Motor Calibration

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rexcsmith
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Motor Calibration

Post by rexcsmith »

I'm running CNC 10 on a Millport Rhino IV. My measured DRO accuracy on the x axis appears to be in the neighborhood of .0009"/inch. I cannot find a spec on this mill, but I suspect the accuracy should be better than that. Can anyone tell me what I should be able to expect? I don't want to fiddle with motor calibration if my expectation is too high. thanks much
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Re: Motor Calibration

Post by cnckeith »

hi, are you talking about measuring backlash of the mechanical system? or calibrating the positioning (commanded vs. actual)?
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rexcsmith
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Re: Motor Calibration

Post by rexcsmith »

I'm talking about calibrating the positioning. I want the DRO to accurately represent a known length of travel.
polaraligned
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Re: Motor Calibration

Post by polaraligned »

You already have an active thread on this issue and you received a proper answer in that thread.
rexcsmith
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Re: Motor Calibration

Post by rexcsmith »

This was a new question. I'm asking what accuracy would be spec'd on my Millport Rhino IV mill. I cannot find a spec for it. I realize that no-one may have the answer, but I thought I'd ask.
cncsnw
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Re: Motor Calibration

Post by cncsnw »

Here is one reference to accuracy grades:
https://tech.thk.com/en/products/pdf/en_a15_011.pdf

I have not had time to fully absorb it, but the table on page 2 and the example on page 3 seem to indicate that linear errors are allowed to be significant, as long as they are consistent.

Your machine should have precision ground ballscrews. It is probable that Millport didn't spend any more than they had to, so assume they are supposed to be grade C5.

Note, however, that even the cheapest rolled screws -- grade C10 -- are not supposed to have more than 210um/300mm error (.0007 units per unit).

Did you measure the same 0.0009" per inch linear error at multiple places along your X axis, with multiple indicator setups?
rexcsmith
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Re: Motor Calibration

Post by rexcsmith »

I haven't done that yet. I'm waiting to receive a new indicator I've ordered before I put much time into the investigation.
I'll take a look at the table; thanks!
polaraligned
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Re: Motor Calibration

Post by polaraligned »

rexcsmith wrote: Fri Jul 16, 2021 4:04 pm This was a new question. I'm asking what accuracy would be spec'd on my Millport Rhino IV mill. I cannot find a spec for it. I realize that no-one may have the answer, but I thought I'd ask.
You do the best you can on the calibration, and then it is what it is.

Short of rebuilding the machine mechanically, you will probably never meet the original accuracy/repeatability specifications anyway.

.001" sounds like a lot, but then again we have no idea how you are measuring that. You should be able to get a whole lot better than that over short distances. What kind of a distance are you measuring this over and importantly what is your reference? I used a 15" Invar micrometer calibration rod when I did my X axis.

What is the backlash on the axis as it stands? A machine with a lot of backlash is usually well worn.
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Re: Motor Calibration

Post by rexcsmith »

So far, I mic'ed a couple of 1-2-3 blocks and put them end to end with a vertical stop block at the end. I assume that would suffice for making an accurate measurement?
I'm finding calibration rods to be very expensive for something I won't use very often. What's the least that can be spent for a reliable rod and where should I look?
polaraligned
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Re: Motor Calibration

Post by polaraligned »

rexcsmith wrote: Mon Jul 19, 2021 11:21 am So far, I mic'ed a couple of 1-2-3 blocks and put them end to end with a vertical stop block at the end. I assume that would suffice for making an accurate measurement?
I'm finding calibration rods to be very expensive for something I won't use very often. What's the least that can be spent for a reliable rod and where should I look?
1-2-3 blocks are fine to get you real close, and will get you way better than nearly a thousandth that you say you are off. I can't help you with the rods as I used one that came with my 12" to 16" micrometer set.
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