Introduction to CAPP Flashcards
Reasons why CAPP is useful
- In future, industry may need more people to do process planning than the available ones
- Different people produce different process plans for the same problem
- Process plan performed by humans, in general, is not optimal wrt some performance criteria
- > computerized systems to automatically perform process planning (CAPP) can solve these problems
Possible types of CAPP
- Variant CAPP
- Generative CAPP
Variant CAPP systems: general approach
- preparatory stage:
- Existing products are grouped into families by the group technology method
- For each family, a standard process plan is made - production stage
- The process plan for a new product is decided by classifying the product in a family, and applying the standard process plan of the family with some possible modifications
Generative CAPP systems: general approach
- On the base of the human knowledge, the system should provide a process plan for a new product, independent from existing designes, without human intervention
- Designing a generative CAPP is a problem in expert systems field
- Difficult to implement due to the large amount of data required
Group technology: purpose
To classify manufactured components into part families
Definition of part family
A group of parts that have similarities in shape and size, or in the processes used to produce them
How to identify part families
- visual inspection
- coding systems and production flow analysis algorithms
A code is associated to each product, basing on its shape and initial form;
a process plan is made for each product, and code is associated to each operation, indicating the necessary machine too;
a component-operation matrix is formed;
a Clustering algorithm finds the families from this matrix
What is a composite part
For a given family, a composite part is an ideal product which includes all the design and manufacturing elements of the family.
- A process plan for the composite part includes all the possible operations for each component of the family
- Omitting some operations the process plan for any part of the family can be obtained
Components of a generative CAPP expert system
- Computer-compatible part description, that contains a representation of the part, from which features can be extracted
- Representation of the knowledge and logic used by process planners
- Algorithm that apply the knowledge and logic to the part description, generating a process plan
Graph-Based approach for the computer-compatible description of a part
- A graph based representation of a part is suitable for applying heuristic algorithms for feature extraction
- face - > node
- edge - > arc
- weight t = 0 if the edge is concave
weight t = 1 if the edge is convex
Basing on the properties of the graph, the features of the part can be recognized
How to choose between variant and generative CAPP systems
One of the most critical factor that leads the choice of the approach is the number of part families:
- for low number of part families, each one with a high number of pieces, the variant approach gives good results
- for high number of part families, containing a low number of parts, a generative CAPP is Worth using
A solution for the problem of the difficul implementation of generative systems is the semi-generative approach
semi-generative CAPP approach
- Generative CAPP, in the true sense of the definition, does not exist
- semi-generative approach is an advanced application of variant technology using some generative fetures, for example:
- Generative approach for standard process plan creation, variant approach for process plan retrieval
- generative approach for creating as much as possible of the plan, variant approach fot the details of the plan
Benefits of CAPP
- process standardization and optimization
- better re-use of previous knowledge
- increased productivity of process planners
- decreased time and cost for process plan maintenance (generative approach)
- interface with other programs, like for cost estimating
