Chapter18GroupTechnologyandCADPlanning

CAD/CAM Principles and Applications by P N Rao, 2nd Ed

1

CAD/CAM Principles and Applications

Ch 18 Group Technology and Computer Aided process Planning


2

Objectives• Understand the need for group technology (GT) as a means of

bringing the benefits of mass production to the relatively smaller production that is required in a majority of the present day manufacturing industries.

• Learn about the coding and classification methods and schemes used in manufacturing.

• Use production flow analysis as a method of applying GT for manufacturing applications.

• Appreciate the need for computer aided process planning (CAPP)

• Understand the different approaches used in CAPP application• Learn in detail about the techniques utilised in developing CAPP

systems


3

18.1 Group Technology

• Group Technology (GT) is a manufacturing philosophy which can be used to group parts based on similarities in design or manufacturing process so as to reduce the overall manufacturing cost.


4

Fig. 18.1 Examples of part families that can be

grouped by geometry or processing methods.


5

Fig. 18.2 Layout of machines with process grouping in a conventional job shop, T - Turning, M - Milling, D - Drilling, CG -

Cylindrical grinding and SG - Surface grinding

T CG

SG

M D

T T

T T T

CG

SG

M

MM

D D

D D D


6

Fig. 18.3 Layout of machines with functional

grouping as per product requirements.

T

T CG

M D D

M D SG

M D

T T M CG


7

Fig. 18.4 Composite part and its variants possible.

(a) (b)


8

Fig. 18.5 Opitz coding system.


9

18.2 Computer Aided Process Planning


10

Fig. 18.6 Architecture of a CAPP system


11

Fig. 18.7 Variant approach to CAPP

GT Code

Process planProcess PlanRetrieve/Edit

Type of the product: ILA750

Drawn By : Prasad AVSRK

Title: XYZ Drawing No:240248

MasterPlans


12

Fig. 18.8 Generative approach to CAPP

Manufacturing Facility Information

Part Descriptive System

Process Plan Logic Process Plan

MachineTool

CuttingTool Jigs Materials


13

Fig. 18.9 Decision table and tree as used in CAPP

Actions

C1

C2

A B C D

Y

Y

Y

YN N

NNConditions

N

N

N

Y

Y

Y

A

B

C

D

Conditions ActionsC1 C2

Decision table Decision tree


14

Fig. 18-10 Essential elements of a retrieval type CAPP

system


15

Fig. 18-11 Essential elements of a generative type

CAPP system

Selectionrules

MachineTool Fixture Cutting

ToolProcessparameters

OperationSelection

USER

Cutter PathGeneration

MachineTooldata

FixtureData

CuttingToolData

MaterialDataDatabases

KnowledgeRules

MachinabilityData

Knowledge base

Geometricrules

Roughplanning

Finishplanning

Processcapability

StandardTime

OperationSequencing

Inference Engine


16

Fig. 18.12 Interaction of various elements in

Computer Aided Process Planning (CAPP)

PDIR PPIR

MRIR

C A P PSystem

Manufacturing system

MRIR: Manufacturing Resources Internal RepresentationPDIR: Part Design Internal RepresentationPPIR: Process Plan Internal Representation


17

Fig. 18.13 An example rotational part

KnurlGroove

TaperFilletArc

Chamfer

Turn

Thread

Face

Blank


18

Fig. 18.14 Steps in pocket identification

procedure

a

c

d

f

g

h

i

e

j

k

l

m

n

o

b

p

Recording the pocket Intermediate part


19

Fig. 18.15 Down face - Turn - Up face pattern forming

a pocket

(a) Pattern (b) Record (c) Modify


20

Fig. 18.16 Determining the number of setups in a

'Chuck Only' component

Chuck onlycomponent

external dia.at one end?

Min. internal diaat the same

Is blank

a long bar?

external dialess than

spindle bore?

Single setup

Two setupsRougn and finishReverseRough and finish

Three setupsRoughReverseRough and finishReverseFinish

Determinethe demarcation line

toleranced featureslying on either side of

the demarcation

any tolerances presenton the features?

Rough and finish

N

Y

N

Y

Maximum

end?

Max.

Are there

Are all

line?

Y

Y

Y

N

N

N

N

Y


21

Fig. 18.17 Pocket splitting based on demarcation line

AB CD: External demarcation lineEF GH: Internal demarcation line

DL sets(AB,EF)(AB,GH)(CD,EF)(CD,GH)

1: External left pockets2: External right pockets3: Internal left pockets4: Internal right pockets

External profileBlank

A

B

C

D

E

F

G

H

1 2

3 4

Internal profile


22

Fig. 18.18 Pocket splitting based on demarcation lineA

B

C

D

Max

imum

exte

rnal

dia

.

(a) If AB is the demarcation line (b) If CD is the demarcation line


23

Fig. 18.19 Operation sequencing constraints for multi-level pocket

sorting

DL

( a ) Setup level

( b ) Operation level

( c ) Pocket level

1st setup before 2nd setup

Turning before grooving(in 1st setup)

Pocket X before pocket Y(in Rough turning)

Machining span infirst setup

Machine span insecond setup

X

Y


24

Summary• GT as a philosophy started in early 1950’s brings the

advantages of mass manufacture to small batch manufacture by combining batches based on certain similarity principle.

• GT employs product layouts to reduce the travel time and waiting time of parts unlike the process layouts.

• GT utilises coding and classification system of parts to be ableto group them into similar groups by means of certain characteristics such as similarity in shape, size or processing method used. There are a number of coding schemes in use such as Opitz, MICLASS, KK-3, etc.

• To organise the processing machines into product layouts, production flow analysis, which identifies the similar operations into a single group of machines to form a cell.

• It is necessary to follow the guidelines to develop efficient manufacturing cells utilising GT.


25

Summary• CAPP is a method to develop efficient process plans directly from the

CAD model of the part by considering all the manufacturing resources unlike the manual process planning methods.

• CAPP utilises two major approaches, viz variant approach using GT for classifying process plans, and generative approach to develop process plans directly from scratch every time.

• Implementing a variant CAPP system is straight forward since it makes use of the existing process plans that are already proven in the form of a catalogue from which the required process plan need to be selected and edited.

• Implementing a generative CAPP system requires far more effort. This involves the codification of the various knowledge related to the manufacturing resources of the enterprise and build the algorithms for identifying the resources and plans based on the component geometry.

• Artificial intelligence techniques have often been found to be useful in generative CAPP system development.

Documents

Chapter18GroupTechnologyandCADPlanning