Manufacturing Planning and Control MPC 6th Edition Chapter 8a

Advanced Scheduling Detailed scheduling of individual jobs through work centers in a shop has been the focus of numerous researchers. Focusing on some basic concepts and results and relating them to some newer manufacturing approaches can show how to apply results in different operating situations.

Agenda Overview of Scheduling Research Basic Scheduling Research Advanced Procedures

Scheduling Research Framework A schedule is a plan for the sequence of time allotted for and operations necessary to complete an item The schedule has several inputs Sequential constraints Time estimates for each operation Required resources for each operation

Basic Scheduling Research The One-Machine Case The Two-Machine Case Dispatching Approaches Sequencing Rules

The One-Machine Case Problem of how to best schedule a fixed set of jobs through a single machine All jobs are available at the start of the period Setup times are independent of the sequence Objectives Minimize total time–all sequences are equal Minimize average time–process shortest jobs first Minimize average number of jobs in the system–process shortest jobs first

The Two-Machine Case More complex than the one-machine case Must consider job routings Minimum total time (make span) depends upon sequencing Two-machine case can be used as an approximation of the M-machine problem (where M > 2)

Dispatching Approaches Dispatching approaches allow analysis of dynamic problems (where randomness in inter-arrival times and service times are considered) Uses logic rules to guide the prioritizing of jobs at a work center (sequencing rules) Often uses simulation methodology to study realistic problems

Sequencing Rules Sequencing rules provide guidelines to the wide range of possible sequencing options With n jobs and m machines, there are (n!)m possible ways to schedule the system

Sequencing Rules Random–pick any job with equal probability. Used as a benchmark for other rules. First-come/first-served–jobs are processed in the order they arrive Shortest processing time–reduces work-in-process, average completion time, and average lateness Earliest due date–often works well to reduce job lateness Least work remaining–considers all remaining processing time Least setup–minimizes changeover time on the machine and maximizes capacity utilization

Due Date-Setting Procedures Dynamic Due Dates Labor-Limited Systems Group Scheduling and Transfer Batches Advanced Procedures

Due Date-Setting Procedures Establishing order release and due dates is critical in many firms Due dates often must be set at the time of order entry Estimating manufacturing time is a key function Calculating lead times on the basis of total work content is often best

Dynamic Due Dates Maintaining valid due dates as system parameters change is also important Dynamic due dates allow updating of open order due dates May lead to system “nervousness” Can cause shop floor employees to distrust the system Can improve customer service and reduce total inventory

Labor-Limited Systems Useful when labor is the bottleneck resource Three major elements for controlling workflow Determining which job to do next at the work center Determining when a person is available for transfer to another work center Determining the work center to which an available person is to be assigned Research shows the importance of cross-training and labor assignment flexibility Both labor and job dispatching can have a major impact in controlling work flow

Group Scheduling and Transfer Batches Theory of Constraints scheduling uses different batch sizes for bottleneck and non-bottleneck work centers Repetitive lots concept–original order quantities released to the floor can be split into smaller transfer batches Gives work centers the flexibility to start producing an order before the previous work center is completely done with the job Reduces flow times, improves utilization, cuts setup times, and smoothes work flow

Principles It is important to determine the objective(s) of scheduling before selecting an approach. Shortest processing time sequencing rule can provide effective performance and should be considered a standard in designing shop-floor systems. Flexibility is introduced into scheduling through alternative routings, labor assignment adjustments, the use of transfer batches, and overlap scheduling. Flexibility can lead to great improvements. Setting and managing due dates is an important scheduling activity. Due date filtering procedures should be used to reduce shop-floor nervousness.

Quiz – Chapter 8a In the one-machine case, the order of job processing is not an important factor in performance? (True/False) The shortest processing time (SPT) sequencing rule performs well to minimize overall job lateness? (True/False) One means of evaluating the effectiveness of due date-setting is _________ __________ _____________. In TOC, original orders may be divided into smaller quantities called _____________ batches.