M3 Screw Hole Thread Tapping Machine

Need to tap 50 threaded holes in printed parts by hand? I put the tap bit in this machine, powered by a 9V tape cassette DC motor, to make the work quicker and more precise. The bit goes down and up while rotating, guided by an M3x50mm screw fixed on it, up to 2 cm distance. Currently there is no safety mechanism, if going too far up or down it can get stuck and something may break.

Note that there are different sizes of the unthreaded part of the tap bit, this design will fit to only one size.

###Printed parts
top, bottom, motorholder, nutholder, gear1, gear2, gear3 x1
side x2, mirroring not necessary
The blocks are optional, i design them to accomodate various sizes of the workpiece.

###Additional parts
9-12V tape cassette DC motor, stripped of the electronics. Open the back cover, detach the controller board, attach cables to motor terminals. I used a DPDT ("on off on") rocker switch to apply 12V DC with reversible polarity to the motor.
M3x10mm screws are used to assemble the pa