Friday, 16 June 2006

Hybrid Assembly (3/5)


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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