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ComputerAided_Design_Engineering_amp_Manufactur.pdf

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DL is needed to be set only if the part type is 2 (i.e., part containing through internal features). For other<br />

types of parts (3, 4 and 5), the internal pockets can be accessed only from the end to which the internal<br />

features are opened.<br />

If the part contains more than one feature with the same maximum external diameter or minimum<br />

internal diameter, the demarcation lines give rise to different alternative arrangements. (In Figure 5.29,<br />

four different sets of DL are marked.) These alternatives can be evaluated based on certain guidelines:<br />

• The features with relational tolerances (such as concentricity, run out, etc.) should be machined<br />

in the same setup.<br />

• The features with high surface finish requirements should be in on one side of the demarcation<br />

line as far as possible. This helps in reducing the number of setups since all the finishing operations<br />

can be carried out in the last setup, as shown in Figure 5.29.<br />

• The DL should be nearer to the center of the part to balance the component stiffness in different setups.<br />

As shown in Figure 5.29, the number of setups needed to machine a part can be known only at this<br />

stage after DL is fixed.<br />

Once the demarcation line is fixed, the pockets are attached to different setups. If any of the pockets spread<br />

across the demarcation line, these are split so that they can be placed in different setups (Figure 5.30). While<br />

attaching the pockets to the setups (by filling in the slot “pocket.setup”), certain fields in PPIR are<br />

referred. These include the fields (a) “pocket.type” which specifies whether the pocket is external or<br />

internal; (b) “pocket.operation” and “pocket.nature” which give the operation details; and (c) describing the<br />

relative position of the pocket with respect to DL.<br />

Determination of the Cl<strong>amp</strong>ing Surfaces<br />

Automatic selection of the cl<strong>amp</strong>ing surfaces depends on the work-holding method and the setup number.<br />

Some general guidelines in the form of rules are included in the system for selecting the cl<strong>amp</strong>ing surface.<br />

Some typical rules of cl<strong>amp</strong>ing are<br />

• If the surface is conical, it cannot be used for cl<strong>amp</strong>ing.<br />

• The distance between the cutting point and the cl<strong>amp</strong>ing point should be minimized.<br />

• The search span, within which the cl<strong>amp</strong>ing surface must lie, should not extend into the machining<br />

span in which the pockets will be machined, etc.<br />

FIGURE 5.30 Pocket splitting based on demarcation line.

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