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.
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.