By Shaun Griffiths
Creating a keyway in a shaft can be completed using work planes. However, you do not need to create a lot of geometry to build a tangent plane for your sketch.
You can use the option to create a plane “Tangent to Surface and Parallel to Plane”. However, this may appear odd as there are no visible planes parallel to the tangent face.
However, if you turn on your origin planes, you should have planes that are appropriate running through the shaft.
This will then allow me to create a plane tangent to a surface and parallel to a plane.
Draw an appropriate keyway sketch.
Then extrude the final feature.