Mill DRO Errors?

[BobRock]

A few suggestions:
1. Are you seeing it on all axis or just the X axis? If just on one axis did you check the scale resolution settings and/or change any of the error compensation parameters? Sometimes there is a reset procedure for the DRO.

2. The error seems to be a cumulative error, is it a linear cumulative error or is it random.

3. Error compensation is usually over a fixed distance in increments over the length of the scale, not based on the total length. A 5 micron scale should track within 1 digit, so something like +/- 0.0002" over something like 0.5 to 1"

4. If the error is not linear/cumulative at the same amount it is possible that it is either a read error due to alignment or possibly contamination. Is this occurring on a new install/machine or is this something new with an older machine.

5. I had a similar issue on a mill, and the error was a lead screw error because the pitch was 3 mm, so I got a fixed cumulative error and required a DRO to get an accurate measurement readings. The error was very similar to what you are experiencing.

I would take a look at this thread, you need to quantify what is in error and in more detail:

Ideas for proving DRO calibration

I am installing a nifty Chinese DRO and set of glass scales on my grizzly 12x36. I am done , except the z axis chip shield. I am pausing here to check the accuracy of the set up, as I am running out of time to complain if there is a problem. I don't need a chip shield to test these, so it can...

www.hobby-machinist.com

1) I see it in X and Y of about the same amount.

2) It sure seems repeatable and consistent at around .016" per 1.000" - X and Y.

4) This is a pretty new mill and might have been there since I set it up in December.

 

[davidpbest]

I had a similar issue with my 1340GT, but it turned out to be the cross slide dial, not the DRO. In your case, it sounds to me like a DRO calibration or read-head alignment issue. See attached.

 

[BobRock]

I had a similar issue with my 1340GT, but it turned out to be the cross slide dial, not the DRO. In your case, it sounds to me like a DRO calibration or read-head alignment issue. See attached.

That was an interesting read, and quite a mess. But I agree with you, it appears that I have the right hand wheel dials. 10 full careful, non-backlash turns on the x-axis or y-axis hand wheels show 1.016" on the DRO.

Today I am going to double check the actual travel of the table with some formal measurements using a dial indicator. But informal hole position results show the hand wheel dials to be matched to table movement.

I'm hoping to hear back from Precision Matthews on my support email soon.

 

[BobRock]

OK, I made some dial indicator measurements.

I set up a 1-2-3 block and a dial indicator as shown:

I measured the length of the block at 3.001”.

I removed the backlash, zeroed the dial indicator, hand wheel dial, and DRO. With the 1-2-3 block removed I turned the hand wheel exactly 30 turns which resulted in 3.0484 on the DRO, 2.910 and on the dial indicator.

Bringing the dial indicator to 0 gave me 3.0566 on the DRO and 3.007 on the hand wheel.

Is there any other information I can gather to help diagnose this?

 

[higgite]

Do not use the dial indicator to read absolute distance traveled in this process. If it is not absolutely dead nuts perpendicular to the X-axis, you have cosine error in its reading. It’s only function is to show relative zeros so you know when you have traveled the exact length of the 1-2-3 block. You want to compare only the handwheel with the DRO. Actually, for DRO calibration purposes, you only want to compare the DRO reading with the known length of the 1-2-3 block. This may help.

Tom

 

[BobRock]

Do not use the dial indicator to read absolute distance traveled in this process. If it is not absolutely dead nuts perpendicular to the X-axis, you have cosine error in its reading. It’s only function is to show relative zeros so you know when you have traveled the exact length of the 1-2-3 block. You want to compare only the handwheel with the DRO. Actually, for DRO calibration purposes, you only want to compare the DRO reading with the known length of the 1-2-3 block.

That's a good point that I didn't take into account.

So my comparison is between the DRO and the length of the 1-2-3 block: 3.001 for the block, 3.048 on the DRO. Big problem. My DRO is not as nice as the TouchDRO appears to be and the calibration method, if I understand the poor Chinese to English translation, is to enter a correction measured in mm/m.

But the manual suggests that the maximum compensation is 1.5mm/m and mine is out 15.67mm/m. I'll give it a try, anyway.

 

[Mini Cooper S]

How old is the mill? It almost seems counter intuitive but a worn lead screw actually makes the table move more than the dial shows! Think about it, if the thread is worn narrower in the most used part of the thread, as you turn the screw, the nut moves to a thicker less worn part of the thread. That thicker thread pushes the nut farther than the worn part did. The worn thread effectively steepens the inclined plane of the thread. Years ago I had a worn out mill with a readout on it that did exactly that.
My Dad explained it to me, we checked travel with 1,2,3 blocks, the readout was right, the dial was wrong!

Richard

 

[BobRock]

How old is the mill? It almost seems counter intuitive but a worn lead screw actually makes the table move more than the dial shows! Think about it, if the thread is worn narrower in the most used part of the thread, as you turn the screw, the nut moves to a thicker less worn part of the thread. That thicker thread pushes the nut farther than the worn part did. The worn thread effectively steepens the inclined plane of the thread. Years ago I had a worn out mill with a readout on it that did exactly that.
My Dad explained it to me, we checked travel with 1,2,3 blocks, the readout was right, the dial was wrong!

The mill was new in December. My check of the 1-2-3 block compared the block to the DRO. The block is 3.001, but the DRO showed 3.0566.

 

[Mini Cooper S]

The mill was new in December. My check of the 1-2-3 block compared the block to the DRO. The block is 3.001, but the DRO showed 3.0566.

I see that now looking at your pictures, guess my suggestion wouldn't apply. I think I would concur with Davidpbest, calibration or head alignment.

 

[qualitymachinetools]

Did you change the resolution setting after you received it, to something other than 5.0? Just want to check, many people do that thinking the resolution is what is displayed on the display, but that needs to be set to match the scales. Make sure that is on 5.0