1

Computer Aided Process Planning (CAPP)

1. INTRODUCTION

Technological advances are reshaping the face of manufacturing, creating paperless manufacturing environments in which computer automated process planning (CAPP) will play a preeminent role. The two reasons for this effect are: Costs are declining, which encourages partnerships between CAD and CAPP developers and access to manufacturing data is becoming easier to accomplish in multivendor environments. This is primarily due to increasing use of LANs; IGES and the like are facilitating transfer of data from one point to another on the network; and relational databases (RDBs) and associated structured query language (SQL) allow distributed data processing and data access.

.

With the introduction of computers in design and manufacturing, the process planning part needed to be automated. The shop trained people who were familiar with the details of machining and other processes were gradually retiring and these people would be unavailable in the future to do process planning. An alternative way of accomplishing this function was needed and Computer Aided Process Planning (CAPP) was the alternative. Computer aided process planning was usually considered to be a part of computer aided manufacturing. However computer aided manufacturing was a stand alone system. Infact a synergy results when CAM is combined with CAD to create a CAD/CAM. In such a system CAPP becomes the direct connection between design and manufacturing.

Moreover, the reliable knowledge based computer-aided process planning applicationMetCAPP software looks for the least costly plan capable of producing the design and continuously generates and evaluates the plans until it is evident that non of the remaining plans will be any better than the best one seen so far. The goal is to find a useful reliable solution to a real manufacturing problem in a safer environment. If alternate plans exist, rating including safer conditions is used to select the best plans

1.1 COMPUTER AIDED DESIGN (CAD)

A product must be defined before it can be manufactured. Computer Aided Design involves any type of design activity that makes use of the computer to develop, analyze or modify an engineering design. There are a number of fundamental reasons for implementing a computer aided design system.

  1. Increase the productivity of the designer: This is accomplished by helping the designer to visualize the product and its component subassemblies and parts; and by reducing the time required in synthesizing, analyzing, and documenting the design. This productivity improvement translates not only into lower design cost but also into shorter project completion times.
  2. To improve the quality of the design: A CAD system permits a more thorough engineering analysis and a larger number of design alternatives can be investigated. Design errors are also reduced through the greater accuracy provided by the system. These factors lead to a better design.
  3. To improve communications: Use of a CAD system provides better engineering drawings, more standardization in the drawings, better documentation of the design, fewer drawing error, and greater legibility.
  4. To create a database for manufacturing: In the process of creating a the documentation for the product design (geometries and dimensions of the product and its components, material specification for components, bill of materials etc), much of the required data base to manufacture the product is also created.

Design usually involves both creative and repetitive tasks. The repetitive tasks within design are very appropriate for computerization.

1.2 COMPUTER AIDED MANUFACTURING (CAM)

By the time computer use in design began, numerical control technology (NC technology) had matured to become cost effective for applications in machining. An important in numerical control is part-programming. A part-program is simply a set of statements comprehensible to the machine control unit (MCU), that oversees slide and tool movements and other auxiliary functions. In the case of components with complex geometries, part-programs had to carry out lengthy calculations for which it was logical to use computers. This gave rise to machine control units (MCU’s) with built in microprocessors- the building blocks of computers. The use of computers in extending the applications of NC technology, especially to part-programming was earlier termed Computer Aided Machining (CAM) and the associated technology was called Computer Numerical Control (CNC). Later Computer Aided Machining became an acronym for Computer Aided Manufacturing (CAM). Earlier Computer Aided Manufacturing used to denote computer use in part-programming only. Today it means any non design function of manufacturing that is computer aided.

1.3 CAD/CAM

As the use of computers in design and manufacturing broadened under CAD and CAM, it became evident that certain tasks were common to both, eg:-both design and manufacturing require data on tolerances. Part geometries created during CAD can readily be saved in the database for latter use. The forward slash (/) between CAD and CAM was meant to reinforce the shared functions of design and manufacturing.

2. PROCESS PLANNING

The product design is a plan for the product and its components and subassemblies. To convert the product design into a physical entity, a manufacturing plan is needed. The activity of developing such a plan is called process planning. It is a link between product design and manufacturing. Process planning involves determining the sequence of processing and assembly steps that must be accomplished to make the product. It is concerned with the engineering and technological issues of how to make the product and its parts. What types of equipment and tooling are required to fabricate the part and assemble the product. It involves determining the most appropriate manufacturing and assembly process and sequence in which they should be accomplished to produce a given part or product according to the specifications set forth in the product design documentation. All the related information is documented on a Route Sheet .The planning begins with engineering drawings, specifications, parts or material lists and a forecast of demand. The scope and variety of processes that can be planned are generally limited by the available processing equipment and technological capabilities of the company or the plant.

Process planning is usually accomplished by manufacturing engineers. Based on process planner’s skill, knowledge, and experience, the processing steps are developed in the most logical sequence, to make each part.

The following are the list of many decisions and details usually included within the scope of process planning.

  • Interpretation of design drawings:The part or product design must be analyzed (materials, dimensions, tolerances, surface finishes etc) at the start of the process planning procedure.
  • Processes and sequences: The process planner must select which processes are required and their sequence. A brief description of all processing steps must be prepared.

00105145S/U COLLET2.00 0.173

ROUGH TURN M/C PER TAPE NO: LS982A

0 .440 DIA BY1.7500 LENGTTH

0 .300 DIA BY0.8120 LENGTTH

0 .275 DIA BY0.4375 LENGTTH

FINISH 3/64 GROOVES (TYP) AND CHAMFERS

0.270 DIA.BY 0.375 LENGTH

CHAMFER CUTOFF TO 1.960

0015 1026#2 CENTERS BOTH ENDS0.250.004

0020 9401 CARBURIZE AND HARDEN0.50

0030 4063 S/U BETWEEN CENTERS1.250.0983

GRIND OD HOLD CONCENTRICITY

HOLD 0.4200 DIM. HOLD 0.2600 DIM.

HOLD 0.2815 DIM. HOLD 0.2712 DIM.

0040 9501BLAST TO CLEAN0.001

0050 9201CHROME PLATE PER PRINT0.38

0060 9805FINAL INSPECT

ROUTE SHEET GENERATED BY MIPLAN
  • Equipment selection: In general, process planers’ must develop plans that utilize existing equipment in the plant. Otherwise, the component must be purchased or an investment must be made in new equipment.
  • Tools, Dies, Moulds, Fixtures and gauges: The process planner must decide what tooling is required for each processing step. The actual design and fabrication of these tools is usually delegated to a tool design department and tool room or an outside vendor specializing in that type of tool is contracted.
  • Method analysis:Workplace layout, small tools, hoists for lifting heavy parts even in some cases hand and body motions must be specified for manual operations. The industrial engineering department is usually responsible for this area.
  • Work standards: Work measurement techniques are used to set time standards for each operation.
  • Cutting tools and cutting conditions:These must be specified for machining operations often with reference to standard handbook recommendations.

The results of planning are:

  • Routings which specify operations, operation sequences, work centers, standards, tooling and fixtures. This routing becomes a major input to the manufacturing resource planning system to define operations for production activity control purposes and define required resources for capacity requirements planning purposes.
  • Process plans which typically provide more detailed, step by step work instructions including dimensions related to individual operations, machining parameters, set-up instructions, and quality assurance check points
  • Fabrication and assembly drawings to support manufacture.

Manual process planning as mentioned earlier is based on a manufacturing engineer’s experience and knowledge of production facilities, equipment, their capabilities, processes and tooling. Process planning is a time-consuming process and the results vary based on the person doing the planning.

3. COMPUTER-AIDED PROCESS PLANNING ( CAPP)

Process planning translates design information into the process steps and instructions to efficiently and effectively manufacture products. As the design process is supported by many computer aided tools, computer aided process planning has evolved to simplify and improve process planning and achieve more effective use of manufacturing resources.

3.1 CAD/CAM INTEGRATION AND CAPP FEATURES

A frequently overlooked step in the integration of CAD/CAM is the process planning that must occur. CAD systems generate graphically oriented data and may go so far as graphically identifying metal etc to be removed during processing. In order to produce such things as NC instructions for CAM equipment, basic decisions regarding equipment to be used, tooling and operating sequence need to be made. This is the function of Computer aided process planning. Without some elements of CAPP there would be no such thing as CAD/CAM integration. The CAD/CAM systems that generate tool paths and NC programs include limited CAPP capabilities or imply a certain approach to processing.

CAD systems also provide graphically oriented data to CAPP systems to use to produce assembly drawings etc. Further, this graphically oriented data can then be provided to manufacturing in the form of hardcopy drawings or work instruction displays. This type of system uses work instruction displays at factory workstations to display process plans graphically and guide employees through assembly step by step. The assembly is shown on the screen and as a employee steps through the assembly process with a footswitch, the components to be inserted or assembled are shown on the CRT graphically along with text instructions and warnings at each step.

If NC machining processes are involved, CAPP software exists which will select tools, feeds, and speeds and prepare NC programs.

3.2 COMPUTER-AIDED PROCESS PLANING : TYPES

Computer aided process planning systems are designed around two approaches. These approaches are called:

  1. Retrieval CAPP systems or Variant Approach
  2. Generative CAPP systems or Generative Approach

Some Computer aided process planning systems combine the two approaches in what is known as Semi Generative Approach.

  1. Retrieval CAPP System or Variant Approach

The retrieval type is suitable for a family of parts. This system draws a standard process plan and stores it in the database. Whenever a different part from the family is to be processed, the standard process plan is retrieved and appropriately modified – hence the retrieval to this system. The retrieval system relies on the concept of group technology for part coding and classification. It is also compatible with the concept of cellular manufacturing in which cells are designed and laid out for family-of-parts production. In this type, as mentioned earlier a standard process is stored in computer files for each part code number called the Route Sheet.

The retrieval CAPP system operates as given in figure 2 . Before the system can be used for process planning, a significant amount of information must be compiled and entered into the CAPP data files. This is referred to as the “preparatory step”. It consists of the following steps:

  1. Selecting an appropriate classification and coding scheme for the company
  2. Forming part families for the parts produced by the company
  3. Preparing standard process plans for the part families

Steps (ii) and (iii) continue as new parts are designed and added to the company’s design database.

GENERAL PROCEDURE FOR RETRIEVAL CAPP SYSTEMS

After the preparatory phase has been computed, the system is ready for use. For a new component for which the process plan is to be determined, the first step is to determine the GT code number for the part. With this code number a search is made for the part family file to determine if a standard route sheet exists for the given part code. If the file contains a process plan for the part, it is retrieved (hence the word “retrieval” for this CAPP system) and displayed for the user. The standard process plan is examined to determine whether any modifications are necessary. It might be that although the new part has the same code number, there are minor differences in the process required to manufacture it. The user edits the standard plan accordingly. This capacity to alter an existing standard process plan is what gives the retrieval system its alternative name: “variant” CAPP system.

If the file does not contain a standard process plan for the given code number, the user may search the computer file for a similar or related code number fro which a standard route sheet does exists. Either by editing an existing process plan or by starting from scratch the user prepares the route sheet for the new part. This route sheet becomes the standard process plan for the new part code

The process planning session concludes with the process plan formatter, which prints out the route sheet in the proper format. The formatter may call other application programs into use. For eg: -To determine machining conditions for the various machine tool operations in the sequence, to calculate standard time for the operations or to compute cost estimates for the operations.

One of the commercially available Retrieval CAPP systems is MultiCapp, from OIR, the Organization for Industrial Research. It is an online computer system that permits the user to create new plans, or retrieve and edit existing process plans as explained earlier.

  1. Generative CAPP System or Generative Approach

The generative method of developing process plans involves starting from scratch every time a different part is to be processed; no plans are available as the baseline. The basic requirement for a generative process planning system is that the given component model/drawing is to be interpreted in terms of manufacturability. Here instead of retrieving and editing an existing plan contained in the computer database, generative system creates the process plan based on logical procedures. In a fully generative CAPP system the process sequence is planned without human assistance and without a step of predefined plans.

A generative CAPP system is usually considered part of the field of expert systems, a branch of artificial intelligence. An expert system is a computer program that is capable of solving complex problems that normally require a human with years of education and experience. Process planning fits within the scope of this definition.

There are several ingredients required in a fully generative process planning system:

  1. First the technical knowledge of manufacturing and the logic used by successful process planners’ must be captured and coded into a computer program. In expert systems applied to process planning, the knowledge and logic of human process planners’ is incorporated into a so called “knowledge base”. The generative CAPP system then uses that knowledge base to solve process planning problems (ie create route sheets)
  2. Second ingredient in process planning is a computer compatible description of the part to be produced. This description contains all the pertinent data and information needed to plan the process sequence. Two possible means of providing this description are:
  3. the geometric model of the part that is developed on a CAD system during product design and
  4. a GT code number of the part that defines the part features in significant detail.
  1. The third ingredient in a generative CAPP system is the capability to apply the process knowledge and planning logic contained in the knowledge base to a given part description. In other words, the CAPP system uses its knowledge base to solve a specific problem – planning the process for a new part. This problem solving procedure is referred to as the “inference engine” in the terminology of expert systems. By using its knowledge base and inference engine, the CAPP system synthesizes a new process plan from scratch for each new part it is presented.

4. GROUP TECHNOLOGY

Group technology is a manufacturing philosophy in which similar parts are identified and grouped together to take advantage of their similarities in manufacturing and design. Similar parts are arranged into part families. Each family would possess similar design and manufacturing characteristics. Hence processing of each member of a given family would be similar and this results in manufacturing efficiencies. These efficiencies are achieved in the form of reduced set-up times, lower in-process inventories, better scheduling, improved tool control and the use of standardized process plans. The design retrieval system is a manifestation of group technology principle applied to the design function. To implement such a system some form of parts classification and coding is required.