Thursday, 15 June 2006

Hybrid Assembly (4/5)


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.

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