Announcement

Collapse
No announcement yet.

Digital scale accuracy

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Digital scale accuracy

    I have the generic digital scales, with the simple display, mounted on my RF 45 type mill.

    If I mount a dial indicator on the mill and move the table in the x or y directions and check the movement with the indicator I get a bit of a discrepancy, maybe a couple of thou up to .005" and even .010" after a while.

    Today I put in a wiggler, the type with the ball on the end of a shaft, and ran it into a parallel mounted in the vise, zeroed it and then ran it back and forth several inches, jogging it in both directions so there would be lots of backlash added into the movement, and back into the parallel again. I repeated this 5 or 6 times in the x and y directions and every time I got a 0.000 reading against the parallel.

    These scales are suppose to be accurate to .001" which is good enough for my shop, mostly.

    Now dial indicators are comparators and not really meant for measuring because they can hang up a bit when the plunger is returning to rest so there could be some error there and a bit of tangential error in the way it is mounted.

    So which is right the scales or the indicator or is there something wrong with my methodology in checking this out. It would seem to me that if it returns to the same place with the same readings the scales should be right. :
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

    Southwestern Ontario. Canada

  • #2
    If You get the same reading every time on the scale, with the wiggler doing the same... then it would be the dial ind. that is misleading. Unless of course , the wiggler and indicator are in conspiracy against You.

    Steve

    Comment


    • #3
      When was the last time you saw a CNC machine with a mechanical measurment system on it??
      Most electronic scale systems use the same technology, the only differences being the resolution of the display system.

      Comment


      • #4
        More Info, Please

        Originally posted by loose nut
        If I mount a dial indicator on the mill and move the table in the x or y directions and check the movement with the indicator I get a bit of a discrepancy, maybe a couple of thou up to .005" and even .010" after a while.
        I can't tell if you're asking about an error that happens when you :
        1) Move the table a distance then move the table back to 0, the scale doesn't read 0 but reads 0.005. Repeating this adds a bit to the measurement each time;

        or

        2) Move the table 6.000 inches and the scale reads 5.995 inches

        or

        3) Move the table 6.000 inches and the scale read 6.005 inches

        For 1, the batteries in the scales might be getting low. I have had this happen on my digital caliper. When the battery is low it starts missing counts. When the battery is really low it's wildly innacurate.

        For 2, the scales aren't exactly parallel to the axis. The law of cosines is getting you.

        For 3, I have no idea how this could happen, except for excessive backlash in the scale mountings.

        More info, please.

        Comment


        • #5
          Get a set of gauge blocks and use them to check the scales, indicators, etc. Then you will know.
          Paul A.
          SE Texas

          Make it fit.
          You can't win and there IS a penalty for trying!

          Comment


          • #6
            Now dial indicators are comparators and not really meant for measuring because they can hang up a bit when the plunger is returning to rest so there could be some error there and a bit of tangential error in the way it is mounted.
            A dial indicator is not a comparator. It is incapable of making a comparative measurement. A DI provides an absolute measurement of variation. That is an entirely different matter. Even with a tangential contact the measurement is still absolute subject to a cosine error that is also absolute. If the reading varies then either the indicator is malfunctioning, the mounting is not secure or the position being measured is really varying.

            It is easy enough to check the indicator. Set it up to measure to a non-varying reference surface. Introduce a known thickness of shim stock (spark plug gap gauge is good) between the indicator and the surface. Remove the shim stock and repeat several times. Try different thicknesses of shim stock, do not stack them. It will be quickly apparent if the indicator is accurate or not.
            Free software for calculating bolt circles and similar: Click Here

            Comment


            • #7
              A dial indicator will measure distance within it's range, as will a DRO scale. Both can have cumulative error within their limits, usually expressed as +-something per unit of measurement.

              Dial indicator accuracies are usually +- one graduation over the full range. The full range is usually considered to be 2-1/2 turns of the pointer. Extended range indicators will have wider tolerances. Dial indicators are most accurate between the 10 o'clock and two o'clock positions from zero. A dial indicator, should be preloaded, about 1/8 to 1/4 of a turn before setting the zero point to ensure it will return to zero.

              The accuracy of the DRO will depend upon the manufacturer, but probably approaches that of a good digital caliper, something on the order of +- 0.001" per 6 inches of travel or so.

              Both can be out in different amounts and different directions and still be within their specification. If one is out in the plus direction and the other in the minus, the error will be doubled. Both can be out at some distance of travel and still return to the same zero point with dead accuracy. For these reasons, checking the two instruments against each other does not prove if either is out of spec. The best method of checking is with known standards such as gage blocks, assembling a series and checking the travel of the DRO against them.
              Jim H.

              Comment


              • #8
                The opinion seems to be that my experiment won't test the degree of accuracy of ether measuring device, which isn't what I intended, so I better rephrase the question.

                If the digital scales come back to the same point with the exact same reading (within the accuracy level of the device) it would seem that they have a accurate degree of repeatability (not accurate measurement of distance traveled) where the DTI (Mitutoyo) which, for what ever reason, didn't have the same readings. It would appear that the scales are accurate for finding a set point in X,Y coordinates, IE: setting the center of a rotary table and being able to go back to that point after an operation is complete, on a mill table.

                It would seem that they are repeatable unless the amount of error in my use of the wiggler and the error in the scales are exactly the same every time which would give the same erroneous readings. After repeating the trial half a dozen times that is almost impossible.

                This is different than testing the accuracy of the distance that they traveled, my mistake, that's what I have to do next.

                Hope this is less confusing, have I got it right this time.
                The shortest distance between two points is a circle of infinite diameter.

                Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                Southwestern Ontario. Canada

                Comment


                • #9
                  I think the problemo lies in the way your using your DI, they are flimsy set-ups the way they mount - they just are - they also have fairly stiff little springing on the ball levers -- once you get them preloaded they can be somewhat accurate but only in the preloaded direction of travel, If your trying to measure fore and aft with one its a total Fuqe as now your releasing the tension from the entire "wet noodle" mounting rig, this is where your getting your discrepancy -- your seeing the battle between the ball lever "sticktion" and the mounting hardware flex, its a no-no, the hardware has no sticktion - its just flimsy length levers - when you back up its ALWAYS the first to move -- your DI ALWAYS takes more energies to compress than it does to spring back BUT its still an internal mechanism with frictional qualities -- your variance in readings is directly proportional to the difference between these two systems.

                  Comment


                  • #10
                    Are you using a dial indicator (DI) or a dial test indicator (DTI)? They are two different instruments.

                    Regardless, either should return to the same zero point if applied properly. Mechanical indicators can get gummed up or mechanically damaged to the point that they will not return to zero reliably, this is not an inaccuracy in the gage, but the result of abuse or neglect. As AKB points out, the rigidity of the setup will also play a large part in the repeatability of the system.
                    Jim H.

                    Comment


                    • #11
                      It was mounted on one of the "mighty mag" mounts stuck onto the mill table pushing against the column. Could slip I suppose, I don't really care about the DI, I was only using it as a check against the scales (which isn't much good if the DI is moving), my concern is the repeatability of the scales which seems pretty good BUT????
                      The shortest distance between two points is a circle of infinite diameter.

                      Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                      Southwestern Ontario. Canada

                      Comment


                      • #12
                        Originally posted by Evan
                        A dial indicator is not a comparator. It is incapable of making a comparative measurement. A DI provides an absolute measurement of variation. That is an entirely different matter. Even with a tangential contact the measurement is still absolute subject to a cosine error that is also absolute. If the reading varies then either the indicator is malfunctioning, the mounting is not secure or the position being measured is really varying.
                        Maybe a dial gauge is not, in itself, a comparator but they are commonly used as part of a comparator.

                        see ebay 220390513653 or 250360000736 for examples.

                        Tim

                        Comment


                        • #13
                          Maybe a dial gauge is not, in itself, a comparator but they are commonly used as part of a comparator.
                          The essential difference is that the comparator has a built in point of reference. A DI does not.
                          Free software for calculating bolt circles and similar: Click Here

                          Comment


                          • #14
                            Originally posted by Evan
                            The essential difference is that the comparator has a built in point of reference. A DI does not.
                            But surely the OP's setup included a point of reference?
                            I think you're splitting hairs

                            Tim

                            Comment


                            • #15
                              Splitting hairs? Hardly. How can you use a DI as a comparator? The fact that it has a point of reference in the setup is of no value. It isn't dependable or portable or even referenced to the dial indicator, all of which must be true for it to become a comparator. A dial caliper is also a comparator and I have never seen anybody here confuse it with a dial indicator even though it has an indicator dial.
                              Free software for calculating bolt circles and similar: Click Here

                              Comment

                              Working...
                              X