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Stepper-Driven Power Drawbar
Last night, thanks to some prodding from a friend, I finally got off my lazy a$$ and designed up the stepper-driven power drawbar I've been wanting to build for the last several years. Today, between interruptions to help a neighbor build a dog run, I got most of the parts machined. All that's left is the coupling between the gearbox and the drawbar, a trivial lathe-turned part, and the spindle brake mechanism, which is also very simple. Then comes all the electronics (Gecko G201 and optical encoder I/F) and software to make it all work.
For anyone who hasn't kept up with the plan here, I've been wanting a power drawbar that would give me full drawbar tension required for TTS tooling under the worst case conditions (about 2500-3000# drawbar tension, which equates to about 25-30 ft-lbs drawbar torque), and would make it possible to "quick-change" both TTS tools (unscrew drawbar one turn), or full R8 tools (unscrew drawbar compeltely). Belleville washers were out, due to space constraints, and they would not allow R8 tools to benefit from the power drawbar functionality. So, my solution was a stepper motor, driving a planetary gearbox with a high reduction ratio (55:1 in my case). With the 400 oz-in NEMA34 stepper I have, this gives me up to 75 ft-lbs of torque to play with.
The air cylinders will provide ~200# of down-force, to pop the taper free, while springs will return the assembly to the up and disengaged position. The existing mechanical spindle brake will be mechanically activated by the PDB assembly coming down into engagement. The stepper will then unscrew the drawbar the required amount, as confirmed by feedback from the shaft encoder on the stepper. Stepper driver current limit will be controlled to provide maximum torque when loosening the drawbar, and limited torque, equating to 25 ft-lbs, then tightening the drawbar. The stepper will be driven by a spare axis of the CNC controller, under control of a macro.
With luck, I'll get the drawbar coupling done tomorrow, and be ready to give it a try as soon as I can get the electronics updated.
Regards,
Ray L.
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A Completely New And Different Power Drawbar
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:
Stepper Motor Power Drawbar - YouTube
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.
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Re: Stepper-Driven Power Drawbar
Nicely done. I was just given the link to this thread. I've been working on the same thing for my combination R8 and TT holders. The Butterfly Wrench approach sucks! I've got a hall sensor to see a magnet on the socket and another to know when it's all the way down.
My system is designed to run manually or with LinuxCNC which sends my controller board CANopen messages. Manually I tap the release button and the system tries to do only two turns. Most of the time that works. If I hold the release button it runs the butterfly wrench for a longer time to release the R8 tool.
I've ordered a 25:1 planetary drive. I have a small DC servo that I will run with an HP_UHU drive.
Attachment 483984
And here's the controller.
Attachment 483986
There's more in this thread.
https://www.cnczone.com/forums/uhu-s...ovement-3.html
Re: Stepper-Driven Power Drawbar
Take a look at the old thread for the Novakon Power drawbar. I think the title is something like Novakon Torus Pro as a production machine. The PDB design that Ray created for Novakon was (at least in my opinion) one of the best designs on the market. It seems a bit over complex but is quite simple when broken down into its key parts. I built one which is close to the Novakon (my own interpretation of the design) and have never looked back. It took about six months working only with the information provided in the thread but was a fun and rewarding. Ray is very talented with his designs.
Re: Stepper-Driven Power Drawbar
Thanks. The only threads I can find with that subject use the belville washers compressed which then release the tool.
Ray's version in this thread is very similar to what I designed other than I used a butterfly wrench. I need to be able to automatically release/capture R8 and TT holders.
I have a planetary reduction on order. When it arrives I can continue. Currently working on modifying the HP_UHU so I can dynamically change torque much like Ray used a relay on the Gecko to do this.
Ah.... Hang on. Didn't find it in this forum but did with google for $1495.
https://www.novakon.net/products/pow...-for-torus-pro
And yes essentially that's what I'm building. Hopefully today I'll have the spindle lock made. I do like how moving the drawbar down locks the spindle with a pin. I wonder if they park the spindle to do that?
Here's the thread on the HP_UHU.
https://www.cnczone.com/forums/uhu-s...ovement-3.html
Re: Stepper-Driven Power Drawbar
The Novakon design features a spindle lock works with a simple hall effect sensor to sense a magnet located in the locking collar when it is aligned with the pin/collar slot. The VFD roates the spindle in slow motiion until the magnetic is sensed which then fires the air solenoid to lower the PDB into position for the 100:1 Planetary to Lock/Unlock the drawbar. Quite a nifty little circuit and not very expensive!!! Just one of the circuits designed by Ray!!! Again, this is my guess at how the PDB was designed. I've never had access to a production unit so I man be wrong but I built the thing and it works flawlessly.
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Re: Stepper-Driven Power Drawbar
Mine has two hall effect sensors. One for the socket to detect number of turns before it is told to stop. In hindsight more than one PPR will be more useful. The second sensor tests if the socket has made it all the way down. I've had issues with it jamming on top of the draw bar half way between flanks. So if it doesn't make it down I send it up and then down to try again.
Mine is designed to function without the LinuxCNC system running. So no access to spindle which is driven by AC Servo rather than VFD. That means if I did insist that LinuxCNC was running, I could step it around to an index position since the spindle is run with step/dir signals. I may find I have to be able to do this. I'm trying to avoid it.
Here's a photo of my controller. Uses a dsPIC30F5011 which talks to the LinuxCNC system via CAN bus. Not the final wiring or mounting position by any means.
Attachment 484084
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Re: Stepper-Driven Power Drawbar
Quote:
Originally Posted by
bill south
The Novakon design features a spindle lock works with a simple hall effect sensor to sense a magnet located in the locking collar when it is aligned with the pin/collar slot. The VFD roates the spindle in slow motiion until the magnetic is sensed which then fires the air solenoid to lower the PDB into position for the 100:1 Planetary to Lock/Unlock the drawbar. Quite a nifty little circuit and not very expensive!!! Just one of the circuits designed by Ray!!! Again, this is my guess at how the PDB was designed. I've never had access to a production unit so I man be wrong but I built the thing and it works flawlessly.
Bill,
I do have a question about how yours operates. I can see how the spindle is locked with the toothed wheel which is bolted to the nut which captures and probably even sets the bearing preload on the spindle. But of course the draw bar rotates independently inside that and therefore the nut position isn't predictable.
I've had problems with the socket jamming onto the nut. Especially when I was using the original equipment really really soft draw bar. Here's a photo of the 12 point socket jamming (and sticking tight) onto draw bar.
So I have a few questions:
1. Is the air pressure on the cylinder that pushes down the motor assembly quite low so that the force down is minimal?
2. Is it a 12 point or 6 point socket?
3. Does the stepper motor that turns the socket through the 100:1 drive turn until it lines up to the draw bar? If so how does it know. If the draw bar is loose with no R8 installed yet then there's nothing to prevent it from turning with the socket sitting on top of it.
Attachment 484088
4. Is it a custom socket? How is it connected to the planetary gear assembly? Pinned or set screws?
Thanks
John