microstep uniformity

[leadinglights]

How uniform are the steps at various microstep sizes on a stepping motor? For that matter, what is the accuracy of the steps and microsteps?

The reason that I ask is that similar techniques used in ADCs for example, result in some steps disappearing at the smallest sizes, while others are over large - this is called differential non-linearity.

Thanks,

Mike.

[irving2008]

On a typical stepper the linearity of the main 1.8deg steps is about 1.5%. Halfstepping is about the same. Once you get into microstepping below that then its down to the driver and the load. A typical 8bit driver will give a further 1 - 1.5% non-linearity on an unloaded motor but the load will determine the accuracy to a great extent as the magnetic forces act as a spring against the load. Microstepping (other than halfstep) should never be used to improve resolution only smoothness of motion.

[leadinglights]

Thanks again Irving. As I willl be using ballscrews with a 5mm pitch, I think that halfstepping at 0.0125mm per half step is perhaps a bit lumpy so I am now looking at using a toothed belt drive for the ballscrews.

Mike.

[Robin Hewitt]

5mm pitch, I think that halfstepping at 0.0125mm per half step is perhaps a bit lumpy so I am now looking at using a toothed belt drive for the ballscrews.

Hi Mike

I use a 4:5 tooth belt simply to drop the steppers below the top of the table and stop faffing about with motor alignment.

4:5 effectively reduces the 5mm pitch to a 4mm pitch so 400 steps/rev = 0.01mm. As the machine weighs less than a ton this is hardly achievable because it will bend under the tool pressure, but I still microstep to smooth it out a bit :naughty:

Don't skimp on the pulley size, I use 32:40XL

Robin

[HiltonSteve]

Thanks again Irving. As I willl be using ballscrews with a 5mm pitch, I think that halfstepping at 0.0125mm per half step is perhaps a bit lumpy so I am now looking at using a toothed belt drive for the ballscrews.

Mike.

I have 16mm x 5mm pitch ballscrews on x,y and z on my machine running at 1/4 step, runs very nice and not noticed any lumpyness. Did try them at 1/8th and 1/16th step just to see how they ran but did not seem to make too much difference.

Mine are direct drive but belt driving them will give you more mounting options so definitely worth thinking about.

Just my two-penneth..!

[leadinglights]

I was probably spoiled by the first CNC that I built - based on a Proxxon MF70, it worked it's little heart out to give almost unmeasurable errors, really smooth curves etc.. I would like to get a step resolution of 0.0025mm but would settle on 0.005mm. I am thinking of 2.5:1 (1000 half-steps per leadscrew revolution) with 16T to 40T Syncroflex 2.5mm pitch pulleys and 6mm wide belt - would that work with a 3Nm stepper motor?

[HiltonSteve]

Probably don't have to tell you but remember belt driving at 2.5:1 will increase your resolution but will reduce your max feed rates.

My machine uses 3Nm steppers and I am driving them at 36V 5A, absolute maximum feed I can get without losing steps is 7500mm/min but have reduced it to 5,000mm/min so that I am not pushing it to the limit all the time and this is driving the screws direct without any belt reduction.

So 5,000mm/min divided by 2.5 will only give you 2,000. Does depend on voltage's, driver type etc but I would say that 2,000mm/min may not be enough.

[leadinglights]

2000mm/min sounds like the speed of sound to me, my Proxxon manages about 160mm/min - going by Youtube videos of similar machines this is average to good for this type of conversion.

[HiltonSteve]

Did I read that correctly....

Your maximum feedrate was 160mm/min????

That is seriously slow, are you sure about that?

[John S]

Bloody hell you could eat a packet of Hob Nobs in that time..............