Position Bonus Tolerancing

[Robaloy]

Hi

I am having trouble with a Position Bonus Tolerance

As you can see in the drawing attached the Basic is 5.185" and TP = 0.010" MMC
The Hole Diameter is 0.158"

Note 4. Does not apply as this is just a clearance screw hole and these Notes seem to appear on all of this customers drawings, e.g. Note 5. which refers to threads when there are no threads on this drawing!

Note 1. says General Linear Tol +/- 0.010" and General Radii Tol +/- 0.005" which of these do I apply to the Hole Diameter 0.158"?

Regards

Rob

 

[mkcski]

ROBALOY

First: There are no standards for interpreting General Notes. I would normally assumes Note 2 applied to fillet and edge radii/chamfers, but this would be redundant with Notes 6 and 7. So, you will have call the customer or contact your engineering dept for clarification.

Second: There are not datum references. The 1994 (proceeding) and 2009 (current) versions require datum references with position. What version of Y14.5 standard are you applying to this drawing? Even older versions 1982 and 1973) allowed this. No datums may be allowed with ISO standards, but I am not versed in the details of ISO. The position FCF as shown only controls the position between the pattern of 2 holes and does not define where the pattern is located to other features on the part - this is critical too. You might want to consider Composite Position see paragraph 7.5.1 page 127 in 2009

Third: Calculating Bonus tolerance: Assuming Note 1 provides the size tolerance one the holes, the MMC size is .158 - .010 = .148. So there is .010 position tolerance at MMC of .148. As the actual hole size gets larger there is additional position (bonus) tolerance available. For example: an actual hole size of .160 would provide .160 - .148 = .012 bonus. This is added to the specified of .010 (at MMC .148) to equal .022 allowed position tolerance.



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[drawoh]

Robaloy,

This is why I like applying tolerances to each and every dimension I apply. If you don't understand the notes, don't use the notes. How is the shop supposed to understand them? It is not like you can answer their questions.

--
JHG

 

[3DDave]

Datums are not required for positional tolerances, but there has to be some position relation. See '2009 figure 4-24 and note that it constrains the features to each other with no datum reference.

As to the tolerance - none of them seem to apply. It's obvious the customer has no idea what they are doing and they have been doing this for a while. Whoever is managing the contract should tell the customer that no responsibility will be accepted for dimensional checks as the drawings are defective.

 

[greenimi]

Okay, so what is wrong with the way the customer wanted to communicate that the relationship between the holes must be within Ø.010 at MMC?
We agreed that none of the notes are applicable.

Wherever the holes are located on the plate, they have to be within Ø.010 @MMC from each other. That's all.

 

[mkcski]

3DDave:

My understanding of Pare 7.2, third paragraph, is that datums are required. One exception is made to this: position without datums is strictly limited to coaxial features as per para 7.6.2.3 and as shown in figure 4-24. This coordinates with CF (continuous feature)concepts as related to establishing a single datum from multiple features.

To my knowledge, not requiring datums was eliminated to remove the guesswork and ambiguity associated with implied datum ref frames.

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[greenimi]

Latest ASME version (2009) datum features are not required on the positional callout(s).
1994 : were required
2009: not required

Ø.158 size tolerance seems to be the one in question, right?

Ø.158 ±.005?
Ø.158 ±.010?
Ø.158 ±.005?

Location (within each other) is clear, IMHO

 

[greenimi]

Ø.158 ±.015? --last line---or any other tolerance you can imagine

 

[mkcski]

3DDave and greenimi:

Considering the shear quantity of posts to this forum, your depth of GDT knowledge/exposure/synthesis is much deeper than mine. Besides figure 4-24 and the lower frame of a Composite FCF, I have never seen an example of the use of position without datums. I do understand the feature-to-feature control concepts like you mentioned. Can you please provide paragraph references and/or published examples to point me in the right direction help me improve my understanding?

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[pmarc]

mkcski,
Although it is not a part of the standard, this may interest you:

http://www.geotolmeadows.com/newsletters/2013/sep2013.htm

Subject: Questions regarding Simultaneous Requirements and Position without Datum References
Subject:GD&T for Pipes, Hoses or Tubing

In short, any pattern of features functionally working as primary datum feature is allowed to receive datumless position callout.

That, of course, does not mean OP's example shows a good use of position tolerance.

As for the main topic, I agree with 3DDave - the drawing is defective. But in my opinion that should not lead to a conclusion that tolerances should always be directly applied to each and every dimension. General notes can do the job too. They "just" have to be constructed properly - much more wisely than it was done here.

 

[3DDave]

mkcski,

You don't often see it because it is not a useful thing to do. In this case one would have to look at the default angle tolerances on the implied 90 degree angles between the hole axes and both the top and bottom surfaces and any other nominally perpendicular surfaces and inspect each angle to see if the part conforms. Inspectors never do this check and instead assume some frame of reference to verify the locations. Most buyers only care if the part fits and if it does, they also don't care that the drawing would accept parts that don't fit.

I can't point to anything that helps, because I really don't know what the committee is thinking. It's like the rules that people add to Monopoly about landing on certain squares. The math behind it all is trivial, but the rules keep being changed to include or exclude or make certain interpretations that don't reflect the math. The math should always be the same - Does the part meet the requirement? and What range of parts are expected based on the requirement? Discussed in this forum earlier was the ambiguity of '2009 figure 4-16(c) calculation which was beaten to a pulp. The underlying math doesn't support the value given in the standard because they did the math wrong.

For example of varying interpretations - it seems like symmetry is on the way out again. Was there, was removed, was replaced, maybe will be removed again. It's one of the simplest math concepts - equal distances from a datum, that can't be easily replicated by existing terms in the standard. No doubt some other term will get redefined, like flatness was for center planes, or not. I don't have the latest draft, and even if I did only those pledged not to tell the meeting results have any clue as to symmetry's fate.

Another example - in inspection a frequently held opinion is that the gage should be allowed 10% of some tolerance. This is handy for the gage maker, but is not handy for the engineer designing the part. It doesn't clarify if the actual parts will be held 10% tighter or allowed to be 10% over the limit.

 

[mkcski]

3DDave:

Thanks much for the insight. I only "discovered" this forum last summer and have not followed as many posts as you to see the "inconsistencies" in the Standard and the ensuing discussions that present different interpretations of the same thing.

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