Weak stepper motors with Arduino CNC Sheild

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murray484
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Weak stepper motors with Arduino CNC Sheild

Post by murray484 » Tue Jul 07, 2015 3:46 am

Hello,

I am currently building a CNC using an Arduino and 3 Pololu A4988 stepper drivers. I'm using 1.7a nema 17 motors from SparkFun, and am using the GRBL firmware. I was hoping you could help me troubleshoot an issue I've been having, since I know that a lot of you use similar electronics on your machines.

Simply put, the motors seem to be extremely weak. The machine can easily be stopped by lightly pressing against it with your finger. We are using a belt drive system, and have adjusted all of the axes to have little to no friction. We are using the same motors and drivers on a 3d printer, and they seem to be much stronger. I know that the ShapeOko also uses a belt drive system, and am wondering if it stops easily as well, or if our electronics are not working correctly.

Thank you.

WillAdams
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Re: Weak stepper motors with Arduino CNC Sheild

Post by WillAdams » Tue Jul 07, 2015 4:09 am

The ShapeOko is supposed to use motors w/ at least 58 oz./in. torque (for an SO2) — what’s the torque rating for the motors which you’re using?
Shapeoko 3XL #0006 w/ Carbide Compact Router w/0.125″ and ¼″ Carbide 3D precision collets

murray484
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Re: Weak stepper motors with Arduino CNC Sheild

Post by murray484 » Tue Jul 07, 2015 4:13 am

These are the motors I am using. They are 68oz-in. https://www.sparkfun.com/products/10846

cvoinescu
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Re: Weak stepper motors with Arduino CNC Sheild

Post by cvoinescu » Tue Jul 07, 2015 10:34 am

Those should give a decent amount of resistance when you push against the carriage. You will be able to force it, but not easily.

Have you adjusted the motor coil current? Do you have enough cooling on the drivers to allow decent current without going in thermal shutdown? Heatsinks help, but a fan helps even more. What power supply are you using?

Also check that your wiring is correct, because a motor with a disconnected wire can still turn sometimes, but very weakly.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

murray484
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Re: Weak stepper motors with Arduino CNC Sheild

Post by murray484 » Tue Jul 07, 2015 12:21 pm

Thanks,

I have adjusted the motor current so that it was around 1.7a (by measuring the vref and multiplying it by 2.5 to find the current). The drivers have heat sinks, but no fans. I am using a 12v 30a power supply from ebay.

This machine uses a belt loop, rather than a fixed belt. Could this be a factor into why it seems so weak?

Thanks again for all the help.

WillAdams
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Re: Weak stepper motors with Arduino CNC Sheild

Post by WillAdams » Tue Jul 07, 2015 12:32 pm

How many teeth on the pulleys?
Shapeoko 3XL #0006 w/ Carbide Compact Router w/0.125″ and ¼″ Carbide 3D precision collets

cvoinescu
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Re: Weak stepper motors with Arduino CNC Sheild

Post by cvoinescu » Tue Jul 07, 2015 2:29 pm

Belt loops aren't much different from open-ended belts. All that matters is the pitch circumference of the pulley (number of teeth times pitch); the larger, the weaker. Still, it should be quite firm with those motors.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

murray484
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Re: Weak stepper motors with Arduino CNC Sheild

Post by murray484 » Fri Jul 24, 2015 11:55 pm

After putting the finishing touches on the machine, we were finally able to actually try cutting with the machine today. We tried foam and it cut very accurately, but as soon as we tried wood it immediately stalled out, despite the fact that we were running it fairly slow and with a cut depth of only 0.5mm. I can't figure out how it can be so weak considering that we are using 68oz in motors. The pulleys are 20 teeth, so that shouldn't be a problem either. The machine uses plastic bushings not bearings like the Shapeoko, but I've made sure that the axis moves with very little friction. I also had a rotozip bit and not a proper endmill (don't have one just yet). Could these be the only issues? Can the Shapeoko be stopped easily by putting your hand in the way? Any other ideas on what could cause this?

Thanks

Gadgetman!
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Re: Weak stepper motors with Arduino CNC Sheild

Post by Gadgetman! » Tue Aug 04, 2015 10:46 am

I believe some Rotozip bits are endmills, the rest...

A ShapeOko can NOT be 'easily stopped' by a hand. But we use 24V instead of 12V to drive out motors... Trust me, it makes a difference.

It would also help if you posted the GRBL settings since some combinations can result in a weak system.
Weird guy...
Shapeoko 2014F: 1000mm X/Y, 300W Quiet Cut spindle, Arduino Uno/G-shield and GRBL 0.9i
15x30 drag chains, custom spindle mount, 9mm belts, 8mm endplates, 6mm motor plates.

Fablicator
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Re: Weak stepper motors with Arduino CNC Sheild

Post by Fablicator » Tue Aug 04, 2015 11:05 am

Have you made sure the pulleys on the motors are tight?

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