Suggested Technique for Creating a Cam with Surfaces Using a Variable Section Sweep and Trajpar in Pro/ENGINEER

This procedure outlines the creation of a cam with cycloidal motion using a variable section sweep driven by the "trajpar" function. This technique may also be applied to other models whose geometry is dictated by an equation. The optional module Pro/SURFACE is required to complete this exercise. The goal of this procedure is to create the cam illustrated in Figure 1.


Figure 1

Figure 2


Figure 3

• Begin by creating the trajectory of the cam follower. The trajectory will be represented by a set of surfaces.
• First create two extruded surfaces representing the dwell areas of the cam.
• Select DTM1 as the sketch plane and specify the depth of each extruded surface up to datum plane TOP. This allows the thickness of the part to be completely driven by the offset dimension of DTM1. Figure 4 below shows the dimensions and dimensioning scheme used for the 150 degree dwell area.
• Use the same dimension scheme for the 60 degree dwell area.
• After the two surfaces are created, modify the dimension symbols of the overall angles and radii in order to clearly identify the dimensions for use in a relation in a later step.
• To modify the dimension symbols, select the feature, then hold down the right mouse button and select Edit.
• Next, select the dimension to modify, hold down the right mouse button and select Properties. Modify the dimension name from the Dimension Text tab of the Dimension Properties window. Figure 5 below, shows the dimensions for each surface with the desired dimension names.

Figure 4


Figure 5

• This and the next step will create the surface representing the cycloidal motion area of the cam.
• First create sketch features to be used as the origin trajectory and x-vector of the variable section sweep (see Figure 6, below.) Create the first sketch on DTM1. The section is comprised of two centerlines. Each is aligned to both a vertex of one of the extruded surfaces and the axis A_1. The arc is sketched from centerline to centerline with an arbitrary radius (1.0 in this example.)
• Once the sketch is created, modify its dimension symbol to "traj_radius" for use in a relation in the next step.
• Create the second sketch on the TOP datum plane with a section which is simply a Sketch > Edge > Use of the first sketch.

Figure 6

• Now create the cycloidal motion surface using a variable section sweep. Select Insert > Variable Section Sweep... to begin creating the feature.
• Select the curve on DTM1 as the Origin trajectory, and the curve on TOP as the x-trajectory. The section of the sweep, shown in Figure 7, is comprised of two vertical centerlines (one through the origin trajectory and one through the x-trajectory) and a horizontal line with a vertical dimension.

Figure 7 

• Drive the single section dimension by a sketcher relation.

• While still in the sketcher mode, select Tools > Relations... to open the Relations window.

• Enter the following relation (substitute the dimension symbol in your section for the left hand side of the equation):

sd4=(R1-TRAJ_RADIUS)+(R2-R1)*(trajpar-1/(2*pi)*sin(360*trajpar))

 This is a standard equation for generating cycloidal motion. Here are the meanings of the symbols in this relation:

• R1 - Radius of the extruded surface at the start of the sweep
• R2 - Radius of the extruded surface at the end of the sweep
• TRAJ_RADIUS - Radius of the trajectory curves
• pi - A system parameter for the mathematical pi constant
• sin - The mathematical sine function
• trajpar - This is the key to the relation. "trajpar" is a system generated parameter which linearly increases in value from 0 to 1 as the section is swept along the length of the spine trajectory.

See Figure 8, below for the completed surface feature (highlighted in red.)


Figure 8

• Because this cam is symmetrical, the cycloidal surface can be mirrored using a Surface Transform.
• Select the cycloidal surface, then select Edit > Mirror. Select datum plane FRONT as the mirror plane and finish the feature.
• Next create a composite datum curve feature from the edges of the four surfaces where they lie on DTM1. This curve will serve two purposes: one, the curve can be referenced in the next step, and two, the curve could be referenced as the location of the center of the follower in the cam-follower assembly.
• To create a composite datum curve select one of the surface edges then hold down the <Shift> key and one-by-one select all four surface edges. Then select Edit > Copy followed by Edit > Paste and finish the feature.
• Lastly, create an extruded protrusion from DTM1 to TOP to form the cam surfaces. The cam surface should be offset inward from the follower trajectory by the follower radius. For this example we will assume the offset to be 0.5 inches. While in sketcher mode, select Sketch > Edge > Offset... and select Loop from the Type window.
• Select the composite curve created in the step above and enter 0.5 as the offset dimension (or enter -0.5 depending on which way the offset arrow is pointing). With this feature, the cam is complete.

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