Blank GCD for Hard Tooled Sheet Metal
The Blank GCD is specific to the Sheet Metal process groups. It represents the flattened part. Its properties (such as Perimeter, Thickness, SER Length, and SER Width) are listed in the Geometric Cost Drivers pane, and are all independent of the blank’s orientation on the stock sheet or coil.
 |  |
You can display the flat pattern head on, in a separate window, together with its associated SER, by selecting Flat Outline from the Analysis menu in the Viewer toolbar.
Blank SER/SWER Lenth and Width Versus Blank Pitch and Coil Width
It is very important to distinguish between the following two rectangles that enclose the flattened part:
• Blank’s SER: this is the smallest possible rectangle that contains all of the flattened part. Its dimensions are independent of the blank’s orientation on the stock sheet or coil, that is, SER Length and SER Width do not change if the blank’s nesting or orientation change.
• Nested blank’s extent rectangle: this is a rectangle that
o Contains all of the flattened part
o Has sides that are each either parallel or perpendicular to the pitch direction (direction of coil flow) or sheet length.
o Encloses extra material around the flattened part for various purposes, such as carrier strips and cutting strips.
The dimensions of this extent rectangle do depend on the blank’s orientation on the coil; these dimensions can change if the orientation changes.
Some users might be used to using the term “blank” to refer to the extent rectangle described above; but this is not how the term “blank” is used in aPriori. In the aPriori interface and documentation, “blank” refers to the flattened part, whose SER Width and SER Length are independent of nesting and orientation.
The figure below shows nested blanks oriented on the stock coil. The blank SERs are shown in yellow.
Flattening for Hard-tooled Sheet Metal
aPriori provides two alternative methods for determining the outline of the flattened part, finite element analysis (FEA)-based flattening and geometric flattening:
• FEA flattening:
o Analysis is based on a forming simulation which uses a finite element analysis derived from the part’s CAD model and the properties of the selected material.
o More accurate than geometric flattening for formed and drawn automotive components and similar complex stampings.
o Generally slower than geometric flattening (very complex parts sometimes require more than one minute to flatten, although the great majority require far less time).
• Geometric flattening:
o Analysis is based on unfolding of CAD model geometry, and neither takes into account material properties nor simulates the forming process.
o Accurate for “mostly developable” parts, that is, parts consisting primarily of flat planar surfaces and simple bent or rolled surfaces, with a few small, isolated deformed features like gussets and stiffening beads. These types of parts are common in the agriculture/construction equipment and hi-tech industries.
o Generally faster than FEA flattening.
FEA flattening is used by default in starting point VPEs. VPE administrators can customize this default with the site variable flatteningSolverType.
For a given part, you can override the default flattening method, or configure FEA flattening, with the Flattening Options dialog, which is available from the Viewer toolbar.
The dialog provides the following options:
• Solver Type:
o FEA – Plastic and elastic material behaviors considered: takes into account both the elasticity-related material properties (including Young’s modulus and Poisson’s ratio) and the plasticity-related material properties (including K, the strain-hardening coefficient, N, the strain-hardening exponent, and R, the Lankford parameter, average) . This is the default in starting point VPEs.
o FEA – Only elastic behaviors considered: takes into account only the elasticity-related material properties (including Young’s modulus and Poisson’s ratio). Because only elastic behavior is modeled, blank size estimates are slightly larger with this option than with the “Plastic and elastic” option. Some users prefer this approach as it is slightly more conservative for estimating material usage and costs.
o Geometric Unfolding: Select this if you don’t want to use FEA flattening.
The Flattening Method property of the Blank GCD indicates the method used for the most recent geometry extraction. It’s possible values are FEA_PLASTIC, FEA_ELASTIC, GEOMETRIC_UNFOLDING.
VPE administrators can customize the default solver type with the site variable flatteningSolverType.
• Surface for Flattening: applies to FEA flattening only. FEA flattening proceeds by imposing a mesh on an overall surface of the part. The mesh serves to divide the part into a large but finite number of discrete elements.
o Mid-Surface: specifies that the mesh should be imposed midway between one side of the part material and the other side of the part material. A mid-surface mesh basically represents the neutral surface of the part. This is the default in starting point VPEs.
o Larger Area Side: specifies that the mesh should be imposed on the larger-area side of the part material.
o Smaller Area Side: specifies that the mesh should be imposed on the smaller-area side of the part material.
VPE administrators can customize the default surface for flattening with the site variable flatteningSurface.
• Fill Holes in Blanks: applies to FEA flattening only.
o No: specifies that the mesh (see Surface for Flattening, above) should not be applied to portions of the material surface that will be removed to form a hole. This is the default in starting point VPEs.
o Yes: specifies that the mesh (see Surface for Flattening, above) should be applied to portions of the material surface that will be removed to form a hole.
VPE administrators can customize the default with the site variable flatteningFillHoleMethod.
• Flattening Direction: applies to FEA flattening only.
o aPriori Selected: specifies that the flattening calculation should be based on the ram direction chosen by aPriori. This option is available only in the Sheet Metal—Transfer Die process group, and is the default in starting point VPEs. See Ram Direction for Transfer Die for more information.
o Average Surface Normal: specifies that the flattening calculation should be based on a ram direction that is the average direction of the normal to the part’s surfaces. This is the only option available in the Sheet Metal process group.
VPE administrators for Sheet Metal—Transfer Die can customize the default flattening direction with the site variable flatteningDirection.
VPE Administrators also can customize the FEA flattening behavior with the following site variables:
• flatteningInitialStrainValue (0.002 by default): for FEA flattening, specifies the strain present in the material before outside forces and loads are applied.
• flatteningTimeoutSeconds (600 seconds by default): determines how long FEA flattening will run before timing out; if FEA flattening times out or fails for any reason, an FEA-failure message appears in the Viewer, and geometric flattening is used instead. A value of 0 indicates no timeout—FEA flattening will run indefinitely.
As mentioned above, FEA flattening takes into account material properties, including the following:
• Young's Modulus
• Poisson's Ratio, K (strain-hardening coefficient)
• N (strain-hardening exponent)
• R (Lankford parameter, average).
Materials in starting point VPEs specify these properties. If your VPE includes a material that doesn’t specify these properties, aPriori uses the following default values based on material type:
Material Type | Young's Modulus (MPa) | Poisson Ratio | K (strain-hardening coefficient, Mpa) | N (strain-hardening exponent, MPa) | R (Lankford parameter, average) |
Aluminum | 69000 | 0.33 | 341.1 | 0.21 | 0.7 |
Copper | 115000 | 0.33 | 317 | 0.54 | 0.99 |
Galvanized Steel | 207000 | 0.28 | 543 | 0.21 | 0.99 |
Stainless Steel | 207000 | 0.28 | 1426 | 0.502 | 1 |
Steel | 207000 | 0.28 | 0.28 | 479.3 | 1.345 |
If the material type is not one listed in the table above, the entries for Steel are used.