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# How can I measure three true positions? GD&T basic question

#### Emmamartsolf

##### Registered
I am more at electronics field. But now I have question about mechanical true position measurement. The drawing is attached. How can I measure three true positions?

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#### Michael_M

Trusted
Re: GD&T basic question

In general (not taking into account max material (m)), measuring true position uses the following formula:

Determine the error (in X).....for example, print calls for a basic (boxed) dimension of 1.000 and you actually measure 1.0015, this gives us an error of .0015.
Determine the error (in Y).....for example, print calls for a basic dimension of .500 and you actually measure .4975, this gives us an error of .0025.

Take the error (in X---.0015) and square it .0015 x .0015=.00000225
Take the error (in Y----.0025) and square it .0025 x .0025=.00000625

Add the two together=.0000085 then take the square root of this--.002915476.

and finally, multiply the final result by 2 (The drawing is in Diameters, the formula above is used for radi, so we have to multiply by 2)

The final results must be less than or equal to the number in the box (.010 in your example).

You most likely will recognize this formula as the pythagorean theorem A squared + B squared = C squared

In your example, the holes on the outside are referenced to a 60 degree angle. Your example does not show the 'radius' to the hole or the bolt circle diameter but if you just calculate where the hole should be and use the same 'where it should be' vs. where it actually is (error) then it should work for you as well.

Hope this helps a little.

#### Emmamartsolf

##### Registered
Re: GD&T basic question

Thank you Michael. Appreciate your help.

The first TP Dia 0.395" -0.384", I think I can get center of Cir 1.812 (Plant A) to be (0,0), compare the center of this small Cir 0.390", use square root method to calculate the error, such as Squ((x-0.390)2+(y-0.390)2).

The second TP0.250 will be the same idea, with position of center line of the notch. It will be (w-0.250)

The third TP 0.148", is that the position of center of hole 0.148" ( M, N), then I use squ((M-0.148)2+(N-0.148))?

Thank you,

Emma

#### Michael_M

Trusted
Re: GD&T basic question

It is hard to read the square and square root in a forum, but if I am reading this correctly, yes. Don't forget to multiply the final results by 2 after the square root is finished.

#### Michael_M

Trusted
Re: GD&T basic question

I have had this PDF for a little while. It may help give a better understanding of Geometric tolerancing.

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#### Emmamartsolf

##### Registered
Re: GD&T basic question

Thank you. You are so helpful. Yes, actually, in equation, I mess radius and diameter. I should use radius, then double the error as the final result. But you are getting my thought.

Thank you for the attachment.