Compound projection tolerance.
Compound projection tolerance.
(OP)
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?
Is this acceptable or justified by the Standard Y14.5?
Any known similar examples?





RE: Compound projection tolerance.
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Compound projection tolerance.
I'm not sure if this is prohibited in Y14.5.
V
RE: Compound projection tolerance.
I don't think the standard specifically prohibits what this guy has done so I guess it doesn't mean it's correct or incorrect but there is a compliant way to do what I think he's trying to do.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Compound projection tolerance.
(I believe it is invalid but have difficulty in convincing the designer.)
RE: Compound projection tolerance.
5.9 (pg.135) has a note (pg.139) A further refinement of perpendicularity within the positional tolerance may be required.
The fixed fastener would still use the positional location. The .750 long Projected Tolerance Zone applied to the .015 dia establishes the virtual condition.
The .750 long Projected Tolerance Zone on the perpendicularity allows the .005 cylinder to float inside the .015 cylinder, at most up to the tangency of the two cylinders, acting only as a refinement.
RE: Compound projection tolerance.
KENAT, probably the least qualified checker you'll ever meet...
RE: Compound projection tolerance.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Compound projection tolerance.
However, thinking about it, I think weavedreamer may be right as is. However I think I'd still look making the bottom one positional as well.
As to applying the fixed fastener calc. Wouldn't this only apply to the top positional tolerance. For this to make any sense to me the perpendicularity would have to be applied for a different reason than simple hole pattern matching, wouldn't it?
KENAT, probably the least qualified checker you'll ever meet...
RE: Compound projection tolerance.
RE: Compound projection tolerance.
RE: Compound projection tolerance.
RE: Compound projection tolerance.
I tried stating that in my first post. However, sometimes it is clearer in my mind than what comes out of my fingertips.
RE: Compound projection tolerance.
I answered you ringman, I may not be right but I did answer.
KENAT, probably the least qualified checker you'll ever meet...
RE: Compound projection tolerance.
My apologiies, and I guess you did answer my question. At least part way. I am still totally confused by the callout as it is.
Your answer is that .015 of the allowed tolerance would be consumed by the projection tol. Do I understand that correctly?
Ron
As a side note:
I've never met you, but I don't believe for a second that you are the least qualified checker. :>)
RE: Compound projection tolerance.
Although now I think I see what you're getting at and I'm baffled too.
KENAT, probably the least qualified checker you'll ever meet...
RE: Compound projection tolerance.
RE: Compound projection tolerance.
If i am not mistaken, when Projection Tolerance is expressed, the tolerance DOES NOT apply to the length of the thread, but rather the projected dimension (interface component thickness)for the thread.
RE: Compound projection tolerance.
In other words, when "Circle P" is applied, the tolerance zone no longer extends into the current part. The tolerance zone begins at the surface of the part and extends in the opposite direction for a MINIMUM of the projected length indicated.
In my opinion, this is why the drawing has a projection modifier on both the positional tolerance and the perpendicularity tolerance. If only one or the other had the projection modifier, you would have one tolerance zone inside the part and another tolerance zone outside the part. To me it makes sense for both tolerance zones to be located together -- either both outside the part (projection modifier) or both inside the part (no projection modifier).
Sincerely,
Josh Church
Quality Manager
Vanderhorst Brothers, Inc.
RE: Compound projection tolerance.
If it is not obvious, I am of the opinion that the callout does not comply with Y14.5. It does not provide a method for what you have stated previously to have the location within the part and refined for location.
I do not find the justification nor explanation of the callout that I attempted to describe. It does not make a connection between the upper and lower callout in my opinion.
The callout also confuses the application of the formula for Fixed Fasteners, and there is no apparent justification for the more restrictive (DIFFICULT TO ACHIEVE).005 perp tol.
Rephrasing it, if the .015 projection tolerance gives the location that works with the formula, what is the possible justification for a further refinement?
RE: Compound projection tolerance.
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?
RE: Compound projection tolerance.
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.
RE: Compound projection tolerance.
Hope this was helpful.
RE: Compound projection tolerance.
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.