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Why does dial test indicator lie ?

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  • Why does dial test indicator lie ?

    I've got an Interapid 312B-3 dial test indicator which reads 0.0001". It consistently reads 0.012" on a 0.010" displacement.

    The Interapid has a 12 degree contact angle and this has been checked. The point length is correct also. Accuracy is way beyond this error (0.00016" per revolution and it's only moving 1/10 of a rev.).

    One other little tidbit, it also reads identically in either direction. If it had a bent rack or something, it shouldn't work this well, should it?

    Den

    [This message has been edited by nheng (edited 06-01-2004).]

  • #2
    I am far from an expert, but I was taught that a DTI is for relative measurements only - not absolute.

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    • #3
      What's the nature of the displacement? THe displacement could be labeled wrong, or it may not be square if you're meaning a gauge block.

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      • #4
        The 0.010" was linear travel of a lathe compound on which I was checking and adjusting backlash. Contact was on a square end and at the proper angle for the Interapid (12 degrees). I verified the travel with another indicator.

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        • #5
          The problem is called cosine error. The only way you get 1 for 1 reading to movement correspondance is when the contact angle is parallel to the surface and perpendicular to the motion. Noodle out the trig on the contact angle problem and maybe you'll come close - maybe not.

          Some pricey electronic indicators have an involute contact point that partly corrects this error but remember a DTI is a convenience instrument.

          If you need hard data that will survive adversarial direct and cross examination before an administrative law judge, use a calibrated dial indicator that works with a linear motion.

          [This message has been edited by Forrest Addy (edited 06-02-2004).]

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          • #6
            Forrest: The Interapid is made for a 12 degree contact angle so that you can more easily poke it down into bores. For the 0.010" movement, I think the angle increases (or decreases) by 0.8xx degrees. The cosine error is very small.

            Cosine error on these is calculated for the angle of contact minus 12 degrees.

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            • #7
              Bad post. Ignore

              [This message has been edited by Forrest Addy (edited 06-02-2004).]

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              • #8
                As may be. You could send it off for calibration. I belive there's an adjustable link inside for that very purpose.

                If didn't you want to bother and you wanted to use it for precision data collection I suggest you calibrate it by comparing its readings to known displacements and making a correction chart from them.

                Otherwise live with it. I hate it when stuff doesn't meet specs but in this case I'd live with it unless you thought it was worth beating up the factory.

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                • #9
                  Forest: I got this one (currently around $250) and about 5 others in very good shape (except for this one's readings) for $50. It included a 312B-1 (0.0005" Interapid), a Gem dual faced dti and several others. They have not seen grinding or coolants and were clean. Not too shabby, eh?

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                  • #10
                    For $50 you got 5 good ones and 1 bad one?

                    I'll take the bad one off your hands if it will make you feel better. I won't charge you anything either!

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                    • #11
                      Actually, it included another 10 or so additional pieces of dti junk and the little gems were hidden from Ebay eyes by a poor image. When I opened them and found the 2 Interapids (worth $400+), almost new (at least in appearance) I nearly died

                      I have since recovered

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                      • #12
                        Are you sure the point lenght is correct ?
                        Since it isn't Brand new..maybe it was changed by previous owner..


                        All Things are Relative..especially DTI's

                        When they stop reading what "I" want, I quit using em!

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                        • #13
                          Just a thought, my DTI has a changeable tip. It came with one that has a ball on the end of the lever ams the other has a teardrop shapped end. Is it possible that a previous owner replaced the tip with one not intended for that make or model?

                          Blast it! I always was aware of the angle/sine thing and therefore never worried about it's absolute accuracy. But now, I GOTTA KNOW. I guess I'll have to test it now.

                          Paul A.
                          Paul A.

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

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                          • #14
                            Problem solved here

                            http://bbs.homeshopmachinist.net//Fo...ML/006765.html

                            Den

                            [This message has been edited by nheng (edited 06-03-2004).]

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