the robot arm.  If the attitude of the part at the feeder exit is different from that required for insertion then extra degrees of freedom are required. Parts which need to be re-orientated from the horizontal to vertical position require an extra roll or pitch axis and parts which are required to be turned end-to-end need an extra yaw axis. Additionally, certain parts may require that final orientation from the feeder is accomplished using a robot with limited sensory capability to define the orientation.  This is applicable to feeders which present the part in a limited number of known orientations. This knowledge can be collated to form a database from which it is possible to predict handling and dexterity requirements for the robot assembly system.  Various organisations have created database software applications for this design process.

ROBOT ASSEMBLY HANDLING EXPERT SYSTEM

It must be possible to describe a part being analysed so that the most appropriate feeding device can be selected.  A standard parts coding system is used to describe a part, as mentioned previously.  The sequence of questions which are asked to describe the part is very important. The response to certain questions may create a need for further questions to fully describe the part.  Alternatively, no further questions may be required. Additionally, a particular response to a question may dictate that only one handling device is appropriate, even before the part has been fully classified. Anybody using the 'selection of parts presentation device technique doesn’t want to be asked a lot of irrelevant questions and so a decision tree has to be developed to ask the minimum number of questions. Statements are presented in a structured format and these statements can be either true or untrue for a particular part. Branching forward only takes place when a particular statement is true, otherwise alternative questions are presented until a correct statement is chosen.  Questions are structured so that if a particular set of statements are untrue then the previous true response to a statement must have been incorrect and that statement is once again presented to the user. By this method, the minimum number of questions are needed to classify a part in terms of its handling suitability.

PRODUCT AND SYSTEM DESIGN FOR ROBOT ASSEMBLY SOFTWARE

The presentation of parts for robot assembly is one section of a product and system design for robot assembly computer software application. It operates on eight screen pages. The first screen page allows the user to enter part numbers and descriptions to the application. The last three screen pages contain economic information and they provide the user with calculated information. The middle four screen pages are all concerned with defining the handling, and to some extent the insertion, requirements of the part under consideration. These four screen pages are displayed consecutively for each part and, when all the parts have been defined, the remaining three screen pages are displayed. In the handling section, the first screen page deals with adverse physical properties of the part. The second screen page deals with the geometrical symmetry features of the part. The third screen page deals with the geometrical asymmetry features of the part. The fourth screen page is used to define the insertion direction of the part and to determine if the part is potentially redundant.

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DATA ACQUISITION FOR ROBOT ASSEMBLY SEMINAR

A series of product design for robot assembly seminars were held at a UK university.  They were well attended and the object of these seminars was to encourage industrialists to analyse their products, using a product design for robot assembly computer software application.  The results of these studies were then investigated by university staff so that handling, gripping and insertion requirements for robot assembly could be recommended. These seminars were funded by the ACME directorate as a means of forging closer links between universities and industry. The results of the studies also gave direction to future research work at the university in the field of robot assembly.  Interested parties were given a copy of a computer software application.  Industrial product data was stored in standard ASCII files and this was easily manipulated by staff at the university.  Statistics were produced that indicated trends in parts geometry and the physical properties of parts.  These statistics showed the relative importance of various pieces of assembly automation for a cross-section of industrial products and they gave indicators for future assembly hardware development.

FUTURE WORK

I strongly believe that industry will only demand products and services if there is a genuine need for either. For this reason, my direction is heavily influenced by the continuously changing needs of my clients. Product information (available to clients) and calculated information (demanded by clients) is monitored through my consultancy contracts.  My approach to the presentation of parts for robot assembly is changed to best suit the needs of the majority of my current clients and future clients. The results of the previously mentioned data acquisition seminars influenced the range of handling devices included in the database. It was necessary to include other devices to cater for particular categories of parts, that were thought to exist in smaller numbers than in reality. The findings also affected the handling expert system format. The sequence of questions was altered so that the minimum amount of information was required for the majority of parts.  Later, a consortium of six companies was being formed to interface the product design for robot assembly software with a conventional CAD system.  The object of this work was to allow a product designer,using a CAD system, to have the benefit of product design for assembly running in the background, which only became active when adverse robotic assembly properties were evident.

CONCLUSIONS

The presentation of parts is a topic often neglected by those considering robot assembly and yet it accounts for the majority of the cost for an installation. It is important to be able to describe the features of a part by the use of a parts classification technique that is sufficiently comprehensive to fully describe the part, without involving undue effort, or understanding, from the user.  Parts presentation devices for robot assembly should have a high general-purpose content and a low special-purpose content.  The orientation of the part during insertion affects the choice of handling device and the number of robot degrees of freedom. The classification of a part for handling can be a tedious process and it is important to only define features that are relevant for the selection of handling devices. This is best achieved by using an expert system approach and decision trees.  The complex process of handling device selection can be carried out by computer software applications, thus eliminating the need to manually carry out many iterative calculations. The types of handling devices which best suit the needs of industry can be chosen by asking current and potential industrial users to specify their particular handling requirements.  Most of the information relating to the design features of products, for robot assembly, can be extracted from a CAD system database.

Robot Assembly (1/4)

I originally presented this article, "Design for Robot Assembly", as a guest speaker at the UK's 2nd National Conference on Production Research ...

SUMMARY

The design of products and systems for robot assembly requires a new approach to that used for manual and automatic assembly.  Robot assembly is only effective if the robot’s flexibility is used to best advantage.  Additionally, peripheral devices supporting the robot must also be adaptable to handle a wide variety of products and product parts.  This is achieved by using equipment that is not designed specifically to handle a particular type of part with minor modifications to tooling, or the use of a different software application, the robot assembly system can be quickly adapted to assemble a different product or product style.  By this method, robot assembly can be economically justifiable in many situations where it would otherwise have been precluded.

This article discusses the development of robot assembly systems and describes how product design plays an important role in the design of the equipment.

INTRODUCTION

There are three categories of system used in product assembly.  These are manual assembly, automatic assembly and robot assembly.  Whilst assembly can be classified in this manner, it is not uncommon to find an assembly system consisting of two or all of these groups to form a hybrid system.  Manual assembly systems account for the majority of applications.  Automatic systems are used in situations where the demand is high and there is no, or limited, change in the product styles being assembled.  Robots have yet to make a significant impact in the field of assembly.  It’s difficult to technically justify the use of assembly robots as the operation time of programmable devices is longer than that of dedicated automatic equipment.  The economic justification of assembly by robots is equally difficult due to the characteristically small batch sizes for which these systems are appropriate.  This has the effect of increasing the handling and insertion costs of the product being assembled.

Manual assembly is still used for more than ninety per cent of all assembly tasks.  This is because many products are required in low volumes and with a high degree of variety.  Robot assembly could account for more than fifty per cent of all assembly tasks if it could be made to be economic for much smaller annual production volumes. This could be achieved by assembling more than one family of products on one system.  For this approach to be effective, two major conditions must be met.  The proportion of re-usable, or general-purpose, equipment must be high and the time taken to re-configure the system for the assembly of the next product must be low.

Using existing technology, the industrial applications where robots can readily be used have been filled.  These applications include paint spraying, spot welding and materials handling.  Only a very small proportion of existing robots are used for assembly.

Robot assembly system equipment is either general-purpose or special-purpose.  A robot assembly system should have a high proportion of general-purpose equipment and a low proportion of special-purpose equipment. The cost of a system, with a high proportion of general-purpose equipment, can be amortised by all the products that are being assembled by the robot.  This is important when trying to economically justify the use of robot assembly for products required in low volumes.  Under these conditions, many products or product styles, each with a low annual volume, can be grouped together and assembled on a single robot assembly station to obtain a high system utilisation.

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THE HANDLING OF PARTS FOR ROBOT ASSEMBLY

Handling device selection for a particular part depends on the size and geometry of a part, as well as the rate at which the part is required.  Each handling device has its own performance characteristics.  This means that it is suitable for dealing with a limited range of parts.  Small to medium sized parts, with features that can be seen in silhouette, can be handled by the most common of devices, the vibratory bowl feeder.  Parts with no useful features for orientation purposes, or parts with adverse physical properties, are expensive to feed automatically and require special automatic feeding devices.  These types of parts need to be re-designed to reduce their cost for automatic feeding.  There are many properties of a part that would prevent it from being handled by vibratory bowl feeders, such as flexibility and stickiness.  Parts with adverse properties such as these, and larger parts, must be handled by other feeding devices like magazine systems or pallet transfer systems.

The multi-part linear vibratory linear feeder can deliver different parts to a robot assembly station.  It consists of two straight and parallel vibratory orientating tracks on a common drive unit.  The rejected parts fall into return tracks and are brought to the start of the orientating tracks by a reciprocating elevator.  The tracks can be CNC machined from a database of designs that are identified by an automated handling code for a particular part.  Only the orientating tracks are replaced to changeover this multi-part feeder to handle other part types.  The vibratory drive unit and reciprocating elevator are completely re-usable and the cost of these devices is divided between the different part types.  The orientating track for this multi-part feeder is straight and it is much less expensive to produce than the curved orientating track of a vibratory bowl feeder.  Applications of this feeder are limited to parts which require orientating devices simple enough to be produced in one set-up on a horizontal machining centre.

Gravity feed track magazines are simply short lengths of track which are loaded manually on-line or off-line.  During off-line loading, a full magazine is substituted for a magazine when it becomes empty.  These magazines are specifically designed for the particular type of part type and cannot easily be re-used for different types of parts.  Although most of the gravity feed track magazine is special-purpose, the cost of these devices is relatively low.  They are useful far feeding large parts and they provide an economic alternative to palletisation.  Parts that are to be handled by this type of device must be stackable, for vertical magazines, and not susceptible to damage when the part is slid into position by the pusher.

The pallet transfer system consists of a walking beam transfer device to load a paternoster, an unload paternoster, and pallets.  Full pallets are elevated by the load paternoster and transferred to the robot working zone by the walking beam transfer device.  Parts are picked from the pallet and the pallets are then indexed to present a new pallet of parts to the robot.  Empty pallets are offloaded from the walking beam by an unload paternoster that produces a stack of empty pallets.  Virtually all of the pallet transfer system is general-purpose, with only the vacuum-formed part retainers being specific to a particular component.  Pallets are loaded by standard means.  Filling of the pallets at the point of manufacture is a very economic way of loading parts, although the cycle time of most manufacturing operations makes it difficult to use this method of loading.  Parts are positively held in position on the pallet by ensuring that they are sandwiched between the underside of one pallet and the top of the one beneath.

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THE ORIENTATION OF PARTS FOR ROBOT ASSEMBLY

An assembly robot will never have the dexterity or intelligence of its human counterpart.  A manual worker is able to pick a part, in random orientation, from a storage bin and orientate it - ready for insertion.  The human senses of sight and touch are used for this purpose.  Whilst a manual worker can perform these tasks with comparative ease, a robot requires a large amount of computer processing power and many feedback devices to achieve any form of intelligence and, even then, the cycle time of the operation is so long as to make it uneconomic to use robots for bin picking.  Automatic feeders for robot assembly must, therefore, present parts to the workhead in a known orientation, or in a limited number of known orientations.  The attitude of the part on the feed track or pallet influences the number of robot degrees of freedom required.  More degrees of freedom are required for those parts which are inserted in a different attitude to which they are presented.  In the case of parts which cannot be presented in one known orientation, the final orientation must be carried out by a robot with extended capabilities.  This involves sensing and part manipulation, to achieve the required insertion orientation.

The robot work envelope poses limitations on the automatic feeder types that can be used in robot assembly.  Each part is presented to the workhhead at the end of a track or on a pallet.  The space occupied by the material of these devices must also be considered when determining the maximum number of parts that can be fed to any one robot.  Robots that can only access parts in the vertical axis must have parts arranged so that they can all be seen in plan view at the part presentation points.  Another problem arises when turret mounted grippers are used.  The grippers can occupy a large volume in space and this makes the avoidance of collision very important.  This situation can be investigated before the design of the robot assembly system is finalised.  The complete assembly process can be studied using computer simulation and there are many three-dimensional graphic simulation packages available that can identify if a collision is likely to occur.

THE INSERTION OF PARTS FOR ROBOT ASSEMBLY

An insertion operation is defined as being the action whereby a part is added to a work fixture, another part, or part-built assembly.  This may involve a simple vertical downwards motion where the part is added to the part-built assembly, without being immediately secured.  Alternatively, it may be a complex motion, such as that required for the application of an adhesive to a part.  Each insertion process may require a different type of end effector and each process takes a certain amount of time to be executed.  It’s possible to categorise each type of insertion process to define the type of end effector required and to estimate the time it would take to carry out the operation.

The end effectors may be accessed by the robot arm in many ways.  The design of end effector, and the method of mounting it onto the arm, influences both the cycle time of the process and the cost associated with the insertion of a particular part.  The simplest, yet most expensive and time consuming, method of accessing an end effector, is to use an
individual gripper, or tool, for each part or insertion process.  The grippers and screwdrivers are stored in a rack within the work envelope of the robot arm.  The relevant tool is picked from the rack, used for the insertion process, and then returned.  The action of picking up the tool, and returning it, can often take longer than the insertion process itself.

Another method of inserting many different designs of parts is to use a multi-functional gripper.  Only one gripper with a multitude of faces is used, for the internal or external gripping of parts.  The time involved with gripper changing is eliminated, but the design of the gripper is complex and other tools cannot be mounted onto the same unit.  Problems may also occur because only one set of jaws is being used for the insertion of many parts.  The gripper designer has to ensure that the gripping force is sufficient to hold the part and yet not too excessive as to cause damage to the part.  The varying force requirements can be met by additional gripper sensing.  This, of course, increases the cost of this design of end effector.

The most efficient method of accessing a multitude of end effectors is to mount them onto an indexing turret.  Between eight and twelve tools can be housed on one unit, depending on their size.  Grippers, screwdrivers and othertools are mounted in a circle.  This may be about a vertical, horizontal or inclined axis.  The use of universal mounting plates, between the turret and the end effectors, allows interchange-ability of grippers and tools for product changeover.  The time lost, due to gripper changing, is minimised because indexing of the turret occurs between movements to, and from, the parts feeders.

Most products, or sub-assemblies, have many possible sequences of assembly and it is important to recognise the most appropriate sequence, particularly in robot assembly.  In all forms of line assembly, where moving work carriers are employed, it is good practice to secure parts as soon as possible because subsequent work carrier movements may cause a part to be displaced. This suggests certain precedences.  If no movement of the part-built product occurs during assembly then the securing of parts is not important and a sequence of assembly can be chosen which involves a minimum number of gripper changes.

Consideration also has to be given to the appropriate action needed when a malfunction occurs.  The decision to scrap, rectify or dismantle depends on the; value of the part-built assembly, frequency of the malfunction, labour cost and sequence of assembly.  In single station robot assembly, an overriding consideration is the cost of gripper changing.  The
optimal sequencing, linked with appropriate product design, can significantly reduce this cost.  Computer software applications are available which, given the precedence constraints, identify the optimal sequence to minimise gripper changes.  The cost of error recovery is important.  The alternative actions need to be examined at each stage in the assembly build and the cost of these actions should be determined for all possible sequences.  This activity is influenced by the chosen criteria of; minimum cost, maximum production or maximum profit.

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PRODUCT DESIGN FOR ROBOT ASSEMBLY

Three factors determine how easy it is to use an assembly robot for a product.  Each product part should be examined with respect to these three important qualities. In order of priority, they are the; necessity of the part to be separate from those which have already been assembled; ease with which the part can be handled, and the ease with which the part can be inserted.  By considering these factors in turn, the most economical design of product can be chosen for robot assembly.  A measure of the assemble-ability of the product is the 'design efficiency', and this is related to the above factors.

A part is considered to be necessarily separate from those previously assembled if one of four conditions apply to the part.  Otherwise, it can be eliminated.  Firstly, if the part or sub-assembly moves relative to its mating part during the normal function of the final assembly then it must be a separate part.  Secondly, if the part or sub-assembly must be of a different material than its mating part (eg. for insulation, vibration damping) then it must be a separate part.  Thirdly, if disassembly of the part or sub-assembly must be allowed for (e.g. servicing requirements, recycling) then it must be a separate part.  Finally, if the part or sub-assembly, when combined with it’s mating part, would prevent the assembly of other separate parts (except where the part's only function is to fasten) then it must be a separate part.

The majority of insertion processes take place along, or about, the vertical axis.  If the action of insertion for a part is not in the vertical axis then the process should be analysed to see if the more complex insertion path is really necessary.  If possible, it should be re-designed to take place in only one axis.  The vertical axis is always the preferred axis because the weight the part acts in this direction and assists, not hinders, the operation.  The robot cost is lower if insertion processes are kept simple.  This is because complex operations need more robot degrees of freedom and each degree of freedom requires an individual pneumatic, hydraulic or DC servo motor which increases the cost of the equipment.  Additionally, the potential profitability of the equipment is reduced because the cycle time of the operation will also be increased.

CONCLUSIONS

The use ofassembly robots will increase in the future if the ancillary equipment, i.e. end effectors and parts feeders, are as flexible as the robot.  The feeding devices should present the parts in a known orientation so that the dexterity required from the robot is low.  The cycle time of the operation would be lowered and, consequently, the assembly rate increased.  The flexibility of the feeders is ensured by using devices with a low special-purpose content.  An indexing turret, used for gripper mounting, minimizes the time lost due to gripper changing.  For any form of gripper mounting, the cycle time can be minimised by using a sequence of assembly which needs the least number of gripper changes.  Operator involvement can be minimised by developing strategies which allow the robot to recover from error situations, without the assistance of manual labour.  The cost of robot assembly can be minimised by designing the product for robot assembly.  This involves using the minimum number of parts and ensuring that the parts can be easily handled and inserted.

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I originally presented this article, "The Design of Hybrid Flexible Assembly Systems", as a guest speaker at the 6th International Conference on Assembly Automation ...

There is a requirement for a special kind of system to assemble products required in modest volumes with a degree of variety.  A system which is as cost effective and efficient as hard automation, whilst providing the flexibility of manual assembly, is called a flexible assembly system. Within such a system, certain product parts may be required at a different rate to other parts. Some operations may require the flexibility and dexterity of a robot, or even manual labour. The resultant system would be a hybrid of many methods of assembly. This article recommends a technique to be used for the design of such a system, with the aid of a case study.

INTRODUCTION

The factory cost of a product is the addition of the manufacturing cost (e.g. casting, moulding, turning) and the assembly cost (e.g. manual, automatic, robotic).  Industrial engineers continually seek new methods to reduce the factory cost of products. The current trend of exploiting cheap labour in developing nations, through “offshoring” creates a challenge for domestic manufacturers in the developed nations.  Between 40 and 60 percent of the factory cost for many products is associated with the labour content. The majority of this cost is incurred during assembly. There are three reasons for this uneven split between labour costs in manufacturing and assembly.

(i) Manufacturing operations are usually done by, or with the aid of, a machine, i.e. turning, milling, drilling, etc. The manufacturing systems designer does not have the wide choice of the assembly systems designer because some degree of mechanisation must be used. It is then a logical extension to further automate the manufacturing process to reduce labour costs.

(ii) New processes have been developed which eliminate many manufacturing operations. Powder metallurgy is an example of such a process.

(iii) Most products are designed to be assembled manually.  This often means that components are of such a design that they cannot be handled by automatic feeders.  Additionally, many assembly insertion operations are too complex to be automated.

The assembly process is one of the last production processes to be successfully automated by theindustrial engineer. However, as much of the factory cost of a product is incurred during assembly, it is this area where great productivity improvements can be made. The design of the assembly system should be undertaken with due consideration of the design of the manufacturing system and of the design of the product. The design of the assembly system, manufacturing system and product should be considered integrally. These three components, when combined, should create a product having the lowest factory cost at the desired level of quality.  The design of a product and it’s associated production system is an iterative process, whereby product design features dictate the design of the production system and the capabilities of the production system determine the product design. The extent to which these actions can be carried out is only limited by the commitment of a manufacturer to a particular production system and product design.

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A company may be already committed to a certain manufacturing system if there is prvious investment in capital equipment and tooling.  Additionally, the external dimensions, performance or appearance of the product may be unchangeable. If a product is only part of a much larger assembly, the effect of changing a critical dimension may have expensive consequences for the rest of the much larger assembly. The performance of the re-designed product must be as good as, if not better than, the original design. The product may be one where visual appearance plays an important part in it’s acceptability in the market place. All of these factors place limitations on the engineer being able to specify the optimum product design and production system for that design.

It is easier to design the most economic assembly system for a product prior to commercial manufacture. In this case, there won’t be an inherited investment in manufacturing equipment or tooling, and the product design won’t have been finalised. If the product is well established, and has been produced for many years, the assembly systems engineer may be limited to a re-design of the assembly system alone. This is because a re-designed product may require expensive design modifications to the tooling used for the manufacture of the product parts. In these situations, a hybrid assembly system is required to meet the product requirements. A hybrid assembly system uses a mixture of methods during assembly of the product.

THE COMPONENTS OF A HYBRID FLEXIBLE ASSEMBLY SYSTEM

There are six methods of assembly and the simplest form is MANUAL ASSEMBLY.  For high volume production, the operatives usually work on an assembly line. Other forms of manual assembly are a single worker assembling a complete product and groups of workers assembling a portion of the product.

For a more limited product range, a MANUAL ASSISTED method may be used, whereby workers are assisted by mechanical devices, such as automated parts feeders. The feeders present the parts to the worker in an ordered manner and the assembly time is reduced by eliminating the time taken to separate the parts from bulk random orientation. The reduction in assembly time is the basis for the economic justification of these devices.

The third form of assembly uses AUTOMATIC INDEXING assembly machines. These are rotary or in-line systems with a number of workstations.  Automatic feeders supply components to workheads and they assemble the part to the fixture or part-built assembly. The workstations are ‘special-purpose’ and are dedicated to the assembly of only one product. Production volumes need to be high for the economic justification of these machines. Component quality must also be high to avoid excessive downtime caused by components jamming, etc.

The efficiency of an AUTOMATIC FREE-FLOW assembly machine is less dependent upon component quality.  Transfer of work pieces between    workstations is non-synchronous. There are small buffer stocks between each workstation and other workstations may operate whilst one is stopped due to a fault caused by, for example, a defective part.

The AUTOMATIC PROGRAMMABLE assembly machine has a non-synchronous transfer line and programmable workstations to assemble the parts, which are presented to the workheads by automatic feeders or, in the case of difficult components, part magazines may be used. The workheads execute one, or a number of, operation(s). Different computer programs, for each series of assembly processes, give the flexibility to assemble a variety of product styles on one assembly machine.

Robotic assembly is used for the assembly of products with large product variety, required in low volumes.  Assembly operations are carried out by a robot which, itself, transfers the completed product onto the next operation.

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THE DESIGN OF HYBRID FLEXIBLE ASSEMBLY SYSTEMS

The assembly process has two constituent parts and these are; the handling of components and the insertion of components. The design features of a part must be examined to decide if it can be automatically handled automatically or if it must be handled manually or placed in magazines.  Similarly, the insertion process must be analysed to decide what type of workhead is required.

Various organisations have developed procedures that help the designer to estimate how easy it is to handle and orientate components by assigning a handling code to each part. The maximum feed rate and relative cost of the feeding method can then be estimated from this code. The parts which would require expensive automatic feeders or which could not be fed at the required feed rate can be identified.  These parts must then be handled manually or in magazines/pallets.  Additionally, certain parts cannot be handled automatically because they have other bad feeding qualities, e.g. they may be flexible or too light. The previously mentioned estimation systems also help the system designer to forecast the relative cost of the workhead required to insert a part into a part-built assembly. Those operations which require a complex path of insertion, or a large thrust, require more expensive workheads than for simpler operations. A list of parts (with their associated automated handling codes) and a list of operations (with their allocated automatic insertion codes) can be constructed from the preceding information.

If the product parts are listed in order of increasing handling difficulty levels then the most economical method of feeding a part to the workhead can be determined. Parts with low handling difficulty levels are fed by conventional vibratory feeders and, as the difficulty level increases, specially designed feeders/magazines/pallets/manual handling are used. The relationship between the handling difficulty level and the type of feeder to be used depends upon the required return on investment for the equipment.

The insertion operations can also be listed in order of insertion difficulty levels to determine the most economical method of insertion of a part into a part-built assembly. Greater difficulty levels can mean that the equipment is more expensive and, for assembly robots, more degrees of freedom are required for an insertion operation. If the difficulty level is too high then it’s necessary to employ manual workers for some operations.

When an assembly system is designed for a new product, the cost of parts handling and insertion can be reduced through re-design of the product. It’s usually not viable for an existing product to be re-designed, because of the tooling modification cost in the manufacture of the parts. Inevitably, therefore, the most economical method of assembly is limited to the existing product design, without design efficiency improvements.

The assembly handling and insertion codes determine which feeding method and insertion device are most appropriate for each part and operation. The part-built assembly has to be transported to each workstation between operations. This will either be synchronous or non-synchronous motion.  Synchronous machines are generally less expensive than non-synchronous types, but they are limited by how many parts can be assembled on one machine. This is due to downtime and the space available.

It is desirable to construct a product from as many sub-assemblies as possible to achieve a high overall efficiency of the assembly system. These sub-assemblies should be common to all product styles, within the family of products. The variety can then be created in the final assembly of the product. If this approach is adopted then sub-assemblies will be required at a rate which is enough to justify the use of automatic indexing machines having dedicated workheads. The output from these machines can then be sent to the final assembly line via free transfer lines, to create a buffer stock of sub-assemblies. The buffer stock is necessary to minimise the effect of any indexing machine downtime.

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CASE STUDY - THE DESIGN OF A HYBRID FLEXIBLE ASSEMBLY SYSTEM FOR SPEEDOMETERS

The case study describes how a hybrid flexible assembly system was designed for the assembly of a mechanical drag cup speedometer. This type of speedometer is the most widely used today and its design has not changed over the last 50 years. If there is already a heavy investment in capital equipment for the manufacture of the individual parts then it is not economical to re-design the product for automatic assembly.

The input shaft of the speedometer carries a permanent magnet. The flexible drive shaft from the engine drives the input shaft, thus setting up a rotating magnetic field. A metallic cup is situated in this field and is continuously connected to the pointer. As the input shaft rotates, a torque is produced at the spindle, which is proportional to the speed of the input shaft. The spindle is free to rotate and yet is restrained by a delicate hairspring. The spring rate is chosen to be linear over the range of the spindle angular deflection, thus providing a pointer movement that is proportional to the input shaft speed. The hairspring returns the pointer to zero when the vehicle is at rest. A series of gears from the input shaft convert the rotation of the flexible drive shaft to a rotation of the odometer wheels. Gear ratios typically vary from 600:1 to 2000:1.

There are 25 parts used in the assembly of the speedometer and more than 50 product styles can be obtained by a variation in the design of six parts. These are the dial, second worm gear, third worm gear, odometer sub-assembly, hairspring and pointer sub-assembly. The total annual production volume for all the styles is in excess of one million units. An individual style may be required in volumes between 200 and 200,000 per year. Clearly, these volumes require an assembly system which has flexibility to handle such large demand fluctuations.

The speedometer consists of four sub-assemblies and twelve parts. The dial sub-assembly has three parts, the first worm sub-assembly has six parts, the speed cup sub-assembly has two parts and the frame sub-assembly has two parts. Each sub-assembly is a self-contained unit and does not require any holding of the parts for stability between workstations.

Synchronous assembly machines are most economical for the high volume assembly of a small number of parts. Each sub-assembly contains six or less parts, making them most suitable for this method of assembly.

A rotary indexing machine for the FRAME SUB-ASSEMBLY is used for the assembly of two components. There are eight workstations on this machine to allow for non-value adding operations in addition to the direct insertion process. The handling difficulty level of the bearing means that it is presented by a specially designed feeder. It is impregnated with oil and this doesn’t allow the part to be handled by a conventional vibratory feeder. The frame cannot be handled by an automatic feeder because it is large and has no symmetry about any axis. The complex shape of the frame means that it cannot be magazined and it is, therefore, palletised. A robot places the frames onto the machine because they are picked from several hundred pallet locations.

The rotary indexing machine for the SPEED CUP SUB-ASSEMBLY uses a simple pressing operation to secure the speed cup to the spindle. There are four workstations for; the assembly of the spindle to the fixture, the speed cup to the fixture, the pressing of the speed cup onto the spindle and an output station. Both parts are fed by vibratory bowl feeders and inserted by dedicated workheads.

The FIRST WORM SUB-ASSEMBLY consists of six components, all of which are fed by vibratory bowl feeders. The indexing machine uses ten dedicated workstations to complete the sub-assembly. The first worm shaft is burnished before final assembly.  This operation is executed after the rotary indexing machine, on a free-transfer line. Two burnishing stations are used, in parallel, to achieve the cycle time. The free transfer line also provides a buffer stock of completed sub-assemblies before the final assembly line.

The rotary indexing machine for the DIAL SUB-ASSEMBLY assembles three parts. Only the pointer stop can be automatically fed and so the dial and label use special feeding methods. Different designs of dials are used to create product variety. However, only the print face and diameter of the dial are variable and the dial is picked from a magazine, on the reverse face, by a dedicated workhead. The label is applied by a conventional labelling device.

All sub-assembly indexing machines are linked to the final assembly machine by free-transfer lines, for overall system efficiency. This also creates space for auxiliary operations to be carried out on the sub-assemblies before final assembly. The speed cup sub-assembly is dynamically balanced before final assembly, and this is done with the aid of two robots. The programmability of a robot is required for the 'decision making' operations of this process. Feedback from the balancing machine determines whether the sub-assembly has to be balanced more than once or, in the case of it being excessively out of balance, it is rejected.

There are twenty six workstations used for the FINAL ASSEMBLY of the speedometer, making it necessary to use a free-transfer linear machine to allow buffer stocks to be created between each workstation, to maintain high system efficiency. Of the twelve parts used during final assembly; seven parts are handled by conventional vibratory bowl feeders, two parts by multiple vibratory feeders, one part by pallet, one part by manual handling and the remaining part by actual manufacture on the assembly line.

The parts which are fed by vibratory feeders are small components with either useable symmetry or definite asymmetry. These are inserted into the part-built assembly by dedicated workheads.

Hybrid Assembly (5/5)

The two parts to be handled by multiple vibratory feeders are the second worm gear and the third worm gear. These parts are changed to produce the various gear ratios used to create different product styles. The disruption to production, during product changeover, is minimised by using a group of vibratory feeders which deliver one particular second or third worm. The pick up point of the workhead is thus quickly changed to the output of a particular feeder for the assembly of a different style.

The jewel--plate sub-assembly is a large and delicate part which cannot be fed by an automatic feeder.  It can, however, be palletised.  A robot picks up the jewel-plate sub-assembly from the pallet and inserts it into the part-built assembly.  The operation is relatively complex and an operator has been retained at this station to assist the robot when difficulties arise.

The hairspring is a delicate part that can’t be handled by an automatic feeder. The insertion process is also difficult because the end of the spring is welded to a stub on the jewel plate. This part is assembled manually by two workers in parallel, because of these difficulties.

The second worm gear retaining pin is manufactured from wire and it is most cost effective to manufacture this part on the final assembly line by a guillotining operation. The bending of the pin is carried out simultaneously to the part being inserted and secured.

CONCLUSIONS

1) Product re-design for ease of assembly creates worthwhile savings in assembly costs.  However, particularly for large products, these cost savings must be offset against the additional tooling modification costs for the manufacture of re-designed components.

2 ) When assembling a product which has :
a) Many parts
b) Many variants in the product family
c) A large annual production volume
d) Many common sub-assemblies

a hybrid flexible assembly system is required and it will combine manual, automatic and robotic assembly methods.

3) Sub-assemblies, having a fixed content, are always best assembled on dedicated automatic assembly machines.

4 ) Variable content sub-assemblies are most economically assembled using either
a) Assembly robots
b) Flexible free-transfer machines

5) Transfer between sub-assembly production units and final assembly need large buffers to de-couple these two activities and reduce downtime.

Assembly Evolution (1/7)

I originally published this article under the title, “Changes in Assembly Work Environments” in the book “Programmable Assembly”, ISBN 0.903608.65.0.

The role of the modern assembly worker is very different now from that of 3 generations ago. Improvements in parts quality consistency has eliminated the previously required skill of the apprentice trained fitter. A new breed of unskilled assembly workers has been created, through the division of labour, to carry out repetitive and mundane tasks.  However, many companies use assembly automation if it can be economically justified, and after the product has been re-designed for automatic assembly.

The development of modern assembly techniques is discussed, together with future trends in manual and automatic assembly. Emphasis is given to the changing needs of the people directly involved in these assembly operations.

Introduction

There has been a rapid increase in living standards in the developed nations throughout the previous century, mostly due to the application of technology to manufacturing. The mass production of goods has made many items available at economic prices. Homemakers now have a multitude of labour saving devices to reduce the amount of time spent on household chores. This has enabled many homemakers to work in factories which produce these goods. Assembly workers can master a simple assembly task and repeat it for more than 1000 times per day; every day. Working with other people on an assembly line can create a sense of cooperation within a joint effort.

However, there has been criticism of the assembly line technique. It is argued that the repetitive work is boring and tedious and that workers no longer gain satisfaction from doing their job.  Workers never see the finished product and the continual repetition of movements creates boredom. Industrial unrest in high volume manufacturing companies has been associated with the job dissatisfaction of assembly line workers. Manufacturers now realise that the economic benefits of the division of labour have to be judged alongside the sociological and psychological disadvantages.

The use of assembly automation during product manufacture eliminates worker dissatisfaction with repetitive work, since most of the mundane tasks are done by machines. Workers are then used to fill magazines/feeders and to maintain the equipment. The reduced labour content often creates a costreduction in the finished goods. The culmination of this desirable process is an increase in leisure time, through a reduction in the working week.  Emphasis must then be placed on how people are to spend their leisure time. This should be the subject of major reform in our training establishments.

Assembly Evolution (2/7)

Technology

Technology is the systematic knowledge of the industrial arts. Industrial engineers have been applying technology to the workplace for over two centuries. Manufacturing systems analysed by method and time studies have been improved by the division of labour, automation and robotics. Large productivity improvements have been achieved by applying technology to manufacturing processes. From the mechanisation of flour production to the robotic assembly of vehicles, process costs have been reduced. The application of technology to the motor industry has resulted in vast increases in productivity.

Method study is concerned with the dissection of a complex operation into it’s single constituent parts, which are then systematically analysed. The method study engineer synthesises the complete operation using components which optimise factors such as symmetry and the rhythm of movement.

The time study engineer measures the time taken to carry out an operation. The analysis is carried out in a systematic manner and it makes this form of study suitable only for simple and repetitive tasks. Often, time study exposes inefficient operations and these can then be analysed using method study.

It was the use of both method and time studies that led to the wide-scale use of the division of labour and the creation of the assembly line concept. Workers grouped on lines achieve productivity levels many times greater than single operatives making the entire product.

Automation has also produced large productivity increases by replacing men with machines. In highly automated manufacturing plants, the operator controls and supervises the process. The main power olders in future societies will not be capitalists or socialists, but people who possess expert technological skills. In this way, power will be passed to the techno-structure.

Automation

Automation in the manufacturing industries covers a whole range of electrical and mechanical equipment. In the field of automatic assembly, devices are used for automatic feeding and insertion. In addition, work transfer is by conveyor or rotating table. The type of system used for the assembly of a product is dependent upon many factors. The local cost of labour affects the economic justification of using automation to replace that labour. The frequency of design changes and the number of product styles dictate how flexible the equipment needs to be. The market life of the product influences the amortisation period of the capital investment. Finally, the annual product volume determines the required cycle time.

In addition to the above economic considerations, another reason for employing automatic assembly may be one of necessity. In certain areas, where labour is scarce, the use of automatic assembly is imperative. Certain operations may be hazardous or they must take place in dangerous working conditions. For example, the handling of toxic chemicals or working in extreme temperature conditions may exclude the use of manual workers. A further reason may be associated with the scheduling of the assembly operations: better control over production can be achieved with automation and product quality will be more consistent.

Assembly Evolution (3/7)

The assembly operation consists of the two basic activities of handling and insertion.  If a product is to be assembled automatically then thought has to be given to the economics of these activities. The automatic feeding of simple parts is usually carried out using a vibratory bowl feeder. Components in bulk random orientation are placed into the feeder and the parts are presented to the workhead in an ordered manner. Difficult parts may be fed by special feeders, hoppers or by magazines. The insertion process is defined as being the action where one part is assembled to another part, or group of parts. High speed operations, where the same parts are inserted for long periods of time, are normally effected by standard pick-and place units. Difficult operations, involving the assembly of a number of different parts with different operations may require assembly robots.  The flexibility of the robot is created by using computer programs to control the robot arm movements. The difference between a robot and a pick-and-place is that the path of the robot arm is not restricted by mechanical means, whereas pick-and-place units rely upon mechanical stops to determine the path they follow.

Division of labour

The division of labour is the process whereby one complex operation is broken down into a number of simpler tasks. These single tasks are carried out using a series of people, each doing one task. In this manner, a complex task performed by one worker is replaced by a number of workers operating in series. This allows operations to be carried out simultaneously, instead of the single operator having to complete one task before commencing another, different task. Unskilled workers can then be used to carry out these simple operations and they soon become efficient at the particular task.

Assembly systems

An assembly method can be classified into one of six types, and most systems may contain a number of different methods.

The traditional form of assembly is manual and, for high volume production, the workers are arranged on an assembly line. Other forms of manual assembly include a single worker assembling a complete product and groups of workers assembling a portion of the product.

When the range of products is more limited, a manual assisted method can be used, whereby workers are assisted by mechanical devices, such as parts feeders. The feeders present the parts to the assemblyworker in an ordered manner.  The assembly time is reduced by eliminating the time taken to separate the parts from bulk random orientation.

The third form of assembly uses automatic indexing assembly machines.  A rotary or in-line machine has a number of workstations with automatic feeders which supply components to workheads for assembly of the part to the fixture, or part-built assembly. The workstations are 'special-purpose' and are dedicated to the assembly of one product only. Production volumes need to be high for the economic justification of these machines. Component quality must also be high to avoid excessive workstation downtime, caused by jamming, etc.

Assembly Evolution (4/7)

The efficiency of an automatic free-flow assembly machine is less dependent on parts quality. The transfer of work pieces between each workstation is non-synchronous. Small buffer stocks are held between each workstation and the other workstations still operate even if one is stopped because of a fault, e.g. a defective part jammed in the escapement mechanism.

The programmable automatic assembly machine has a non-synchronous transfer line with a series of programmable or robotic workstations to assemble components. Parts are presented to the workheads by automatic feeders or, in the case of difficult components, magazines may be used. The workheads can execute one or a number of operations. Flexibility is acheived by using different programs for each product to be assembled.

The final type of assembly system is robotic assembly and it is used for the assembly of products manufactured in low production volumes. This method can also be used when there is large product variety. Work transfer is not by conveyor, as all the assembly operations are carried out by a single robot.  Transfer of the completed sub-assembly onto the next operation may also be done by the same robot.

The direct labour content in assembly is reduced in the progression from manual assembly to robotic assembly. However, the complexity of the equipment increases as workers are replaced by machines.  Indirect labour also increases for the maintenance and computer control of the equipment.

Economic aspects

The application of technology to manufacturing is used to increase productivity and the selection of a system for the economic assembly of a product depends upon a number of factors. The final selection must take into account the following:

- Market life of product - influences the decision of the company on investing in capital equipment. Products with short market lives are usually assembled manually.

- Variations in demand - Automatic assembly machines are designed to operate with fixed cycle times. Low demand leads to increasing stock levels or the machine has to be stopped. Both of these actions are expensive. Flexibility to assemble different types of products is needed if there are large demand variations. This flexibility can only be provided by manual assembly or programmable machines.

- Parts quality- Automatic assembly machines are intolerant of defective parts and they can cause a station to breakdown. Whilst inter-station buffers will reduce the effect on efficiency, manual assembly is necessary for products that use low quality parts.

- Number of products - to be assembled by a system determines how flexible it needs to be. Different products manufactured in high volumes can be assembled using programmable workheads. Smaller volumes require manual assembly.

- Major design changes - Products subject to frequent design changes need flexible assembly systems, in a similar way to systems used to assemble a variety of products.