True Position Measurement

#1
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

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#2
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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#4
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

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#5
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.
 
#6
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.
 

GRP

Involved In Discussions
#8
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
#9
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.
 
#10
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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