4th Axis End Mill Cusp

Trying to use my 4th axis to cut a slot that goes all the way around a shaft (but changes axial position as a function of angle). It's basically a barrel cam. Anyways, I've discovered that trying to make a cylindrical shape using an end mill isn't going to work because the flutes have a relief in the center. Since you are cutting on the center, this is kind of a problem (it's not a problem at all when you cut a flat surface). Has anyone used any truly flat end mills to take care of this problem?

Attached is a picture of what I'm shooting for. On the left is what I want to achieve, on the right is what I'm actually getting.

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