Saturday, 17 June 2006

Hybrid Assembly (2/5)

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.


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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