ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
within the CAPP system. However, the hard coding of manufacturing data has some serious limitations.<br />
For ex<strong>amp</strong>le, if any new machine is added (or removed or breaks down), significant modifications need<br />
to be done either in the data files or in the program to reflect the change on the performance of the<br />
CAPP system.<br />
It is thus desirable to develop MRIR external to the CAPP system. Other advantages of maintaining<br />
MRIR as an independent data base management system (DBMS) are<br />
(a) Flexibility: The same data bases can be used for other manufacturing functions, e.g., for inventory<br />
control;<br />
(b) Customization: The data base can be adapted to the unique character of the manufacturing<br />
environment at a given instant; and<br />
(c) Enhancements: Without affecting the CAPP systems, these data bases can be modified or changed.<br />
5.8 Modeling of the Process Plan for CAPP<br />
Process planning involves the selection of machine tools, setups, machining operations, cutting tools, process<br />
parameters, etc. All experts agree that the decisions about items in this list must be made at some point<br />
during the planning process, but whether the order of the decisions in the list has any relevance is a debatable<br />
question. Furthermore, the list is highly interdependent. The order in which the planning decisions are<br />
made is a factor of a given manufacturing system. Irrespective of this order, there must be some “container”<br />
for keeping the partial plans (or the intermediate results of the planning). In this context, process planning<br />
internal representation (PPIR) serves the purpose of a container which can be referred to in various stages<br />
of process planning.<br />
In addition, the instructions for manufacturing a part can exist in one of several forms:<br />
(a) Textual Process Plans: the plans used by the machinists on a shop floor<br />
(b) Graphical Simulation: the format used by the automated systems for showing the process plan<br />
graphically<br />
(c) Pictorial Process Plans: the status of the component (or machining) after each setup for illustrating<br />
the part, fixtures, setup, cl<strong>amp</strong>ing, etc.<br />
(d) NC Programs: the format used by numerical control machines for executing the process plans.<br />
The basic set of manufacturing instructions from which these formats can be derived is the same<br />
for a given part. To enable a CAPP system to generate the process plans in one or all of these<br />
formats, it is necessary to represent the planning details in a structured format. In this context,<br />
the role of PPIR in a CAPP system can be appreciated (Jasthi et al., 1995).<br />
To represent the process plan, it is first necessary to gain an insight into the manufacturing instructions.<br />
At any instant of manufacturing, the blank (or semi-finished part) is set on a machine tool. In<br />
this setup, some portions of the material will be removed so as to move towards the final part specification.<br />
These chunks of material can be called machinable volumes (or pockets). The parameters for<br />
machining are governed by the part specification and the manufacturing resources. Thus, if (a) the<br />
specification of the machine being used, (b) the details of the setup on the selected machine, (c) the<br />
number and sequence of pockets being removed in each setup, and (d) the parameters (cutting tools,<br />
process parameters, etc.) for machining each pocket are available, then the manufacturing instructions<br />
(or partial process plan) at that instant can be specified (Figure 5.10).<br />
To summarize, the set comprising machine, setup, pocket, and parameters can be considered the core<br />
of the process planning content in the machining domain. If all such sets applicable to a given part can<br />
be obtained, the process plan for that part can be generated. Thus the need to represent these ordered<br />
sets in a structured format forms the basis for PPIR (Figure 5.11).<br />
The implementation of this model is explained in later sections.