Compound projection tolerance. 1

[ringman]

Recently I encountered a drawing that I thought had an unusual callout on it. The feature control frame was for a threaded hole. The upper portion indicated a diametric positional tol of .015 dia with a projection of .750 relative to A, B, C (normal). The block directly below, attached, indicated a perpendicularity of .005 dia projected .750 relative to A.

Is this acceptable or justified by the Standard Y14.5?

Any known similar examples?

 

[weavedreamer]

Section 5.5 clearly does state just that.

I am remiss.

Still, the temptation is to apply the perpendicularity as a refinement of the position, and to apply the previously attached sketch to just the projected tolerance zone. Or is this just not wanting to 'rethink' the understanding I thought I had?

 

[baseballninja]

Weavedreamer: I agree; your sketch describes two fixed tolerance zones and two floating tolerance zones. If they are both projected, then both zones are located outside of the part.

Ringman: Since I'm not an engineer, I can't answer your latest question (why is further refinement of perpendicularity necessary?). But Weavedreamer pointed out a paragraph from the standard indicating that this practice is sometimes necessary. So the question is: what is the most appropriate way to apply this refinement on a drawing?

It could be applied as a composite positional tolerance as vc66 and powerhound suggested. However, the FRTZF (lower segment) would carry the additional implication of a feature-to-feature (within-pattern) positional requirement of .005.

From your initial description, it sounds like this callout applies to a single hole (not a pattern of holes), so perhaps a composite positional tolerance struck the engineer as inappropriate.

My own opinion is that this could be accomplished either way (composite positional tolerance or separate perpendicularity tolerance) and be equally understandable. And to me, that's the spirit of the entire standard: to express requirements in a way that is understandable and unambiguous.

Sincerely,
Josh Church
Quality Manager
Vanderhorst Brothers, Inc.

 

[Xplicator]

If I understand the OP correctly, there is no upper and lower just two single segment feature control frames that are to be verified independently. They are both legit, but the usage of projection on both is suspect. It would be my opinion and suggestion to remove the projection modifier from the position which I believe is being used to locate and place it on the perpendicularity callout as a refinement of the position orientation. Since the whole purpose of the modifier is to control orientation over a projected length. Be aware that the the cylindrical tolerance zone specified in the perp callout with the invoked projection modifier and the basic length comes off the mating surface which must be the datum specified in the FCF not the opposite side. This is a common mistake when invoking this modifier especially on a thru hole.

Hope this was helpful.

 

[baseballninja]

XPlicator,

I think the two methods would be equivalent provided that the hole depth is not 3 times greater than the projected tolerance zone height; in that case the two callouts would be difference (refer to attached sketch). I believe the factor of 3 is coming from the ratio between the tolerance zone widths (.015 / .005 = 3)

I don't think either method is wrong.


Sincerely,
Josh Church
Vanderhorst Brothers, Inc.