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A bit confused on gauge pins...

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  • A bit confused on gauge pins...

    So I have a set of gauge pins I bought on CL a wile back. The label on the set indicates that they are an "M1 -" set. It also specifies "Minus" on the label.

    Does this indicate that the pins are on the "small side" of nominal or that the hole I try them in is smaller than pin nominal?

    For example If i have a good .25" hole, will a
    0.25 minus pin go into it or not?

    I guess I sort of assumed that "minus " pins would be to the small side of nominal but then read this:

    "A go gauge checks the minimum hole diameter. A no-go gauge checks the maximum hole diameter. If the go gauge enters the hole and the no-go gauge is unable to enter, the design specifications of the hole have been met."

    This is from the McMaster Carr descriptions where it calls the minus pins "no-go"

    Color me somewhat confused. Please educate me...



    Sent from my iPhone using Tapatalk

  • #2
    Yep, you need a .250 minus pin to fit a perfect .250 hole. An on-size pin won't have any clearance. It will fit, but only with a hammer!

    So, if you have a good, tight sliding fit with a .250 minus pin, it's a .250 hole.
    George
    Traverse City, MI

    Comment


    • #3
      "Minus" pins (Class ZZ) are .0002" under the nominal size; "Plus" pins are .0002" over the nominal size. As George Bulliss said, you need a "minus" pin to verify a hole size.

      Comment


      • #4
        Thanks guys. That makes perfect sense to me and is the intuitive answer. It was the Mcmaster reference to the no-go gauge being a "-" gauge that confused me. Must be a typo...

        Thanks again, order has been restored to my world.

        Comment


        • #5
          Not a typo. The minus is the no-go. A minus pin is never larger than the specified size (ZZ are no more than 0.0002" undersize). This if how you are sure the hole size is not too large. Imagine a min 0.2490", max 0.2500" hole. If the 0.2500" minus pin does not go in, the hole is at most 0.2498", which is below the max. If you use a plus pin, the pin could be 0.2502", and it would not go in even if the hole is 0.2501", which is oversize, but the minus pin would, showing you htat the hole *could* be oversize. The no-go is to insure that the hole is NOT oversize, so you want the tolerance to insure that.

          The plus size is used for the GO pin, to insure that the hole is AT LEAST the minimum size in the tolerance range.

          Comment


          • #6
            So a no-go gauge pin, which is 0.0002 undersized will have that 0.0002" extra range to ensure that any hole that it does not go into will definitely be UNDER the specified maximum size.

            And within that 0.0002" range the pin will go into a few holes that are still within the specified hole size. But those few rejections are the price of doing business, I guess.

            And likewise, a go pin would be just a bit over the specified minimum hole size.

            It takes a minute to wrap your mind around it, but it makes sense.

            I wonder what the tolerance range on the pins is. Must be at least down to the tens of millionths.
            Paul A.
            SE Texas

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

            Comment


            • #7
              Originally posted by Paul Alciatore View Post
              So a no-go gauge pin, which is 0.0002 undersized will have that 0.0002" extra range to ensure that any hole that it does not go into will definitely be UNDER the specified maximum size.

              And within that 0.0002" range the pin will go into a few holes that are still within the specified hole size. But those few rejections are the price of doing business, I guess.

              And likewise, a go pin would be just a bit over the specified minimum hole size.

              It takes a minute to wrap your mind around it, but it makes sense.

              I wonder what the tolerance range on the pins is. Must be at least down to the tens of millionths.
              The tolerance of gage pins depends on what class they are. "Normal" (shop floor gages) pins are typically Class "ZZ"; Deltronic gage pins are Class "X". This will break it down for you: http://www.meyergage.com/abcs-of-gag...sses-of-gages/

              Comment


              • #8
                The tolerance on a minus pin, class ZZ, is -0.0002, +0.0000. On a plus pin, it is -0.0000, +0.0002. These pins are spec'd in all classes with the nominal size at one end of the tolerance range. Master pins have a symmetric tolerance (class zz would be -0.0001, +0.0001)

                For the example I gave above ( a 0.0010" range for acceptance) the class ZZ pin would likely give excessive rejects of good parts, as the gauge pin tolerance (go and no-go combined) takes 40% of the tolerance range. 10% is common practice, so for the given hole, class X would make sense (0.00004" tolerance) or class Z (0.0001") might be ok for a lot of applications, given that the pins don't usually ride the limit of the tolerance.

                A quick ref chart (first on google search) is http://www.gagesite.com/documents/Ga...hart%20PQI.PDF

                Comment


                • #9
                  Thanks for all of the insightful replies. It does indeed take, as Paul A. said, a minute to wrap your mind around it but it makes sense once it you do. The truly dumb thing I was overlooking is that the "go" and "no-go" pins are generally two different sizes (different pins) at either end of the range for the hole you are measuring. Not sure how I was forgetting that, but there ya go. I suppose it stems from how I generally use these, which is not for checking holes for tolerance (just a hobby guy doing non-critical stuff) but for finding out just how big some particular hole is. So when I have some part with a hole I want to measure, I can simply try successive pins in the hole until I find the one that won't go in. Last one that did is the size, so I can make a part to match or whatever.

                  It is interesting how something that is so seemingly straightforward as poking a calibrated pin in a hole can rapidly become complicated and confusing once you start considering all of the details. As is often the case there is much more than meets the eye.

                  Thanks for helping to set me straight.
                  -Al

                  Comment


                  • #10
                    Originally posted by enl View Post
                    Not a typo. The minus is the no-go. A minus pin is never larger than the specified size (ZZ are no more than 0.0002" undersize). This if how you are sure the hole size is not too large. Imagine a min 0.2490", max 0.2500" hole. If the 0.2500" minus pin does not go in, the hole is at most 0.2498", which is below the max. If you use a plus pin, the pin could be 0.2502", and it would not go in even if the hole is 0.2501", which is oversize, but the minus pin would, showing you htat the hole *could* be oversize. The no-go is to insure that the hole is NOT oversize, so you want the tolerance to insure that.

                    The plus size is used for the GO pin, to insure that the hole is AT LEAST the minimum size in the tolerance range.
                    Thanks for this. It took me a few times reading this through before it sunk in. I'm a bit sluggish at time. Much appreciated.

                    Comment


                    • #11
                      The assumption here seems to be that both the "hole" and the "shaft" ("pin") are straight and without taper within the limits set - but are they?

                      And that if one or both have a total of straightness that is "out" (ie "bent"??) by at last - in this case - 0.0002"?

                      How would you know?

                      Comment


                      • #12
                        Tiffie,

                        You have a good point, but IMHO, this deserves a separate discussion. There is no reason to over complicate a basic explanation that was asked for and given here. A way too many threads on our forum go into an endless counting of angels on a pinhead that has very limited or no practical application.
                        Last edited by MichaelP; 11-20-2016, 02:15 AM.
                        Mike
                        WI/IL border, USA

                        Comment


                        • #13
                          Originally posted by oldtiffie View Post
                          The assumption here seems to be that both the "hole" and the "shaft" ("pin") are straight and without taper within the limits set - but are they?

                          And that if one or both have a total of straightness that is "out" (ie "bent"??) by at last - in this case - 0.0002"?

                          How would you know?
                          The tolerance classes have a maximum geometry deviation as per standards, so yes it can have a taper and yes it can be out of round and out of cylindricity, but the significance of those is very minute compared to the readings you are doing with those pins.

                          If your hole is not staright or cylindrical, well, it ain't the pins fault.
                          Amount of experience is in direct proportion to the value of broken equipment.

                          Comment


                          • #14
                            I am also confused with the go/no-go description. It is my understanding that, when using a go/no-go gage with the red and green ends, that the green go pin is under the desired size, the red no-go pin over. The hole is larger than the go pin and smaller than the no-go.

                            That is, if you want a 0.250" hole, a 0.249" pin should go, a 0.251" pin should not. If a plus pin will pass, the hole is over that size.
                            Jim H.

                            Comment


                            • #15
                              Originally posted by JCHannum View Post
                              I am also confused with the go/no-go description. It is my understanding that, when using a go/no-go gage with the red and green ends, that the green go pin is under the desired size, the red no-go pin over. The hole is larger than the go pin and smaller than the no-go.

                              That is, if you want a 0.250" hole, a 0.249" pin should go, a 0.251" pin should not. If a plus pin will pass, the hole is over that size.
                              You understand correctly, sort of. The go/no-go method is used to confirm that a feature is within a specified range. Yes, if the plus pin will pass, the hole is over the nominal size of the pin.

                              This is why the minus pin is used for the no-go, and the plus for the go. The minus pin will not be larger than the designated size, so, if it does not go, you are certain that the hole is less than the max. If it DOES go, the hole may be less than the max (in tolerance) and there is a false rejection of the hole. The plus pin will be at least the minimum size, so if it does go, the hole is at least the minimum. If it does not, then the hole may actually be large enough, and there is a false rejection.

                              The intent is to insure that there are no false acceptances, at the expense of a small number of false rejections.

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