# True Position Measurement

#### brekke777

##### Registered
Hi all,

I'm fairly new to GD& T and trying to wrap my head around measuring position tolerances. I understand how to measure the tolerance if given an x,y dimension but how would I check the position callout highlighted on this print clip? A is a threaded hole but for the sake of simplicity assume it's just a bore. No CMM, just standard tools. I've come a long way teaching myself the majority of mechanical inspection but true position is something I'm struggling with, have reached out to many an engineer and even machinists who can't seem to explain actually checking it. All the literature and videos I watch give examples of say a hole position tolerance, with x,y dimensions to the center. I understand that. It's when diameters are called out is when I get lost. Any help would be greatly appreciated!

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#### Cari Spears

##### Super Moderator
Staff member
Super Moderator
Take your deviation in X and square it; take your deviation in Y and square it; add them together and find the square root of the sum; multiply by 2. It's the Pythagorean theorem X2 - you're finding the hypotenuse of a triangle and doubling it to get the diameter.

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#### Miner

##### Forum Moderator
Staff member
Ignoring the MMC modifiers for the moment, the shape of your tolerance zone is no longer square, but is circular. See this illustration.

#### brekke777

##### Registered
I know the formula, and I understand the tolerance zone is circular now. I just don't understand the measurement to take. Since these two points align on X, would my X deviation be 0? And I just measure the deviation in Y and plug it into the formula?

#### Ron Rompen

Trusted Information Resource
You have potential deviation in both X and Y axes from the datum - even though there is no B and C datum features, you can still 'assume' an alignment, and measure in the two (X and Y) axes.

#### brekke777

##### Registered
Thank you, yes looking at my question I wasn't clear on what I was asking. I'm confused because there are no basic dimensions called out that I can see, so was struggling to find the actual "true" position to check deviation from.

#### Miner

##### Forum Moderator
Staff member
The datum and the feature are ideally coaxial, so the Basic dimension is (0, 0).

#### GRP

##### Involved In Discussions
Hi all,

I'm fairly new to GD& T and trying to wrap my head around measuring position tolerances. I understand how to measure the tolerance if given an x,y dimension but how would I check the position callout highlighted on this print clip? A is a threaded hole but for the sake of simplicity assume it's just a bore. No CMM, just standard tools. I've come a long way teaching myself the majority of mechanical inspection but true position is something I'm struggling with, have reached out to many an engineer and even machinists who can't seem to explain actually checking it. All the literature and videos I watch give examples of say a hole position tolerance, with x,y dimensions to the center. I understand that. It's when diameters are called out is when I get lost. Any help would be greatly appreciated!
In case it helps, I always liked this website True Position (not affiliated, no promotion intended)

#### Eredhel

##### Quality Manager
Slightly off topic but I'm digging this thread for learning stuff. When using the AS9102 FAI form, it requires the positional results to be recorded when using a CMM, not just a tolerance or pass/fail result. Fortunately Zeiss Calypso software makes that easy, it's just an option and then the X, Y, & Z results are displayed.

#### brekke777

##### Registered
I'm learning all this the, somewhat, old fashion way with mics, comparators, height gauges etc. I have zero experience with a CMM, hopefully soon though. I can see where they'd help drastically on more complex functions of gd&t. It's been rough figuring it all out on web forums, YouTube, and the like on my own. But I think I'll appreciate it more when it becomes second nature.. And when I'm using cmms and software thinking back on my early struggles!

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