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ComputerAided_Design_Engineering_amp_Manufactur.pdf

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FIGURE 5.26 Down_face-turn-up_face pattern forming a pocket.<br />

(b) Take the set of first three elements. Check for the “down_face-turn-up_face” pattern as shown in<br />

Figure 5.26a. If the pattern exists, go to step C; otherwise, go to step E.<br />

(c) Record the pocket (the cross hatched region in Figure 5.26b) and modify the pattern as shown in<br />

Figure 5.26c.<br />

(d) If (pocket_surface � blank_surface) then pocket identification is over. Stop.<br />

(e) Otherwise, skip these three elements and consider the remaining elements. Go to step B.<br />

(f) Skip the first element. Consider the remaining elements. Go to step B.<br />

As soon as the pocket is identified, the corresponding machining operation is also attached to it.<br />

Because of the backward planning strategy employed, the order of pockets is loosely related to the inverse<br />

operation sequence. However, the exact operation sequence can be obtained only after setups are derived.<br />

The above procedure gives rise to a unique set of pockets. No alternate sets of pockets are necessary in<br />

the case of cylindrical components.<br />

Each of the intermediate shapes generated during the pocket processing are stored in different<br />

files as feature-based models. These models are temporary and do not appear on the design.<br />

Nevertheless, they are essential to carry out subsequent processing, as shown in the above procedure.<br />

The same procedure can be applied to identify the internal pockets also. In that case, the bottom<br />

portion of the contour is to be considered. In backward planning, the blank surface is the limiting<br />

element for external features, while the central axis becomes the limiting element for internal<br />

features.<br />

At this stage, PPIR is filled with pocket and operation details only. PPIR is partial since other details<br />

(machine, tool, parameters, etc.) need to be determined in subsequent modules.

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