9. Computer Aided Process Planning (CAPP)

9.1 Overview

• If we had an engine block, how would you manufacture it?

• When deciding how to produce a product there are a number of factors to consider,

Product geometry, material, tolerances, weight, etc

Available processes/machine tools/skills

Available tools and fixtures

Inventory

etc

• Requicha and Vandenbrande [1988] describe the process of process planning as,

“A process planner and a set-up planner (often the same person) examine a part’s blueprint and consult various files and handbooks to produce a process plan. A plan contains process specifications and information on fixtures and clamping devices to be used, and on set-up of the workpiece on a machine tool. Set-up specifications are typically conveyed through annotated sketches or engineering drawings.”

• A process plan will vary from factory to factory, but there are some basic elements to be found on all. An example is shown below.

 

• Obviously a process plan is important when there is a high product mix, because it lets us know were to send the parts, and what to do with them. In a high volume setting, a process plan lets us decide exactly how something will be made before equipment is bought or moved.

• A Process Plan includes,

Part routings (Indication of where to send finished parts)

Bill of Materials (for each operation)

Work Orders (A description of what operations to perform at a work station).

• Every company uses process planning. In smaller companies the process planner may also be the craftsman who makes the product. In larger companies there may be large departments set aside to perform this function.

• As the size of a company grows, and so do the possible methods for manufacturing, and process planning become more difficult.

• A Diagram of the traditional Two-Stage Approach to Process Planning

 

• Depending upon who defines process planning, it may, or may not include operation planning.

• In their purest sense, the definitions are,

Process Planning: Choosing the technological means whereby a feature(s) of a product will be manufactured (e.g. drilling, milling, or casting). Also known as high level process planning.

Operation Planning: Choosing the parameters of the operation which is used to create the feature(s). (e.g. feeds, speeds). Also known as low level process planning.

9.2 CAPP Software

• There are few successful process planning software packages available today.

• There are two main categories of process planners: Variant and Generative.

• Variant process planners use existing process plans, then allow a user to edit the plan for their new parts.

• Generative process planners should create a new process plan, without the use of any existing plans. This does not imply that the process planner is automatic.

• Some of the process planning steps used for machining operations are,

1. Interpretation of part design data

2. Selection of machining processes

3. Selection of machine tools, and fixtures

4. Machining optimization

5. Decomposition of machinable volumes

6. Selection of machinable volumes

7. Generation of precedence constraints

8. Sequencing of machinable volumes

• Most successful variant systems depend upon Group Technology.

• The basic variant approach to process planning with GT is,

1. Go through normal GT setup procedures

2. After part families have been identified, develop standard process plans for each.

3. When a new product has been designed, get a GT code for each part.

4. Use the GT system to look up which part family is the closest match, and retrieve the standard plan for that part family.

5. Edit the standard plan so that values now match the new design parameters, and add or delete steps as required.

• Some benefits of the GT system are,

It is well suited to medium to low product mixes

It can be developed quickly for most companies

Can be used with other CIM

One program can be used in radically different industries

• Disadvantages are,

GT codes can become obsolete quickly

While it is fast to setup, it is slower for planning than generative systems

More prone to error than generative systems

• These systems tend to get exact matches 2-7% of tries. A standard plan is used about 50% of time.

• Each plan is made from scratch

• The generative systems are poorly developed at this point in time, and tend to be research systems, or very limited domain

• These systems rely heavily upon the methods of Artificial Intelligence, or very complex algorithms.

• An example of a Generative system is the development of rules deciding which machines to use.

• Possible sources of input vary from system to system, but they are essentially,

Interpret designs from CAD directly (very difficult)

User defines features then answers questions about them

The user does design directly on the CAPP system.

The users creates a special product description file

• A rule example for a CAPP system called XPS-2 is shown below,

0010 EXECUTE MILL_HOLE FOR EACH BLIND_HOLE IF

BLIND_HOLE.DIAMETER GT 25.,

BLIND_HOLE.DEPTH LT 50. !

• This rule identifies the operation, the feature it is used on, and the two conditions for it to be used. When rules are used, the number of rules in the system becomes very large.

• Advantages,

Runs faster when planning

• Disadvantages,

Requires a more extensive setup

9.3 References

9.1 Ullman, D.G., The Mechanical Design Process, McGraw-Hill, 1997.