Operation Assignment for Machining

As described in Setup Axes and Operation Feasibility, when aPriori costs a part, it traverses the part's GCD hierarchy and performs operation assignment. In order to assign an operation to a GCD, aPriori must identify a setup axis that makes the GCD accessible for the operation (on the machine associated with the operation’s intended parent process). The sections GCD Accessibility and Tool Orientations and GCD Accessibility and Setup Axes describe how aPriori determines whether a given setup axis makes a GCD accessible for an operation.

Each operation’s feasibility module contains rules that determine which setup axes are considered as candidates and the order in which they are considered. If none of the setup axes under consideration makes the GCD accessible, aPriori considers the operation infeasible. The following sections describe these rules for milling operations on surfaces and simple holes:

 3-Axis Mill

 4-Axis Mill

 5-Axis Mill

 2-AxisLathe

 3-AxisLathe

 Mill Turn

In general, the feasibility modules for milling processes use roughly the following strategy:

1 For rotational machines (4- and 5-axis mills), check the rotational axes to see if one makes the GCD accessible. If one does, assign the operation (assuming it is feasible in non-setup-related respects). If not, continue with the steps below.

2 For both rotational and non-rotational machines, check the principal and user-activated setup axes to see if one makes the GCD accessible. If one does, assign the operation (assuming it is feasible in non-setup-related respects). Otherwise continue with the steps below.

3 For holes and axigrooves, check all currently activated setup axes to see if one makes the GCD accessible. If one does, assign the operation (if it is feasible in non-setup-related respects). Otherwise, activate a heretofore inactive setup axis that would (if used) render the GCD accessible, and assign the operation (if it is otherwise feasible).

4 For surfaces, come back later to the feasibility check for this GCD and operation. For now, move on to other operations for this GCD and then to operation assignment for all the other GCDs (to see if one of them activates a relevant setup axis). Then, if the GCD remains unassigned, return to the feasibility check for this GCD and operation.

5 Check all currently activated setup axes to see if one makes the GCD accessible. If one does, assign the operation (assuming it is otherwise feasible). Otherwise, consider the operation infeasible.

In certain special cases for surfaces, instead of considering the operation infeasible, aPriori activates a heretofore un-activated setup axis that would (if used) render the surface accessible and assigns the operation.

In addition, aPriori sometimes executes the last two steps one extra time. If that fails to find a suitable setup axis, the operation is considered infeasible.

Note that this section describes the strategy used during fully automated, not semi-automated, machining—see Semi-automated Costing.

To help implement this strategy, aPriori traverses the GCD hierarchy 3 or 4 times:

• The first time, it skips axisymmetric GCDs such as simple holes, and generally considers principal and user-activated setups.

• In the second pass it considers only axisymmetric GCDs, and principle and activated setups; it activates a setup to accommodate the axisymmetric GCD if necessary.

• In the third pass aPriori considers surfaces again, and considers activated setups. In special cases, it activates a setup to accommodate the GCD if necessary.

• In some cases, aPriori makes a fourth pass to consider surfaces yet again and check any setups activated in the third pass.

aPriori ensures that in-use, feasible setup axes are always preferred to feasible setup axes that are not yet in use by any operation. It does this by doing both the following:

• The strategy above always prefers activated to un –activated setup axes. Other than the initially-activated setup axes, only in-use axes become activated.

• aPriori orders the setup axes by axis key (an ID number for each axis). Among initially-activated setup axes (the principle and user-activated ones), aPriori always picks the earliest of two setup axes that are equally qualified for an operation.

All other things being equal, the feasibility rules generally favor setup axes that provide better access to corners (allowing a larger diameter tool). Of course, feasibility rules also impose various other requirements, such as an acceptable ratio of tool length to tool diameter. See Operation Routing and Feasibility for Machining.