Ballscrew Pitch Correction ?

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Ballscrew Pitch Correction ?

Postby LBayZG » Tue Dec 03, 2019 10:59 pm

We have all heard of backlash compensation.

However, while surfing the web, I came across the concept of 'Ballscrew Pitch Correction'.
This is where software compensation is made for linear inaccuracies of the ballscrew itself.
This is Something I guess would be more valuable for the cheaper ballscrews many of us purchase.

I checked the UCCNC software installation and user's guide, and there does not seem to be any mention of such a capability.

Now, this is (to me) nothing urgent or even necessary for my own use, but I am curious if UCCNC contemplate such a feature in the future.
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Re: Ballscrew Pitch Correction ?

Postby cncdrive » Tue Dec 03, 2019 11:06 pm

Even cheap C7 precision grade ballscrews have a max.linearity error of 0.02mm per 300mm which is a so low value that a hobby mechanics will always do more error with it's elasticness, so there is no reason to have pitch correction.
Furthermore to do ballscrew pitch correction you have to have a measuring method of the pitch error which requires a relatively expensive equippment, e.g. a precise linear scale.

And for an expensive machine you can anytime use precise C0 ballscrews when again no need for pitch correction. :)
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Re: Ballscrew Pitch Correction ?

Postby Arri » Fri Dec 06, 2019 11:17 am

I'm still hoping that Screw Mapping will become a feature in UCCNC. There is much value in having the ability to calibrate the software (UCCNC) to the hardware (machine axis X/Y/Z, etc.).

Not all CNC work is decorative or even a router and sometimes machines are small (under 1m) and need be accurate (to say to 0.0002mm). I design and produce many assemblies and assembly components and I also take in (and must comply with) customer specs/tolerances.

It is certainly true that discovering screw errors requires accurate test measuring (metrology). There are several methods including some that are incremental using precision gauge blocks, for $60 on Ebay a 900mm Digital Readout DRO can be helpful as a cheap start that can be very helpful, though not ideal.
I have found mathematical errors in (dare I say it...) Mach3 that required unique "Steps Per" values in "Motor Tuning" in order to have accurate axis calibration. It is not just screws to consider, but also drive components like timing belts and pulleys, and so on.

Hopefully screw mapping will be a feature someday, or maybe UCCNC will have a method/function that will allow scripting for those who want to tackle this themselves.

For the record I'm a devoted UCCNC user. :)
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Re: Ballscrew Pitch Correction ?

Postby cncdrive » Fri Dec 06, 2019 1:02 pm

I will eat my hat if a hobby/half professional machine can do 0.0002mm accuracy.
Even a HAAS VF4 has a position repetability of 0.0002mm but only on constant 20°C and accuracy is worse than that and that is a 28 tons weight machine. :)
And we did not even talk about the machine frame geometry precision.
So, 0.0002mm precision is a dream for a hobby machine.

Do you think a $60 Ebay DRO will give you a 0.0002mm accuracy? Would be funny. ;)

And you've mentioned timing belts and pulleys, they have some ellasticness which sure makes the 0.0002mm impossible, no matter how accurate you calibrate/map your axis.
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Re: Ballscrew Pitch Correction ?

Postby Robertspark » Fri Dec 06, 2019 2:09 pm

:roll: :lol:

0.0002mm !!!! :shock:

~0.0079 thousands of an inch!!! :shock:

Couple of nice tables here to provide some guidance on tolerance ranges for various (commercial machine?) applications.

https://mae.ufl.edu/designlab/Lab%20Ass ... rances.pdf
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Re: Ballscrew Pitch Correction ?

Postby MattTheNoob » Tue Dec 17, 2019 3:18 am

I may be doing the math wrong (usually I need a pencil to get math right, but I don't have one within reach), but I think that 300mm balls screw changes in length 20 times that amount with each degree C of temperature change.
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