Constraint time buffer determination model

Abstract

In today's competitive environment, reducing Work in Process (WIP) to improve cycle time, delivery and product quality has become the key to maintaining profitability. However, due to conservative attitudes and conceptual mistakes, WIP has spread along the entire production pipe line. Although intended to maximize the utilization of resources, WIP can be caused to become over-sized. This maximum utilization of resources syndrome has been proved incorrect by the Theory of Constraint and WIP should be queued only in front of constraint machine, thereby ensuring the constraint machine not starvation. Although this concept has been proved correct and despite many reported examples of successful implementation in many companies, in practice, the WIP level is still determined by the trial and error approach. Therefore, a WIP computation model is deemed necessary. In this paper, a constraint time buffer determination model is proposed. The model first proposes a machine-view's bill of routing representing a structure that serves as a fundamental structure for formulating and computing the maximum time buffer. The machine-view's bill of routing is a tree structure, the constraint machine is the root node and its feeder machines are entry nodes (or subtrees) of the root node. With this tree structure, the behaviour (or stability) of the constraint machine and its feeder machines can then be studied and formulated. By incorporating the MTTR of each feeder machine, a mathematical relationship can then be formulated and the time buffer computed. Furthermore, to validate the proposed constraint time buffer determination model, an example is illustrated and a simulation model is also developed.