Power Drawbar Getting Stuck

[RJS100]

I have been using a pneumatic power drawbar for about 6 months. It uses a ram to press against a stack of Belville washers that pull up on the drawbar. The drawbar is connected to an Tormach/Lyndex R8 collet that I use for TTS tooling. For the most part it works great.

I have had a problem a few times when I don't use the machine for a few weeks and find that the R8 collet sticks and won't release. To make sure there are no tiny burrs, I very lightly wet sanded the collet and spindle with 2000 paper. I then re-apply ant-seize to the collet and all was good. It worked better than ever!

It worked great for a few months, then I did not use the machine for 3+ weeks and once again it was stuck when tried to use it.

I examined the inside of the spindle and could see some marks where the slots in the R8 collet have been rubbing against the inside of the spindle (Please see below). There are marks like this for each of the three slots on the R8 collet. Does this look normal? If not, any idea as to how to stop these from happening?

Thanks... Richard

[Bham-Tech-RM]

Unless you've made a habit of full-rapid drilling (jk) I am going to guess that your pneumatic drawbar is exceeding the recommended torque for your spindle. Basically draw-forming the spindle bore to match your collets... those pneumatic units are just repurposed impact drivers, and can generate vastly more torque than is required for a drawbar. The mating taper angle does the clamping, the key does the driving, and the draw bar just needs pull the tapers together with a modest force.

The spindle will have a minimum-recommended-maximum torque range or draw-force recommendation. The pneumatic unit should have/come with a chart to relate input pressure with torque output. Make sure your air-pressure regulator is set appropriately.

How's the runout on your tools now?

[RJS100]

Thanks for your thoughts. The chart below shows the torque vs number of turns used to load the Belville stack. Previously I had set it at 10 turn to provide 25 ft lbs of torque. I run my mill with very conservative speeds and feeds and primarily use high helix tools for milling aluminum so this time I set it for 8 turns or 17 ft lbs. of torque.

Precision Matthews did not think these marks were unusual, but I very very lightly wet sanded and smoothed them with 4000 grit paper. My spindle run out is about +/- .0002 and the tooling run out was quite small. I changed to a better brand of anti-seize when I reassembled it and used more of it. It seems to be working much smoother now. I've attached a document that I found that was helpful.

I guess only time will tell if this think locks up again. Thanks again for your thoughts.... Richard

[RJS100]

OK. I may have finally found the culprit. I observed the drawbar working fine, then suddenly freezing up. Then working fine again. This made me think that it was not a lubrication issue at all. Looking closely at the setup, I realized that the shaft that extends out of the pneumatic ram and pushes down directly on the drawbar was not perfectly aligned with the drawbar.

With the drawbar fully frozen in place, I manually aligned the ram shaft, so it was perfectly concentric with the top of the drawbar. I activated the drawbar once again and the tool popped out immediately.

I believe the problem was that if the shaft is not concentric with the drawbar, the actual drawbar probably bows slightly along its length causing the tapered portion of the collet to jam in the spindle. Just my theory. Time will tell.

Best… Richard

[suntravel]

From my experience with collets, last 30 years, if they stick the angle is mismatching, most load should be at the major diameter.

If the spindle gets marks, it is way too soft, its surface should be harder than the collets.

Uwe

[PhilipTrueman]

If you have recently replaced the belville washers they will bed in slightly and will probably need adjusting again. My pneumatic cylinder ram has an adjustment on the end so adjust the travel of the ram. Why has your drawbar got bellville washers if the drawbar screws into the end of the collets?
On my mill the washers are there to pull up the drawbar BT40.