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Digital scale accuracy

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  • #31
    I read most of this but not all . Something to check is . I have seen the nut the screw rides in being loose in its housing and slipping back and forth are the screws are loose that clamp it down. This will give faults readings with a read out are what ever measuring system you are using. So check this before you go any farther.
    Every Mans Work Is A Portrait of Him Self


    • #32
      And as far as this so called cosine area is concerned. Just make sure the stem of your indicator is parallel to the movement in both side to side and vertical and horizontal . If you have some mic standards set a 123 block on the table and clamp down indicated it square lay a long mic standard in a v block and push it up to the 123 block . Pick up end of standard with edge finder ZERO your measuring system remove mic standard and put in one shorter move till it zero out on edge finder check reading on system repeat with shorter standards . You can use any thing of known length . When done move back to 0 and see if numbers return to 0.
      Every Mans Work Is A Portrait of Him Self


      • #33
        "Cosine rule error" garbage

        Originally posted by loose nut
        Boy I think I'll quite asking questions if it's going to stir up this kind of hornets nest.


        All I really wanted to know is, if I get the same reading at the same point every time I try can I consider the repeatability of the scales to be adequate. I believe the predominant answer is yes. Thanks, now play nice.
        Loose nut.

        I think that you've got it right. Your set-up of your DRO scales seems to have no looseness in it. Your repeatability is OK.

        Now just to set the "cosine error" bit to rest - as I think there is more quoting of it than knowledge of its importance or extent.

        Cosine - aka Cos in a right-angle triangle is the ratio of the the length of the side adjacent to the angle to the length of the hypotenuse.

        Or Cos angle = adjacent/hypotenuse

        Or Cos = A/P

        Here is a good explanation:

        In theory, the arm of a dial indicator (with a swing arm)should be at right angles to the surface/part being measured/checked/compared where-as the plunger of an axial indicator should be normal to the part being checked.

        As Cos 0.00 deg = 1.0000 it follows that the adjacent side and the hypotenuse are the same.

        Now let's have a look at the situation at varying angles:

        Cos 0.25 deg = 0.99999
        Error = 0.00001"/inch

        Cos 0.50 deg = 0.999962
        Error = 0.000032"/inch

        Cos 0.75 deg = 0.999914
        Error = 0.000086"/inch

        Cos 1.00 deg = 0.99985
        Error = 0.00015"/inch

        Cos 2.00 deg = 0.9994
        Error = 0.0006"/inch

        Cos 3.00 deg = 0.9986
        Error = 0.0014"/inch

        Cos 4.00 deg = 0.9976
        Error = 0.0024"/inch

        Cos 5.00 deg = 0.9962
        Error = 0.0038"/inch

        Cos 10.00 deg = 0.9848
        Error = 0.00152"/inch

        The lever arm on my "adjustable finger" indicator is about 0.700" long so any so-called "Cosine error" is 0.700 x the values above.

        So if my "arm" was off-set by 3 degrees from "square" or "normal", the cosine error at the end would be:
        Cos 3.00 deg = 0.9986
        Error = 0.0014"/inch x 0.700 = 0.0010" - jep, just a "thou" at 3 degrees.

        For my axial indicator the error will be as above ie
        Cos 3.00 deg = 0.9986
        Error = 0.0014"/inch for each inch of plunger travel.

        So a lot of this "cosine error" is not the problem it is made out to be in some or many circumstances.

        It is easy to set a 90 degree/normal angle to within 2 degrees by eye and most times, that will be good enough.

        So, loose nut, that is why I think you are doing OK, despite some of the adverse, and I suspect largely uninformed, comment.

        I hope this has set your mind at rest.


        • #34
          Tiff, I don't know whose comments you feel are uninformed, and would appreciate you clarifying that statement.

          loose nut has asked a couple of questions, and there have been a couple of sugestions as to how to proceed. Lane suggested using 123 blocks, mic standards or something other of known dimension to measure at intervals, I suggested the same using measured parallels.

          Simply checking that the scale returns to zero or the same number at different points will not tell the accuracy of the scale. Actual measurements of known dimension will have to be taken at several points along the travel of the scale to verify it's accuracy.
          Jim H.


          • #35
            As he sees fit


            I will answer in the negative here in that I do not consider your comments as being uninformed - but they should tell people the extent of the applicability of, in this case, "Cosine error" and by extension the relevance and use of either comparators and direct measurement.

            I hear a lot of stuff quoted here that someone "heard" or "their mentor said" or some long-gone author of some renown said too - much of which may be either or both wrong or inapplicable.

            Indicators versus comparators was a red-herring as well.

            My main point was to show the OP that if his readings on his lead-screw dials was pretty close to that of his DRO scales, then it was near enough to both identify that the indicator was inadequate or faulty and to validate his DRO scales such that he could proceed onward to such other additional measures and measurements as he required to suit his purpose.

            Further, the accuracy required of the mechanical set-up of DRO's relative to the machine are mainly a mechanical requirement. The "errors" of a set-up at or less than o.25 degrees (which if "FAR" better than a good DRO set-up to the manufacturer's specifications will be pretty well irrelevant as the cosine "error" will be pretty well insignificant and certainly within the +/- 0.0005" (or better) for most commonly used DRO's on mills and lathes. Surface grinder DRO's (down-feed) are quite another matter as they are an order of accuracy above those normally used.


            • #36
              The DI is the plunger type, and as for using them as a measurement device, we will have to agree to disagree, 'nuff said.

              The DI is removed from the machine and I'm going to check the accuracy of the scales against gage blocks.

              Tiffie you seem to have it right.

              And that's all she wrote.

              Have a nice day.
              The shortest distance between two points is a circle of infinite diameter.

              Bluewater Model Engineering Society at

              Southwestern Ontario. Canada