The image below represents a task that usually throws people for a loop. It’s an irregular shaped shaft with a keyway slot cut into it. The blue portion is simply the bottom of the keyway, and it’s highlighted in blue to make it easier to see. The keyway is .150 of an inch deep, and it follows the contour of the shaft. So the depth of the keyway is uniform throughout.



The most common mistake in making a keyway like this is to attempt to make a Sweep along a 3D spline. This usually causes more problems than it solves, and it’s really not necessary. All you need to do is offset the profile of the shaft to make the sweep path.

Create an irregular shaped shaft by constructing half the profile, and then revolve the profile about the bottom horizontal line 360-degrees.



Once you’ve revolved the feature, share and edit the sketch. To create the sweep path, follow these steps.

1. Offset the profile toward the inside of the profile, and set the offset to .150.
2. Delete the straight lines of the offset. Only the curved lines of the offset should be left.
3. Draw and constrain vertical lines for the ends of the keyway.
4. Trim the curved lines to the vertical lines.
5. Add .188 radius fillets where the vertical lines intersect the curved lines.

When you’re finished, the sketch should look something like this.



Now draw a horizontal construction line from the end of the highest vertical line, and dimension it.



The construction line will be used to create a Work Plane and Sketch so that you can construct the profile of the keyway. When you’re finished, click the Return button.

Now it’s time to make the sketch plane and profile for the keyway. Follow these steps.

1. Make the XZ Work Plane visible.
2. Create a Work Plane by selecting the XZ plane, and then select the horizontal construction line. Set the angle of the new Work Plane to zero so that it is parallel to the XZ plane.
3. Create a Sketch on the new Work Plane.
4. Project a node onto the sketch plane by projecting one of the vertical lines in the first sketch.
5. Constrain a rectangular profile for the keyway to the projected node.

When you’re finished, the sketch should look like this.



The last step is to cut a sweep of the keyway profile along the path.

Why is this important?
This example uses a procedure I like to call The Partial Offset Method. An offset is created, and then the portions of the offset you don’t need are discarded. This method can be applied to create many different types of profiles that have the same shape as other profiles, and it’s a valuable skill to learn.