cnc planer

I am interested to build a slotting attachment on a milling machine. so what I would like it to do, is to let the y axis move away from the material on the up stroke, and move towards the material before the up stroke is tdc. The tool doesn't need to clear alot, just enough for the tool not to rub on the material, like 0.1mm is enough.

So, I am intending to have a encoder running on the rotating spindle which will drive the ram up and down with a cam, so let's say 0 degrees is tdc, it needs to move the y axis say 0.02mm positive, and stay there up to 180 degrees, then move the y axis 0.1mm negative, between 180 degrees and 200 degrees, then when it reaches say 330 degrees it needs to move 0.1mm positive, up to 0 degrees, then move 0.02mm positive for the next cut, and repeat the cycle, as if the y axis is on a cam, like a planer would operate.

I hope someone has done something similar, anycase I'll go ahead with my mechanicals so long, propably another month before I'll start playing around in linuxcnc,

Please let me know if you have some ideas.

I currently use my z axis and m19(spindle orientation lock) to do internal keys, running a sub program, but I imagine that my bearings and ball screw is eventually going to pack up.