CAD- locating exact 3dpoint on primitive

I was wondering if anyone has experience in CAD projects.
Basically I have a rendered axis system, and I would like to be able to let a user click on any point on the 3 axis to insert a component.
I have used the selection buffer in previous use for picking components in the environment. Now I would like to not only identify the axis (3 primitives for the X, Y, Z line axis), but find out exactly where I click on it. The Minimum depth values in the selection buffer are great, but du to non-linearity, this might be more tricky than I envisioned.

Does anyone have any good tips on any other way to do this?