Lathe DRO Accuracy

I learned lathe work before DROs trickled down to toolrooms. Always check with a mike. This way you will learn soon how trustworthy your DRO is.

Edit: Even after becoming comfortable with the accuracy of the DRO, CHECK WITH A MIKE.

Edit: Correct typo
 
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I took an 11" micrometer standard and used an edge finder in a milling machine vise to check My DRO. I could not certify the standard so it is an unknown, but I know the edge finder repeats easily within a .0005". I found out the DRO read 11.0005 at 11.000". The edge finder eliminated sag and droop with a dial indicator.Asked the seller and He thought it was great. I make wonderful brain surgery and rocket science parts in My own mind.
 
Points to ponder:
1. DRO's need to be calibrated. The manual should have the procedure. I calibrated mine with a spacer measured with a micrometer. The DRO was zeroed against a fixed surface and then the spacer inserted and the new DRO reading compared against the spacer length. A 1-2-3 block can be used but verify the dimension with a micrometer. Even name brand 1-2-3 blocks are slightly oversized to allow for lapping to final dimensions. For the mill, I set up 1-2-3 blocks as a guide and used my 6" parallel measured with my 6" micrometer as a spacer.

2. Asian machines often use metric lead screws for Imperial measure. They will approximate 3mm = .11811 to .12 or 2.5mm =.098425 to .1 to make the dial divisions nice and even.

3. Lead screws and nut wear with use. They may have had perfect [pitch to start but over time, the most used regions will no longer give accurate long distance measurements. Short distances, on the order of a few turns will be OK because the wear pattern will be very similar in any given region. When in doubt, I trust my calibrated DRO before the dial.

4. While it is true that much of the work we do as hobbyists doesn't require extreme precision, I believe that our measurement tools should be calibrated as accurately as reasonably possible. I don't calibrate my tools to NIST traceable standards but I do check them against each other. The assumption being that it is improbable that two instruments will deviate from the true reading by exactly the same amount so if you get identical readings on both, they are most likely accurate. It won't hold up to an ISO audit but it is good enough for my work.
 
I've put an import dro on my grizzly and , I'm sure this is my technique, but when I test travel of either axis against a dial indicator and the handwheels the dro checks out but when I am taking a cut I end up several thousand off...

My process is take a surface cut, measure the part, set the dro accordingly, take a few more cuts to get closer to size, but by now the dro is substantially off ..

I cannot figure out what I'm doing wrong
 
T Bredehoft said:
I learned lathe work before DROs trickled down to toolrooms. Always check with a mike. This way you will learn soon how trustworthy your DRO is.

Edit: Even after becoming comfortable with the accuracy of the DRO, CHECK WITH A MIKE.
Click to expand...

This would seem obvious, but maybe it isn't. Just like the dials, you don't trust the DRO to determine the final dimension, you just have to trust it to get you there from the next to last pass. Since you don't expect to hit a light press fit for a bearing from .100 away, an error of .0005 over 2 inches is near-as-dammit nothing when you make your final pass from .010 over.
 
Driveslayer45 said:
I've put an import dro on my grizzly and , I'm sure this is my technique, but when I test travel of either axis against a dial indicator and the handwheels the dro checks out but when I am taking a cut I end up several thousand off...

My process is take a surface cut, measure the part, set the dro accordingly, take a few more cuts to get closer to size, but by now the dro is substantially off ..

I cannot figure out what I'm doing wrong
Click to expand...

More info is needed. When you say the DRO is subtantially off, do you mean that it no longer agrees with the dials, or do you mean that the part dimensions do not agree with the DRO? How much difference and in which direction (part is larger or smaller than DRO suggests)? On one axis or both?
 
The part doesn't agree with what I would expect it to. For example, I make a trueing cut, and after the cut I measure the part at 1.850", I set the x to 1.850, I don't retract the tool on the x, I only move it back on the z. I dial in two cuts of .030 each then I remeasure. I would expect the part to be 1.730 but it is .020 -.030 off. I could understand ~.005

I guess my confusion is when I cut on the mill the measurements seem to match the dro
 
If your tool is not centered for height, your DRO will become progressively incorrect as you turn down the part diameter.
 
Driveslayer45 said:
The part doesn't agree with what I would expect it to. For example, I make a trueing cut, and after the cut I measure the part at 1.850", I set the x to 1.850, I don't retract the tool on the x, I only move it back on the z. I dial in two cuts of .030 each then I remeasure. I would expect the part to be 1.730 but it is .020 -.030 off. I could understand ~.005

I guess my confusion is when I cut on the mill the measurements seem to match the dro
Click to expand...

You didn't say if the dial and DRO disagree. Pick up the edge of the part, set the dial and DRO at zero. Take your two cuts. Do the dial & DRO disagree? That will tell if there is a problem with the DRO. And what about the y axis?

edit: Wait, are we talking about a lathe? You never said, and for some reason I was thinking a mill.
 
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