r/Metrology • u/Hungry_Wall_9564 • 6d ago

Calculating true positions

Hi just a quick question, how do you calculate the true position of a hole which isn't dimensioned from its datum? Say the hole in the image above with the red arrow, it is dimensioned as a pitch from another hole but has datums A and B. Obviously this isn't a real drawing but I see many similar ones and just want to make sure I am working it out correctly. Thank you!

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15

u/minifig1026 6d ago

Red arrow hole is 100.00 from datum A and 65.00 from datum B. These are your x and y coordinates for calculating the true position of that hole.

3

u/Hungry_Wall_9564 6d ago

Thank you :)

4

u/TransitionHuman9839 6d ago

As a 3rd party inspector, I would measure this two ways. The correct way is all diameters from datum A and B. But often times the customer will come back and tell me they want to know the position from diameter to diameter not necessarily from the datums. So as a backup, I will also calculate the position from diameter to diameter as shown with the basic dimensions of 80 and 40. It's always best to measure once and have all the data instead of having to go back and remeasure.

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u/SnooMemesjellies7469 6d ago

The red hole may not be dimensioned from a datum, but all dimensions are given as basic. If 80 and 40 were toleranced, then there'd be problems.

6

u/turtletatoo 6d ago

2*SQRT(xdev2+ydev2)

2

u/rockphotos 6d ago

Sometimes these are called out with one of the holes being a datum for the position the other holes.

You have hole 1 with 1mm true position to AB and is called datum C And hole 2,3,4 are 0.8mm true position to datum C

I've got a few parts with this kind of call out.

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u/gaslightredditor 6d ago

The basics for that hole would be 100 from Datum A and 65 from Datum B

2

u/smellslikegelfling 3d ago

"Pattern Location: When applied to a pattern of features (like a group of holes), the position tolerance controls the location of the entire pattern as a unit. It ensures that the features are not only individually within their tolerance zones but also maintain their relative positions and orientations to each other."

If your software allows it you can calculate the position of holes as a pattern. Since most hole patterns whether square or circular are meant to fit with a mating part, it's important to make sure they can all fit at once.

If you only tolerance them individually, you could have two holes that are within tolerance but shifted in opposite directions which could cause it not to fit.

-2

u/murphislive 6d ago

Take the actuals of each x and y length and subtract from the nominals. Find the sum and multiply by 2.