ch03 work flow

Work Systems and the Methods, Measurement, and Management of Workby Mikell P. Groover, ISBN 0-13-140650-7.

©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Work Flow and Batch Processing

Sections:1. Sequential Operations and Work Flow2. Batch Processing3. Defects in Sequential Operations and

Batch Processing4. Work Cells

Chapter 3



Some Definitions

Sequential operations – series of separate processing steps that are performed on each work unit

Work flow – physical movement of work units through the sequence of unit operations

Batch processing – processing of work units in finite quantities or amounts



Sequential Operations in Industry

Manufacturing Assembly Construction Mortgage applications Medical services Education Transportation



Work Flow Patterns

Pure sequential – all work units follow the same exact sequence of operations and workstations Work flow is identical for all work units

Mixed sequential – different work units are processed through different operations Different work flows for different types of

work units



Work Flow Patterns

Network diagrams representing (a) pure sequential work flow and (b) mixed sequential work flow



Moves in Sequential Work Flow

In-sequence move – forward transport to operation immediately downstream

Bypassing move – forward transport to an operation beyond the neighboring station

Backflow – transport in a backward direction Repeat operation – operation is repeated at the

same workstation



Four Types of Work Movement



From-To Chart

Indicates any of several possible quantitative relationships among operations in a multi-station work system

Possible variables in a from-to chart: Quantities moving between operations Flow rates of materials Distances between work stations



From-To Chart

To operation j 1 2 3 4 5

1 - 40 15 2 - 30

From operation i 3 10 - 20 4 25 50 5 -



Network Diagram

Network diagram showing same data as in previous From-To Chart



Bottlenecks in Sequential Operations

Bottleneck = slowest operation in the sequence The bottleneck operation limits the production

rate for the entire sequence Terminology:

Blocking – production rate(s) of one or more upstream operations are limited by the rate of a downstream operation

Starving - production rate(s) of one or more downstream operations are limited by the rate of a upstream operation



Batch Processing

Batch processing - processing of work units in finite quantities or amounts Work units can be materials, products,

information, or people Batch processing is common in production,

logistics, and service operations



Examples of Batch Processing

Batch production in manufacturingPassenger air travelCargo transportBook publishingEntertainmentPayroll checksClass action lawsuitsLaundryGrading of student papers



Types of Batch Processing

Sequential batch processing – members of the batch are processed one after the other

Simultaneous batch processing – members of the batch are all processed at the same time



Examples of Batch Processing

SequentialProduction machiningBatch assemblyBook printingPayroll checksGrading of student papers

SimultaneousChemical batch processesHeat treating of multiple partsPassenger air travelCargo transportLaundry



Batch Production

Alternating cycles of setup and production run experienced by a work system engaged in batch production



Why Batch Processing is Important

Work unit differences – different types of work units must be processed separately

Learning curve effect – cycle time per work unit decreases as batch quantity Q increases (apples only to sequential batch production)

Equipment limitations – limits on the quantities that can be processed

Material limitations – the material must be processed as a unit (e.g., processing of integrated circuits on a silicon wafer)



Disadvantages of Batch Processing

Changeovers between batches represent lost productive time Setup changeovers in batch production Airplanes at a terminal unloading and

loading passengers Work-in-process – multiple batches competing

for the same equipment Queues of work units form in front of each

workstation, resulting in large inventories of partially processed units



Economic Order Quantity Model

Inventory level over time in a typical make-to-stock situation




Total annual inventory cost TIC

where Ch = inventory carrying cost, Q = batch quantity, Csu = setup or ordering cost, and Da = annual demand

QDCQC

TIC asuh +2

=




From the total inventory cost equation can be derived the batch size that minimizes the sum of inventory carrying costs and setup costs

h

sua

CCD

EOQQ2

==



Defects in Unit Operations

Input/output relationship for a unit operation in batch processing

Q = Qo(1 – q)

where Q = quantity produced, pc; Qo = original starting quantity, pc; q = fraction defect rate

D = Qoqwhere D = number of defects



Defects in Unit Operations

Processing of Qo starting units to yield Q good products and D defects



Defects in Sequential Operations and Batch Processing

Input/output relationship in a sequence of n unit operations

Qf = Qo(1 – q1)(1 – q2) . . (1 – qn)

where Qf = final quantity at the conclusion of the sequence

Defects Df = Qo – Qf Yield for the sequence Y =

o

f

QQ



Sequential Operations

Compounding effect of fraction defect rate at each unit operation in a sequence of operations



Work Cells

Work cell - a group of workstations dedicated to the processing of a range of work units within a given type

Part family – the range of work units that are processed Members of the part family are similar but

not identical Mixed sequential work flow system Work cells and part families are associated

with group technology



Group Technology

An approach to manufacturing in which similar parts are identified and grouped together to take advantage of their similarities in design and production

Work units are processed individually and continuously, without the need for time-consuming changeovers between part types

Avoids disadvantages of batch processing



Work Cell Layouts

In-line – straight line flow of work units U-shaped – shape of work flow is “U”

Similar to in-line except for shape Better communication among workers

Loop – continuous flow of work units around a loop layout

Rectangular - similar to loop layout



In-Line Work Cell



U-Shaped with Manual Handling



U-Shaped with Mechanized Handling



Work Cell with Loop Layout



Work Cell with Rectangular Layout



Worker Teams

Group of workers who work as a team to achieve common objectives: Meet the production or service schedule Achieve high quality in the goods and

services provided by the cell Make the operation of the cell as efficient as

possible



Success Factors for Worker Teams

Teamwork – the collective skills and efforts of the team members exceed the sum of their individual skills and efforts

Cross-training – workers become trained in more than one job in the cell Allows for job rotations to increase work

variety and job satisfaction Mitigate problems of absences

ch03 work flow

Engineering