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
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