Quote Originally Posted by Clive S View Post
I to am following this with interest. Have you decided on the size and number of the bellevilles? Also could you give more details on the drive arrangement for the PDB as I am making one for my mill but using an intensifier (small jack) to give a 7:1 advantage with 12 belleville washers. From a post on the Zone by SDM
Hi Clive,
I'm only 50/50 on making the spindle+PDB as it is quite a risky project, and could well end up just using the WC spindle as direct drive. But in terms of spec for the PDB system I've not finalised on all the details. It does seem quite hard to find definitive numbers on things like tool pre-load.

I'm away from my home PC at the moment, where all the data is, but from memory I'd put in the same number as the Tormach PDB kit shows (~6?) since I would be using the same R8 collet style. I've read that the load applied using the Tormach PDB kit is OK for general work but could be on the low side and an aggressive cut could pull it out of the taper, so I could need more. They don't want to share the spec details (understandable) for the bellevilles so no more data than this. I can add more than this without a problem.

I've downloaded a calculator to work out loads depending on series or parallel arrangements and also give the total travel until it bottoms out. This is where the tricky bit is to ensure the loads and travel all work out.
I had allowed for 10mm total travel of the drawbar to release the tool, of which 6mm is used to remove the pre-load off the taper, and the remaining 4mm is to push it out and hopefully release the tool.

The mechanical drive system needs a lot of force/torque, so the 7:1 jack you are thinking of is a good start. I currently have a geared down stepper driving a 1605 ballscrew. I did look at a 1204 screw to get more ratio but the axial forces are very high and I didn't want go that route in the end. Again from memory it needed at least 8 Nm to turn the ballscrew. That's all I have for now.