[How do I?] DRO troubleshooting

[WobblyHand]

Interesting. My glass scales were dead on, calibration wise. Tested against two 123 blocks stacked end to end. Also make sure you test return to zero. Use three 123 blocks. The first is clamped to the table. It is your zero point. With a DTI set it to zero (or some value that you write down). Set your DRO to zero. Add a 123 block to the table, against the reference. Move the table past the block and approach the new 123 block to the identical DTI setting. You always have to approach from the same direction, or you will not take up the backlash! It should measure 3.0000", or whatever you have measured your 123 block to be. Then add the third 123 block to make the length 6". Approach the part the same way you did the 3". Write down whatever value you get. If there's error, then you should have the values needed for correction.

Remove both 123 blocks and approach the reference block surface. Stop when the DTI setting is what you had recorded. The DRO should say 0.0000". If it does not, there's an issue.

When my mill scale failed, it would skip counts in one direction vs another. Screwed up a bunch of parts due to that. Cleaning the scale did not help. Cleaning the read head didn't help either. Replacing the read head fixed my problem.

 

[Rex Walters]

@ARC-170 has iGaging EZ-View scales

Fair point.

Still, most error (except jitter) should be quite repeatable. It bugs me that some readings were a thou under and some a thou over.

Your measuring equipment must be better than the tolerances you intend work to. If your DRO sometimes reads a thou over and sometimes a thou under, the best you can really aspire to is the closest 0.005” or so.

The equipment may only have a “guaranteed” accuracy of +/- 0.001”, but I’d be annoyed if an individual installation couldn’t get at least a little better than that.

I’m just a hobbyist, but generally aspire to work to the closest thou. If I was doing production work, I’d generally aspire to a little looser than that. But that means I need to trust my DRO and standards to AT LEAST +/- 0.0005”.

The only time I chase tenths is with measuring tools, bearing seats, high speed spindles and the like. A DRO is a measuring tool. It’s worth fussing over.

Yuriy has an in depth calibration procedure

This is really good. Happy to see it’s consistent with what we’ve written above.

 

[Rex Walters]

Dang. Those Touch DRO resources are REALLY good. Thanks @ChazzC for posting that - I know who I’ll go with the next time I add a DRO to something!

 

[ChazzC]

Still, most error (except jitter) should be quite repeatable. It bugs me that some readings were a thou under and some a thou over.

Hysteresis may explain some errors, but in this case it's more likely just the inherent accuracy of the scale/read head/readout.

 

[ChazzC]

Dang. Those Touch DRO resources are REALLY good. Thanks @ChazzC for posting that - I know who I’ll go with the next time I add a DRO to something!

I guess I'll need to publish my consulting fees . . .

 

[ARC-170]

I used some 1-2-3 blocks and a drill rod in the chuck to check the travel. Started at 0 on the DRO. Got 1.002, 2.003, 3.005 and 4.006. The blocks measured to within 0.001", with two different calipers (over with one, under with another). My calipers are cheap imports, so keep that in mind. I think I'll use some micrometers and try again.

It is possible that the read rail of the DRO is not perfectly parallel with the quill. Over 4", the angle would be about 3.13º on a triangle with a 4.006" hypotenuse , with a deviation at the bottom of 0.219". That seems like something I would notice, though.

I think there's some wear in the gear train that drives the z-axis that's causing most of this. I know there's 0.019" of backlash, hence the DRO's.

When I got some time, I'm going to try some of the other methods described here and see what I get. I'd like to be able to work to the nearest 0.001".

 

[WobblyHand]

I used some 1-2-3 blocks and a drill rod in the chuck to check the travel. Started at 0 on the DRO. Got 1.002, 2.003, 3.005 and 4.006. The blocks measured to within 0.001", with two different calipers (over with one, under with another). My calipers are cheap imports, so keep that in mind. I think I'll use some micrometers and try again.

It is possible that the read rail of the DRO is not perfectly parallel with the quill. Over 4", the angle would be about 3.13º on a triangle with a 4.006" hypotenuse , with a deviation at the bottom of 0.219". That seems like something I would notice, though.

I think there's some wear in the gear train that drives the z-axis that's causing most of this. I know there's 0.019" of backlash, hence the DRO's.

When I got some time, I'm going to try some of the other methods described here and see what I get. I'd like to be able to work to the nearest 0.001".

If you really want to understand what's going on, I advise you to use micrometers, especially for measuring your 123 blocks. Calipers give inconsistent readings depending on contact pressure. Micrometers typically have some ratchet like mechanism to ensure uniform measurement pressure. It's easy, even with analog 0.001" micrometers to estimate to 0.00025" or better, depending on your eyesight.

 

[ChazzC]

If you really want to understand what's going on, I advise you to use micrometers, especially for measuring your 123 blocks. Calipers give inconsistent readings depending on contact pressure. Micrometers typically have some ratchet like mechanism to ensure uniform measurement pressure. It's easy, even with analog 0.001" micrometers to estimate to 0.00025" or better, depending on your eyesight.

Agree: the cheapest 1-2-3 blocks are stated to be within a couple of tenths.