Solution: one Nema-17 stepper motor, a 3-phase drone motor to spin a cut-off disc, an electronic speed controller (esc), a nose hair trimmer, ramps 1.4, atmega 2560, support frame, bluetooth adapter, mobile phone with bluetooth
I saw an old broken nose hair trimmer and had a look inside.
It had a 1.5v powered motor with an eccentric cam.
If a photo-interrupter were somehow moved by the cam then a stepper motor could be used to control the "sweet spot" where the signal from the photo-interrupter has a 50% duty cycle, right on the edge of the abrasive disc, while it spun.
Another stepper motor would control the height of the disc from the work piece.
I thought of combining these to use just one stepper motor but I figured that being able to adjust the inset or outset of the cut would allow the process to be done in stages.
The cog or what would end up being a cog is secured to a shaft that's free to rotate.