David Edward Gaskill offers a method for finding the true center of a slot using working planes in Inventor.

“Have you ever wanted to add an axis or two work planes on a slot so you can have a true center of the slot? Below are instructions for how to do this.

First, open the part with the slot on it.

Pick Plane from the Assemble ribbon.

Select a face that is parallel to the desired work plane. In this case I picked the left-front face.

Then pick as close to the center of the slot as you can. A node should appear and a preview of the new plane should appear. You now have a plane center on the slot. It will update as the slot moves or changes in size.

Repeat the process, this time picking the right-front face and the center of the arc of the slot. You will see the node and the plane previewed. You will now have two perpendicular planes centered on your slot.

Now create an axis from the two planes, and you will have an axis centered on the slot.

Pick Axis from the assemble ribbon. Then pick both planes you just created.

If needed, repeat the process for the other mating piece slot. More often than not, adding them to one part will address your needs.

One last tip: Should you switch to your assembly and not see the work planes, be sure to turn on their visibility.
Go to the View Ribbon tab and check all work features needed.

Notes from Cadalyst tip reviewer Brian Benton: There are many ways to solve a problem. Inventor provides working planes that can be used, as in this tip, to find the center of the slot. Working planes aren’t object parts but planes that provide a place to work with as well as a visual guide.