Lecture by,

J. JEEVAMALAR / EGSPEC

Overview of Group Technology (GT)

Group Technology

Part Family

Parts Classification and Coding

Some of the important Coding systems

Benefits & Problems in Group Technology

MASS PRODUCTION

BATCHPRODUCTION

JOB SHOPPRODUCTIONP

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uct

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Product Variety

Parts in the medium production quantityrange are usually made in batches

Disadvantages of batch production

Downtime for changeovers

High inventory carrying costs

Group Technology is a manufacturing philosophy toincrease production efficiency by grouping a varietyof parts having similarities of Shape, Dimension,and/or Process Route.

GT can be implemented by manual or automatedtechniques

When automated, the term flexible manufacturingsystem is often applied

An approach to manufacturing in which similarparts are identified and grouped together in orderto take advantage of their similarities in design andproduction

Similarities among parts permit them to beclassified into part families

In each part family, processing steps are similar

A group of parts that possess similarities in geometricshape and size, or in the processing steps used in theirmanufacture

Part families are a central feature of group technology

There are always differences among parts in a family

• Grouping the production equipment into machine cells,where each cell specializes in the production of a partfamily, is called cellular manufacturing.

Two parts are identical in shape and size but quitedifferent in manufacturing

(a) 1,000,000 units/yr, tolerance =±0.010 inch, 1015 CR steel, nickel plate

(b) 100 units /yr, tolerance = ±0.001 inch, 18-8 stainless steel

•Ten parts that are different in size and shape, but quitesimilar in terms of manufacturing

•All parts are machined from cylindrical stock byturning; some parts require drilling and/or milling

• There are two major tasks that a company must

undertake when it implements Group Technology.

1. Identifying the part families. If the plant makes 10,000

different parts, reviewing all of the part drawings and

grouping the parts into families is a substantial task

that consumes a significant amount of time.

2. Rearranging production machines into cells. It is time

consuming and costly to plan and accomplish this

rearrangement, and the machines are not producing

during the changeover.

1.Visual Inspection - using best judgment to

group parts into appropriate families, based on theparts or photos of the parts

2.Production Flow Analysis - using information

contained on route sheets to classify parts

3.Parts Classification and Coding - identifying

similarities and differences among parts andrelating them by means of a coding scheme

• The visual inspection method is the least sophisticated and least

expensive method.

• It involves the classification of parts into families by looking at

either the physical parts or their photographs and arranging them

into groups having similar features.

MANUAL VISUAL INSPECTION

Part Family 1

Part Family 2

Systems based on Part Design Attributes

Systems based on part Manufacturing Attributes

Systems based on both Design and Manufacturing

Attributes

Major Dimensions

Basic External Shape

Basic Internal Shape

Length/Diameter Ratio

Part Function

Tolerances

Surface Finish

Major Process

Operation Sequence

Batch Size

Annual Production

Machine Tools

Cutting Tools

Material Type

MANUFACTURING ATTRIBUTE

DESIGN ATTRIBUTE

There are three basic steps in group technologyplanning:

1. Coding

2. Classification

3. Layout.

There are four major issues in theconstruction of a coding system:

• Part (Component) Population

• Code Detail

• Code Structure, and

• (Digital) Representation.

A GT coding is a string of characters capturinginformation about an item.

Used for efficient recording, sorting and retrieval ofrelevant information about objects.

There are three basic code structures used in GT,

1. Hierarchical structure /Mono Code

2. Chain-type structure/poly code/discrete code

3. Mixed-mode structure/decision code/hybrid

• known as a mono-code, in which the interpretation

of each successive symbol depends on the value of

the preceding symbols

(a) Spur gear; (b) HIERARCHICAL CODE FOR THE

SPUR GEAR

A11B2

• known as a poly code, in which the interpretation of

each symbol in the sequence is always the same; it

does not depend on the value of preceding symbols

• which is a hybrid of the two previous codes

Opitz classification

system

DCLASS

MICLASS

KK-3

CODE

Multi-Class

Brisch System

CUTPLAN

Part Analog System

COFORM

RNC system

• Form Code 1 2 3 4 5 for design attributes

• Supplementary Code 6 7 8 9 for manufacturing attributes

• Secondary Code A B C D for production operation type

& sequence

Example 1

Example 2

Metal Institute Classification System

decision-making and classification system

It has 16 digits (0 to 9 and A to F)

To specify the shape is Cylinder, Flat, Block, or Other

It posses a hybrid structure

Facilitates formation of part families

Permits quick retrieval of part design drawings

Reduces design duplication

Promotes design standardization.

Improves cost estimating and cost accounting

Facilitates NC part programming by allowing new

parts to use the same part program as existing parts in

the same family

Computer-aided process planning (CAPP) becomes

feasible

1. Product or Line Layout• m/c’s are arranged in the sequence as required by the product

• Refers to the physical arrangement of production facilitates

2. Process or Functional Layout• keeping similar m/c or operation at one location

3. Mixed or Combination or Group Layout•Combination of product/process layout

Product Design

Tooling and Setups

Material Handling

Production and Inventory Control

Process Planning

Management and Employees

Method for identifying part families and associatedmachine groupings based on production route sheetsrather than part design data

Work parts with identical or similar route sheets areclassified into part families

Advantages of using route sheet data

Parts with different geometries may neverthelessrequire the same or similar processing

Parts with nearly the same geometries maynevertheless require different processing

1. Data Collection – operation sequence and machine routing for each part

2. Sorting of Process Routings – parts with same sequences and routings are arranged into “packs”

3. PFA Chart – each pack is displayed on a PFA chart

Also called a part-machine incidence matrix

4. Cluster Analysis – purpose is to collect packs with similar routings into groups

Each machine group = a machine cell

Before

Clustering

After

Clustering

Its an application of GT in which dissimilar m/c is have beenaggregated into cells, each of which is dedicated to theproduction of a part family.

The procedure of forming cell in known as cell formation

Examples:

1. PFA

2. Rank Order Clustering (ROC)

3. Single Linkage Clustering Algorithm (CLA)

Rank Order Clustering Algorithm is a simple algorithm used to form machine-part groups.

Step 1: Assign binary weight and calculate a decimal weight for each row.

Step 2: Rank the rows in order of decreasing decimal weight values.

Step 3: Repeat steps 1 and 2 for each column.

Step 4: Continue preceding steps until there is no change in the position of each element in the row and the column.

Consider a problem of 5 machines and 6 parts. Try to group them by

using Rank Order Clustering Algorithm.

Part ‘Number’

Mach

ine ID

1 2 3 4 5 6

A 1 1

B 1 1

C 1 1

D 1 1 1

E 1 1 1

Part NumbersDecimal

equivalentRank

Ma

ch

ine

ID

1 2 3 4 5 6

B.

Wt:25 24 23 22 21 20

A 1 1 23+21 = 10 5

B 1 1 24+23 = 24 4

C 1 1 25+22=36 2

D 1 1 124+23+21 =

263

E 1 1 1 25+22+20=37 1

Step 2: Must Reorder!

Part Number

1 2 3 4 5 6

Ma

ch

ine

ID

E 1 1 1

C 1 1

D 1 1 1

B 1 1

A 1 1

Part Number

B.

WT.1 2 3 4 5 6

Ma

ch

ine

ID

E 24 1 1 1

C 23 1 1

D 22 1 1 1

B 21 1 1

A 20 1 1

Decimal

equivalent

24+23 =

24

22+21=

6

22+21+

20=7

24+23=

24

22+20=

524=16

Rank 1 5 4 2 6 3

Step 4: Must Reorder

Part Number D. Eqv Rank

1 4 6 3 2 5

B Wt: 25 24 23 22 21 20

Ma

ch

ine

ID

E 1 1 1 25+24+ 23=56 1

C 1 1 25+24= 48 2

D 1 1 1 22+21+ 20 = 7 3

B 1 1 22+21=6 4

A 1 1 22+20=5 5

Order stays the same: STOP!

Part Number

1 4 6 3 2 5

Ma

ch

ine

ID

E 1 1 1

C 1 1

D 1 1 1

B 1 1

A 1 1

Voids

Part family 1: Part Nos. 1, 4 & 6

Machine Cell 1: E & C

Part family 2: Part Nos. 3, 2 & 5

Machine Cell 1: D, B & A

No. of exceptional Parts: 0

No. of Voids: 3

No. of bottleneck machines: 0

Part Number

Ma

chin

e ID

1 2 3 4 5 6 7

A 1 1 1

B 1 1

C 1 1 1 1

D 1 1 1

E 1 1 1 1

Part Number

1 2 3 4 5 6 7 Equivalent

decimal value

Rank

Machin

e ID

Binary wt. 26 25 24

23 22 21 20

A 1 1 1 41 3

B 1 1 20 5

C 1 1 1 1 105 1

D 1 1 1 82 2

E 1 1 1 1 40 4

Step 1: Assign binary weight and calculate a decimal weight for each row

Part Number

Machin

e ID

1 2 3 4 5 6 7

C 1 1 1 1

D 1 1 1

A 1 1 1

E 1 1 1 1

B 1 1

Step 3: Reorder the matrix according to rank

Part Number

Machin

e ID

Binary wt. 1 2 3 4 5 6 7

C 16 1 1 1 1

D 8 1 1 1

A 4 1 1 1

E 2 1 1 1 1

B 1 1 1

Equ. Decimal

Value24 20 11 22 3 10 20

Rank 1 3 5 2 7 6 4

Step 4: Assign binary weight and calculate a decimal weight for each Column

Part Number

Machin

e ID

1 4 2 7 3 6 5

C 1 1 1 1

D 1 1 1

A 1 1 1

E 1 1 1 1

B 1 1

Step 5: Reorder the matrix according to rank

Part Number

1 4 2 7 3 6 5 Equivalent

decimal value

Rank

Machin

e ID

Binary wt. 26 25 24

23 22 21 20

C 1 1 1 1 120 1

D 1 1 1 70 2

A 1 1 1 56 3

E 1 1 1 1 39 4

B 1 1 5 5

Repeat Step 1&2: Assign binary weight and calculate a decimal weight for each row

Order stays the same:

Part Number

Machin

e ID

Binary wt. 1 4 2 7 3 6 5

C 16 1 1 1 1

D 8 1 1 1

A 4 1 1 1

E 2 1 1 1 1

B 1 1 1

Equ. Decimal

Value24 22 20 20 11 10 3

Rank 1 2 3 4 5 6 7

Repeat Step 4 & 5

Order stays the same: STOP!

Part Number

Machin

e ID

1 4 2 7 3 6 5

C 1 1 1 1

D 1 1 1

A 1 1 1

E 1 1 1 1

B 1 1

Part family 1: Part Nos. 1, 4, 2 & 7

Machine Cell 1: C, D & A

Part family 2: Part Nos. 3, 5, and 5

Machine Cell 2: E & B

Exceptional

parts

No. of exceptional Parts: 3

No. of Voids: 5

No. of bottleneck machines: 2(Machines D & E)

Solutions for overcoming this problem?

• Duplicate machines

• Alternate process plans

• Subcontract these operations

Voids

Part Number

Machin

e ID

1 4 2 7 3 6 5

C 1 1 1 1

D 1 1 1

A 1 1 1

E 1 1 1 1

B 1 1

Part Number

Machin

e ID

1 4 2 7 3 6 5

C 1 1 1 1

D 1

A 1 1 1

E 1

E 1 1 1

B 1 1

D 1 1

No. of exceptional Parts: 0

No. of Voids: 9

No. of bottleneck machines: 0

No. of duplicate machine: 2(Machines D & E

No. of exceptional Parts: 3

No. of Voids: 5

No. of bottleneck machines:

2(Machines D & E)

Part Number

Machin

e ID

1 4 2 7 3 6 5

C 1 1 1 1

D 1 1 1

A 1 1 1

E 1 1 1 1

B 1 1

Part Number

Machin

e ID

1 4 2 7 3 6 5

C 1 1 1 1

A 1 1 1

E 1 1 1 1

B 1 1

D 1 1 1

No. of exceptional Parts: 2

No. of Voids: 3

No. of bottleneck machines: 2(Machines D & E

No. of exceptional Parts: 3

No. of Voids: 5

No. of bottleneck machines:

2(Machines D & E)

Standardization of tooling, fixtures, and setups is encouraged

Material handling is reduced

Parts are moved within a machine cell rather than entire factory

Process planning and production scheduling are simplified

Work-in-process and manufacturing lead time are reduced

Improved worker satisfaction in a GT cell

Higher quality work

Identifying the part families (the biggest problem)

- If the plant makes 10,000 different parts, reviewing all of the part drawings and grouping the parts into families is a substantial task

Rearranging production machines in the plant into the appropriate machine cells

- It takes time to plan and accomplish this rearrangement, and the machines are not producing during the changeover

Standardization of tooling, fixtures, and setups is encouraged

Material handling is reduced

Parts are moved within a machine cell rather than the entire factory

Process planning and production scheduling are simplified

Work-in-process and manufacturing lead time are reduced

Improved worker satisfaction in a GT cell

Higher quality work

Design retrieval systems

Industry survey: For new part designs,

Existing part design could be used - 20%

Existing part design with modifications – 40%

New part design required – 40%

Simplification and standardization of design parameters such as tolerances, chamfers, hole sizes, thread sizes, etc.

Reduces tooling and fastener requirements in manufacturing