# DRO Accuracy



## Merlin123 (May 24, 2014)

Hello all,
I have been using my Precision Matthews 1236 for about 3 years and when originally purchased, I had the seller include the Easson ES-8A DRO.  When I first got the machine I tested the DRO (which was set up with glass scales) to see if there was any offset to the readings.  As it turned out the readings would grow by .002 to .003 each time you would move either axis away from your cutting point and then back again.  Very frustrating.  
I did the measuring and the math to try reducing the offset error and entered that into the setup program.  The changes seem to work but only intermittently so I am still double checking my positions manually to ensure I don't damage a part.  This really adds to my shop time and would like to be able to trust my DRO.
I have only been setting the linear error adjustments as yet but have considered attempting to figure out if the non-linear error correction scheme might be worth a try.
Any one out there have a tip or two on how to improve my setup?
Be great to hear from ya!
Thanks,
Merlin123


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## Senna (May 25, 2014)

I can't help you with the non-linear compensation thing because the computer stuff is all Greek to me but from your description of the problem it seems to me that the issue might be a lack of rigidity in the mounts for the scales or the reader head.
I'm sure you've checked that everything is tight countless times but are the mounting brackets themselves robust enough to preclude any movement at all? 
A decent glass scale should not be that unpredictable.

Just a thought which occurred as I read your post.

Good luck.


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## xalky (May 25, 2014)

The first thing that I would do is make sure that all the attachment points of the scales and slide are very secure and rigid. Any flex in those locations will give an unreliable reading. The next thing to check is that your gibs are as tight as practically possible. Flex there can throw off your measurements. The other thing with a DRO is that the DRO is set on the carriage and not on the tip of the cutting tool, which will always produce a slight anomaly dependent on the depth of cut and the rigidity of your machine. The heavier the cut is the more this will come into play... It is my experience that glass scales are very accurate but they will only show you slide movement at the scale, anything beyond that is flex related and is unavoidable. I usually bring my passes in to about .020-030" to final measurement, and make a spring pass( thats a pass without advancing the carriage)then I check the dimension on the part with a caliper or micrometer and take my final pass from there.  Tool post, tool and carriage flex, all play a role, therefore, the final accuracy always rests on the operator. The glass scales are probably dead on.

Marcel


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## Ray C (May 25, 2014)

As far as I know, the only time you need to consider non-linear compensation is when the scales are not (or cannot be) attached perfectly flat and in-line with the sensors.  This is akin to what Marcel mentioned about making sure the jibs are set properly as, the effect is the same.  What's happening is there is actually a triangular path being taken covering one leg of the triangle in one direction and the other leg of the triangle on the return path.  If any of the scales are not mounted flat or, if the movement of the sensor is not perfectly in-line with the direction of scale then, your motion has two vector components.    ... Check relative alignment of the scales and sensors -and make sure there is not a lot of slop in any of the jibs.


Also, how are you determining that the values are growing relative to some point?  The only way to know you are returning to a known location is by measuring with some other external dial indicator fixed at some location or by using a carriage stop -and both methods have their flaws in being totally repeatable...

Ray


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## Merlin123 (May 25, 2014)

Thanks all for the input!!
I'm going to recheck all the scale mountings as I didn't install the scales.  I have my gibs set fairly tight and checked the motion manually with my dial indicator.  The backlash is only about 5 thou. but I always pay attention to the direction I'm cutting to account for the backlash.  I don't have any triangular motion on the movement that I can detect.  I placed a .0005 dial indicator on the carriage and cross slide and all was happy so I will continue to check for other anomalies.
I'll pull, clean and reset my scales and pay really close attention to the levels and travel linearity.
Thanks for the suggestions.  I appreciate how quickly I got answers on what to look for.

Merlin123


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## Ray C (May 25, 2014)

Merlin123 said:


> Thanks all for the input!!
> I'm going to recheck all the scale mountings as I didn't install the scales.  I have my gibs set fairly tight and checked the motion manually with my dial indicator.  The backlash is only about 5 thou. but I always pay attention to the direction I'm cutting to account for the backlash.  I don't have any triangular motion on the movement that I can detect.  I placed a .0005 dial indicator on the carriage and cross slide and all was happy so I will continue to check for other anomalies.
> I'll pull, clean and reset my scales and pay really close attention to the levels and travel linearity.
> Thanks for the suggestions.  I appreciate how quickly I got answers on what to look for.
> ...



Now hold on partner...  Backlash is a whole different matter.  Virtually all lathes and mills have backlash -it's to be expected.  The whole purpose of a DRO is to measure actual movement/travel regardless of how much backlash there is.  In other words, don't worry about the backlash unless it's really excessive and bugging you.  The real important issue is that the DRO is telling you how much things moved.

If you want to remove some backlash, that's adjustable too so, check back with us if you want tips on how to deal with that...


Ray


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