metalworking pillar drill experiences

[Robin Hewitt]

I am not good at explaining things and my drawings are functional rather than informative...

The knee originally had a 6mm pitch, left-hand screw and a 3:2 bevel gear reduction to give 4mm vertical travel per rev.

I fitted a 5mm pitch, right hand ball screw. First it connects to the stepper motor with a toothed belt, then it turns through 90deg using a 1:1 spiral tooth form bevel gear to reduce cogging. Then a 5:4 spur gear reduction. The new gear shaft, goes forwards, where it gets the rather nice 4mm satin chrome dials the mill came with and a Bridgeport style dog clutch and crank. Going backwards it has the sprag bearing keyed on to it. The outside of the sprag bearing has a friction brake. I used bits of timing belt for brake shoes.

When I wind the table up the sprags turn freely and do nothing.

When I wind the table down, or when it tries to wind the table down, the sprags dig in and it resists.

I try to adjust the brake so it feels the same going down as it does for up.