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?
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.
Simple, the true position is within a diameter, and you're calculating the radius.
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.
Originally Posted by CatMan
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.
OK. The diameter reference makes sense. Thanks guys.
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.