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# GD&T help

Builder

Hi,

I need some help figuring out this gdt. I never seen something with two true position tols on the same hole. Can some one explain this to me and how this part should be oriented to measure it. I am a noob, forgive me.

8 REPLIES

# Re: GD&T help

Gears Esteemed Contributor

It seems unusual to me too. Looks to me like he wants to allow the hole to float more in one axis than the other. The hole in one axis forms the C datum. I'm not sure the reference to the C datum in the .08 tolerance makes any sense or adds any information so to me the callout is weird because of that too.

Bruce Shand
ST10 MP7 - Insight - Win10 - K4200

# Re: GD&T help

PLM World Member Phenom
@bshand, you're exactly right. It's not a circular tolerance, which most people are accustomed to seeing for holes.

In the Y-direction, there is .04 at MMC and it sets up Datum C along the X-direction only at the centerline of the hole.
In the X-direction, there is .08 at MMC.

Inspection is a different matter than I'm not good at explaining via written word. I need pictures and conversation.

____________________________________
--Scott Wertel, P.E.

# Re: GD&T help

Builder

Ok. I had the mind set that the .04" true position pretty much made the .08" true position void. Thanks for the input guys.

# Re: GD&T help

Phenom

The C datum in the second 0.08 callout establishes rotational positioning of the part. This allows you to 'float' the 0.08 parallel to a line drawn from the center of B to the center of C. They're trying to treat it like a slot- which is probably the exact feature this hole interfaces with. However, the way the print is dimensioned, you will always be restricted by the 0.04 true position anyway. The 0.08" true position is extraneous.

-Dylan Gondyke

# Re: GD&T help

Gears Esteemed Contributor

I see what you're saying. It's just that C doesn't exist until the hole exists. So referencing C in the .08 tol seems circular and self-referential.

For machining anyway, inspection not so much.

Bruce Shand
ST10 MP7 - Insight - Win10 - K4200

# Re: GD&T help

Genius

ASME Y14.5 - 2009, Section 7.4.4, BIDIRECTIONAL POSITIONAL TOLERANCING OF FEATURES OF SIZE

# Re: GD&T help

PLM World Member Phenom
@bshand, you are right. For machining, datum C doesn't really matter, but for inspection it does.

For the 0.04 tolerance, since it is only relative to A and B, there isn't any clocking mechanism and therefore the directional tolerance could theoretically be in any direction, but it is implied to be in the Y-direction because of the orientation of the drawing view and dimension line.

For the 0.08 dimension, since there wasn't any clocking before, this could also theoretically be in any direction aside from the assumptions implied above. By creating Datum C and making the 0.08 dimension relative to C as the tertiary datum, clocking is achieved and the 0.08 tolerance has to be "perpendicular" to the 0.04 tolerance because once you inspect the 0.04 tolerance, you lock the rotation of the part and measure the 0.08 tolerance 90 degrees from however you clocked the part. (Which probably sounds confusing in written text which takes me back to my last sentence from my earlier post.)

____________________________________
--Scott Wertel, P.E.

# Re: GD&T help

Gears Esteemed Contributor

Thanks for the link. I thought the ASME manuals weren't available free.

Bruce Shand
ST10 MP7 - Insight - Win10 - K4200