Shop 1

1. Tasks of a Shop Control Manager.

2. Do Shop Control Decisions Vary ?

3. Terminology

4. Performance Measures

5. Scheduling Performance Measures

6. Scheduling Work

7. Theory of Constraints

8. Principles of OPT

Shop 2

Tasks of a Shop Control Manager

What stage of decision making would you classify those decisions performed by a shop floor control manager?

What characteristics should a shop floor control manager have, in terms of:• Level of Technical Expertise * Interpersonal

Skills• Level of Control * Knowledge of

workers task

What are the Main Responsibilities of a Shop Floor Control Manager 1. Sequencing of jobs. (Priorities)2. Reporting level of WIP information3. Conveying status of a shop order4. Reporting actual output data for capacity control5. Reporting measurement of efficiency, utilization &

productivity of the workforce and machines.6. Establishing and communicating improvements in

processes or equipment that result in improved metrics.

Can many of these responsibilities be automated?

Shop 3

Do Shop Control Decisions Vary ?

What decisions would a shop floor control manager make for the following process types ?

• Continuous Process• Maintaining status quo * Product & Process design• Common reporting mechanism * Minimizing quality issues• Variations in decision is in how to prevent breakdowns the plant.

• Repetitive Operation • Prioritization based on demand schedule (lumpy demand)• Detailed scheduling of activities * Flexibility in Order Sizes• mixed model assembly * Flexible manufacturing processes• Synchronized scheduling

• Job Shop Operations• Broad level of expertise * Define scheduled completion date• Define sequence of events• Coordinate priority with other on-going jobs

Shop 4

Terminology

Transfer Lot - Batch size of units that are sent on (transferred) to the next workstation.

• Batch size is typically less than the size of the job

Operation Processing Time - estimated time that is required at an operation.

Queue - a waiting line or buffer.

Planned Lead Time - estimated time that it will take to complete an order.

• Including processing time & delays

Makespan – time from beginning the first operation on the first part to completing the last operation on the last part in a batch.

Flow Time - time that the order actually takes to flow through the shop.

• Also referred to as cycle time. (Processing Time + Wait Time)

FIFO - First In - First Out; a method of scheduling

Shop 5

Performance Measures

Work -In-Process (WIP): Should be maintained at a minimum level.• Commitment of raw material & processing time• Indication of long flow times ( too much waiting)• Contributes to shop congestion & confusion

Throughput: number of units completed in a given time period.• Not as relevant in job shop

Utilization: Percent of time a resource (workforce or machine) is operating.• Can lead to incorrect decisions

Bottlenecks are a limiting factor in a performance measure.

Actions to improve Performance.• Reduce setup time * Reduce queue time• Reduce material handling

Processing time, in most cases, represents only a small percentage of the total time.

Shop 6

Loading

Def: the assignment of jobs to processing centers

Infinite Loading: Jobs are assigned to work centers without regard to the capacity of the work center.

• result is queues when work exceeds capacity

Finite Loading: Jobs assigned to work centers taking into account the work center capacity and job processing times.

• Frequent updating required to ensure capacity is maintained.

Vertical Loading: Loading a work center, job by job usually according to some priority criterion, using infinite loading. (localized priority)

Horizontal Loading: Loading each job on all work centers required, before scheduling any lower priority jobs, using finite loading. (Global priority)Forward Scheduling: How long will it take to complete the job?

Backward Scheduling: What is the latest the job can be completed?

Shop 7

Measures of Scheduling Performance

1. Average Flow Time• (Processing time + Wait time) / number of

jobs

2. Average number of jobs in system

• (Flow time ) / Processing times

3. Average lateness

• (Days Late)/ Number of jobs

4. Maximum lateness• MAX (|Due Date (i) – Completion Date (i)|)

Note: Flow time equals processing time plus wait time plus any setup time.

Scheduling Performance Measures

Shop 8

Scheduling Work

Sequencing Rules for Single Machine1. First Come - First Serve

• Least confusing; most fair• Long jobs delay shorter jobs; no consideration for due date.

2. Shortest Processing Time• Minimizes the number of jobs in work; fewer late jobs• Some large jobs can wait forever.

3. Earliest Due Date• Less late jobs * Does not consider processing time

4. Slack Time Remaining• Needs to be reevaluated after every job

5. Critical Ratio- Difference between the current time and the due date divided by the remaining processing time.- (td - t0 ) / tp

Shop 9

Scheduling Example

Processing times ( including setup times) and due dates for six jobs waiting to be processed at a workcenter are given in the following table. Determine the sequence of processing according to each of these rules:a) FCFSb) SPTc) Due Dated) Critical Ratio

Use Average completion time; Average job lateness; & Average number of jobs at the work center as performance measurements.

Processing DueJob Time (Days) Date (Days) A 2 7 B 8 16 C 4 4

D 10 17 E 5 15 F 12 18

Shop 10

JobSequence

Processing Time

Flow Time

DueDate Lateness

Avg # Of JobsIn Work

Avg Lateness

Scheduling Setup

Shop 11

Johnson Rule: Used to minimize the throughput (completion) time for a group of jobs with a fixed sequence and are processed on two machines.

Assumptions:• Job times are known• Job times are independent of sequence• All Jobs have same sequence• Job priorities can not be used

Process:• List the jobs and their times at each work center.• Select the job with the shortest time. If the shortest time is at the first

machine schedule the job first, if the time is on the second machine the

schedule the machine last.• Repeat for remaining jobs

Scheduling - Johnson Rule

Shop 12

Johnson Rule Example

A group of six jobs are to be processed through a two -step operation. The first operation involves degreasing , and the second involves painting. Determine a sequence that will minimize the total completion time for this group of jobs.

Processing TimeJob Workcenter 1 Workcenter 2 A 5 5 B 4 3 C 8 9

D 2 7 E 6 8 F 12 15

WC 1

WC2

Finished Good

Shop 13

Johnson Rule Example

Processing TimeJob Workcenter 1 Workcenter 2

A 5 5

B 4 3

C 8 9

D 2 7

E 6 8

F 12 15

D – A – E – C – F - BJob Sequence;

Shop 14

Personnel Scheduling in Services

Objective: Find the schedule that minimizes the number of five day workers with two consecutive days off, subject to the demand of the daily staffing schedule.

Step 1: Assign the first worker to all the days that require staffing.- Copy all the number of workers required each day into this row.

Step 2: Circle the two consecutive days with the lowest numbers.- Circled days are the days off for this employee- The circled days could be on opposite ends of the table.

Step 3: For worker 2, subtract 1 from each of the days not circled with positive numbers.

Step 4: Repeat Steps 2 & 3 until all workers have been assigned days or there are only zero requirements for each day.

Shop 15

# of Workers needed

M T W R F SA SU

4 3 4 2 3 1 2Matt

Jean

Burt

Hazel

Sean

Scheduling Days Off

Shop 16

# of Workers needed

M T W R F SA SU

4 3 4 2 3 1 2Matt 4 3 4 2 3 1 2Jean 3 2 3 1 2 1 2Burt 2 1 2 0 1 1 2Hazel 1 0 1 0 1 0 1Sean 1 0 0 0 0 0 0

Scheduling Days Off

Shop 17

Scheduling Difficulties center around variability in:

Setup

Processing time

Interruptions

Changes in set of jobs

Aspects of Good Job Scheduling

1. Realistic Job Due dates

2. Focus on bottlenecks

3. Consider lot splitting for large jobs

Scheduling Difficulties