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View Full Version : Drawbar Springs - Belleville or Die Springs



Davidimurray
05-01-2017, 06:15 AM
Hey everyone.

I see lots of info around on homemade drawbars and most people seem to use Belleville washers. I wondered why as Die springs seem surprisingly cheap - https://www.springmasters.com/diesprings/extra-heavy-rectangular.html with a great range of spring forces etc.

Is anyone out there using die springs or any reason not to use them?

Cheers

Dave

spumco
05-11-2017, 01:23 AM
Spring rates available with Belleville/disc springs are significantly higher than die springs given the same space limitations.

It depends on your drawbar tension requirements. I'm using an R8 spindle with the TTS system, and I torque the 7/16-20 drawbar to 25ft-lbs which gets me about double the 'standard' retaining force published by Tormach. To get the preload and travel I need Belleville springs were required. The major limitation, of course, is that I wanted to get the spring stack inside the spindle adapter thingie I made.

With coil (die) springs, you could make it work, but it'd look like you had a car strut on top of your spindle. More weight, bigger OD, etc. Not good for something that is spinning at 5-10kRPM.

On the other hand, if you're using a pull-stud CATxx system you may not need as much spring force as in my case, but you'd need quite a bit more travel to permit the balls to retract or petals to release (or whatever is retaining the stud). I only need about 0.050" of travel (or less) to release the tool holder. The pull-stud arrangements need much more travel - you can see photos of some of the grown-up VMC drawbars with 110 disc springs stacked up.

The alternative is to do away with springs entirely and use a device to tighten the drawbar to a given torque/bolt strain. However, since you're asking about this in the Auto Tool Changer section, my guess is that you're interested (as am I) in eventually buying or making an ATC. This kind of rules out the air-ratchet thing used frequently on Bridgeports and similar R8 spindles.

The only alternative I've seen compatible with an ATC is one that uses a gear-reduction stepper motor to tighten-loosen the drawbar. Stepper is disengaged from the spindle while it's running, and then it moves down and loosens the drawbar the correct number of rotations (steps) to release the collet without completely unthreading the R8 collet. I think I read that he even sets the stepper so that it always has enough torque to loosen the drawbar.

One of the forum members here came up with it and is selling them for Novakon (or Syil, can't remember) and it's unbelievably cool.

For me, just a big stack of Bellevilles, a 8:1 ratio hydraulic intensifier, and an 80mm single stage air cylinder running about 50-60psig. Been working well so far.