Diametrical True Position - need clarification

• 12-20-2018, 11:53 AM
acncguy
Diametrical True Position - need clarification
I am confused by a drawing of part that needs to be checked ... instead of explaining the drawing, I've attached a .pdf of the drawing with my questions. I hope that's ok. If not, I will try something different.

Thanks for any help!

[ATTACH]2251[/ATTACH]
• 12-20-2018, 01:41 PM
Kelly Bramble
Datum "C" is the center plane of the associated surfaces (left and right side only.

The basic dimension .875 appears application to be undefined and would be considered redundant.
• 12-22-2018, 02:04 PM
acncguy
Hello Kelly,

Thank you for the reply. One thing I don't understand is that this is a diametrical true position callout (the diameter sign before the 0.005) ... what does that mean to this drawing? I'm not the greatest at G D & T, does that mean the position of the lug has to fall within a 0.005 diameter? My question then is, from where on this drawing? I would think it is from the center of the associated surfaces (Datum C) as you mention (left and right), but where is my up and down point coming from? Or don't you need one? If "up and down" position is controlled by the 7.985 +/- 0.005 length, if we held the datum C "perfect" and the 7.985 at one of its limits, the diametrical position of 0.005 would be out of tolerance (it would be 0.010, from my understanding).

I'm also confused at that .875 basic dimension because I would guess the engineer would design the lug on center, but with the 1.7500/1.7495 width requirement, that would mean they want one surface of the lug to be .875 from centerline and the other .8745. Seems a little odd to me, unless I'm not understanding this correctly.

Thanks again for any clarification.

Regards,

Paul
• 12-24-2018, 08:00 AM
Kelly Bramble
The drawing is missing a lot of information but with what is specified the tolerance specified is dia. .005 tolerance boundary that is oriented and located back to the specified datums A and B. The center of the two associated surfaces derives into a plane (at the center) which clearly cannot fit into a cylinder.

The drawing requires clarification from the originator.

Your confusion is warranted.
• 12-29-2018, 06:35 AM
acncguy
Hello Kelly,

Thank you for the reply.

Just to be clear, when tolerance of position is used to control this lug, no diameter symbol is allowed to precede the tolerance in the feature control frame, like there is on this drawing, correct? The diameter symbol is restricted to the use of controlling circular features?

Thanks for the help.

Regards,

Paul
• 12-29-2018, 07:40 AM
Kelly Bramble
[QUOTE=acncguy;15489]Hello Kelly,

Thank you for the reply.

Just to be clear, when tolerance of position is used to control this lug, no diameter symbol is allowed to precede the tolerance in the feature control frame, like there is on this drawing, correct? The diameter symbol is restricted to the use of controlling circular features?

Thanks for the help.

Regards,

Paul[/QUOTE]

No, the diameter symbol used in the feature control frame defines a cylindrical tolerance boundary within which the derived feature median element is to lie.

It is very much possible and in practice to define a cylindrical tolerance boundary for non-cylindrical features. In this particular example there is a center plane and confusing associated basic dimensions.