Help Interpreting Drawing

[Chaldaean]

I'm looking for an explanation of the feature control frame outlined in red within the attached.

I'll take a stab at the meaning: The position of the axis of the shaft (along the entire length of the shaft) must be within a 1.1 diameter circle after the part is first fixtured relative to datum A and then to datum B.

Is this accurate?

Also, what does the "L" (least material condition) mean as applied to datum B? Does that mean when the inspection fixture is produced, the fixture should be at max material condition (4.52 as drawn)?

Thanks for taking the time!

 

[powerhound]

I'm having a tough time figuring out the design intent given the LMB modifier. I'm pretty sure the fixture won't work since the hole could be anywhere above LMC value and not even fit onto it.

What the feature control frame is saying is that the axis of the long rod must be located within a 1.1 diameter tolerance zone that is perpendicular to A and centered around B when B is at 4.52. As B gets smaller your cylindrical tolerance zone grows by that amount.

This does not make sense to me since the tolerance is applied to the rod and not the hole that the rod is pressed into. If this tolerance were applied to the hole that the rod is pressed into, then it would make sense.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

A short answer is that the "L" means you can't use a physical fixture. This is because the datum is to be taken at LMC, which is the largest inside diameter of 4.52 (that's the LMC, not MMC). When a fixture is made to that, per specification, then anything less than 4.52 wouldn't fit on the fixture!

But numerically, it means that a part with datum feature B at something smaller than 4.52 will get that much extra when checking the position of the needle/shaft.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[pmarc]

Chaldaean,
1. As powerhound and J-P said, presence of (L) modifier after datum A reference means that hard fixture cannot work well. Such datum feature simulation can be done using virtual datum simulator, but not physical one.

2. Besides, I think it is quite unclear which feature the dim. 4.50+/-0.02 applies to. Looking at the value, I guess this dimension has nothing to do with the threaded hole, so it is probably diameter of the hole reaching 12.1 depth. But that is only my assumption.

3. Which GD&T standard governs the print? Is it ASME Y14.5 or ISO? I am asking because if ASME, then the straightness callout is simply illegal, i.e. straightness tolerance value cannot be greater than 0.012 - shaft's size tolerance, unless it is stated anywhere on the print that in any manner Rule #1 does not apply. I think it would make much more sense from functional point of view to put this callout right below existing position FCF. This would mean that straightness of derived median line of the shaft (this term is not equal to 'axis' of the shaft!) would be limited. And in this case it could stay at 0.25 because straightness of derived median line overrides Rule #1.

If the print is intended to be according to ISO GD&T, then I have no remarks to the straightness callout.

 

[Chaldaean]

Thanks powerhound, Belanger and pmarc for taking the time to pass along your knowledge; I appreciate it.

We couldn't figure out how the fixture would work if the part were above LMC either. The only thing we can figure is the the needle "hub" is plastic (a bit compliant) to accommodate the 0.04mm tolerance.

The drawing is states that it is "based on an interpretation of ASME T14.5M-1994", so perhaps the straightness callout is illegal, pmarc.

 

[Belanger]

Yes, a straightness tolerance value must be less than the size tolerance value, unless Rule #1 is overridden.

But if that needle/shaft is non-rigid, then Rule #1 is automatically nullified, so we can't say right now that it's illegal. It depends on the flexibility. See Y14.5M-1994 paragraphs 2.7.1 and 2.7.1.3(b).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[powerhound]

The straightness is probably illegal. I'm trying to visualize how this thing is screwed over a 1/4-20 nipple. At least from the perspective of an outsider like me, I don't see how the OD of the needle is dynamically related to the ID counterbore in a way where the LMC modifier proves beneficial. The smaller the 4.50 hole gets, the more it can shift relative to the axis of the needle? It doesn't sound right to me.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[dgallup]

The straightness needs to be applied to the centerline. Then Rule #1 is overridden and the tolerance value may be greater than the size tolerance.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[ewh]

The straightness needs to be applied to the centerline.

I have been under the impression that centerlines are not toleranced, features are. Are there exceptions that I have yet to learn about?

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dgallup]

Centerlines can have geometric tolerances applied to them. In this case it totally changes the legality and effect.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[dingy2]

dgallup:

Centerlines are imaginary lines so could you please give an example of a centerline with an applied geometrical tolerance.

Dave D.

http://www.qmsi.ca

 

[powerhound]

Centerlines cannot have geometric tolerances applied to them.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[axym]

Let's make sure that we're all using the same definition of "centerline".

Imaginary entities on the drawing, such as centerlines or centerplanes, should not be toleranced or specified as datum features. These entities are not clearly defined on the actual part.

Certain derived entities representing the center geometry of the actual part can be toleranced. These might be loosely described as centerlines, but have specific names and definitions. When Straightness is applied to a feature of size, the Derived Median Line (locus of cross sectional centers) is controlled. When Position is applied to a cylindrical feature, the axis (perfectly straight centerline of the Actual Mating Envelope) is controlled.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[powerhound]

I was speaking strictly in terms of what was originally proposed...a geometric tolerance cannot be applied to a centerline. As in, the arrow points to the centerline on the drawing. I was not talking about a centerline derived from a feature such as the examples you gave.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[dingy2]

I agree with you on this one powerhound. That is the reason that I asked for an example.

Dave D.

http://www.qmsi.ca

 

[ewh]

Semantics can be important.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Chaldaean]

To recap from above:

• The inspection fixture probably wouldn’t work (would be too big to fit in the bore if the part was at anything bigger than LMC). The only way the fixture would "fit" is if the hub were plastic and flexed a bit.
• “As B gets smaller your cylindrical tolerance zone grows by that amount.” Or, in other words “with datum feature B at something smaller than 4.52 will get that much extra when checking the position of the needle/shaft.”
• If the shaft is rigid, the 0.25 straightness tolerance on the shaft is illegal as it must be less than 0.12 (the tolerance on the shaft itself). If the shaft is flexible, the straightness is legal per Y14.5M-1994 paragraphs 2.7.1 & 2.7.1.3(b).

Am I missing any additional points or are any of the bulleted points incorrect?

Again, thanks, to all, for your time and wisdom!

 

[fcsuper]

I intentionally didn't read any other responses in this thread until I was done typing. My guess is the L was used based on a fundamental misapplication of Less Material Condition. The originator likely thought that the tips of the threads (the thru diameter) counted as the less material condition. Of course LMC doesn't work like that (on several levels). Though marginally legal, it not practically inspectable and likely produces the exact opposite of what is necessary for design intent.

OK, now I'm going back to read other responses. Yes to Clakdaean, the B datum actually would vary more in location as material approaches MMC. I iterate my opinion that L is causing this callout to mean the exact opposite of what I think it intends to mean.

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

 

[pmarc]

"As B gets smaller your cylindrical tolerance zone grows by that amount." Or, in other words "with datum feature B at something smaller than 4.52 will get that much extra when checking the position of the needle/shaft."

This is not exactly true, I mean the first part of this statement. (L) modifier after B brings datum shift into play, but the datum shift itself does not increase a size of positional tolerance zone - it is always 1.1. When the diameter of datum feature B gets smaller than 4.52 towards 4.48, you have a possibility of moving the part in datum feature simulator B (max 0.04) to fit the toleranced axis of the shaft inside the position tolerance zone. For this particular case the interpretation I am giving now is the same as interpretation given by powerhound, but if geometry was more complicated this wouldn't have to be equal.

If the shaft is rigid, the 0.25 straightness tolerance on the shaft is illegal as it must be less than 0.12 (the tolerance on the shaft itself). If the shaft is flexible, the straightness is legal per Y14.5M-1994 paragraphs 2.7.1 & 2.7.1.3(b).

I was not commenting J-P's statement about part's flexibility vs. straightness callout legality before, but in my opinion there is something not OK with this. It cannot be that a drawing reader (inspector, machinist or whoever) is free to interpret such callout depending on part's rigidity. In my opinion there should be a clear indication on a drawing that rule #1 applies to the long shaft or not. A note like PERFECT FORM AT MMC NOT REQD would do the thing if rule #1 is intended to be overriden, otherwise I still think the straightness callout is simply illegal.

 

[Belanger]

Interesting, pmarc. I agree that using the "I" modifier or the "perfect form..." note would make it clearer, but paragraph 2.7.1.3 of 1994 says that a non-rigid part subject to distortion after manufacturing forces are removed is automatically exempted from Rule #1.

On an engineering drawing for a rubber band, would I really have to say "perfect form at MMC not required"?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com