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Thread: GD&T - True Position

  1. #1
    CatMan is offline Hot Rolled
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    Default GD&T - True Position

    Let's say you're trying to hit dimension 0,0. But you really hit dimension 3,4. Through the Pythagorean theroem and such you do the formula SQRT(x squared + y squared) to find the hypotenuse and you end up off 5 from your intended 0,0 target.

    However, the way true position callout works, the formula is 2*SQRT(x squared + y squared), thus putting you off 10 (even though we just established that you were really only off 5).

    Anyone know why they double the number to get true position?

  2. #2
    9100's Avatar
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    If you bored a hole .005" off center, then indicated it from the correct position, you would read .010" TIR. Somehow, though, that doesn't sound like what you are describing.

    Bill

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    Bobw's Avatar
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    Simple, the true position is within a diameter, and you're calculating the radius.
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    toastydeath is offline Hot Rolled
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    Quote Originally Posted by CatMan View Post
    Let's say you're trying to hit dimension 0,0. But you really hit dimension 3,4. Through the Pythagorean theroem and such you do the formula SQRT(x squared + y squared) to find the hypotenuse and you end up off 5 from your intended 0,0 target.

    However, the way true position callout works, the formula is 2*SQRT(x squared + y squared), thus putting you off 10 (even though we just established that you were really only off 5).

    Anyone know why they double the number to get true position?
    Look on the true position callout - see if the true position tolerance number is a diameter or a radius. If it's a diameter, there you go - a .010 diameter true position is only .005 in radius. So you can either A) half the tolerance when you do the math, or B) just double your number.

    So, if the magnetude/hypotenuse of the errors comes out to be 5, and the tolerance is a .010 diameter, you've just taken up all the allowable error.

    Edit: Ooops, didn't see bobw's answer.

  5. #5
    CatMan is offline Hot Rolled
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    OK. The diameter reference makes sense. Thanks guys.

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    Don't forget that the hole tol. can add to or tighten your true postion needed also. i.e. on a hole sized .5" +/- .010" making the hole .510 adds adds that .010" as a bonus to your true postion number.
    Steve

  7. #7
    CEM@DELVATOOL is offline Plastic
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    Quote Originally Posted by scadvice View Post
    Don't forget that the hole tol. can add to or tighten your true postion needed also. i.e. on a hole sized .5" +/- .010" making the hole .510 adds adds that .010" as a bonus to your true postion number.
    Steve
    A Hole size tolerance can only give you bonus tolerance If and ONLY IF you're given MMC. Boring a hole .5" +/- .010" to .510 with MMC adds .020 to your TP (not .010).

    In other words, for a hole .500 +/- .010 has TP of Dia. .010 at MMC (.490) If you bore the hole to LMC (.510) you get .020 bonus giving you a grand total of TP Dia. .030

    Side note: +/- .010 on a hole of .500 in my shop is like throwing a hot dog down a hallway. Bonus tolerance is more important with closer size tolerances. (+/- .002 or less)

    -CEM

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    Quote Originally Posted by CEM@DELVATOOL View Post
    Boring a hole .5" +/- .010" to .510 with MMC adds .020 to your TP (not .010).

    -CEM
    No.

    Whatever bonus you get from MMC diameter is added to the diameter positional tolerance, not the radial tolerance. So if the hole is .010 above low limit, there is another .010 on the diametral positional tolerance.

    Beware of LMC, where the hole size above low limit subtracts from the hole positional tolerance.
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    rsegriggy is offline Plastic
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    Quote Originally Posted by CEM
    Boring a hole .5" +/- .010" to .510 with MMC adds .020 to your TP (not .010).

    -CEM
    In the example above, at .510, wouldn't he be .020 above the lower limit of .490?

    Quote Originally Posted by gbent View Post
    No.

    Whatever bonus you get from MMC diameter is added to the diameter positional tolerance, not the radial tolerance. So if the hole is .010 above low limit, there is another .010 on the diametral positional tolerance.
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