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I recently rebuilt my machine and now I'm having some issues with repeatability on one axis. In the video you can see the long axis has a motor on each side. The Gecko drives are slaved off the X axis from the controller so they track. There are anti-backlash nuts on all screws. I believe my problem is that the motors are missing steps because there is a lot more 'friction' than with the old system. Before I lubricated the slides I could actually see it missing steps.

After the rebuild, the length of the cables has increased. My questions are:

Can the added cable length cause me to miss steps?
Could the wire gauge be causing me to miss steps? It is currently 22 awg for each wire in the four-wire bundle.
Should I increase the power of the motors? They are currently KL23H251-24-8B wired in parallel, so about 156 once-in.
Should I change the acceleration in Mach3?

https://www.dropbox.com/s/cfdm69m0gggg9cx/CNC.mp4

Typically speaking, stepper motors don't just miss like a step or two or even 10 or 100, they usually stall and stay stalled until the axis stops.

Have you checked for loose coupling connections or some slop elsewhere in the drive train? Lots of guys on CNC zone thought they had lost steps when it was actually the stepper shaft to flex coupling connection that was slipping.

Your acceleration sounds pretty slow on the video you linked to but you could always try decreasing it by half and running some test programs to see if the problem persists.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

If I jog the axis (which is at a reduced speed), I move it a few inches, then move it back, and it is perfect. When I ran a program, it was off in one direction. I'll check the couplers, but before I lubricated the slides I could actually see it skipping steps. I'm just wondering if it is skipping steps when I run the program and I can't see it because it is moving back and forth instead of just in a straight line.

I'm not saying it's impossible but just that typically, when an axis stalls, it stalls for good until the motor has an opportunity to re-accelerate to whatever speed the controller is commanding. Once an axis is stalled, you're trying to get it from zero to full speed with infinite acceleration - which usually leads to more stalling.

I want to recommend going into the motor tuning pages to play with the settings to see if you can get it to stall but I don't have any experience with a double drive axis and don't know if that could do permanent damage to your machine. If your machine can handle it, do it. Raise your top speed until you're stalling and then back off like 25%. Do the same for accel and see how it compares with your current settings.

I initially had my heavy axis set at 450ipm but after a bunch of ruined parts, I found that to get it 100% reliable I had to go down to 400ipm as a max speed and had to drop my accel as well. Even now I have to be mindful to oil my ballscrew from time to time to keep it reliable.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Ok, great, thanks! Could too small a gauge wire also cause it to stall? I'm worried because the wires from the Geckos to the motors are about 10 feet long and only 22 awg.

Well, theoretically wiring could be a bottleneck but I doubt it is in your case. Check out this chart here: http://www.powerstream.com/Wire_Size.htm

10 feet of 22 gauge wire would have about .16Ω of resistance which is not a heck of a lot. I believe your gecko is supplying 3.5A or less which should fit in line.

Again, not impossible but doubtful in my opinion.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Thanks Andy!

One more thing I should mention - In my rebuilding, I replaced the single-start screws with 5-start screws. This increase in torque required might be contributing to my problem. When I went home to test the motor contouring I found that my PC gave up the ghost. The new one should be online today.

If you have a G540 and a 48v supply, spring for the 381 oz-" motors from Automation Technologies - This is the one you want: http://www.automationtechnologiesinc.co ... -381-oz-in

Low inductance and matches perfectly to a G540 @ 48v. That's the best performing motor you can get for a G540 / G251 setup. Will kick the crap out of the 156oz motors you've got now. Now that I've looked more closely at your motors, there's a good chance they just don't have the torque you need at the speeds/accel you're commanding.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

My Geckos are G201s, power supply is 30 volts.

With 201's you've got even more options. However to take full advantage, you need a bigger PSU. G201s are way overkill for your motors.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Ya, the power supply is pretty simple - a big transformer, a bridge, a giant cap, and a bleed-off resistor. It's built right into the electrical box that has the Geckos. I would have to get a new transformer, and that should do it.

I found some Chinese motors with more torque that I can put in without having to make new brackets:

http://www.aliexpress.com/item/Stepping-Motor-57BYGH76-401A-Free-Shipping-Best-Price-on-Aliexpress/328414408.html

Good advice from Andy, as usual. Almost any Nema 23 motor will have the same mounting face and shaft, so the 381s should mount, too. The 57BYGH76-401A is a decent motor but is 3.6mH inductance, a bit higher than the 2.8mH of the 381, so won't run as fast (or needs more voltage, 60V is optimum for it but that's an odd power supply). For the small difference in price, I'd get the 381s from a US seller.

If swapping the transformer, also check that the cap and bridge are rated for the higher voltage. Opinions vary, but I'd say the cap should be rated for at least 20% more than the nominal voltage.

Or, 48V switching supplies (commonly used but not as bulletproof as a transformer type) are readily available:
7.5A: http://www.ebay.com/itm/171119405142
13.5A: http://www.ebay.com/itm/330686893345 (We built a machine with 4 G201s that use a server supply like this... just needs 3 pins jumpered together to make it work.)

4 of the 381s running at their max 3.5A would need at least a 9.5A supply (4 * MotorCurrent * 2/3), but if you don't need all their torque, less amps will run cooler.

Or, you might daisy-chain an 18V supply on your 30V, but that has some safety issues if you don't get the grounds right:
http://www.cnczone.com/forums/general_e ... eries.html

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David Malicky

Update - I tuned the motors and it runs pretty repeatable. I had to slow it way down, which was one of the reasons for the upgrade, but I can live with it until I can afford new motors.

Update : After the new A.B. nuts broke in I haven't had any problems.