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Author Topic: Advice on TTS Power Drawbar Design.  (Read 4397 times)

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Offline tjhj

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Advice on TTS Power Drawbar Design.
« on: March 30, 2010, 11:36:28 PM »
Ok So in consideration of using the Tormach Tooling System, I have embarked on the journey of some sort of a drawbar for a quill type machine. In my case it is an X3.
The unknown question is how much force is actually needed on the drawbar. So I have taken a different approach to how much force. Instead of trying to figure out how much torque is needed then the problems with the screw calculations. I solved how much force it would take to yield the drawbar. I think most of us don’t pull our drawbar our and find it necking from yielding. Referring to the AISC Manual for yield strength and data.

Assuming that the drawbar is of average quality steel. A36. The yield strength is 36 KSI. (if it works for skyscrapers, Ill take it)  My Drawbar has a nominal diameter of 7/16”. (Is this always the same? Or Close?) And a ф 0.9 Factor of safety /error. The resulting force to cause permanent deformation is 5411 lbs. Now taking the assumption we are not riding on this limit. 50% should suffice. So 2705 lbs, lets round to 3,000lbs of force and say no more than 5,000lbs. It can be increased later.

So here is the idea, a stepper motor, because it can hold the torque for prolonged period of time. I have a 420 oz-in Nema 23 Stepper motor (just an extra). Use the motor to apply the force to the drawbar while cutting, vs something like spring washers.

Now how to do it?
Basic Idea, the two primary plates would be pushed apart pressing on the top of the quill and bottom of the drawbar. The drive “screws” would be 5tpi Thompson ball screws. 0.625 Track Diameter. One screw on each side to ensure a symmetrical application of force. In the Force calcs from the ball screws I used an assumed efficiency of .8
Torque(lb-in) = Force(lbs)*Pitch(in)/(2*pi*eff)

Version 1. Use a series of timing belts to reduce the speed for more torque. Common SDP timing pulleys allow for a two step total ratio of 10.24 multiplication. Through the ball screws the motor would be able to well exceed the required force, and produce a maximum of 6755 lbs. Should cover it.

Version 2. Instead of using a complex scheme of timing belts use of a worm gear drive to create the torque multiplication, which can easily obtain the required torque or even use a much smaller motor.

It does require some guide to stop it rotating, but can be added later.

So here is the idea in more of a whole. Attached are a series of Solid Works Pictures to help portray my idea.

Is it feasible? Will it Work? Comments? Suggestions? Ideas?

Thank you for your time,
TJ

PS. The CAD is just a rough idea so it can be portrayed and seen better than I can explain.

Offline tjhj

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Re: Advice on TTS Power Drawbar Design.
« Reply #1 on: March 30, 2010, 11:41:23 PM »
Here are the cad pictures of the belt drive version/ and an video of explode to help make it easier to see what is going on.

Thank you,
TJ
« Last Edit: March 30, 2010, 11:43:57 PM by tjhj »

Offline tjhj

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Re: Advice on TTS Power Drawbar Design.
« Reply #2 on: March 30, 2010, 11:45:24 PM »
And finally the Video file.

files.me.com/tjhj/ cyozek

Thank you for the time,

Any suggestion would be greatly appricated.

TJ

Offline Hood

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Re: Advice on TTS Power Drawbar Design.
« Reply #3 on: March 31, 2010, 03:17:44 AM »
I would be very wary of relying on a stepper, or for that matter any other means such as air or hydraulics , to keep the drawbar held. It is just not a safe way to do things as if the power to your stepper fails your drawbar will release, this could be nasty if the spindle is rotating at the time.
 The common way to do things is have a belleville washers to keep the tension on the drawbar and only use the power drawbar (motor, hydraulic, air etc etc) for release.
 If you are determined to proceed I would say you should think of some mechanical means of locking it in place so that once the stepper has tensioned the drawbar  it wont matter if the stepper loses power, it will complicate things but may just stop an injury.
Hood