ComputerAided_Design_Engineering_amp_Manufactur.pdf

In spite of these rules, more than one clamping surface may qualify. Sometimes, internal surfaces are
preferred to external surfaces for the clamping purpose (especially in the case of thin-walled components,
in order to avoid the compressive stresses). It is also possible to orient the work-holding devices in
different styles, such as reversing the jaws, etc. In the machining domain, the number of valid plans
quickly increase with the number of clamping surfaces. In such cases, selection of the best clamping
surface will be based on the evaluation of all these alternatives. Some constraints that limit the clamping
possibilities, such as cutting and clamping forces and accessing limits of the clamping devices and their
availability, will have to be taken into consideration while evaluating these alternatives.
Operation Sequencing
The aim of operation sequencing is to apply sequencing constraints in order to arrive at a feasible plan.
In the first instance, it appears that the operation sequence will be the reversed-sequence of the pocket
identification since it is based on backward planning strategy. However, it is true only to some extent
because the pockets identified earlier are spread over different setups and are placed in different groups
to satisfy the relational tolerances and the accessibility constraints. Therefore, these changes will have to
be reflected during the operation sequencing.
Operation sequencing is a complex task in CAPP since a great deal of computation is required to
evaluate all possible sequences. For example, consider that there are N pockets. In a strict theoretical
sense, these can be machined in N! different sequences. In a simple part with 16 pockets (16 in number),
it is theoretically possible to have 2.09227 10 machining sequences. The evaluation of all these
alternatives will take years. However, in practice, it is possible to cut this seemingly large search space by
applying some constraints. If, for example, a single constraint (that pocket X needs to be machined before
pocket Y) can be applied, then the search space is reduced to (N!/2). Similarly, if one operation (e.g.,
rough turning) needs to be carried out before another operation (e.g., finish turning), the search space
will be further reduced.
13
� ( 16! )
Multi-level Pocket Sorting
Operation sequencing can be considered as the sequencing (or sorting) of the pockets because the
operations are already attached to the pockets. Pocket sorting is attempted at three levels:
1. Setup level: At this level, the precedence among the setups is established. For example, the pockets
of the second setup cannot be machined until those of the first setup have been machined.
Pockets are sequenced (sorted in ascending order) based on the setup to which these are attached
(Figure 5.31a)
2. Operation level: When identifying the pockets, the threads, grooves, etc. are identified first and are
stored at the top of the pocket file. However, these operations are normally the last operations
carried out in the sequence. This is resolved by establishing the precedence relationships between
pockets belonging to the same setup based on the machining operation (Figure 5.31b). Based on
the work shop practices, certain precedence between the operations can be established. A few
typical examples are listed below:
• The machining sequence should prevent destruction of features that have been created by a
previous operation. By this yardstick, a thread should be machined only after finishing the adjacent
chamfer/groove and parent diameter.
• Facing operation would normally be the first operation to get a reference for subsequent
machining.
• If internal features are present, as much material as possible is removed by the drilling operation
prior to external turning operations.
• All roughing operations should be performed first, followed by finishing operations.
• Based on accessibility constraints, grooves will be cut after turning operations.