A REVIEW OF FLOWSHOP SCHEDULING RESEARCH WITH SETUP TIMES

Flowshop scheduling problems with setup times arise naturally in many practical situations. This paper provides a review of static and deterministic flowshop scheduling research involving machine setup times. The literature is classified into four broad categories, namely sequence independent job setup times, sequence dependent job setup times, sequence independent family setup times, and sequence dependent family setup times. Using the suggested classification scheme, this paper organizes the flowshop scheduling literature involving setup and/or removal times and summarizes the existing research for different flowshop problem types. This review reveals that, while a considerable body of literature on this subject has been created, there still exist several potential areas worthy of further research.     

PRODUCTION SCHEDULING OF CONTINUOUS FLOW LINES: MULTIPLE PRODUCTS WITH SETUP TIMES AND COSTS

The problem of production planning and setup scheduling of multiple products on a single facility is studied in this paper. The facility can only produce one product at a time. A setup is required when the production switches from one type of product to another. Both setup times and setup costs are considered. The objective is to determine the setup schedule and production rate for each product that minimize the average total costs, which include the inventory, backlog, and setup costs. Under the assumption of a constant production rate, we obtain the optimal cyclic rotation schedule for the multiple products system. Besides the decision variables studied in the classical economic lot scheduling problem (ELSP), the production rate is also a decision variable in our model. We prove that our solutions improve the results of the classical ELSP.     

A COMPARISON OF RULES FOR ALLOCATING SETUP-REDUCTION INVESTMENTS IN A CAPACITATED ENVIRONMENT

This paper builds on a recent empirical study of the setup-reduction process that suggests setup-reduction proceeds through three major stages and that each stage is dominated by a particular type of investment function. Specifically, it examines the question of how to best prioritize investments during the stage that emphasizes standardizing setups across a work center. We compare different investment-allocation rules in a multi-item, capacity-constrained, dynamic demand environment under a variety of cost, demand, and investment assumptions. This analysis shows that significant differences in benefits can be achieved depending on the way setup-reduction investments are prioritized.     

THROUGHPUT TIME REDUCTION: TAKING ONE'S MEDICINE

Managing throughput time and its relation to work-in-process (wip) inventory and customer service is the focus of this paper. This research combines theory, simulation results, and the analysis of corporate data in an effort to address the issues associated with how one company (Eli Lilly) managed a reduction in their throughput times and an improvement in their delivery reliability. The results for this company suggest that production control decisions—expediting and de-expediting—can lead to a vicious circle of decisions, which in turn can lead to increased levels of WIP inventory and higher and more unpredictable throughput times.     

SETUP REDUCTION AND MACHINE AVAILABILITY

This paper is an extension of Billington, who used the framework of the economic production quantity (EPQ) to model setup cost reduction. In the present paper, we use the EPQ model as a starting point to investigate the nature of setup costs and the effect of setup time reduction on the increase in available capacity. Reducing setup is vital to a company's success because a lengthy changeover of machinery is expensive: it demands long production runs to justify its cost, and these, in turn, lead to excessive inventory and to a slow response to customer needs. As in Billington, setup reduction is modeled as a function of an annual amortized investment. The paper examines the behavior of the setup time, the inventory cost, the lot size, and the freeing up of machine time in the face of a capacity constraint. A solution algorithm is provided to find setup times that minimize the sum of setup and holding cost, subject to a constraint on machine availability. The analysis sheds light on the true nature of setup cost and on the opportunity cost of not reducing setups. In the constrained optimization, the Lagrangian multiplier gives an estimate of the marginal value of adding one time unit of machine capacity, or, alternatively, of reducing one unit of setup time.     

EVALUATING ORDER RELEASE MECHANISMS IN A JOB SHOP WITH SEQUENCE-DEPENDENT SETUP TIMES

Job-release mechanisms in job-shop control are receiving increased recognition in recent research. However, there is considerable disagreement over the benefits of sophisticated job-release mechanisms, and they remain to be tested in various operating environments. In our study, we extend previous job-releasing research by examining several job-release mechanisms in a different operating environment-a sequence-dependent setup job shop. We tested these release mechanisms, originally developed for a machine-constrained job shop, in conjunction with two groups of sequencing rules-ordinary and setup-oriented sequencing rules. We measured shop performance in terms of total cost, the sum of inventory-holding and late-penalty costs. We developed a simulation model of a nine-machine job shop for our experiment.     

MODELS FOR OPTIMAL LEAD TIME REDUCTION

Most inventory and production planning models in the academic literature treat lead times either as constants or random variables with known distributions outside of management control. However, a number of recent articles in the popular press have argued that reducing lead times is a dominant issue in manufacturing strategy. The benefits of reducing customer lead times that are frequently cited include increased customer demand, improved quality, reduced unit cost, lower carrying cost, shorter forecast horizon, less safety stock inventory, and better market position. Although the costs of reducing lead times in the long term may be relatively insignificant compared with the benefits, in the short term these costs can have a significant impact on the profitability of a firm. This article develops a conceptual framework within which the costs and benefits of lead time reduction can be compared. Mathematical models for optimal lead time reduction are developed within this framework. The solutions to these models provide methods for calculating optimal lead times, which can be applied in practice. Sensitivity analysis of the optimal solutions provides insight into the structure of these solutions.     

Making Employee Involvement Work: Cooperative Goals and Controversy to Reduce Costs

Organizations are involving employees to helpreduce costs to succeed in the intensely competitivemarketplace. However, many researchers have beenskeptical of the value of participation. This study uses Deutsch's theory of cooperation and competitionto examine the critical mediating variable of theinteraction as employees discuss how to reduce costs.Results of structural equation analyses on datacollected from interviews when employees participated inreducing costs indicate that cooperative goalscontributed to constructive discussion among employeesthat resulted in productive work and strongerrelationships, which in turn led employees to be committed toreducing costs. Competitive goals were negativelycorrelated with dynamics and outcomes. These resultssuggest that researchers and managers can agree that employee involvement has potential butemployees need cooperative goals and skills to discussissues open-mindedly and constructively to realize thesebenefits.     

REDUCING FLOW TIME IN AIRCRAFT MANUFACTURING

The assembly of aircraft is a labor-intensive process that exhibits a significant learning-curve effect and that requires long flow times and costly work-in-process inventories. This paper describes the production context, the cost of flow time in this context, and some of the causes for the long flow times. We then develop an argument for a firm to use improvements in labor productivity to reduce flow times. Boeing has implemented the recommendations from this research and has obtained significant benefits from reducing flow times.     

OPTIMAL AND NEAR OPTIMAL CONTROL OF A TWO-PART-TYPE STOCHASTIC MANUFACTURING SYSTEM WITH DYNAMIC SETUPS

This paper deals with a manufacturing system consisting of a single machine subject to random failures and repairs. The machine can produce two types of parts. When the production is switched from one part type to the other, a random setup time is incurred at a constant cost rate. The objective is to track the demand, while keeping the work-in-process as close as possible to zero for both products. The problem is formulated as an optimal stochastic control problem. The optimal policy is obtained numerically by discretizing the continuous time continuous state opti-mality conditions using a Markov chain approximation technique. The discretized optimality conditions are shown to correspond to an infinite horizon, discrete time, discrete state dynamic programming problem. The optimal setup policy is shown to have two different structures depending on the parameters of the system. A heuristic policy is proposed to approximate the optimal setup policy. Simulation results show that the heuristic policy is a very good approximation for sufficiently reliable systems.