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A Power Drawbar Like No Other....
« on: May 12, 2012, 09:27:13 PM »
I had a crazy idea for a completely different kind of power drawbar a couple of years back, and yesterday I finally had the time to build one. I specifically wanted to be able to use both TTS tooling, as well as R8. This meant Belleville spring designs were out, as it is very difficult to provide enough drawbar tension to properly retain TTS tooling under worst-case conditions (about 2500+ pounds drawbar tension, or 25+ ft-lbs drawbar torque is required). To change out a TTS tool, the drawbar only needs to be backed off about one turn, while for R8 tooling it needs to be backed out 8-10 turns.

So, here's my crazy idea - use a stepper or servo motor, driving through a high-ratio planetary gearbox. This provides very high torque capability, and makes it easy to control how much the drawbar is turned, by driving the stepper as another axis from the CNC controller. A little E-Baying got me a 55:1 NEMA34 gearbox capable of 75 ft-lbs. I picked up a 400 oz-in NEMA34 stepper, which lets me use the whole 75 ft-lbs if necessary. Mechanically, the thing is dead simple - The stepper and gearbox are mounted to a floating plate which is moved up and down by air cylinders. A 3/4" socket is attached to the gearbox output shaft by by a simple home-made coupling, and either a mechanical or pneumatic mechanism is used to actuate the spindle brake. I got a dual-shaft stepper so I can put a rotary encoder on the shaft, to monitor actual movement, if needed. Everything can be controlled by a simple macro (or, in my case, a simple KFlop C program).

Here's how it will operate:

1) Spindle stops, spindle brake is applied
2) The PDB is dropped down, with low air pressure, and the stepper is rotated slowly, until the socket and drawbar hexes align and engage. A micro-switch will signal that the socket is fully engaged.
3) The stepper is turned at high current/torque to loosen the drawbar by the required amount - one turn for TTS tools, 8-10 turns for R8 tools.
4) Once the new tool is mounted, the stepper is turned at reduced current/torque (corresponding to 25 ft-lbs drawbar torque), until motion stops.
5) The PDB is raise up, and the spindle brake is released.

The photos below show the complete mechanical assembly, missing only the engagement microswitch and spindle brake actuator. Here's a short video showing the PDB moving up and down while mounted to the mill:

http://www.youtube.com/watch?v=saGv_deZmL0&feature=youtu.be

I just spun up the stepper for the first time. On Monday, I hope to get the software going, and be able to "use it in anger" for the first time.

Regards,
Ray L.
Regards,
Ray L.

Offline BR549

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Re: A Power Drawbar Like No Other....
« Reply #1 on: May 13, 2012, 11:29:21 AM »
HIYA RAY interesting approach but it is not new. The problem was getting enough torque to consitantly tighten the collet and having enough torque to LOOSEN it back up. SOMETIMES it takes more torque to loosen than tighten and THAT is the reason everyone WENT to an impact wrench (air or power) to ensure it got tighten enough AND that it would loosen it back up consistantly.

Even saw one that used the spindle motor to rotate a NUT that tensioned the drawbar. The machine shifted into low range for more torque and A fixed plate dropped down on the nut and held it and then the spindle rotated to loosen it then retighten it. Then shifted back into high range.

Just a thought, (;-) TP
« Last Edit: May 13, 2012, 11:34:08 AM by BR549 »
Re: A Power Drawbar Like No Other....
« Reply #2 on: May 13, 2012, 08:48:48 PM »
Terry,

That will not be a problem at all with my scheme.  The motor/gearbox combination is capable of 75 ft-lbs - about 3X the max required torque.  Tightening torque will be limited to ~25 ft-lbs by current limiting in the stepper driver.  Current will be max for loosening, so the full 75 ft-lbs will be available.  So, I will have ~3X as much torque when loosening as when tightening.  There's no way the loosening torque requirement will be 3X the applied tightening torque.

Regards,
Ray L.
Regards,
Ray L.

Offline BR549

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Re: A Power Drawbar Like No Other....
« Reply #3 on: May 14, 2012, 12:02:46 AM »
SOunds like you have it figured out. Please keep us up to date on how it is working.

(;-) TP
Re: A Power Drawbar Like No Other....
« Reply #4 on: May 14, 2012, 06:01:05 PM »
Well, I'm happy to report the power drawbar seems to work *perfectly*!  I got it all plumbed and wired, and did the 20 or so lines of macro code to make it work, and it grabbed, and released, the tools 25-30 times in a row, without so much as a single hiccup!  Limiting current when tightening seems to work perfectly, tightening the drawbar very consistently, and no problems whatsoever loosening it again with max current.  This is going to be REALLY nice, and gets me one giant step closer to an ATC!

Regards,
Ray L.
Regards,
Ray L.

Offline Sam

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Re: A Power Drawbar Like No Other....
« Reply #5 on: May 14, 2012, 11:33:31 PM »
Nice craftsmanship, Ray.
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."
Re: A Power Drawbar Like No Other....
« Reply #6 on: May 15, 2012, 12:33:34 AM »
Nice craftsmanship, Ray.

Thanks, Sam!  I'm really pleased with how it came out, and how easy it was!

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: A Power Drawbar Like No Other....
« Reply #7 on: May 16, 2012, 03:36:33 PM »
Interesting arrangement. Proving the old addage 'more than one way to skin a cat'.

Being one of the guys with a 'toe in the water' on this topic, I have a box of parts waiting for me to find the time to put together my own PDB. I am taking an entirely differnet approach (BT30) and what has me backed up is the cost of drawbar collets; $400 to $500 US.  Ouchies. 

Many moons ago I had a furnace and did my own heat treating. I may have to go back to that in order to be able to make some of the parts I want . . . or . . . I could just stop  :'( and spend the money and get on with it.

Re: A Power Drawbar Like No Other....
« Reply #8 on: May 16, 2012, 03:52:57 PM »
Pretty neat Ray, nice concept.
One question ... When tightening, do you run the motor for a specific time to assure proper tightening and let the motor stall ? ? or are you sensing the load as an input to Mach ?

Thanks,
Russ
Re: A Power Drawbar Like No Other....
« Reply #9 on: May 16, 2012, 04:22:14 PM »
Pretty neat Ray, nice concept.
One question ... When tightening, do you run the motor for a specific time to assure proper tightening and let the motor stall ? ? or are you sensing the load as an input to Mach ?

Thanks,
Russ

Russ,

Right now, I have only partially implemented software to drive it, so I'm taking a brute-force approach - To loosen, I command a move equivalent to 1.25 turns of the drawbar.  To tighten, I command a move equivalent to 1.5 turns of the drawbar, ensuring the motor will stall before it goes that far.  Seems to work just fine like that - I've been using it all day today without a single hiccup (well, other than Mach3 weirding out one me twice....).  When I move it over to the KFlop, and ditch Mach3 (Mach3 is currently running ONLY the drawbar, everything else is running on the KFlop), I will put an encoder on the stepper, so I can detect when the drawbar stops moving, and stop the stepper right there.

Regards,
Ray L.
Regards,
Ray L.