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Geometric Dimensioning Tolerancing

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  • Geometric Dimensioning Tolerancing

    This question maybe a little heavy for the HSM. I bought the book "Machine Shop Trade Secrets".

    The author has about a half-page on this subject. I remember having "Geo Tolerancing" training/class in late 1980's or early 1990's. And have pretty much "forgot", working in a place that didn't (I think)use it.

    Was this a "flash in the pan" of that time, and it lingers on in "old" prints?

    OR:

    Is this something that has been widely adopted?

    My job searching hasn't brought GDT up. But I am wondering if this should be "prioity learning" for 21st century CAD, CAM, or CNC.




    ------------------
    Today I will gladly share my experience and advice, for there no sweeter words than "I told you so."
    Today I will gladly share my experience and advice, for there no sweeter words than "I told you so."

  • #2
    GTD is alive and well (used) in automotive
    design and part checking. I see it as permanent and will be with us a long time

    old school

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    • #3
      GDT is simply a concise way of decribing unambiguously all relevant dimensions, finishes and properties of parts and assemblies using standard terms with clear unambiguous documented meanings understood by all familiar with the practice.

      When applied correctly it leaves no question whatsoever what is required when making a part or assembly. It has been around for a long time and will remain so.
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        In college drafting and machine shop the books both had a chapters on geometric tolerance and dimensioning, it was fun and makes a hell of a lot of sence once you get all the nomenclature and symbols down.

        scroll down this page to see a nice chart.
        http://en.wikipedia.org/wiki/Geometr...nd_Tolerancing

        other then the fancy looking crap, everyone here uses all of the main tolerances all the time even if they dont know it.

        when you get a shaft running strait in a lathe with an indicator you are working on the concentricity. or maybe flatness and parallelism and perpendicularity.

        when you put a part in a mill vice with an indicator you are working on flatness,parallelism, location,perpendicularity or angularity

        some tolerances like "total indicater runout" over a group of surfaces can be realy tight and make the part a total PITA.

        All it does is set limits on how sloppy a part can be made and the relationship of the surfaces.

        fun stuff

        [This message has been edited by tattoomike68 (edited 01-17-2006).]

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        • #5
          I have been using GD&T for the past 16 years. I work for an automotive supplier and have run a CMM and have also instructed about 50 people on the proper use of GD&T and how to understand it. It is my opinion that most people that design parts don't have a clue on GD&T. My thoughts are they take all the symbols they have and dump them on the print and push them to the nearest dimension.

          If done correct, it leaves little guessing on the design and function of the parts.

          It seem they are getting better but still have a long way to go.

          Just my thoughts..

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          • #6
            I had a whole semester of it in college. I also worked for awhile at a shop making Tombstone fixtures and I used it there. As near as I know, anyplace that does Government work uses GD&T.

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            • #7
              It was a required course for first yr students at the JC college machinist training program I went to last year.
              Steve
              Steve

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              • #8
                "It is my opinion that most people that design parts don't have a clue on GD&T. My thoughts are they take all the symbols they have and dump them on the print and push them to the nearest dimension. "

                What Jeff says is true.

                Very few of the customers I've worked with over the years use this system.

                Many who do cause lots of problems. When we really dig into what they were trying to convey it doesn't make sense.

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                • #9
                  Yep. As Dave says, it's used in automobiles quite heavily. Even in collision repairs.

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                  • #10
                    I see it a lot where I work, mainly automotive parts, but sometimes on other stuff too. I would guess that it is used correctly for about the first 75% of the stuff I see it on, but often, when read correctly, I find the last 25% will be wrong, or sometimes even contradict itself with the earlier specifications.

                    The unfortunate part is that others see it being used like that, then 'copy' it when designing something similar, so the cycle keeps going, and going....


                    Mark

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                    • #11
                      Just to satisy my curiosity, how could anyone come up with an idea such as Most allowable material on ANY dimension? Either a particular dimension is Correct, or it is scrap! Simple as that. This BS is starting to turn to there is no right or wrong way to machine a piece of anything. That's the cool thing about machining; it's right or WRONG.

                      ------------------
                      Dave da Slave

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                      • #12
                        You are joking, right? Nothing can be made to exact dimension, ever. Even telescope mirrors have tolerances measured in wavelengths of light, but they have tolerances.

                        Since nothing can ever be made to an exact dimension it is mandatory to specify allowable deviation from nominal. Without this the calculation of tolerance stacking is impossible.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Evan, maybe we worked in different environments. Geometric tolerancing is TOO vague

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                          • #14
                            You are free to dimension to an exact value if you wish but nobody can make it. It doesn't matter what machine you use.

                            There are single point CNC lathes now that can cut "billet" aluminum to an accuracy of 1/4 wavelength of green light. That's good but not good enough for a visible light mirror for exacting use. They use these machines to make infrared laser mirrors.

                            Even at that level of accuracy there are still tolerances.

                            Please explain further what you have in mind.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Yes, there are tolerances, such as when 2 pieces of steel are perfectly lapped , what is between the 2 pieces. The tightest I had to make were 10 millionths in. using a microscope so you got me on the green wavelength. All I mean is tolerance structure, Ely Whitney held tolerances never before heard of. It either is right or wrong. Which is scrap, do you see what I mean?

                              ------------------
                              Dave da Slave

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