GCD Extraction for Sand Casting
aPriori extracts the geometry of a CAD model in the form of geometric cost drivers (GCDs) that include Surfaces, Holes, Voids, and Combo Voids, as well as the Parting Line. As usual, there is a special GCD, Component, that represents the part as a whole. Following are some of the important properties of each of these GCD types:
• Component:
o Volume (mm3)
o Surface Area (mm2)
o Length, Width, and Height (millimeters)
o Number of Surfaces
o Min Thickness, Max Thickness, and Avg Thickness (millimeters)
o Length Direction, Width Direction, and Height Direction: Each of these is a unit vector, two of whose components are 0 (the other component is 1 or -1). These vectors can be compared with the Direction vector of another GCD (such as a Perimeter or Setup Axis) in order to determine whether the GCD’s direction is aligned with the part’s length, width, or height.
• Parting Line:
o Draw Direction
o Height (millimeters)
o Parting Type
o Projected Area (mm2)
• Surfaces (Curved Surface, Curved Wall, and Planar Face):
o Finished Area (mm2)
o Bend Diameter (millimeters)
o Perimeter (millimeters)
o SER Length and SER Width (smallest enclosing rectangle dimensions in millimeters)
o Corner Diameter (millimeters)
o Inside Diameter (millimeters)
o Number of Surfaces
o Fillet Min Angle and Fillet Max Angle (degrees)
o Round Radius (millimeters)
o Round Length (millimeters)
Holes (Simple Hole, Multistep Hole):
o Diameter (millimeters),
o Length and Depth (millimeters)
o Volume (mm3)
o Surface Area (mm2)
o Maximum diameter and Minimum diameter (millimeters—for multistep holes)
o Step count (for multistep holes)
o Hole type (THROUGH or BLIND)
o Blind, Sleeve Pin, Threaded, Countersunk, and Flanged (TRUE or FALSE)
o Accessibility (THROUGH, OBSCURED, SIDE_A, SIDE_ B, or UNDERCUT--determines operation sequence and tooling requirements.)
• Volumes (Void, Combo Void):
o Box Length, Box Width, Box Height (millimeters)
o Exposed Area (mm2)
o Number of Surfaces
o Number of Openings
o Perimeter (mm)
o Surface Area (mm2)
o Tool Reach (millimeters)
o Volume (mm3)
o Maximum Tool Diameter (millimeters)
o Parting Line Draw Distance (millimeters)
o Accessibility (THROUGH, OBSCURED, SIDE_A, SIDE_ B, or UNDERCUT--determines operation sequence and tooling requirements.)
The GCD extraction algorithm creates voids when it finds a probable core because either a volume is inaccessible in the parting direction, or the flow of the metal may break away a green sand mold. Whenever a hole overlaps such a volume, a ComboVoid can result.
Each ComboVoid consists of either multiple voids or else one or more holes and one or more voids. GCD extraction algorithms presume that holes that are children of ComboVoids are manufactured mostly by the casting processes (coring, green sand, and so forth). Any machining on these holes is limited to surface finishing operations.
You can signal to aPriori that a hole needs to be fully machined by unmerging the hole from the ComboVoid (see below). Note that this means that the GCD extraction algorithm fills in these holes before computing thicknesses.
You can also unmerge a void from a ComboVoid, so that the void is assigned a separate operation.
Unmerge a GCD as follows: in the Geometric Cost Drivers pane, right click a GCD that is a child of a ComboVoid, and select Unmerge. (Alternatively, click the GCD in the Geometric Cost Drivers pane, and then right click the model in the Viewer and select Unmerge.)
To re-merge a GCD that previously was unmerged, right-click the GCD in the Geometric Cost Drivers pane, and select Auto-Merge (or click the GCD in the Geometric Cost Drivers pane, right-click the model in the Viewer, and select Auto-Merge).
Note that the scenario must be CAD connected in order to access these options; GCD extraction occurs as soon as Unmerge or Auto-Merge is selected.
The GCD extraction algorithm also fills in uncastable holes before computing thickness, based on two site variables, minCastableHoleDiameter and minCastableHoleWallThickness. Uncastable holes are those whose diameter is below minCastableHoleDiameter and those whose wall thickness is below minCastableHoleWallThickness. Uncastable holes are never included in ComboVoids. Note that VPEs may have variables with the same name as the site variables. The VPE variables change the cost model behavior but have no effect on GCD extraction.
In some rare cases the algorithm that fills in holes fails. In this case aPriori does not create a hole GCD at all.