Creating a Cylindrical Cam Using Mechanism Design Extension

by Dana Coombs, SYNTHES CMF

While Mechanism Design Extension (MDX) in Pro/ENGINEER allows users to create planar cams, there is no built-in functionality for cylindrical cams. This technique shows you how to create a cylindrical cam with a follower. This cam will move the follower between two dwells.

 The general approach is to have a cylindrical representation of the cam geometry and a planar representation of the cam geometry. When the mechanism is defined, the planar cam moves in translation and has a cam connection to the follower. The geometry is hidden from the assembly. The cylindrical cam moves in rotation to represent the motion, but will not drive the follower.

 The key is to drive the geometry of the cylindrical and planar cam with one reference, using a family table of one cam part. In the following example, graph features are used to drive the geometry of both the cylindrical and planar cam. Note that graph features can be applied to predefine geometry of any variable sections sweep, which has proven useful for aerospace applications.

 1.  Define the Cam Profile

The easiest way to define a cam profile is in a 2D representation. This can be done with a graph feature in the cam part, which can then be used to define a variable section sweep to create the profile around a cylinder. The graph feature is defined according to Figure 1. 

Figure 1

2.  Define the Cam Geometry

A cylinder is created and the groove is cut around the cylinder using a variable section sweep. The trajectory is a curve around the perimeter of the cylinder and the orientation is defined as normal to the trajectory. A variable section sweep is used because it can be controlled by the graph feature using a sketcher relation and the “trajpar” parameter. The section is defined in Figures 2 and 3. (For more information on using “trajpar” to define cam geometry, see the PTC customer support document “Creating a Cam With Surfaces Using a Variable Section Sweep and Trajpar.”)

Figure 2

Figure 3

The resulting cam geometry is shown in Figure 4.

Figure 4

3.  Define Planar Cam Geometry

To create a mechanism, the cam groove will be represented in a planar fashion. The same model will be used to create the planar geometry so that the same graph feature can drive the geometry. A surface is made as a variable section sweep using the “trajpar” parameter in a section relation to drive the surface. The variable section sweep trajectory is a datum curve whose length is driven by the perimeter of the cylinder. The perimeter is measured using an analysis feature. Figure 5a–b shows the relation to drive the curve length.

Figure 5a

Figure 5b

Figure 6 shows the section to define the surface, and Figure 7 shows the sketcher relation.

Figure 6

Figure 7

This method provides a robust way to represent the planar cam geometry. The shapes of the cylindrical and planar cams are defined by the same graph feature. This one feature can be adjusted to drive both geometries. The length of the planar cam is driven by the perimeter, which allows the diameter of the cylindrical cam to be modified and automatically drive the planar geometry.

4.  Create a Family Table to Separate the Planar and Cylindrical Geometry

The planar cam will become an instance of a family table by adding the solid geometry features (the cylindrical protrusion and the variable section sweep cut) to the table and suppressing them in the table. Figure 8 shows the family table.

Figure 8

The generic, named Cylindrical_cam, and the instance, named Planar_cam, can both be assembled in a mechanism assembly. 

5.  Assembling the Mechanism

It is a good idea to use a part as ground for a mechanism instead of the assembly. In this case, the ground part will be a skeleton model that will provide the needed references for the moving parts. Figure 9 shows the skeleton model. Notice the planes and axis that provide references for pin and slider joints.

Figure 9

The first moving part to be assembled is the cylindrical cam, named Cylindrical_cam. This is assembled using one pin joint. Figure 10a–b shows the joint connection.

Figure 10a

Figure 10b

The second moving part is the instance of the family table, named Planar_cam. This part is placed using a slider joint. This part will slide and represent the “unraveled” cylindrical cam. Figure 11a–b shows the slider connection.

Figure 11a

Figure 11b

The third moving part is the follower. The follower will be driven by the cam motion. Figure 12a–b shows the slider connection.

Figure 12a

Figure 12b

The last remaining joint is a cam-follower joint. To create this joint, the mechanism functionality must be accessed from the applications menu. The cam-follower joint is created by selecting the surfaces of the cylinder on the follower and the cam surface on the Planar_cam. Figure 13a–b shows the cam-follower connection.

Figure 13a

Figure 13b

6.  Defining the Mechanism

Now that the components are assembled using mechanism connections, the mechanism must be defined with motors and joint zero positions. The first motor is defined on the pin joint of the Cylindrical_cam part and ground, defining how the cylindrical cam rotates. The other motor is defined on the slider joint of the Planar_Cam, defining how the planar cam slides. Notice that the functions are tied together. The planer cam will translate the distance equivalent to the perimeter of the cylindrical cam. Figure 14a–d show the setup of the motors.

Figure 14a

Figure 14b

Figure 14c

Figure 14d

Next, zero positions are defined for the connections. The zero position for the planar cam is defined by aligning the end of the cam to the assembly datum plane (located at the center of the cylinder) for the slider joint. The zero position of the cylindrical cam is defined by aligning the front datum planes of the cylindrical cam and the skeleton part on the pin joint as shown in Figure 15a–d.

Figure 15a

Figure 15b

Figure 15c

Figure 15d

7.  Analyze the Mechanism

The mechanism is fully defined and now it is time to analyze the motion. An analysis is created as a kinematic analysis with a start time of 0 and a run time of 1 second. This corresponds to the drivers so that the cylindrical cam will rotate 1 revolution. The frame count is set to 100. Figure 17 shows the analysis setup.

Figure 16

The resulting animation shows the cam follower moving in relation to the cylindrical cam. Select the image below to see the animation.

Conclusion

This technique provides a method to animate a cylindrical cam and gather measured information from the model. Since the cylindrical and planar cams are tied to one generic part, the mechanism model can be taken further. The profile of the cam, which is driven by the graph feature, can be changed and optimized based on kinematic or dynamic measures.   

Dana Coombs is a product development engineer at SYNTHES CMF in West Chester, Pennsylvania, USA. He can be reached by email at Coombs.Dana@synthes.com.