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Accuracy of typical digital gear calipers

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  • Accuracy of typical digital gear calipers

    I was thinking of buying one of the lower-priced gear calipers to use occasionally for figuring unknown tapers. There are the old standby pricy vernier ones like Starrett, the cheap vernier ones from Russia, then the new cheap digitals that all look alike and look like they come from China no matter what seller's name is on it. All these have two perp. Axes each with readout for depth and width resp.

    Anyone tried one of the cheap (under $100) digitals enough to guess at accuracy? It seems the older vernier ones advertise 0.01 or 0.02mm accuracy depending.

    Has anyone used these successfully to get the measurements to calculate tapers?
    Last edited by Cannonmn; 10-20-2016, 06:08 PM.

  • #2
    I've used a cheap digital gear tooth caliper and it wasn't far off what a benson gear tooth caliper said, good enough in other words, it cost about £30.
    Mark

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    • #3
      Thanks, will steer toward the digitals then, even though the old vernier types are much cooler looking!

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      • #4
        I have an old pal;;;

        Originally posted by Cannonmn View Post
        Thanks, will steer toward the digitals then, even though the old vernier types are much cooler looking!
        He is a very good machinist. He calls all the calipers, digital, dial or old fashioned engraved ones " Near Enoughs" In his hands the results always seem " Near Enough' to make good sound working equipment. Regards David Powell.

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        • #5
          Originally posted by David Powell View Post
          He is a very good machinist. He calls all the calipers, digital, dial or old fashioned engraved ones " Near Enoughs" In his hands the results always seem " Near Enough' to make good sound working equipment. Regards David Powell.
          Dave its not that vernier calipers are old fashioned its just our eyes that are old. I've always call then very-nears. Still have a place in my shop in the larger size, I mean 24" Starretts, Etalon and Mitutoyo vernier calipers are out there in abundance but a 24" digital is an expensive thing
          in Toronto Ontario - where are you?

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          • #6
            Originally posted by Cannonmn View Post
            Thanks, will steer toward the digitals then, even though the old vernier types are much cooler looking!
            What kind of reasoning is this? No details given on accuracy of either type to base your decision on, but you make it anyway.

            Based on my experience, a good dial caliper will repeat within .0003-.0004. Cheap digitals are usually no better than +/- .001. Really good (expensive) digitals might be better than good dial calipers.
            North Central Arkansas

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            • #7
              Curious as to how one uses gear tooth calipers to measure tapers, unless the taper is very short.
              Jim H.

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              • #8
                I haven't tried any of the digital ones, but from what I can see of them in ads, they are made in the same manner, from the same components as the $10 to $25 import, six inch, digital calipers. So I would expect about the same level of accuracy. Talking in metric units, these calipers have a least count of 0.01mm and +/- that amount (the least count) is always ADDED to the rated accuracy of any digital instrument.

                On top of that, these inexpensive calipers will have some basic scale accuracy which should be expressed as a percentage of the reading (of the distance across the scale). So it would be more at the longer readings and less at the lower ones. This rating is rarely given in any ads, especially in ads for the inexpensive digital instruments. So we must make some kind of estimate.

                For the inexpensive calipers a reasonable estimate of that number would be +/- 0.05mm at the full 150mm distance. That's about +/- 0.002" for you English system types. So the overall accuracy rating should be around +/- 0.06mm at 150mm. But at smaller distances, as you would be measuring with a gear tooth caliper, it would be more like 1/6 or 1/10 of that amount or about +/- 0.005mm. That may not be completely reasonable and I would make a guess of about +/- 0.01mm at distances of around 10 to 20 mm.

                Adding those two figures together you get +/- 0.02mm which is in agreement with the larger of the two figures that you gave.

                Another factor here is that these inexpensive calipers are not as well made as the more expensive ones. I know this from personal experience. I can often see a +/- 0.025mm change in the readings on my inexpensive digital calipers resulting from the amount of wiggle that the moveable jaw has on the beam of the caliper. Thus, in order to get consistent readings, you must have a very consistent technique in your measurement. This variation is above and beyond the two figures that I gave above and would bring the total error up to about +/- 0.045mm. For you English measure types, that is almost +/- 0.002". Gear tooth calipers are more complicated than standard 150mm/6" calipers as they have two scales at 90 degrees to each other. This will increase the errors by some additional factor so we may be up to +/- 0.08mm (0.003") or more.
                Paul A.
                SE Texas

                And if you look REAL close at an analog signal,
                You will find that it has discrete steps.

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                • #9
                  Yea, I wondered about that too. I guess you could use them to take two diameter readings at two different distances from an end. But it will be hard to ensure that the jaws are at the actual diameter position. They could easily be off to one side or the other.



                  Originally posted by JCHannum View Post
                  Curious as to how one uses gear tooth calipers to measure tapers, unless the taper is very short.
                  Last edited by Paul Alciatore; 10-20-2016, 10:20 PM.
                  Paul A.
                  SE Texas

                  And if you look REAL close at an analog signal,
                  You will find that it has discrete steps.

                  Comment


                  • #10
                    Originally posted by JCHannum View Post
                    Curious as to how one uses gear tooth calipers to measure tapers, unless the taper is very short.
                    I got the idea from an oxtools video on turning short tapers on a lathe. Now that I think of it, he took the numbers with a vernier gear caliper, then did it again with a dual-diameter brass ring gauge he made. The distance between diameters he measured was about 3/4 inch, if I remember correctly.

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                    • #11
                      I see he does it all the way to the end of this video;

                      https://www.youtube.com/watch?v=OfrRFCzGNTw

                      It sort of works if you have a geartooth caliper, but I would hardly recommend rushing out to purchase one just for that purpose. Tapers are tricky to measure and there are too many sources of error in that method to make it anything more than a rough estimate.
                      Jim H.

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                      • #12
                        Thanks, I'm going to wait till next time I have to read a taper precisely then reinvestigate how best to do it. After getting the inputs here I'm thinking it would be best in my case to take it to a machinist who's better equipped and skilled and have him do it; I'm not going to lose any sleep over being taper-challenged.

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                        • #13
                          The digital ones seem to be in 1/10 of a degree, that's 6 minutes of angle.

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                          • #14
                            Since you are turning it on a lathe anyway, the easiest way to measure a taper is to mount a DI (not a DTI) on the cross slide and zero it at one end of the taper. Then move the carriage a given distance and see how much change there is in the reading.
                            Paul A.
                            SE Texas

                            And if you look REAL close at an analog signal,
                            You will find that it has discrete steps.

                            Comment


                            • #15
                              That's not very accurate. This is why people use trig. to get better accuracy.

                              Of course, it may be good enough for many purposes.



                              Originally posted by old mart View Post
                              The digital ones seem to be in 1/10 of a degree, that's 6 minutes of angle.
                              Paul A.
                              SE Texas

                              And if you look REAL close at an analog signal,
                              You will find that it has discrete steps.

                              Comment

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