Newbie mill DRO installation question

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Sep 14, 2019
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I’ve recently acquired a Bridgeport mill and am installing a DRO. So far everything is going well but I have a question.
Just how parallel to the axis of travel does the scale need to be. I’ve seen YouTube videos where it’s dialed in (over many iterations) to within a thou or two over its length. With initial installation I’ve got it parallel to 10 or 15 thou over it’s length (in both axes) Do I need to fiddle with it to get it closer or is this good.
Thinking about it, if the scale is 24 inches long and is out of parallel by 20 thou the error along its length should be less than .00001 (given 24 squared - .0020 squared = near enough to 24 as to make no difference to the accuracy of my calculator).
I am aware that theory and actual practice are sometimes at odds, so l am asking the combined wisdom of the many with more experience, is this a case where what is theoretically insignificant is actually something I need to account for?

Any decent scales should have installation instructions which you should follow. It's important that the scales are aligned parallel with the travel of the machine slide, not only for accuracy but for reliability and long life. If the scale head rubs at the extremes of travel you will eventually have big problems. The scale mounting surfaces also need to be parallel as well and a proper amount of clearance between the reader head and scale must be maintained.

With the Easson glass scales on my mill, for scales less than 950mm length (37"), the maximum allowed error is 0.1mm to 0.15mm (.004"-.006"). So yeah, you have more work to do.

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