# DRO Install Question HELP!



## mws (Jun 2, 2014)

OK, so I bought one of these SINPO 3 axis DROs for my mill.  Installing scales went nice, DRO reads consistently/repeatably.  

Determined error and input Linear Correction Factor into DRO. No change in readout.  Checked DRO memory, correction factors ARE saved.  
Went through power cycle, resetting basic params, unplug overnight, make entry in Metric Mode, you name it. I cannot get the displayed numbers to read correctly (or differently) no matter what numbers I store in the Correction Factor, (and they're definitely being stored). 

I have corresponded with JJ at the DRO store and he's asking about the basics of my data entry and calculations so far. I was hoping I'd get an "Oh yeah, that model needs you stand on one foot and pat your head after entering the numbers and it will work".  But not yet. 

Anybody with one of these devices have any advice? 

Regards,
Mark

PS: Errors are 0.0161/12" in X and -0.0061/6" in Y Corrections are .125 and .024 (mm/M) respectively. No big deal but certainly not trivial.   It ain't Rocket Science!


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## xalky (Jun 2, 2014)

Why are you using linear error correction? Are the scales not mounted parrallel to the machine slides? 

If the scales are mounted parrallel to the slides on the machine, theres no need for error correction. The slides themselves will read exactly how much they move. The error correction on the DRO is for instances where it is not practical to install the scales perfectly parrallel to the slides on the machine.


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## mws (Jun 3, 2014)

xalky said:


> Why are you using linear error correction? Are the scales not mounted parrallel to the machine slides?
> 
> If the scales are mounted parrallel to the slides on the machine, theres no need for error correction. The slides themselves will read exactly how much they move. The error correction on the DRO is for instances where it is not practical to install the scales perfectly parrallel to the slides on the machine.



These glass graticule aren't perfect, and these Chinese ones may be less so. Even my Heidenhain have an error specification based on temperature, which can be corrected for if needed but nobody ever does on such precision equipment.  Nevertheless, the graticules do have a continuous error over several inches that needs to be corrected for.  

And I'd like to see how these slides could possibly be mounted out of parallel with the ways and still slide at all unless they moved in two (or more) planes.  But yes, the slides are mounted "perfectly" parallel to the ways, certainly within a few thousandths over 18".  Optical reader cant was one of the first things I checked and the two count states seem to change dead on at the reference marks, like they should.


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## mws (Jun 3, 2014)

*Re: DRO Install Question Problem Solved!*

OK, simple solution!  

The equation in the book was wrong.  To determine the error per meter the actual test distance must be multiplied by its part of a meter.  Ex: using 300mm for a test length the final error would be multiplied by 1000/300 to yield error per meter, which IS the correction factor. 

The manual had this as test length/1000, exactly the inverse of what was needed.  Since I was coming up with correction factors much too small (below the programmed level of precision*) these factors were ignored.  

Had I thought about the equation, knowing full well what it was supposed to be doing, I might have noticed the error.  But I didn't think to question the formula. After all, how could they have gotten that SO wrong in a user manual?!! 

*Level of precision:  Apparently, in these devices, being "inexpensive", they have chosen a level of calculation precision below that to which we have become accustomed.  I noticed this first when using the CALCulator function. It only goes to four decimal places.  Hence, converting inches to meters (dividing by 0.03937) is not possible, at least not accurately, but I digress.

*When entering the correction factor there are discrete steps in correction that do not correspond with changes in the correction factor. Ex.  1.126~1.129 all result in the same correction, then, 1.130~1.134 result in a similar correction.  So you may not be able to hit your target exactly but you will be +/- the resolution of the scale.  Had I to do this again I would get 2.5 or 2.0 u scales for greater resolution. I bought the standard 5u being +/- 0.0002" 

So there you go.  Hope my not doing my due diligence with the math helps somebody.  

Thanks,
Mark


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