Tip #1 - Part Morphing

(Note: This tip will work in both versions 2 and 3 of Mechanical Desktop®.  The new table driven feature suppression feature added in version 3 could be effectively utilized here to hide the inactive profiles (all that is except the "base" or "dummy" feature).  It is my intention to update this tip in the future to utilize this feature.

You can download an MDT 3.01 file demonstrating this technique using Table Driven Feature Suppression at the File Download Page.  R.W. 5/23/99

Hi! and welcome to the first in our series of Mechanical Desktop® tips and tricks, Part Morphing. In this example, we will create a part that can either assume the shape of an equal leg structural angle or a AISC W (Wide Flange) structural shape. This is an example from the "real world" and can be easily adapted to other similar design problems. This technique was born from the need to have different structural shapes available for stiffening as the base size of a vessel changed due to varying design parameters. Our first approach was to join several parts into a subassembly, and control visibility of the "inactive" part from the browser. Without getting into too much detail, this approach was rejected because it was not fully parametric, very hard to assembly constrain and resulted in inaccurate mass property command outputs (a big no no if you plan to use the models for CG or FEA analysis. The Part Morphing approach that follows solves all these problems. The part is fully parametric, easy to assemble and the mass properties command will yield the correct results regardless of the active profile.

Mechanical Desktop® has always allowed you to use 2 or more profiles in a part in order to build complex shapes out of simple ones. What we will do in this example is use this feature to effectively hide the "inactive" profile shape within the "active" shape definition.  Since Mechanical Desktop® does not allow you to enter zero (0) dimensions (except for offsets), we must hide the "inactive" profile by making it very small and hiding it within the "active" profile.

Before starting, be sure you are familiar with creating Work Planes. Work Planes are a very powerful and under-utilized tool in Mechanical Desktop® in my opinion. These examples make extensive use of Work Planes throughout. Also be sure you can create and manipulate table driven Local Part Variables. Although I prefer to use Global Variables in my assemblies (along with a utility that imports the values from Excel and updates the assembly), I have used Local Variables in the example to utilize the built in Excel to Mechanical Desktop link.  (Note: Global part variables can now be linked in version 3)

First let us examine our shapes and determine exactly what we want when we are finished. The angle shape profile will need to be constrained to an assembly using the horizontal top edge and vertical left side edge. The wide flange shape will need to be constrained to an assembly using the vertical centerline and the horizontal top edge. We must provide a set of edges, common to both parts so that the part is accurately constrained regardless of which profile is "active". (I have not put in an edge to control the z axis (depth) positioning of the part in the assembly. This is something you will have to add in a "production" part) Each profile must be controlled by a distinct set of variables so that we have independent control of each. We will use a common length parameter to control the depth of part, regardless of which profile is active.

You should now have Mechanical Desktop® open and be ready to create a new part. Following are step by step instructions to create the Angle/W beam part.

Step 1 - Create the Variables
Table 1 shows the variable names and values to create.  Figure 1 is a screen shot of the AMVARS dialog showing all the variables required.

Figure 1.

Step 2 - Create the Base (Dummy) Feature
The first profile that we will create is a small cube that will act as the base (or dummy) feature.  The purpose of this feature is to provide an anchor point for the other profiles and to provide surfaces to hang the work planes used for assembly.   Other purposes for this feature will become apparent in future tips.  Draw the cube, profile it, dimension it (0.01 wide x 0.01 high in this example) and extrude to a depth of 0.01.  Now, we must create the 2 work planes that we will use to assemble this part to other parts.  Create the first work plane planar-parallel with a zero offset to the left side face of the cube.  Create the second work plane planar-parallel with a zero offset to the top face of the cube.  To allow me to identify these 2 important work planes after the part is complete, I change the default color (magenta in the example).  This can be done easily by right-clicking on the work plane in the browser and selecting Properties.  Lastly, we need to create the centerline offset plane for the W beam.  this work plane will be used to center the W beam profile.  When the W beam profile is active, this plane will be flush with the left side anchor work plane.  When the W beam profile is inactive, this plane will be offset slightly from the left side anchor so that the W beam profile is safely hidden within the angle profile.  Create this work plane planar-parallel/offset to the left side work plane.  Enter the parameter plane_off as the offset value.   When you are done, your screen should look similar to Figure 2.

Figure 2.

Step 3 - Create the angle profile.
The next step is to now create the profile for the angle (see Figure 3).   Dimension as shown using the ang_leg and ang_thk variables.  Please note that the 3 fillet radiuses are equal to ang_thk.   The cleanest way to accomplish this is to constrain the fillets to have equal radii and to project constrain the centers of the leg fillets to their respective leg edges.   The profile should then be constrained to the base (dummy) feature.  This is accomplished by by setting the left angle edge line collinear to the left edge work plane and setting the top angle edge collinear to the top edge work plane.  Your model should now look like Figure 3.  Extrude (using the JOIN option) in the same direction as the base (dummy) feature.  Set the extrusion depth equal to the length variable.

Figure 3.

Step 4 - Create the W beam profile.
The next step is to now create the profile for the W beam (see Figure 4).   Dimension as shown using the i_width, i_height, i_web, i_flange and i_rad variables.  Please note that we have used a construction line to act as the center line of the W beam.  The profile should then be constrained to the base (dummy) feature.  This is accomplished by by setting the construction line (center line) collinear to the center line offset work plane and setting the top flange edge collinear to the top edge work plane.  Your model should now look like Figure 4.  Extrude (again using the JOIN option) in the same direction as the base (dummy) feature.  Set the extrusion depth equal to the length variable.

Figure 4.

Step 5 - Link the variables to the spreadsheet.
Using the AMVARS command, link the local parameters to an Excel spreadsheet.   Edit the spreadsheet as shown in Figure 5.  We have renamed the "generic" values to "build" and added values for 4 possible versions for the part, a 4x4x3/8 and 3x3x1/4 angle and a W8x25 and W6x12 W beam.  Update the link and return to Mechanical Desktop®.

Figure 5.

Step 6 - Morph the Part!!!
The browser will now show 5 possible configurations for the part (you can add as many as you like!!!), see Figures 6 and 7.  It is usually a good idea to turn off the visibility of the center line offset work plane at this point because it is no longer required to be visible and it may lead to confusion as you constrain this part to an assembly.  All you need do now is double-click on the version name that you wish to activate in the browser and the part shape will update automatically.  Note that the left constraining work plane is located at the center line of the W beam and at the left leg edge when configured as an angle.

Figure 6.

Figure 7.

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