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Compound projection tolerance.

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?

RE: Compound projection tolerance.

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

RE: Compound projection tolerance.

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

RE: Compound projection tolerance.

(OP)
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.)   

RE: Compound projection tolerance.

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.

   

RE: Compound projection tolerance.

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...

RE: Compound projection tolerance.

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

RE: Compound projection tolerance.

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...

RE: Compound projection tolerance.

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.

 

RE: Compound projection tolerance.

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

RE: Compound projection tolerance.

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



 

RE: Compound projection tolerance.

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.

RE: Compound projection tolerance.

Quote:

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...

RE: Compound projection tolerance.

(OP)
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.  :>)

RE: Compound projection tolerance.

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...

RE: Compound projection tolerance.

(OP)
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.

RE: Compound projection tolerance.

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.

RE: Compound projection tolerance.

(OP)
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?

RE: Compound projection tolerance.

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?

RE: Compound projection tolerance.

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.

RE: Compound projection tolerance.

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.

RE: Compound projection tolerance.

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.

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