Improved extrusion model in Geometer/MetaMesh

MetaMesh (our 3D automatic mesh generator) accepts geometric information from a wide variety of sources (see Automatic meshing of printed circuits). For relatively simple systems, the quickest path to a mesh is through the native combinatorial solid modeler of MetaMesh. A variety of shape models are available. One of the most useful is the extrusion, the projection of a arbitrary shape over a given length. We have added new features that make it much easier to create extrusions with a desired position and orientation.

Figure 1. Outline generation utility in Geometer

Figure 1. Outline generation utility in Geometer

I’ll start with a review of the old procedure for extrusions. The basic principle in MetaMesh is to define basic parts (i.e., set model types and dimensions) in a reference workbench space and then to move them to the model space by SHIFT and ROTATE operations. In the old convention, an extrusion extended along z in the workbench space with its cross-section outline defined in the x-y plane. The outline consists of a set of line and arc vectors that form a closed shape. The outline editor of Geometer (a full-featured 2D CAD utility shown in Fig. 1) is a convenient way to create vector sets. Once created, an outline may be used to define multiple extrusions at different positions or orientation by adding SHIFT or ROTATE parameters. An extrusion using the outline of Fig. 1 has the following TYPE form in the MetaMesh script:

Type Extrusion
  L    5.2912 -1.000 7.500 -1.000 S
  L    7.5000 -1.000 7.500  1.000 S
  L    7.5000  1.000 5.291  1.000 S
  A    5.2912  1.000 3.000  2.500  3.000  0.000 S
  A    3.0000  2.500 0.500  0.000  3.000  0.000 S
  A    0.5000  0.000 3.000 -2.500  3.000  0.000 S
  A    3.0000 -2.500 5.291 -1.000  3.000  0.000 S
End

The data lines contain the x-y coordinates of the start and end points of lines and the start-end-center points of arcs.

Because the outline may specified at absolute positions in the x-y plane, it is easy to create desired extrusions that point along z. In most cases, SHIFT and ROTATE operations are unnecessary The problem arises if you want extrusions that extend along x or y. It is not immediately obvious to most people (myself included) how the x-y coordinates of Fig. 1 map to y-z or z-x coordinates when 90° rotations are applied about different axes. Positioning x or y extrusions often requires considerable experimentation in Geometer.

Figure 2. Add/Edit part dialog in Geometer

Figure 2. Add/Edit part dialog in Geometer

We have modified both Geometer and MetaMesh so that you can directly define extrusions along x and y. For extrusions along x, the outline coordinates are interpreted as absolute values in the y-z plane. For an extrusion along y, the outline data gives z-x values. It is easy to apply the feature. Figure 2 shows the modified Add/Edit part dialog in Geometer. The red arrow shows new radio buttons that are active only when the part type is set to EXTRUSION. The choice of extrusion axis determines how the path coordinates are interpreted. In the MetaMesh script, the TYPE command has a modified form:

Type Extrusion [X,Y,Z]

Some usage notes:

  • If no direction option appears in the TYPE EXTRUSION command, the default is Z. Therefore, existing scripts are interpreted correctly.
  • The new capabilities affect the extrusion shape and direction in the workbench frame. SHIFT and ROTATION operations may still be applied in the transfer to the assembly frame.
  • The same outline may be used in Geometer for multiple extrusion parts with different extrusion directions. Figure 3 shows an example.

For more information on Geometer and MetaMesh, please see http://www.fieldp.com/geometer.html and http://www.fieldp.com/metamesh.html.

Figure 3. Multiple=

Figure 3. Multiple extrusions with different directions from the same outline

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