# Mill DRO Errors?



## BobRock (Mar 23, 2020)

I have a PM-727M w/DRO that I like a lot. But I’ve found, after mis-drilling a hole several times, that the DRO does not agree with the hand wheels.

If I zero the x-axis on the DRO, take the backlash out of the x-axis hand wheel and then turn the hand wheel 10 full revolutions (1.000”), the DRO displays 1.016”. The same is true of the y-axis. 20 turns (2.000”) shows 2.032”.

Has anyone seen such a problem or have any ideas how I can go about fixing this?


----------



## dpb (Mar 23, 2020)

How far is the table actually moving?


----------



## BobRock (Mar 23, 2020)

dpb said:


> How far is the table actually moving?



The table moves correctly with reference to the hand wheels. A hole drilled 10 hand wheel rotations from another ends up 1.000” away.


----------



## JimDawson (Mar 23, 2020)

That's interesting.  Try checking both against an indicator.  Maybe you'll get 3 different values.

Does the manual have any instructions for calibration?


----------



## Lo-Fi (Mar 23, 2020)

Sounds like you need to verify both the hand wheels and DRO. Usual method is to clamp a stop to the table, pop a 123 block (or two) on against it and indicate against the spindle. Set zero, remove the blocks, move the table until the indicator reads zero against the stop. You then know your actual move against the hand wheels and DRO readings.


----------



## BobRock (Mar 23, 2020)

JimDawson said:


> Does the manual have any instructions for calibration?



The manual is a poor translation. As best I can tell, there is a linear correction of 1.5mm/m available. But my error is 16.199mm/m.


----------



## BobRock (Mar 23, 2020)

Lo-Fi said:


> Sounds like you need to verify both the hand wheels and DRO. Usual method is to clamp a stop to the table, pop a 123 block (or two) on against it and indicate against the spindle. Set zero, remove the blocks, move the table until the indicator reads zero against the stop. You then know your actual move against the hand wheels and DRO readings.



I will try that to be exact. But holes drilled in a plate come out spaced equal to the hand wheel and not the DRO. That’s how I found the problem in the first place - a correctly placed hole per the DRO was not in the correct place when measured on the part.


----------



## tweinke (Mar 23, 2020)

Might be a good idea to contact PM and see what they say?


----------



## mksj (Mar 23, 2020)

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


----------



## BobRock (Mar 23, 2020)

tweinke said:


> Might be a good idea to contact PM and see what they say?



Yes. I have an email into them. If I don't hear back soon, I'll give them a call. I don't know what they are doing around quarantines and closures.


----------



## BobRock (Mar 23, 2020)

mksj said:


> 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.
> ...



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 (Mar 24, 2020)

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 (Mar 24, 2020)

davidpbest said:


> 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 (Mar 24, 2020)

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 (Mar 24, 2020)

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 (Mar 24, 2020)

higgite said:


> 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 (Mar 24, 2020)

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 (Mar 24, 2020)

Mini Cooper S said:


> 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 (Mar 24, 2020)

BobRock said:


> 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 (Mar 24, 2020)

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


----------



## qualitymachinetools (Mar 24, 2020)

Be sure to click the CC option at the bottom of the player to see the captions. (Video sucks yes new videos for everything are in the works, but it will give you an idea if you pause it step by step)          Set all scales to 5.0 res and you will be set.   The math is working out with the .2 setting and then Metric on the display.                   5.0 in the parameters, and Inch on the display should get you there.


----------



## RJSakowski (Mar 24, 2020)

Head alignment will not impact the measurement process.  I think that you have shown that the error is in the scale calibration.  Dial errors occur if metric lead screws are used and the dial reading is an approximate inch value.  A ".100"/turn" dial on a 2.5mm lead screw would move 25 mm or .984" in ten turns.  Curiously, this is a .016"/1" error but it is in the wrong direction based on what you have experienced.

As far as I know, DRO scales are inherently metric, either 1micron, 5 micron, or 10 micron; and inch readout is done by a conversion factor.  I would try using the metric readout.  For 1 inch of travel, you should be reading 25.400 mm.  If you are reading correctly, the conversion factor is wrong.  If you are reading 25.806 or thereabout, I would suspect the scale calibration.


----------



## mksj (Mar 24, 2020)

See below, do a reset of the DRO, make sure the shrinkage is off and redo the scale resolution.


----------



## hwelecrepair (Mar 24, 2020)

One thing to look at, is that you have multiple places where this accuracy can be lost.  Rock in the table/saddle, head movement, and actual knee movement/rock could also be causing this.

Was the DRO installed at the factory or by you?  An improperly mounted scale can also cause accuracy issues.  

Jon


----------



## BobRock (Mar 24, 2020)

@qualitymachinetools figured it out. My DRO setup was for .2um scales (instead of 5um scales) and in metric (not imperial) mode. The wild thing is that with that combination of incorrect parameters, the display and the increment in movement was at least plausible for inches!


----------



## BobRock (Mar 24, 2020)

Also, kudos to @qualitymachinetools for very fast and accurate technical support. If you are thinking about a mill or a lathe, don't forget the importance of good US-based help.


----------



## hwelecrepair (Mar 25, 2020)

@BobRock To clarify, did you mean 2um or .2um?  Never heard of .2um.  

Jon


----------



## BobRock (Mar 25, 2020)

hwelecrepair said:


> @BobRock To clarify, did you mean 2um or .2um?  Never heard of .2um.



The "resolution for encode" DRO parameter was set at 0.2um. Mine has a large number of possibilities including .2, 2, and 5um.


----------



## hwelecrepair (Mar 25, 2020)

@BobRock Dang, never heard of a .2um.  I mean, it is just resolution, but its just not one I have seen.

Thanks 

Jon


----------

