# How to Calculate a True Position on a Square Slot - GD&T

N

#### Nuno Costa

Hi to all,
i need help.
i need understand how can i calculate a true position on a square slot.
can you help me?
thanks.

PS: If it's possible with formulas.... thanks

#### Jim Wynne

Staff member
Re: GD&T - How to Calculate a True Position on a Square Slot

Hi to all,
i need help.
i need understand how can i calculate a true position on a square slot.
can you help me?
thanks.

PS: If it's possible with formulas.... thanks
Welcome to the Cove, Nuno.

Without more information, it's hard to tell what you're after. You find the position of a slot the same way you would any other feature, which involves finding the center of it. Do you need the formula, or is it just the nature of the feature you're having trouble with?

N

#### Nuno Costa

Re: GD&T - How to Calculate a True Position on a Square Slot

Thanks Jim,
my question is because i think that i can't calculate a TP for a square slot with the same formula that i use for calculate on a hole or a pin.
my doubt is: I'm wrong?
thanks once again

T

#### True Position

Re: GD&T - How to Calculate a True Position on a Square Slot

If I were asked to check the true position of a square feature, I would just find the center point and calculate it like a pin or hole, same method. You may want to specifically mention to whoever requested this measurement that it does not control the rotation of the feature though.

Adding a perpendicularity or parallelism tolerance can fix this concern.

Good luck.

#### Jim Wynne

Staff member
Re: GD&T - How to Calculate a True Position on a Square Slot

Thanks Jim,
my question is because i think that i can't calculate a TP for a square slot with the same formula that i use for calculate on a hole or a pin.
my doubt is: I'm wrong?
thanks once again

#### Stijloor

Staff member
Super Moderator
Hi to all,
i need help.
i need understand how can i calculate a true position on a square slot.
can you help me?
thanks.

PS: If it's possible with formulas.... thanks
Hello Nuna,

The position (location) of square or rectangular features of size is typically controlled with a tolerance of position in two directions. Because the shape of the feature is not cylindrical (round) a cylindrical tolerance zone can not be applied. So two feature control frames may be required.

Not knowing what fits in the square hole (does something actually fit in this hole?); the actual position tolerance can be calculated based on the clearance between the two features.

If you can provide us with a simple sketch of the part (or assembly), I should be able to advise you further. It may not be as simple.

Stijloor.

T

#### True Position

To follow up, ASME Y14.5M - 1994, Page 139 (on my copy):

5.10 Noncircular Features
The fundamental principles of true position dimension and positional tolerancing for circular features, such as holes and bosses, apply also to non-circular features, such as open-end slots, tabs, and elongated holes. For such features of size, a positional tolerance is used to locate the center plane established by parallel surfaces of the feature. The tolerance value represents a distance between two parallel planes.

Perhaps if you included a scan of the drawing I could help further.

#### Stijloor

Staff member
Super Moderator
To follow up, ASME Y14.5M - 1994, Page 139 (on my copy):

5.10 Noncircular Features
The fundamental principles of true position dimension and positional tolerancing for circular features, such as holes and bosses, apply also to non-circular features, such as open-end slots, tabs, and elongated holes. For such features of size, a positional tolerance is used to locate the center plane established by parallel surfaces of the feature. The tolerance value represents a distance between two parallel planes.

Perhaps if you included a scan of the drawing I could help further.
Hawat,

BINGO!

The OP is from Mexico. They may subscribe to ISO 1101 and associated GD&T Standards.

Stijloor.

D

#### David DeLong

Positional tolerances on a square hole without MMC is pretty easy. If you have a positional tolerance of 0.4 mm, one has +/- 0.2 mm on both the X and also Y direction for center movement from the theoretical center (called "true position"). Features must be confirmed on both ends of the square hole controlling orientation.

Now, MMC on the positional of a square hole; it is best to have a checking fixture with a positional pin the virtual condition size. The pilot pin for this checking pin must not be round. It could be rectangular or a double round pins so that the square positional pin could only be inserted one way. This controls orientation (clocking).

If one was to figure the actual tolerance, it is the difference between that actual size of the square hole and its virtual condition size. It if calculates out to , say 0.6, the tolerance is then +/- 0.3 in both directions.

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