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Harbor Freight Calipers inside and out

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  • Harbor Freight Calipers inside and out

    This is a restart of a thread about repeatability and accuacy of the HF caliper.

    Two articles are presently available that describe my best understanding of these calipers. The first

    http://rick.sparber.org/Articles/PAC/PAC3.pdf

    deals with just the caliper. I felt compelled to retract some of my conjecture related to extrapolted accuracy presented in versions 1 and 2 but added more related to repeatability and how it interacts with accuracy at values near zero.

    These retractions may later be reversed when the work from ckelloug is done. He has Jo blocks and the background to clearly see the absolute accuracy of these calipers in the range of 0 to 4".

    The second article is a 28 page monster which was coauthored by Larry Gill of the Yahoo group "shumatech" and myself.

    http://rick.sparber.org/Articles/JB/JB4.pdf

    It does talk a lot about the caliper but more from an internal standpoint. The shumatech DRO directly accesses the caliper's internal count. There is a display mode that permits the user to see this count.

    One thing that should generate a nice little "fire storm" is the section on repeatability. The experiment suggests that if you use this internal number you can get a short term (i.e. less than 10 minute span) repeatability of ± 2 tenths rather than the adverstised ±5 tenths.

    I welcome your questions and comments. No reason to stop at the present versions.
    Rick Sparber

    [email protected]
    web site: rick.sparber.org

  • #2
    Rick, even if that is so (resolution)
    They only display to 0.0005" or 0.01 mm
    How does that help the user?
    I don't wish to denigrate your work.
    Maybe I'm just having a blonde night?
    Just got my head together
    now my body's falling apart

    Comment


    • #3
      Originally posted by rgsparber
      The experiment suggests that if you use this internal number you can get a short term (i.e. less than 10 minute span) repeatability of ± 2 tenths rather than the adverstised ±5 tenths.
      That's interesting Rick -- I don't have time right now to read the latest version of your document, but how far are you moving the caliper when you're measuring +/- 2 tenths repeatability?

      Are you still seeing +/- 1 thou accuracy during the same time-frame/conditions?
      "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

      Comment


      • #4
        Originally posted by Swarf&Sparks
        Rick, even if that is so (resolution)
        They only display to 0.0005" or 0.01 mm
        How does that help the user?
        I don't wish to denigrate your work.
        Maybe I'm just having a blonde night?
        I was refering to the internal number. A single count here ideally equals 0.5 tenths. The Shumatech software filters this number to eliminate jitter so I am really looking at the average value.

        If you don't have a Shumatech DRO, then you are right. It does not help you. Sorry for the confusion.
        Rick Sparber

        [email protected]
        web site: rick.sparber.org

        Comment


        • #5
          Originally posted by lazlo
          That's interesting Rick -- I don't have time right now to read the latest version of your document, but how far are you moving the caliper when you're measuring +/- 2 tenths repeatability?

          Are you still seeing +/- 1 thou accuracy during the same time-frame/conditions?

          My repeatability test uses a recently factory rebuilt Starrett Last Word finger DTI to sense touch down on the Z axis. I move the quill up about 0.2" and then lower it until the DTI reads zero as viewed through my Optivisor. I have an ultra fine feed on my RF30 that permits me to move a lever 2" and get a 2 thou down feed (it is just a dumb 15" long rod threaded 1/4-20 on the end that screws into the Z axis collar).

          At touch down I read the internal number in HEX on the DRO. This operation was repeated 10 times in less than 10 minutes. Past experience has shown me that I can hit TD within ± 1 tenths with the DTI. About 5 hours later I did one more TD and saw a drift of 2 tenths. No surprise there.

          The overall accuracy is still, at best, ± 1 thou. As you get above 100 mm of movement, the error goes up.
          Rick Sparber

          [email protected]
          web site: rick.sparber.org

          Comment


          • #6
            Fair enuff Rick.
            I was not aware of your DRO application.
            I just use em as calipers and height gauge.
            Cheers,
            Lin.
            Just got my head together
            now my body's falling apart

            Comment


            • #7
              Originally posted by rgsparber
              I move the quill up about 0.2" and then lower it until the DTI reads zero as viewed through my Optivisor.

              At touch down I read the internal number in HEX on the DRO. This operation was repeated 10 times in less than 10 minutes.
              The overall accuracy is still, at best, ± 1 thou.
              Ah, got it Rick -- interesting results.
              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

              Comment


              • #8
                Ummm, perhaps I am missing something, But ......IT IS A HARBOR FREIGHT CALIPER, WHAT DO YOU EXPECT?

                Tenths? yea right on a Cheap DRO?

                Comment


                • #9
                  Originally posted by IOWOLF
                  Ummm, perhaps I am missing something, But ......IT IS A HARBOR FREIGHT CALIPER, WHAT DO YOU EXPECT?

                  Tenths? yea right on a Cheap DRO?
                  It is not what I'm expecting, it is what I saw. The subject is short term repeatability, not accuracy.

                  I see a short term repeatability of ± 2 tenths. This repeatability is good for at least 10 minutes.
                  Rick Sparber

                  [email protected]
                  web site: rick.sparber.org

                  Comment


                  • #10
                    So Rick, are you making turbine parts for the military or rocket parts for NASA? What kind of machines are you making these parts on?
                    It's only ink and paper

                    Comment


                    • #11
                      Originally posted by Carld
                      So Rick, are you making turbine parts for the military or rocket parts for NASA? What kind of machines are you making these parts on?
                      Nope, just trying to understand my error sources. That is part of my hobby. I own a little RF30 mill/drill but with maximum care it can hold ± 4 tenths. I plan to continue to scratch my head and find out where at least some of this ±4 tenths comes from. Only then can I find a way to reduce it.

                      I use a sine bar to set depth of cut. This procedure requires me to touch down twice with my DTI. If each touchdown adds ± 1 tenth, then I just bought ± 2 tenths of error worst case. That is 50% of my overall, measured error. If I can get the TD error down to ±0.5 tenths, then I should be able to cut my overall error by ± 1 tenth which is a reduction of 25%.

                      That ± 4 tenths is the "effect". I just need to undertand all of the tiny "causes".
                      Rick Sparber

                      [email protected]
                      web site: rick.sparber.org

                      Comment


                      • #12
                        I Don't know about you , but I figure if I can hold +_ .001 on any mill you are doing good +- .0001 you are on a Jig borer of some kind. I don't care what kind of DRO you have the mill ain't capable of it.
                        Every Mans Work Is A Portrait of Him Self
                        http://sites.google.com/site/machinistsite/TWO-BUDDIES
                        http://s178.photobucket.com/user/lan...?sort=3&page=1

                        Comment


                        • #13
                          Originally posted by lane
                          I Don't know about you , but I figure if I can hold +_ .001 on any mill you are doing good +- .0001 you are on a Jig borer of some kind. I don't care what kind of DRO you have the mill ain't capable of it.
                          That has been my experience on my mill/drill. I've never actually seen a jig borer although I've read about them. Not exactly "hobby" equipment for most of us.

                          When I get to within 15 thou of my final value, I start to use my sine bar rather than my DRO. I attempt to make 3 equal cuts of 5 thou each. The first cut defines my reference surface, the second cut tells me how much I'm off from 5 thou, and the third cut uses the correction factor from the second cut to get as close as possible. At each step I use my mic to measure the thickness so can figure the actual depth of cut.

                          To a first order approximation, cuts of the same depth into the same material at around the same time should have about the same error. At least that is what I've found in my shop.
                          Rick Sparber

                          [email protected]
                          web site: rick.sparber.org

                          Comment


                          • #14
                            Originally posted by rgsparber
                            The first cut defines my reference surface, the second cut tells me how much I'm off from 5 thou, and the third cut uses the correction factor from the second cut to get as close as possible.
                            That's basically how Moltrecht describes how to finish to a specific diameter on a lathe cut.
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                            Comment


                            • #15
                              Originally posted by lazlo
                              That's basically how Moltrecht describes how to finish to a specific diameter on a lathe cut.
                              This approach also works for boring as can be seen in HSM July/August 2002, page 34.
                              Last edited by rgsparber; 06-10-2007, 08:10 PM.
                              Rick Sparber

                              [email protected]
                              web site: rick.sparber.org

                              Comment

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