Steppers skipping steps on circles at 2x microstepping

[killersoundz]

I finally have my 4x8 machine together for the last week or so and have successfully used it a few times on smaller parts. I originally had the driver's on 4x microstepping (800 steps/rev) and after having it skip steps from lack of torque in the midrange on rapids (130-170ipm), (which I understand is expected from steppers for them to have midrange problems), I then switched the drivers to 2x microstepping (400 steps/rev) which is the lowest my driver's go, thinking I'd pick up some torque.

Granted I slowed my rapids down to 100ipm before this next issue just to be careful to not skip steps when doing a big job and only running at 40-70ipm when machining. I have 10mm pitch ballscrews for the record.

When this job got to 11" diameter circles the x axis started running roughly and skipping steps, if you've ever cut circles you know how the steppers rev up and down to make the circle. It kinda kicked instead of just silently skipping steps...but regardless I knew it was skipping. Pretty upset because I just wasted a sheet of mdf not having homing switches or anything on the machine yet.

I figure the low 2x microstepping was causing resonances at very slow speeds. I then switched back to the 4x microstepping and ran those circles again to see. It definitely ran smoother and didn't do the kicking/skipping thing.

I have 1200oz in steppers on 60v 5.6a. There certainly isn't a lack of holding torque regardless. I just learned the hard way that more microstepping is better as it gets rid of certain resonances that can cause skipped steps. My question to you, am I right? And would it be worth it to take it to 8x microstepping? Does that significantly reduce my torque? I didn't consider that as an option before because I was hoping to run the machine faster but clearly now it's going to be slower than I had imagined regardless. Any extra advice to keep from skipping steps in my situation is appreciated.






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[ger21]

You've discovered that with stepper motors, bigger is not always better.
You'd be far better off with 400oz steppers, and would probably see much higher speeds.

1/8 (or 1/10) microstepping is what's most commonly used, as it provides the best balance of smoothness and power.

There certainly isn't a lack of holding torque regardless.

The issue here is that holding torque, is the torque when the motor is not spinning. AS soon as it starts spinning, torque drops off rapidly. And the larger the motor, the faster is drops. Larger motors also have high detent torque, which makes them less smooth.

You didn't mention what drives you are using, but with good drives, and low inductance 400-450oz motors, you should see 4-5x higher speeds than what you are getting now.

[killersoundz]

You've discovered that with stepper motors, bigger is not always better.
You'd be far better off with 400oz steppers, and would probably see much higher speeds.

1/8 (or 1/10) microstepping is what's most commonly used, as it provides the best balance of smoothness and power.



The issue here is that holding torque, is the torque when the motor is not spinning. AS soon as it starts spinning, torque drops off rapidly. And the larger the motor, the faster is drops. Larger motors also have high detent torque, which makes them less smooth.

You didn't mention what drives you are using, but with good drives, and low inductance 400-450oz motors, you should see 4-5x higher speeds than what you are getting now.

Thank you for the reply!!

The first thing I said to myself when I first tried to rapid this machine at 500ipm when I first fired it up was....."these motors are too dang big" because they just straight clapped out and stalled. I'm certainly knowledgeable on the effects of high inductance and as you say the detent torque. I'm tapped out on this project though money wise for now so I'm going to try to make these work for now, but you really think 400oz steppers would do better on this machine? My gantry must be 200lbs. I do have ballscrews though it doesn't take much to move it.

I am using some "dm860a" drivers that I got this all as a kit on ebay a few years ago. "longs motor" lol. I'm sure you'll say they suck but I will say I don't believe those to be my issue here. Since I've slowed the machine and my expectations down for now I may definitely try the 1/8 microstepping, I had it at that once when I was playing around and it certainly did seem to run very smooth but limited the speed. I guess I'm going to switch to that and run the toolpath without actually cutting and then see if it skips at all. I'm pretty sure the issue was a resonance problem as the stepper was slowing down to a very slow speed while cutting the circle it was resonating at the natural resonance of the machine and just gave up or something. Hopefully the higher resolution microstepping will help

[ger21]

I would buy one of these, and try it on your gantry. It's only $50, and will give you a good idea if they will work for the rest of the machine.

https://www.automationtechnologiesin...or-dual-shaft/

[Jim Dawson]

I assume that your steppers a direct coupled to the lead screws? You would be better to overdrive the lead screws by 2X and slow the motor down using a timing belt drive. The would allow you to operate the motors at a higher torque range. If your gantry is only 200 lbs, you might be able to overdrive by 3X given the torque of your motor.

Here are the specs on my machine for comparison. I have a 4x8 moving table machine, but the principal is the same.

Stepper motor 1280 oz/in, 6.3 amp, 80V power supply
Table weight ~700 lbs
Lead screw 40mm x 40 mm pitch
Drive ratio 1.5:1
Table movement / motor rotation 26.6 mm (1.049 inch)
Microstepping set to 100x
Absolute max rapid speed 600 IPM, normal rapid during operation 500 IPM.
Motor RPM at max rapid ~560 RPM

To reach 600 IPM the acceleration has to be set a bit low, but at 500 IPM normal acceleration works fine. At 60 volts and your motors you may have to live with slightly lower acceleration and a little less top end.

I hope this helps

[louieatienza]

Motor specs? Usually steppers this large need a lot more voltage to "push" the juice fast enough though those larger motor coils to have sufficient torque at speed. 1200oz-in holding torque is great - but you want your machine to MOVE, not just stand still.

Also we'd need to know what pitch screws you're using. These large steppers typically lose torque at a faster rate, and there are countless threads here where someone switched to a stepper half or even third that rating and INCREASED speed and performance.

[ger21]

Imo, you'd be better off with the smaller motor.

[killersoundz]

Motor specs? Usually steppers this large need a lot more voltage to "push" the juice fast enough though those larger motor coils to have sufficient torque at speed. 1200oz-in holding torque is great - but you want your machine to MOVE, not just stand still.

Also we'd need to know what pitch screws you're using. These large steppers typically lose torque at a faster rate, and there are countless threads here where someone switched to a stepper half or even third that rating and INCREASED speed and performance.

hi Thank you, i dont have specs of the motors on hand, they're chinese 1232oz in I believe though. on 60v and 5.6a. My ballscrews are 10mm pitch

- - - Updated - - -

I assume that your steppers a direct coupled to the lead screws? You would be better to overdrive the lead screws by 2X and slow the motor down using a timing belt drive. The would allow you to operate the motors at a higher torque range. If your gantry is only 200 lbs, you might be able to overdrive by 3X given the torque of your motor.

Here are the specs on my machine for comparison. I have a 4x8 moving table machine, but the principal is the same.

Stepper motor 1280 oz/in, 6.3 amp, 80V power supply
Table weight ~700 lbs
Lead screw 40mm x 40 mm pitch
Drive ratio 1.5:1
Table movement / motor rotation 26.6 mm (1.049 inch)
Microstepping set to 100x
Absolute max rapid speed 600 IPM, normal rapid during operation 500 IPM.
Motor RPM at max rapid ~560 RPM

To reach 600 IPM the acceleration has to be set a bit low, but at 500 IPM normal acceleration works fine. At 60 volts and your motors you may have to live with slightly lower acceleration and a little less top end.

I hope this helps

that does sound like a great idea...I would love to do this. It does sound like reinventing the wheel to me at this point in the build though lol.

[Jim Dawson]

.........that does sound like a great idea...I would love to do this. It does sound like reinventing the wheel to me at this point in the build though lol.

Or follow Gerry's suggestion of replacing the motors, or maybe just cut slower.

I agree that changing the drive system would be a bit of a project.

[louieatienza]

JerryBurks thread is one example, he went from similar steppers to 400oz-in and nearly doubled his performance.

[QuangDo]

JerryBurks thread is one example, he went from similar steppers to 400oz-in and nearly doubled his performance.

New here.
Just want to share my experience with motor step skipping and coordinate skipping. Here is how I fix it,
My machine is 3040 with 500 W spindle using Mach3 for milling

1. Change kernel speed from 250000 to 450000 so that computer can keep up with Mach3. Doing that I got rid of motor grinding and skipping.
2. Set feed rate to lower still coordinates are no longer skipping.

After doing those 2 things my milling is now perfect. No more skipping, grinding, coordinate skipping.

For pine wood and quality result (not interested in fast moving), I set my machine at +/- 9500 RPM and 42 IPM. (straight flute, ball nose, V60).

I am very happy with this set-up.