ComputerAided_Design_Engineering_amp_Manufactur.pdf

make this approach realistic in conventional machining. While it is possible to accommodate changing
depths of cut in the case of NC machines, the usefulness of employing varying depths of cut will depend
upon whether there is a significant improvement with respect to objective function. It is therefore safe
to assume that equal depths of cut will be used for all roughing passes.
The solution procedure can proceed with finding optimal parameters for the finishing operation first.
In case of finishing operations, it can be assumed that the finishing operation is a single-pass operation.
This is a reasonable approximation as only very little material will be left to be removed in the finishing
operation to achieve the required finish or tolerance on the surface being machined.
The parameters can then be determined for roughing operations, based on the remaining depth of
material. The optimization for multipass rough turning operations is carried out in two stages. In the
first stage, a set of feasible depths of cut, based on the total depth of material to be removed and the
permissible limits of depths of cut (resulting in an integral number of roughing passes), can be calculated.
To determine the optimal number of passes and hence the depth of cut to be used, optimization will be
carried out for each feasible subset of depth of cut and number of passes. Finally, the subset which results
in the least production time is chosen to be the optimal pair. The corresponding cutting speed and feed
rate are chosen to be the optimal parameters for the roughing operation. The solution methodology is
shown in Figure 5.33.
The individual optimization problem is solved using the geometric programming technique. The
dual of the geometric programming (GP) problem is solved by using the techniques of separable
programming (SP) and linear programming (LP) (Kochenberger et al., 1973). Simplex algorithm is
used for the solution of the LP problem. The primal variables are determined from the dual solution
using the method given in Beightler and Phillips (1976). The steps in GP solution procedure are given
in Figure 5.34. Details on the solution procedure of GP dual problem can be found in Kochenberger
et al. (1973) and Prasad (1994).
Similar formulation can be carried out for facing and boring operations (Prasad et al., 1994a, 1994b).
The explicit optimization of cutting parameters for secondary operations such as chamfering, grooving,
filleting, knurling, and thread cutting is not necessary. Instead, the parameters for these operations are
chosen as a fraction of turning conditions. Formulation for milling operations in a similar approach can
be found elsewhere (Manidhar, 1995).
Time and Cost Calculation
The time required for machining a component is equal to the sum of cutting, setup, and handling times.
Handling time includes the time involved in tool change, tool resetting, and component loading and
unloading. Based on the outputs obtained so far (machining operation, pocket dimensions, and process
parameters), the cutting times can be calculated using the machining formulas. However, the calculation
of setup times and the handling times is a data-intensive activity since each company has its own set of
standard elemental times.
5.15 Report-GIFTS: Report Generation Module
When the control passes through all of the above modules, PPIR contains the necessary information a
CAPP system is supposed to generate. The PPIR generated for the example part shown in Figure 5.18 is
given in Table 5.6.
As explained earlier, PPIR is basically a set of setups, with each setup pointing to a set of pockets.
However, PPIR is an internal model and its format is meant for computer processing. Hence, it is not
useful for any practical purpose unless it is converted to the required form, comprehensible by the people
involved in using the CAPP system.
A report generation module, called Report-GIFTS, is developed to serve this purpose. This module
can be regarded as a postprocessor which transforms the data from one format to another format. When
one looks at various formats of the process plans, four possibilities can be visualized: