# Ideas for proving DRO calibration



## LEEQ (Mar 16, 2020)

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 wait. Currently I have a one inch standard used to calibrate micrometers and such. I have no Jo blocks. I have an er40 collet chuck mounted to the spindle with a nice ground flat face on it. It's the kind that came as a plate with an er40 chuck protruding from it, around the chuck is the ground flat. I have .0001 Interapid test indicator on a mag based noga arm.  I have an Aloris knockoff qctp on the compound or I could also attach a vertical milling slide in place of the compound. I'm hoping to use this gear (preferably without making parts) to check the cross slide and carriage scales against the 1" standard. Something like hold the standard in a repeatable fashion against the chuck face and bring the indicator in till preloaded and zero it as well as the dro. Then remove the standard, bring the indicator in until it again reads zero and check the readout. I'm hoping to pick other peoples brains for solutions to holding the standard, indicator and general set up. Thanks for making my puzzle your puzzle.


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## RJSakowski (Mar 16, 2020)

What do you have in the way of a micrometer or calipers?


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## RJSakowski (Mar 16, 2020)

I ask this because from your description, the only article of reasonable accuracy is your 1" calibration standard which is not the best for determining the calibration of a DRO scale.  (too short)  I would prefer something like 4 - 6".  The longer the better.

For the z axis on my lathe, I used the 3" length of a 1-2-3- block.  I had measured the actual length with a micrometer.  I clamped a block to the ways of  my lathe and I ran the carriage to the block and zeroed the DRO.  Then I moved the carriage and inserted the 1-2-3 block between the carriage and the clamped block, ran the carriage up to the 1-2-3 block and compared my DRO reading to the 1-2-3 block dimension.


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## LEEQ (Mar 16, 2020)

I do have 123 blocks, but no mics big enough to measure 3". They are china or india cheapies


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## LEEQ (Mar 16, 2020)

I like your set up for z. I also have 6" Mitutoyo calipers I trust to .0005.


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## DavidR8 (Mar 16, 2020)

I’m a bit confused. Are you trying to ensure that the DRO results match the results from your metrology setup? 


Sent from my iPhone using Tapatalk


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## mksj (Mar 17, 2020)

You can just start out by comparing their overall accuracy to the dials on the lathe. Take out any play in the dial, zero it and the DRO and then compare to the DRO over each turn, and over something like 10 turns. This will tell you incremental and overall travel information in relation to the dials. If they compare then good enough.


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## bill70j (Mar 17, 2020)

mksj said:


> You can just start out by comparing their overall accuracy to the dials on the lathe. Take out any play in the dial, zero it and the DRO and then compare to the DRO over each turn, and over something like 10 turns. This will tell you incremental and overall travel information in relation to the dials. If they compare then good enough.


Maybe it's just my lathe, but I had a hard time using dials as a check for DRO accuracy.  It appeared to me that wear in the screws led to differing dial accuracy along the length of the screw.  I ended up using a 4" gage block as RJ describes to measure accuracy of the DRO.

As a FYI, I have attached plots of dial readings vs. DRO output for two of the three axes on my Smithy 3-in-1.  You can see that wear on the X-Axis lead screw at the headstock end led to higher inaccuracy relative to the tailstock end.

Since the DRO looked very accurate by the gage block method, I conclude that over the entire length of travel, there is an average of 0.0004"/inch of inaccuracy in the lead screw dial and 0.0008"/inch in the crossfeed screw dial.  But again, it varies along the length of travel.


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## burdickjp (Mar 17, 2020)

Gage blocks and a dial indicator?
If you don't have gage blocks you should be able to borrow some from someone locally or on the forums.


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## RJSakowski (Mar 17, 2020)

My Grizzly G0602 lathe has pseudo inch dials.  i.e., .4000" by the dial is actually .3937".  As long as you  realize this, the dials could be used as an indicator.  If I were using the dials, I would make my calibration points at either end of the lead screw travel to avoid the region of highest use and most wear.  One must realize that there will be uncertainties in manufacturing of the lead screw which could affect a DRO calibration though..

I would prefer to use a block, "calibrated" with a caliper as a "standard" over the lead screw dial. A 5" block can be measured to within a thou or two which would provide a DRO calibration to within 40 ppm.  This is essentially five digits in the DRO calibration factor.

An understated fact is that DRO's are not necessarily calibrated out of the box. @Bob Korves brought this to light in another thread. https://www.hobby-machinist.com/threads/dro-error.64376/#post-534589 post#8

 I was unaware of this myself and had used my mill DRO "as received" for for fourteen years.  The calibration procedure is not particularly well documented, especially in the Chinese manuals.  In my case, the error wasn't great to cause problems with most machining but would lead to machining errors over longer intervals.


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## mksj (Mar 17, 2020)

On newish lathes and mills you can use the dials to see if there is cumulative error and how well they track. I have checked a number of DRO scales against 123 and 246 blocks as well as use them to check the electronic calibration of electronic edge finders, I have yet to find one that wasn't in spec. These type of measurements if one stacks a series of blocks may lead to more error than inherent in the scales,  but in some cases the scale resolution/parameters are not set correct. There are also calculation errors due to rounding errors based on the DRO software and scale resolution.  I did have a problem on my previous mill that I was always off on what the dials indicated vs. the cut, when I measured the dial accuracy I found that "I go 10 turns on the X axis (1.200") I get a readout on the DRO of 1.1808", and on the Y axis I get 1.1782" for the same 10 turns. It is reproducible at various points in the travel. So a discrepancy of about ~0.02" per 10 turns". Evidently the manufacturer (Optimum BF30) of the mill at the time used metric leadscrews with imperial dials, all of which were useless for any type of accurate work. It was also never corrected under warranty, so without the DRO you had a fancy drill press.




My point is I would use whatever you choose to verify the accuracy of the DRO scales, but I would be hesitant  to calibrate them over the full travel distance unless you have the equipment to do it accurately.  On a 5μm (0.0005”) scale you would need measurement blocks and/or a system that exceeds this accuracy to calibrate it. If you feel the scale is not preforming accurately than I would contact the manufacturer.


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## RJSakowski (Mar 17, 2020)

mksj said:


> On newish lathes and mills you can use the dials to see if there is cumulative error and how well they track. I have checked a number of DRO scales against 123 and 246 blocks as well as use them to check the electronic calibration of electronic edge finders, I have yet to find one that wasn't in spec. These type of measurements if one stacks a series of blocks may lead to more error than inherent in the scales,  but in some cases the scale resolution/parameters are not set correct. There are also calculation errors due to rounding errors based on the DRO software and scale resolution.  I did have a problem on my previous mill that I was always off on what the dials indicated vs. the cut, when I measured the dial accuracy I found that "I go 10 turns on the X axis (1.200") I get a readout on the DRO of 1.1808", and on the Y axis I get 1.1782" for the same 10 turns. It is reproducible at various points in the travel. So a discrepancy of about ~0.02" per 10 turns". Evidently the manufacturer (Optimum BF30) of the mill at the time used metric leadscrews with imperial dials, all of which were useless for any type of accurate work. It was also never corrected under warranty, so without the DRO you had a fancy drill press.
> 
> View attachment 317037
> 
> ...


It sounds like you have metric leads screw which approximate inch measure.  If your lead screw has a pitch of 3mm, you would traverse 30mm in ten revs. or 1.1811".  I have a similar problem which was one reason for adding a DRO.


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## DavidR8 (Mar 17, 2020)

My recently acquired LC-30A mill has a hybrid approach. 
The marking says |-| = .0254” but the actual graduations are .001. 
Smart folk here reminded me that 1 cm x 2.54= 1 in. 
Doh!


Sent from my iPhone using Tapatalk


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## Suzuki4evr (Mar 17, 2020)

mksj said:


> You can just start out by comparing their overall accuracy to the dials on the lathe. Take out any play in the dial, zero it and the DRO and then compare to the DRO over each turn, and over something like 10 turns. This will tell you incremental and overall travel information in relation to the dials. If they compare then good enough.


That's what I did.


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## pacifica (Mar 17, 2020)

LEEQ said:


> 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 wait. Currently I have a one inch standard used to calibrate micrometers and such. I have no Jo blocks. I have an er40 collet chuck mounted to the spindle with a nice ground flat face on it. It's the kind that came as a plate with an er40 chuck protruding from it, around the chuck is the ground flat. I have .0001 Interapid test indicator on a mag based noga arm.  I have an Aloris knockoff qctp on the compound or I could also attach a vertical milling slide in place of the compound. I'm hoping to use this gear (preferably without making parts) to check the cross slide and carriage scales against the 1" standard. Something like hold the standard in a repeatable fashion against the chuck face and bring the indicator in till preloaded and zero it as well as the dro. Then remove the standard, bring the indicator in until it again reads zero and check the readout. I'm hoping to pick other peoples brains for solutions to holding the standard, indicator and general set up. Thanks for making my puzzle your puzzle.


how about a gage rod-here's one thats not too expensive:  https://www.ebay.com/sch/tvwtool/m.html?item=253681296790&ssPageName=STRK:MEBIDX:IT&rt=nc&_trksid=p2047675.l2562 . scroll down to lufkin 15" rod


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## higgite (Mar 17, 2020)

Tom


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## LEEQ (Mar 17, 2020)

ok, let's see, lot's of responses. Thanks. As stated my time is running out, and I have no Jo blocks. So I can't use Jo blocks or order tools in time. I got high resolution scales and wish to test better than with my igaging  scales, handwheels, calipers, ect. My z axis handwheel measures to .010" per division, which has never been handy . I don't know what my 123 blocks measure, but I used them. My good calipers say 3.000 and they read down to .0005. So if I trust them one more digit than I should ( and I do) the blocks are between 3.0004 and 2.9996 or so. The Z reads 3.0002" pretty reliably. I'm figuring that's not too bad, not knowing my blocks.  That set up was good and solid though. The x setup not so solid and tests didn't repeat well. The results I got varied, but showed from abt.0005 to .0015. I'm going to get a metric bolt tomorrow so I can replace the apron fill screw with said bolt and use my mill clamp set to secure a 123 block to the way. Then we'll see about more trustworthy readings.


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## LEEQ (Mar 17, 2020)

I hope my ways aren't bad enough to give me issues. There is little wear. As for screws I don't understand how they would cause issue. I got linear scales, not rotary encoders, so I don't know how they would affect my scales/readout. Somebody might have to educate me there.


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## LEEQ (Mar 17, 2020)

My hopes are that tomorrow I can achieve results in the x similar to those I got in the z. If so, no complaining for me. I should have had something here ready and waiting to test them out when they arrived. Oops. Shame on me.


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## LEEQ (Mar 18, 2020)

This is what I came up with.  I bought 2 metric bolts to cobble together into 1 that was long enough for the job. I pulled the apron fill plug and used the metric cobbled bolt in its place. Then I could use my toe clamp to secure the 123. Once I got that to happen I achieved the same results in X axis as with the Z. I thought that was pretty promising. This evenings test along with your  experiences reassures me about keeping them, no complaints. I look forward to finishing this install so I can test drive the new set up. Thanks for all the help and ideas.


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## astjp2 (Jun 18, 2020)

using a round lathe test bar can get you the same results, just put it in, mount your TDI in cross feed, move to the other side and set it back to zero, if the DRO matches the diameter, you are good for that measurement.  DRO's get into offsets and biases if you really get into how they work.  Tim


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## hwelecrepair (Jun 19, 2020)

I think my recommendation to check for accuracy is to check where your tool will be.  What I mean by that is that every added "layer" where there can be play is another place where you can get added slop.  To check for accuracy of a DRO scale and to make sure it is reading properly, you should indicate the actual scale/reader head movement.  However, that isnt always the exact travel of the tool.  Once you add in the cross slide, the carriage, the tool post, etc (depending on what type of machine and accessories you have on it) then the 10 thou you moved is actually now 4 thou because of play, wear, etc.  

As an example, I was troubleshooting a Bridgeport with no repeatability on the X.  When I got there, the scale was repeating perfectly and reading accurately.  I found the issue was when the table went to the extreme ends of travel and then back to zero, and the column/knee was worn enough that the entire knee/saddle/table was moving.  Obviously the scale couldnt pick up that movement, but the movement was seen in the part.

Just some food for thought, since it was a puzzle that took me longer than I want to admit to piece together lol

Jon


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