A standard rotating-cam is a closed shape. The cam-coordinates move uniformly along the cam-profile as the MMA increases from 0 to 360. The XYZ Points do not self-intersect or repeat (unless there is undercutting).
However, with a Slot-Cam* the contact-point may/will not move along the cam steadily. The Cam can be stationary, asnd often will retrace its path with the roller moving in the opposite direction. Thus the coordinates will repeat the same profile.
You cannot create a sketch from these coordinates.
This topic describes how to solve this problem.
* Any Cam in which the contact-point does not move steadily along the cam.
There are two possible complications:
A.The Cam-Follower changes its direction, or dithers*.
If the cam-follower changes its direction, the contact-point of 'Cam 1', as calculated by MechDesigner, moves from one cam-flank to the other cam-flank
*'Dithers' : small directional changes
B.The Cam-Follower becomes Stationary
If the cam-follower becomes stationary relative to the cam - usually at each end of the Slot-Cam - the contact-point also becomes stationary. While the contact-point is stationary, the cam-coordinates continue to be calculated as the MMA increases.
When the cam-coordinates have complications as described in A or B, or A and B, SOLIDWORKS cannot create the Curve feature.
Example Calculations with Slot Cams
Example of a cam with the two complications.
Refer to the image to the left:
Sliding-part moves horizontally. A 'connecting-rod' joins the sliding-part to a point that has a complex 'looping' motion' as identified by the Trace-Point.
Thus, the sliding-part moves to the right and left, then right again but with a complex motion, rather than move with a simple motion that is to the right and then to the left.
Each time the sliding-part changes its direction, the contact-point moves from one cam-flank to the other cam-flank.
A line is drawn between the cam-flanks each time the contact moves across.
SOLIDWORKS will NOT be able to create a Curve Feature with this Cam.
Before we export the cam to SolidWorks, we must remove the two 'complications'.
There are two ways:
•Manually edit a text file of the cam-coordinates - which can be difficult, but possible.
•Linearise the motion between the cam and cam-follower.
Linearise is the best option.
Linearise the Independent Input to the Cam.
We need two models - the Original-Model and a Dummy-Model.
STEP 1: Measure the full range of the sliding-part in the original-model.
Original Sliding Range = Maximum – Minimum
We can use a Measurement FB and a Graph FB to measure the Sliding-Part Range - just note the Minimum and Maximum values.
STEP 2: Build a Dummy-Model with a Dummy-Slider and a Dummy-Rocker.
Important: The input to the Dummy-Slider should increase at a constant rate ( Constant-Velocity ).
In one machine cycle, the Dummy-Slider should move the same distance and range as the Original-Slider
To do this:
1.Connect a Linear-Motion FB to the input of a Gearing FB. The output from the Gearing FB is connected to the input of the Motion-Dimension FB of the Dummy Slider.
2.Edit the Gearing FB and enter for the Gearing Ratio : Original Sliding Range / 360 (the numerical sliding range, not the words!)
The output from the Gearing FB will increase steadily: From 0 to Real Sliding Range (mm) in one machine cycle.
If, with the original model, the Maximum Measurement is 340mm, and the Minimum Measurement is 20mm, then the Range is 340-20 = 320mm.
Thus enter the actual equation for the Gearing Ratio : 320/360 . The Gearing Ratio will equate to 0.88889. (You can also, if the Range is an integer value, use the Pulley Teeth parameters: Enter enter 320 as the output-pulley and the 360 as the input-pulley).
Connect wires from the output-connector of the Linear-Motion FB to the Gearing FB, and to the input-connectors of the Motion-Dimension FB.
Now the Dummy-Slider will move with Constant-velocity from 0 to 320mm
Now, the Cam-Coordinates will not change flanks.
When you export the cam, SolidWorks can create the Curve entity and the Cam feature.
Note: You may need to delete the last point in the saved cam-coordinate data or from the Curve feature in SolidWorks.
You can also save the Cam-Coordinates as a text file. and Manually edit the values.