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?

 

[fcsuper]

If you are questioning the perpendiculairy, so am I.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[vc66]

The only thing that I can think of is that the person was trying to tighten up the perpendicularity so they called out the perpendicularity tighter than the positional. I would've just said positional of .015 to B, C... Then directly below it I'd use a projected positional to A of .005, projected to .750.

I'm not sure if this is prohibited in Y14.5.

V

 

[powerhound]

It's probably a misguided attempt to do the same thing as a composite FCF with the PLTZ having a value of .015 WRT A, B, and C and the FRTZ having a value of .005 WRT A.
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

 

[ringman]

If this were to be accepted as a valid callout, how would the formula for fixed fasteners be applied?

(I believe it is invalid but have difficulty in convincing the designer.)

 

[weavedreamer]

5.5 (pg. 115) references Figure 6-38 (pg. 184) applying a Projected Tolerance Zone to a Perpendicularity callout.

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.

 

[KENAT]

I'm still not sure the projected makes sense on the perpendicular though weavedreamer. While not perhaps the best or most elegant way of doing it the basic perpendicularity makes sense, it's just the projection of it that gets me.

KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

So going by weavedreamers post, does it seem that if the projected tolerance zone were removed from the positional FCF and left in the perp, this would be valid? I think this would be okay. The positional locates the hole and the perp with P controls the amount of perp error.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

powerhound, I was thinking the opposite. Keep the projection on the position but lose it on the perpendicular.

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...

 

[weavedreamer]

If the Projected Tolerance Zone were removed from the Positional tolerance, the Perpendicularity would still only move within a .015 dia zone, and effectively yield the same envelope due to the Projected Tolerance Zone applied to the Perpendicularity. Therefore, the P.T.Z. on the T.O.P. is superfluous.

 

[weavedreamer]

Just in case the abbreviations are ambiguous, a reiteration would be: the Projected Tolerance Zone on the Tolerance of Position is extraneous.

 

[ringman]

All inputs appreciated. However, I did not see one that responded to my later question with regards to the formula. Did I miss it?

 

[weavedreamer]

ringman - the fixed fastener calculation would use the positional tolerance for resolution.

I tried stating that in my first post. However, sometimes it is clearer in my mind than what comes out of my fingertips.

 

[KENAT]

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?

I answered you ringman, I may not be right but I did answer.

KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

Kenat,

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. :>)

 

[KENAT]

Not sure I quite get your phrasing but yes, I'd use the .015 in the calculation.


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...

 

[weavedreamer]

Perhaps this will help illustrate it . . .

 

[ringman]

weavedreamer,

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.

 

[baseballninja]

I agree with Ringman. ASME Y14.5M-1994 is clear in stating that projected zones exist OUTSIDE the part only; the projected zone is meant to be in the space that will be occupied by the mating part.

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.

 

[ringman]

Baseballninja,

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?