Messages

351

General Mach Discussion / Re: A Power Drawbar Like No Other....

« 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.

352

General Mach Discussion / 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.

353

General Mach Discussion / Re: Mach Mill adjusting feed to overcome inconstant spindle rpm

« on: May 10, 2012, 09:16:55 PM »

You're forgetting that FRO will only affected NEW commands, not those that are already in the buffer.  This means it could take several seconds for the FRO to change anything.  That will create a feedback loop with a VERY long, and VERY variable time constant, which will make it extremely difficult to control.

Regards,
Ray L.

354

General Mach Discussion / Re: 2 router heads one machine

« on: May 04, 2012, 11:22:46 AM »

Sounds like a perfect application for fixture offsets.  Use G54 for the router head, G55 for the drill head.  Set the offset between them in your start-up script, and you're all set.

Regards,
Ray L.

355

General Mach Discussion / Re: Hour Meter DRO

« on: May 04, 2012, 11:21:14 AM »

I have NEVER seen the timers in Mach3 operate reliably.  They seem to start and stop on their own, with no relation to whether the machine is actually running or not.  I ended up buying a $5 digital Kitchen timer, and using that.

Regards,
Ray L.

356

General Mach Discussion / Re: Acceleration set the same for all axes?

« on: May 04, 2012, 12:03:42 AM »

Ok, I got it, thanks.

I found the section in the manual that states mach will create the motion based on the acceleration and velocity of each axis, supporting linear rates in all axes, and arc based in your choice of two. So it was a bug, that in theory is fixed.

I agree that X and Y should always be the same, and mine are. Its Z that wants lower acceleration. I plan on keeping with my  35 for X and Y, and 20 for Z.

Thanks for the infoz.

That was exactly the setup I had, and when it transitioned from a 2-axis move in X/Y to a 3-axis move in X/Y/Z, it ignored the lower Z acceleration limit, which caused the Z axis servo to fault every time.

Regards,
Ray L.

357

General Mach Discussion / Re: Acceleration set the same for all axes?

« on: May 03, 2012, 10:41:56 PM »

No, there was a bug in Mach3 that would cause it to violate the acceleration limit on an axis under certain circumstances, which could cause position loss if one axis had lower acceleration than the others.  I was the one who found and reported it about two years and a half ago.  Brian has said it has since been fixed, but I'm not sure if anyone has verified that.  I certainly have not, since I no longer use Mach3.  But, certainly there is little or nothing to be gained by having either X or Y having higher acceleration than the other, since there will be few opportunities to take advantage of the higher acceleration on that one axis, even without the bug.

Regards,
Ray L.

358

General Mach Discussion / Re: servo encoder ?

« on: April 30, 2012, 03:53:08 PM »

Well.....  Assuming this is your motor:  http://www.arceurotrade.co.uk/projects/stepper/220Ncm%20Stepper%20Motor%20160-010-00400.pdf  I'm afraid if you want high speed the first thing you need to do is get new motors.  Those have VERY high inductance, which makes their high-speed performance very limited.  A good motor in that size will have an inductance of around 2mH, rather than the 10mH those have.  You also probably should go up a step in torque, to something on the order of 300-400 N-cm, to make sure you have plenty of torque at high speed.

The Gecko 201s are fine.  You could increase the speed with the existing motors some by increasing your power supply voltage.  Given the high inductance, you could run it up to the max on the Gecko drives, about 75-80V.  But if you get some good 2mH motors, you'll only want about 48V for optimum performance, and you'll get considerably better performance.  In either case, power supply current rating should be about 70% of the combined rated motor currents.  So if you have three 5A motors, you'd want about 10-11A.

Regards,
Ray L.

359

General Mach Discussion / Re: servo encoder ?

« on: April 29, 2012, 04:53:15 PM »

I use gecko drives for stepper motor and 24 volts

And there, in a nutshell, is your real problem.  A machine that size should be able to easily run several hundred inches/minute on steppers, IF they're properly driven, which yours are definitely not.

Getting good high-speed performance from a stepper motor requires MUCH higher voltage.  Most steppers require on the order of 50-70V to get optimum high-speed performance.  At 24V, you'll be severely limited, due to the coil inductance, and will never actually reach full current except at very low speeds.  Increase your supply voltage to the appropriate value (based on the inductance of your motors), and your performance will be improved dramatically.  Do some reading of the Gecko FAQs for steppers, and proper power supply sizing.

Regards,
Ray L.

360

General Mach Discussion / Re: servo encoder ?

« on: April 29, 2012, 03:13:27 PM »

What kind of machine is this?  Unless it's very large, you should be able to get all the speed you need with the proper steppers, and do it for a lot less money.

What motors, controllers, and power supplies do you have now?  Speed problems are most often the result of using motors that are too small, and/or controllers that have limited voltage/current capability, and/or power supplies that do not provide enough voltage/current.  Switching to servos will not solve your problem any more than steppers will, if the whole system is poorly designed.

Regards,
Ray L.