J Cecil IE 590 NMSU

IE 590 INTEGRATED MANUFACTURING

GROUP TECHNOLOGY

J Cecil IE 590 NMSU

CAM-I Automated Process Planning System

• One of the most well-known systems is the CAM-I Automated Process Planning (CAPP) system

• DO NOT CONFUSE THE CONCEPTUAL TERM ‘CAPP’ (COMPUTER AIDED PROCESS PLANNING) WITH THE system‘CAPP’, WHICH IS FOR THE PROCESS PLANNING SYSTEM BUILT BY CAM-I

• CAM-I stands for Computer Aided Manufacturing - International, which is a non profit industrial research organization

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In CAPP, previously prepared process plans are stored in a databaseWhen a new component needs to be planned,

a process plan for a similar component is retrieved ANDsubsequently modified by a human process planner to satisfy specific requirements

CAM-I Automated Process Planning System

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VARIANT PROCESS PLANNING SYSTEMS (VPPS)

• Variant process planning– uses similarity among parts or components to

retrieve existing process plans• Standard Plan (SP):

– A process plan that can be used by a family of parts – SP is usually stored permanently in the DB– Each SP has a FAMILY NUMBER as its KEY– No Limitation on the level of detail in an SP– AT A MINIMUM, AT LEAST A SEQUENCE OF

OPERATIONS

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VARIANT PROCESS PLANNING SYSTEMS (VPPS)

• When an SP is retrieved, a certain min degree of modification is usually required to use the plan to manufacture a part

• The retrieval method and the logic used in Variant Systems depend on the grouping of parts into families

• Common manufacturing methods can be then identified for each family

– Such common manuf methods can be rep by SPs

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VPPS ......contd

• Mechanism of standard plan retrieval is based on part families

• A family can be rep by a family matrix, which includes all possible members

• VPPS HAVE 2 OPERATIONAL STAGES:

– PREPARATORY STAGE

– PRODUCTION STAGE

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VPPS ......contd

• PREPARATORY STAGE:

• Preparatory work is reqd when a company first starts implementing a VPPS

• During this stage:

– existing parts are coded

– classified

– then grouped into families

• FIRST STEP IS TO CHOOSE AN APPROPRIATE CODING SYSTEM

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PART CODING SYSTEMS AND ISSUES

• CODING SYSTEM must cover the entire spectrum of parts produced in your shop

• it must be UNAMBIGUOS and easy to understand

• Special features on the parts MUST BE CLEARLY IDENTIFIED BY THE CODING SYSTEM

• An Existing Coding system can be adopted and then modifications can be made for the specific manufacturing shop or facility

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PART CODING SYSTEMS AND ISSUES

• CODING REQUIRES DETAILED STUDY OF INVENTORY OF DRAWINGS / MODELS AND PROCESS PLANS

• PERSONNEL INVOLVED IN CODING MUST BE TRAINED

– they must have a precise understanding of the coding system

– test: they must generate identical code for the same part, when they work independently

– Note: inconsistent coding will result in redundant and erroneous data in the DATABASE DB

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PART FAMILIES

• After coding is completed, PART FAMILIES can be formed

• Our interest is in grouping parts which may require similar manufacturing processes or operations

• NOT NECESSARILY SIMILAR IN SHAPE

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PART FAMILIES

– Such a Set of similar parts can be called a PRODUCTION FAMILY

– Since SIMILAR PROCESSES are needed for ALL FAMILY MEMBERS, A MACHINE CELL can be built or used to manufacture this family of parts

– This makes production planning and control far easier.

– Such a cell oriented layout is called a Group Technology layout or CELL LAYOUT.

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GROUP TECHNOLOGY

Please refer to chapter 13 of CAM book

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GROUP TECHNOLOGY (GT)• In 1958, Mitrofanov (Russian engineer) formalized this

concept in his book, The Scientific Principles of Group Technology

• GT can be defined as:

• “ the realization that many problems are similar, and that by grouping similar problems, a single solution can be found to a set of problems thus saving time and effort”

• This definition is broad; however, usually engineers relate GT only to manufacturing or production applications

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PART FAMILIES

• Design families: In part design, many parts may have similar shape (FIGURE 12.1)

– similar parts can be grouped into design families

– new design can be created by modifying existing part design from the same family

– using this concept, composite parts can be developed

– These parts embody all the design features of a design family

– See FIGURE 12.1

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GROUP TECHNOLOGY (GT)

• Production families :

– are families formed because they require similar mfg operations to be produced

– SEE FIGURE 12.3

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Coding and Classification

• Three types: Hierarchical, chain and hybrid codes

• Hybrid is widely used

– Eg: Opitz code

• Other examples are Vuoso-Praha and KK-3 (Japan) coding systems

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HIERARCHICAL OR MONOCODEIn a monocode, each code number is qualified by

the preceding characters

• SEE FIGURE 12.4

• The fourth digit indicates threaded or not threaded for family 322X

• Advantage: large amount of info with few code positions

• Disadvantage: potential complexity of coding system

–all branches in hierarchy must be defined

–hence, difficult to develop

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Chain or PolyCode• Every digit in the code position rep. a distinct bit of

info, regardless of previous digit• In Table 12.1, a chain coding scheme is given• A ‘1’ in the 3rd digit position always means an

axial hole (no matter what numbers are assigned to digits 1 and 2)

• Advantage: Compact and easy to construct / use• Disadvantage: They cannot be as detailed as

hierarchical structures with same number of coding digits

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Hybrid Code

• mixture of chain and hierarchical code structures eg. Opitz code

• Will discuss Opitz code in next section

• Advantages of both can be obtained

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FURTHER READINGS

• Read the Vuoso Praha system (short code), KK-3 system (long code)

• OPITZ CODE is most widely used and will be discussed

• Also review MICLASS and DCLASS Systems

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THE OPITZ CODE• BEST KNOWN AND MOST WIDELY USED

• has 2 sections

– geometric code

– supplementary code

• Geometric code

– rep. Parts of following variety

– Rotational, flat, long, cubic

– Dimension ratio used to classify geometry

• l/d ratio (for rotational)

• l/width or l/height ratios (for non rotational / prismatic)

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GEOMETRIC CODE

• 5 Digits

• Digits:

– 1 - component class

– 2 - basic shape

– 3 - rotational surface machining

– 4 - plane surface machining

– 5 - auxiliary holes, gear teeth and forming

• see table 12.4 (pages 483 and 484)

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SUPPLEMENTAL CODE

• 4 Digits APPENDED TO GEOMETRIC CODE• DIGITS:

– 1- major dimension (dia or edge length)• range: 0.8 to 80 inches• < 0.8 rep by 0 and > 80 rep by 9

– 2 - material type– 3 - raw material shape– 4- accuracy

• clearance tolerances or surface quality (eg: 32 microinches)

• see pages 485

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OPITZ CODE EXAMPLE

• SEE FIGURE 12.7

• CODE: 1 1 1 0 2

• Supplementary code: review part given

– code:?

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• Coding/classification provide few benefits if it ends there

• coding:a means to quantify part geometry, content

• One use: to code potential designs before formally designing them

– designers sketches a concept, then codes it

– using code, similar designs are retrieved from DB

– if existing part can be used to satisfy design new design needs, then process ends and time saved

– if existing part cant be used, perhaps a variant can be used (simply modify existing design)

GT BENEFITS

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• In both cases, no or minimal changes in process plan and production plans may be needed

• Many companies have found that they produce identical parts with different names

– duplicate tooling, fixtures and engineering time are required when this occurs

GT BENEFITS

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• Common characteristic of US industry (Chang) is under-utilization of expensive processing equipment

• Takes 2 forms:

– Much of the machine time is idle and unproductive

– Many of the parts assigned to a specific machine are far below the capacity of the machine

• Approach: By grouping closely matched parts into a part family, machines can be more fully utilized from both a scheduling as well as a capacity standpoint

OTHER PROBLEMS

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Application of GT concepts• Major benefit includes part family formation for

efficient workflow

• Efficient workflow can result from grouping machines logically so that:

– material handling and setup can be MINIMIZED

• Parts can frequently be grouped so that the same tooling and fixtures can be used:

– this enables a major reduction in setup times

• Machines can be grouped so that the amount of handling time between machining operations can be minimized

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LAYOUT ISSUES• See figures 12.6 and 12.7 : Process type layout

Versus GT based layout

• M/c’s are clustered by function Vs M/cs that produce part family form a cell

• BASIS FOR GT LAYOUT is part family formation

– family formation is based on part features viz. manufacturing features.

• No rigid rules for part families ; user sets own definition

– General rule: all parts in a family must be related

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Part families

• For production flow analysis, all parts in a family must have similar routings

• Family size will change depending on criteria

• If criteria is: only those parts having exactly the same routing

• then few parts will qualify for this family

• If criteria is: group all parts requiring a common machine into a family

• large part families will result

• Before grouping can start, collect info of design and processing of all parts

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Part families• Each part is then rep as a coded form, called an

Operation Plan code (OP code)

• OP code rep a sequence of operations on a machine and/or a workstation

• Eg: DRL01 can rep the sequence:

–load the workpiece onto a drill press

–attach a drill

–drill holes

–change the drill to reamer

–ream hole & unload workpiece from drill

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Part families

• An Operation Plan (OP Plan) is a plan where operations are rep using OP codes

• OP codes SIMPLIFIES REPRESENTATION OF PROCESS PLANS

• see table 12.7

• Next Focus: Clustering Approach