Grouping machines for effective workload control

Workload control (WLC) allows the release of new orders to the shop floor as long as workload norms for capacity groups, generally a number of functionally similar machines, are not exceeded. Effective WLC requires a profound decision on the grouping of machines as well as on the norm levels for the respective capacity groups. Also a routeing decision has to be taken in case of several machines allowing to perform the same kind of operations. In practice, some intuitive rules are used to define the capacity groups for WLC and to make a routeing decision; the norm levels are often determined by a trial and error approach. This paper aims at providing a theoretical starting point for these decisions. Queuing theory provides some insights in possible pooling synergies and losses when grouping machines, but only with rather restrictive assumptions. Up to now, little attention has been paid in the literature to machine grouping within WLC. Additionally, the question of how to relate norm levels to the composition of the capacity groups and the appropriate routeing decision rules still remain unanswered. Supported by a simulation study, this paper points out that pooling synergy insights can be translated to situations with controlled workloads. Absolute performance can be strongly improved by appropriately defining capacity groups in combination with suitable routeing decision rules. Besides, the choice of norm levels appears critical for both grouped and non-grouped parallel machines, and affects other control parameters as well.