Torque driver for setting correct tension on 6mm belt on 3d printers.  It is designed to limit torque- hence pulling force, on an M3 screw.  Tension is calibrated using a fixed weight (1.25kg).  The weight is based on the figure described in Lost in Tech's video

If you wish to tension the belt through the idler pulley,  double the weight in calibrating the driver.

Build guide video

Hardware required:
- Spring from a retractable ball pen (clicky pen)  4.5 mm diameter x 20 mm long (relaxed)
- Steel ball x 2- 6.35 mm or 1/4 inch diameter.  Soft air 6mm BB pellets can also be used although the sound will be softer.
- M3 Allen Socket head screw - 25mm or longer with full thread
- M3 nut
- For calibration piece, M3 Allen socket head screw 25mm or longer, flat washer and nut.

Belt tensioner used is available here