I'm considering building an auto tool changer for my home built CNC mill (based on an Alexander engraving machine) which takes ER32 collets directly in the spindle.

The taper on the ER32 is very similar to an R8 taper which is commonly used with the Tormach TTS toolholders.

I am thinking of making a special collet with an 8 degree taper (as ER32) but with a plain section and 12mm thread for a drawbar which will be pulled up with Belleville washers.

Attachment 304476

I would make the collet out of a decent tool steel but don't contemplate hardening it as I don't have the facilities.

The spindle has a 3 step poly-v pulley arrangement and with a nominally 0.5 hp 3-phase motor and VFD can run anywhere between 200 and 10,000 rpm.

Can anyone think of any good reason why this wouldn't work?

I don't like the idea of a pneumatic actuator for depressing the drawbar as my compressor is deafeningly loud and it frightens the life out of me every time it starts. I do like the idea of a ballscrew driven by a stepper with e.g. a 1:4 toothed belt drive or even a worm gearbox to sufficiently amplify the available force. I haven't tried to do any calculations but I'm thinking I will need a force of around 5,000N (roughly 1,200 lbf) to compress the Belleville washers. I'm not too worried about speed of operation as long as it doesn't take minutes to open the collet. Is this a dumb idea?

Cheers.

Phil.