Batch Size Selection for Group Technology

Summary

Of all the potential advantages offered by group technology (G.T.), a reduction in throughput time and a corresponding fall in work in progress are the most frequently quoted. It is commonly held that further savings of investments in stocks and improvement in response from the manufacturing system can be achieved by a decrease in batch quantities, made economic by the setting-time reductions achieved by G.T.

In practice the opinions of major companies indicate that there is no single answer to the question of whether G.T. should lead to a reduction in batch sizes. Mr Ranson, Managing Director of Serck Audco Valves Limited, speaking of his own company states: ‘Piece parts will only be manufactured for an assembly programme for products, and not held in store’¹. F. W. Craven, Director of Herbert Machine Tools, Coventry, making more general comment has said: ‘Batch size reduction is certainly not essential to G.T. application, but can be a further added benefit in certain circumstances’².

Clearly any apparent difference of opinion on this matter should be based upon fundamental operational differences between the companies involved.

The present work is the result of an investigation, into various types of companies operating G.T., in which factors affecting the ability of a given company to reduce its batch sizes are indicated. Initially, from an analysis of cost parameters involved, a general examination is detailed relating various types of industry to an ability to achieve economically justifiable batch quantity reductions. The distinction between a reduction in optimum batch size and a reduction in cost-sensitivity to batch size changes is made. A cost sensitivity factor, S_f, is developed to express the ability of a company to change its batch sizes without financial penalty, and the operational benefits of a reduction in S_f is stressed. The theory is then examined against a background provided by some of the major G.T. implementations in Great Britain. This confirms that reductions in batch size are not in fact a natural sequel to implementing G.T., and proceeds to indicate factors outside the general theory, which in certain cases can greatly modify the arguments for or against a reduction in batch size.