26-05-2012, 12:59 PM
INTEGRATED APPROACH IN COMPUTER AIDED PROCESS PLANNING
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General context
An important link of a CIM system’s chain is the CAPP module, within which take
place the preparation, design and coordination of the manufacturing processes. CAPP is the
linkage between the CAD module and the CAM module, which confers it a special place in
the CIM system. Thus, a performing CAPP module has to meet following requirements:
· it must be capable of «understanding» and analysing the parts’ characteristics;
· it must posses multiple knowledge about machine-tools, cutting tools and their
characteristics;
· it must posses the capability of analysing the interdependencies between part - machining
process - quality - cost;
· it must posses multiple calculus possibilities.
Starting from these requirements and after analysing the information streams there
were defined the main objectives for conceiving a CAPP module which could be integrated in
the chain of a CIM system.
The utilisation of the CAPP module does not impose as an absolutely necessary
condition the existence of the previous process (CAD), neither that of the next process (CAM)
and thus the existence of automated manufacturing systems (FMS) /6/.
If the goal is to conceive a CIM system, the two processes become absolutely
necessary and the linkage between them is assured by CAPP.
Conceiving of the CAPP module
In conceiving the CAPP module, the possibility of using different methods was taken
into account, two of them being basic /5,11,12/:
· the method through variants,
· the generative method.
The method through variants (VPP - Variant Process Planning) is conceptually based on
the idea that similar parts are being manufactured in similar ways. Therefore, one of the main
components of the CAPP module is that of part coding, which uses the principle of group
technology. In a consistent database we must search for the variant which is closest to the
needed part. Creating and modifying the typified process is the job of the engineer.
The first activity carried out is that of coding, classifying and grouping the parts on
families, which represents the preparation state, followed by the production state, which refers
to the usage of the CAPP module during actual production.
The database which was created during the preparation state undergoes a process of
continuous completing with new part types which will be machined.
Their regrouping will be made based on the group technology, by modifying and
adapting the configuration of the new part type as needed.
The realisation of the CAPP module
Combining the three solutions presented above has led to the creation of a CAPP
module based mainly on the TechCIM program /2/, which allows the optimisation of the
machining process according to specific algorhytms, allowing also the intervention of the
engineer for the final determination of the machining sequences, by modifying the defined
objects’ attributes, according to the access levels in databases. The TechCIM program for the
automatised realising of the machining process for cardanic transmissions uses the principles
of group technology. The correspondence between surface coding and technical conditions for
machining is achieved through the graphical files, realised in the CAD module.
The processing of graphical information from CAD files is made together with that of
the technological information, thus realising as a first step an association between geometrical
shapes and machining procedures. The program has been realised and structured in Microsoft
Excel, because the corresponding database has been realised with the same program.
Interfacing the CAPP module with other modules of the CIM system
Starting from the existing model, which intends to define the machining process for
the parts (part drawings) taken from the CAD module, there is to be realised an expert CAPP
system.
The structure of the expert system comprises following main modules:
· knowledge acquiring module;
· consulting module for the analyse of the CAD models’ shapes;