Operations Scheduling

Scheduling in a Process-Focused Environment

3MTSU Terminology F Job shop - a functional organization whose departments or work centers are organized around particular types of operations F Order - quantity of a unique part number F Routing - network of processes or work centers through which the order must pass before it is completed F Schedule - allocate resources to be used to complete each job, when, and for what period of time. Often performed in two phases - loading of jobs and sequencing of jobs

4MTSU Possible Objectives of Job-Shop Scheduling F Meet promise or due dates F Minimize lead time or make span F Minimize setup time or cost F Minimize work-in-process inventory F Maximize resource utilization

5MTSU Elements of the Job-Shop Scheduling Problem F Job arrival patterns F Number and variety of machines in the shop F Ratio of workers to machines in the shop F Flow pattern of jobs through the shop F Priority rules or methods for allocating jobs to workers or machines F Schedule evaluation criteria

6MTSU Shop-Floor Control Activities F Assigning a priority to each order F Issuing dispatching lists to each work center –Orders to be produced –Order priorities –Order due date F Keeping the work-in-process inventory updated F Providing input-output control F Measuring the efficiency, utilization and productivity

7MTSU Scheduling and Shop-Floor Decisions MPS MRPCRP Planned order release report Work center loading and overtime plan Routing plans Assignment of orders to work centers Design and processing plans 1. Set priorities of orders 2. Assign resources to orders 3. Shop-floor control-dispatching

8MTSU Some Approaches to Developing Job Shop Schedules F Priority rules F Forward scheduling F Backward scheduling F Finite and infinite scheduling

9MTSU Examples of Sequencing (Priority) Rules F First-come, first served (FCFS) –sequence is based on the order jobs arrive F Shortest processing time (SPT) –job with the shortest processing time among waiting jobs is next F Earliest due date (EDD) –job with the earliest due date among waiting time is next ò more

10MTSU Rules (continued) F Start date –Due date minus normal lead time F Slack time remaining (STR) –due date minus processing time remaining F Slack time per operation F Critical ratio (CR) –job with the least critical ratio (time to due date divided by total remaining production time) among the waiting jobs is next

11MTSU Forward Scheduling F Jobs are assigned to the earliest unassigned time slots in work centers F Excessive in-process inventories usually result because jobs tend to sit and wait for their next work-center assignments

12MTSU Backward Scheduling F Starting with an order’s promised delivery date, schedulers work backward through each stage of production using lead times to determine when the order should pass through each stage F An order is assigned to the latest possible time slot in work centers to allow the promised delivery date to be met

13MTSU Finite and Infinite Loading F Infinite loading approach –jobs are assigned to work centers without considering the capacities of the work centers F Finite loading approach –capacities of work centers are allocated among a list of jobs –no more work is scheduled to a work center during any hour than the capacity of the work center

14MTSU Possible Criteria for Evaluating a Sequence F Average flow time –average amount of time jobs spend in the system F Average number of jobs in the system F Average job lateness –average amount of time that a job’s completion date exceeds its promised delivery date

Shop-Floor Control

16MTSU Gantt Charts F A plan of resource commitments by each work center to orders is plotted on the chart F Actual performance is then plotted on the same chart to identify deviations from the plan F This type of chart assists the manger in coordinating the orderly flow of jobs between work centers

17MTSU Input-Output Control F The amount of work flowing to a work center is compared to the amount flowing out F Identifies such problems as insufficient capacity, excessive capacity, and production difficulties F For example if too much is flowing into the work center compared to its capacity, then excessive WIP inventory precedes the work center

18MTSU Minimizing Total Production Time For a Special Case F Sequencing n jobs through two work centers 1Select the shortest processing time in either work center 2If this time is on the first work center, place that job in the first open slot 3If this time is on the last work center, place that job in the last open slot 4Eliminate the job assigned and go back to Step 1

Synchronous Manufacturing

20MTSU Synchronous Manufacturing and the Theory of Constraints F The Goal of the firm F The hockey-stick phenomenon F Performance measurement F Capacity and flow issues

21MTSU The Hockey Stick Phenomenon F The end-of-period rush! Period 1234 Output ($)

22MTSU Goldratt Proclaims The goal of a firm is to make money.

23MTSU Performance Measurement Financial F Net profit – an absolute measurement in dollars F Return on investment – a relative measure based on investment F Cash flow – a survival measurement

24MTSU Performance Measurement Operational F 1. Throughput – the rate at which money is generated by the system through sales F 2. Inventory – all the money that the system has invested in purchasing things it intends to sell F 3. Operating expenses – all the money that the system spends to turn inventory into throughput

25MTSU Productivity F Does not guarantee profitability –Has throughput increased? – Has inventory decreased? – Have operational expenses decreased? F PRODUCTIVITY IS ALL THE ACTIONS THAT BRING A COMPANY CLOSER TO ITS GOALS

26MTSU Unbalanced Capacity F Earlier, we discussed balancing assembly lines. – The goal was constant cycle time across all stations F Synchronous manufacturing views constant workstation capacity as a bad decision. F Why is this the case?

27MTSU The Statistics of Dependent Events F Rather than balancing capacities, the flow of product through the system should be balanced Process Time (B)Process Time (A)

28MTSU Some Capacity Related Terminology F Capacity – Available time for production F Bottleneck – Capacity is less than demand placed on resource F Nonbottleneck – Capacity is greater than demand placed on resource F Capacity-constrained resource (CCR) – Capacity is close to demand placed on resource

29MTSU What’s Going to Happen? XY Market Case A

30MTSU What’s Going to Happen? YX Market Case B YX Market Case B

31MTSU What’s Going to Happen? XY Assembly Market Case C

32MTSU What’s Going to Happen? XY Market Case D

33MTSU Saving Time Bottleneck Nonbottleneck What are the consequences of saving time at each process?

34MTSU Drum, Buffer, Rope ABCDEF Bottleneck (drum) Inventory buffer (time buffer) Communication (rope) Market

35MTSU Quality Implications F More tolerant than JIT systems – Excess capacity throughout system F Except for the bottleneck – Quality control needed before bottleneck

36MTSU Inventory Cost Measurement Dollar Days F A measurement of the value of inventory and the time it stays within an area Dollar Days = (value of inventory)(number of days within a department) Example

37MTSU Benefits from Dollar Day Measurement F Marketing – discourages holding large amounts of finished goods inventory F Purchasing – discourages placing large purchase orders that on the surface appear to take advantage of quantity discounts F Manufacturing – discourage large work in process and producing earlier than needed

38MTSU Comparing Synchronous Manufacturing to MRP F MRP uses backward scheduling F Synchronous manufacturing uses forward scheduling

39MTSU Comparing Synchronous Manufacturing to JIT F JIT is limited to repetitive manufacturing F JIT requires a stable production level F JIT does not allow very much flexibility in the products produced

40MTSU Comparing Synchronous Manufacturing to JIT F JIT still requires work in process when used with kanban so that there is "something to pull." F Vendors need to be located nearby because the system depends on smaller, more frequent deliveries

41MTSU