A mill isn’t really ideal, but you can read this thread about how someone else attempted to do this:
You can measure the bearing seat of a yoyo you own to create the drawing in your designs. There’s many ways to approach the guts/yoyo design, I talk about one method here: https://www.youtube.com/watch?v=krm_Jlv9g-c
There are several other videos on my channel that may interest you.
