1.2 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
1.2.1 Variant Process Planning
• 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.
1.2.2 Generative Process Planning
• 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