Saturday, 10 June 2006

Assembly Evolution (4/7)

The efficiency of an automatic free-flow assembly machine is less dependent on parts quality. The transfer of work pieces between each workstation is non-synchronous. Small buffer stocks are held between each workstation and the other workstations still operate even if one is stopped because of a fault, e.g. a defective part jammed in the escapement mechanism.

The programmable automatic assembly machine has a non-synchronous transfer line with a series of programmable or robotic workstations to assemble components. Parts are presented to the workheads by automatic feeders or, in the case of difficult components, magazines may be used. The workheads can execute one or a number of operations. Flexibility is acheived by using different programs for each product to be assembled.

The final type of assembly system is robotic assembly and it is used for the assembly of products manufactured in low production volumes. This method can also be used when there is large product variety. Work transfer is not by conveyor, as all the assembly operations are carried out by a single robot.  Transfer of the completed sub-assembly onto the next operation may also be done by the same robot.

The direct labour content in assembly is reduced in the progression from manual assembly to robotic assembly. However, the complexity of the equipment increases as workers are replaced by machines.  Indirect labour also increases for the maintenance and computer control of the equipment.

Economic aspects

The application of technology to manufacturing is used to increase productivity and the selection of a system for the economic assembly of a product depends upon a number of factors. The final selection must take into account the following:

- Market life of product - influences the decision of the company on investing in capital equipment. Products with short market lives are usually assembled manually.

- Variations in demand - Automatic assembly machines are designed to operate with fixed cycle times. Low demand leads to increasing stock levels or the machine has to be stopped. Both of these actions are expensive. Flexibility to assemble different types of products is needed if there are large demand variations. This flexibility can only be provided by manual assembly or programmable machines.

- Parts quality- Automatic assembly machines are intolerant of defective parts and they can cause a station to breakdown. Whilst inter-station buffers will reduce the effect on efficiency, manual assembly is necessary for products that use low quality parts.

- Number of products - to be assembled by a system determines how flexible it needs to be. Different products manufactured in high volumes can be assembled using programmable workheads. Smaller volumes require manual assembly.

- Major design changes - Products subject to frequent design changes need flexible assembly systems, in a similar way to systems used to assemble a variety of products.

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