Just stubled across this thread . . .interesting project. The arm is a work of art . . . I want one just to put on my desk and look at!
I don't see where a momentary 2k lb static load on the spindle bearings will do any harm, but the mechanism to prevent that is pretty clever. A sizzor arrangement would also work and not require an additional actuator. Just thinking out loud . . .
The Tormach setup is slick. First I've seen it. Tormach most certainly does know the drawbar force (or range) required . . it's a liability issue to advise on that, I would imagine. Certainly, the quality and accuracy of the spindle taper and the collet are going to have a significant effect on the force required, as will the presence and type of oil on the holders.
2,000lbs seems like a reasonable number to me for R8.
It will be fun to follow this thread . . . very project.
"A sizzor arrangement would also work and not require an additional actuator" - I considered that, but couldn't see a way to avoid the extra actuator, as the drawbar must be aboe to move down when not changing tools. If you see a way to do without, can you describe it?
Here's my question to Tormach tech support:
I have a Bridgeport-clone knee mill for which I want to make an automatic tool changer, using Tormach toolholders in an R8 collet. Can you advise me on the optimum drawbar tension?
And here's the response:
Funny you should ask that question. We have been trying to determine the optimum draw bar tension for a R-8 collet and have not been able to put an accurate description together for it yet. If I had something I would be happy to give it to you but I don’t. If you find something we would appreciate you letting us know. It seems to be a mystery of the industry that no one can put a real number on, but develop a feel for it working with it every day.
I have been completely unable to find *any* credible information on drawbar tension for R8. I have found several references to drawbar torque (typically 25 ft-lbs, or thereabouts), but nothing about tension, other than guesses. For real ATC systems (CAT/BT/etc), the numbers are in the 1000-3500# range, depending on which one but that's such a different animal, it's really not helpful. The only issue is getting enough tension to keep the tool from slipping. Since I rarely work with anything over 1/2", other than face mills and flycutters which are pretty low torque, I doubt I ever exceed perhaps 1/2-1HP, which is pretty low. I think about 1000# would be adequate for that. So, if I can make 2000, I'll have plenty of margin. First thing I'll do is take some really heavy test cuts, maybe up to tool breakage, and see if I get any pull-out.
Regards,
Ray L.
Regards,
Ray L.