ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
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Machine Selection<br />
The choice of the most suitable machine tool is an important task since the characteristics of the chosen<br />
machine tool will reflect on subsequent planning functions, such as setup planning, and the selection of<br />
process parameters. The details of the machine tools can be stored in a data base (see s<strong>amp</strong>le given in<br />
Table 5.4).<br />
Machine tool selection is done by matching the part requirements with the machine tool capabilities<br />
(Figure 5.27). A candidate machine tool is selected based on the following four aspects:<br />
Physical aspects: These are concerned with the physical characteristics of the machine tool and the<br />
workpiece. For ex<strong>amp</strong>le, the maximum diameter (“swing over carriage” for lathes) and maximum<br />
length (“admit between centers” for lathes) that can be accommodated on a machine tool should<br />
be greater than those of the part. In case of bar feeding, the machine tool spindle bore must be<br />
greater than the diameter of the bar stock.<br />
Technological aspects: As the name suggests, these are concerned with the technological attributes<br />
(surface finish and tolerances) present on the part. The achievable accuracy on a machine tool is<br />
compared with the required accuracy and only the capable machines are short-listed.<br />
Scheduling aspects: Only available machine tools need to be selected for processing the part. If a<br />
machine tool is scheduled to do any other job, it can be made non-available to the CAPP system<br />
by simply changing its status in the machine tool data base to either “Reserved” or “Breakdown.”<br />
In that case, regardless of its capabilities, the machine tool will not be selected. Sometimes, the<br />
part may be reserved to a particular machine tool on the shop floor. Recall that such information<br />
can be found in the global data of PDIR. If the part is reserved, the machine tool search will no<br />
longer be necessary.<br />
Optimization aspects: In spite of the above constraints, it is not unusual to have a large number of<br />
qualified machine tools for machining a given part. In such cases, different machine tool selection<br />
strategies can be applied, based on the company policy (either minimum production time or<br />
minimum production cost), to choose one from the alternatives. The power available, machine<br />
tool costs, material removal rates, range of speeds and feeds, are the key parameters used to evaluate<br />
the alternatives.<br />
Setup Planning<br />
Once the operations are decided and the machine is selected, the next logical step is to find out how the<br />
part is to be set on the machine and what operations are to be carried out in each of these settings. It is<br />
also necessary to determine the number of setups, sequencing of setups, and cl<strong>amp</strong>ing parameters in<br />
each setup.<br />
The procedures presented here are based on the works reported by Hinduja and Huang (1989b),<br />
Arora (1991) and Jasthi et al. (1994). The execution of these procedures results in some changes in<br />
PPIR such as filling up the setup details into the relevant slots and attaching the pockets to different<br />
setups.<br />
Deciding the Holding Method<br />
The first task in setup planning is to find a method of holding the job. A particular work-holding<br />
configuration will directly affect the operation sequence and must fulfill the technological requirements<br />
(like roundness, relational tolerances, etc.) specified on the component. For rotational components, the<br />
following methods of holding are possible:<br />
(a) Holding between centers and using face plate and dog as driver (between centers method, BC)<br />
(b) Holding between centers and using chuck as driver (chuck and center method, CC)<br />
(c) Holding in chuck (chuck only method, CO)<br />
(d) Cl<strong>amp</strong>ing in special fixtures and collets.