TP of threaded diameter

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  • TP of threaded diameter

    Hello All,
    I have been reading up on the correct way to measure a threaded diameter but I am still unclear on what the correct way is to do this.

    I have created a program using an auto cylinder 2 levels, 3 hits per level. It is not coming out good. When I go to double check it with the height gage it is right on the money.

    For what I have read I need to adjust the pitch so that it follows the thread correctly.
    So If I am checking a .086-56UNC-2B, do I input 56 into the pitch field?

    Also, has anyone successfully used the Cylinder Centering Thread Scan Strategy?

    Thanks for any help!

  • #2
    I'll be honest with you Famous, I wouldn't use the pitch thing, no 2 threads are alike. Every Program runs the same. you might be better off putting a tight gage pin and finding the location of that, or use a shank probe (cylinder styli) or bigger ruby end. I'm sure some people will say different.
    Last edited by KIRBSTER269; 04-12-2016, 05:59 PM.
    (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
    They say "Nobody's Perfect." I must be Nobody.

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    • #3
      Some of these guys can tell you how to scan a thread pretty easy, but a threaded hole is usually a projected tolerance zone. So you should get some Lavezzi True position gauges. very easy to use and most cases better than scanning a thread. Some of you may disagree, but as a tap and drill wear out in production, the thread becomes increasingly less reliable to gauge the position of a hole. unless it is thread milled and you can guarantee no burr along the thread form. I bought a set of the Lavezzi gauges a few years ago and did some testing and I like them, so long as you don't have a very short measurement time and can remember to thread it in before you run the part....my 2 cents
      sigpicTAU ALPHA PI INDIANA DELTA CHAPTER
      "Due to the highly confidential nature of my job, I am not allowed to know what I am doing" - author unknown

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      • #4
        Current most-accurate way: purchase expensive thread plug gages, put in Operator probe comments to instruct operators to put them in and take them out.

        Current less-accurate less-trouble way: Auto Cylinders with spiral scan set to thread pitch.

        Future high-accuracy high-speed no-plugs: stay tuned.

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        • #5
          The Lavezzi gauges are actually inexpensive and very accurate. But, Josh is right! The best way would be to have the machine do it. However I am not sure how you could guarantee the scanning , no matter how fast or precise can extract reliable numbers from a poorly tapped hole unless you have a special v shaped disk probe...?
          sigpicTAU ALPHA PI INDIANA DELTA CHAPTER
          "Due to the highly confidential nature of my job, I am not allowed to know what I am doing" - author unknown

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          • #6
            Idea of Pitch diameter is that you will follow the pitch of the thread while probing, therefore probing approximately the same place along the thread for all points to minimize error for points measured along probe path.

            If possible using locators as mentioned earlier post probing on threads goes away but there is a cost and does your GD&T say "minor diameter" on threaded ID?
            Try a circle with the 56 pitch and see how that looks, this is a fairly small size minor dia watch your speeds as prehit/retract may become issues inside that small threaded ID, probe size along thread matters too...larger probe dia is more forgiving and I will guess that this size thread is a 1mm tip......

            also I was always taught to use at least 3 levels on a cylinder if you stay with the cylinder
            Last edited by Rich P; 04-12-2016, 06:26 PM.

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            • #7
              Originally posted by Josh Carpenter View Post
              Current most-accurate way: purchase expensive thread plug gages, put in Operator probe comments to instruct operators to put them in and take them out.

              Current less-accurate less-trouble way: Auto Cylinders with spiral scan set to thread pitch.

              Future high-accuracy high-speed no-plugs: stay tuned.
              That's good news!

              So the Cylinder Centering Thread Scan Strategy is not very reliable?
              I have not tried it yet but I will once I get back to the machine.

              When you say set to thread pitch. if the pitch is 56 do I input 56 in the pitch field?
              When I tried that offline, the probe goes up to the sky lol.

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              • #8
                The 56 means 56 TPI which is 0.0178" or 0.453mm one of those is what you put in the pitch field

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                • #9
                  56 threads per inch: Pitch = 1/56 or 0.017857
                  Lately, it occurs to me
                  What a long, strange trip it's been.

                  2017 R1 (Offline programming)

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                  • #10
                    I agree the gages would be the most accurate but I would like the machine to do the work.
                    I guess I need to figure out how to correctly use the pitch field.

                    The Cylinder Centering Thread Scan Strategy seems useful. From what I have read in the help file, I guess the probe centers itself in the thread and spiral scans inside the valley of the thread.
                    The diameter of the probe needs to be larger than the width of the thread to avoid shanking. I personally have not tried it yet though.

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                    • #11
                      56 threads per inch: Pitch = 1/56 or 0.017857
                      The 56 means 56 TPI which is 0.0178" or 0.453mm one of those is what you put in the pitch field
                      Thanks for clarifying!

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                      • #12
                        60% ratio from Ø of hole to ruby used is a good start. AS someone said already, to small of ruby and your just using a shank probe instead. I personally never use cylinders and use multiple auto circles and different depths. Set the pitch and then I change the setting from 0-360 to 0-720 ( how ever many rotations you want ) . We did have some "PLUGS" here that we bought for a job a few years ago. 2 of them were actually bad and would give some seriously inconsistent readings. Don't just think cus you bought a thread plug you are golden. Some brands are better than others. Some brands, when screwed tight to the face of the plug may tip 1 way or another. They are not all created equal. Depending on what type of tolerances you are dealing with, My 1/2 cent is to not waste your time with inserts and use the ruby. ( I understand customers may drive this).

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                        • #13
                          I used to have a lot of true positions with MMC to threaded holes. I was lucky that the customer wanted them to the minor diameter and I scanned five lines (from bottom of thread to top) and created a minimum circumscribed cylinder out of the line. Worked well for our customer.
                          DeWain Hodge

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                          • #14
                            You can go online and find different diameter for ex. Metric thread. You can find list of all metric thread diameter in Google search. I am not sure but they consider diameter at 60*. So select Female inside diameter for threaded hole. I printed list to find Female inside diameter for M8 size threaded hole which was I think 6.647 mm or 6.648 mm in the list. You would have to consider this diameter as nominal size.

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                            • #15
                              Can anyone give a rough estimate of how accurate measuring a threaded hole for true position should be using just a 2mm ball on a 1.25 pitch thread?

                              I am currently measuring a part which varies hugely and we keep bouncing the parts back to the supplier saying they are out of spec. They however keep saying that we are not capable of measuring a threaded hole accurately and their parts are good.

                              Their parts are measuring anything upto 2.5mm out of spec when tolerance using true position. I will fully admit that measuring a thread isn't the most accurate thing to measure but surely measuring as a cylinder with the pitch input would yield accuracy with at least 0.1-0.2mm? or am I wrong, could the error of over 2mm be coming from the measurement strategy?

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