Virtual condition 1

[3DDave]

Suppose there is a basic brick shape - Datum feature A uses the largest face, datum feature B uses the next largest face, and datum feature C uses one of the smallest faces.

In the same face identified as datum feature A is a hole through the part of diameter E, positioned at MMB with [A|B|C] as the DRF with a tolerance of dia. X. Assume the basic dimension to B is b1, and the basic dimension to C is c1.

Now the face opposite of C is identified as datum feature D and is toleranced with a profile tolerance to [A|B|C] with a zone width of Y. Assume the basic dimension from C to D is d1.

The desire is to make a bracket sharply bent of a rectangular piece that mates to datum feature A, will have one edge coincident to datum feature B plane, and hook around the end of the part to mate with datum feature D. A bolt will pass through both parts.

What is the virtual size of that hole in the [A|B|D] DRF?

 

[greenimi]

Suppose that since rule #1 does not apply, the largest envelope that the actual surface of the feature does not violate is 4.75, as a result of form error

May I ask where 4.75 is coming from? Edit: Random number?

 

[Burunduk]

Just an example for an actual feature that was produced that way.

 

[greenimi]

So, are we confident that VC name/term is correct (I mean standardized) to be used in the CONTEXT of the "no rule#1" design?
There are some different opinions about this "no rule#1" and I've been asked this question recently. However, I am sure about the correct answer.

 

[3DDave]

greenimi, That is a good question since, on its own, the feature might have a zero size MMC envelope, even though there is a limit to the local size. Rule #1 is what sets the smallest MMC envelope; when that rule does not apply then neither does the limit to the size of the MMC envelope.

 

[chez311]

The calculated VC boundary is the same for MMC position with or without rule 1 - the only thing that has changed is the limitations of form for the feature however the same VC may not be violated.

If the boundary remains unchanged in the two cases why would we not use the same term to describe said boundary?

 

[Burunduk]

The virtual condition and rule #1 boundary are two completely separate boundaries that impose different requirements. One requirement can be in force while the other is not.
The rule #1 boundary results from actual local size and form limitation imposed by the size dimension and tolerance. It is not constrained to the DRF.
The VC boundary is constrained to the DRF and results from the location and orientation limitation of the position tolerance applied at MMC (or LMC).
The bottom line is: even when the hole is allowed to violate the boundary of perfect form at MMC, it can still be limited by the VC boundary effectively.

 

[greenimi]

The bottom line is: even when the hole is allowed to violate the boundary of perfect form at MMC, it can still be limited by the VC boundary effectively.

If the boundary remains unchanged in the two cases why would we not use the same term to describe said boundary?

Maybe because VC name is not valid/ per the standard. You have only the two points size and not the AME/ UAME/ MMC envelope, therefore no VC...
Hmmm....I don't know …….

 

[Burunduk]

The VC term is valid wherever a position tolerance with a material condition modifier is applied.
From the practical point of view: would rule #1 not being in force prevent you from being able to use a fixed size gage for the position verification?
If you understand that the answer is "no", reverse it into theory and explain to yourself as you see fit.

 

[axym]

Hi All,

This is what we sometimes refer to as a "swamp".

Y14.5 indirectly weighed in on this question in the 2018 standard. There is an updated version of what was Fig. 4-16 in 2009, with example calculations for MMB. The new version in 2018 (Fig. 7-22) adds a straightness tolerance at MMC to feature D, but this tolerance has no effect on the MMB's. This is not mentioned in 7.11.6.1 on Determining the Appropriate MMB, but it is clarified in 8.4.1.3 on Derived Median Line Straightness:

"When the straightness tolerance at MMC is used in conjunction with an orientation or position tolerance at MMC, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value and does not contribute to the IB or OB of the position or orientation tolerance. The collective effect of the MMC size and form tolerance produces a VC, OB, or IB resulting from the form tolerance but does not affect the IB or OB created by any orientation or position tolerances on the feature. See Figure 7-22."

So we're not allowed to specify a straightness tolerance at MMC that exceeds the position tolerance at MMC. The boundary resulting from the straightness tolerance must somehow get absorbed by the boundary for the position tolerance. Does this mean that specifying Independency, which would allow even more straightness error, would not be allowed?

There are new examples of boundary calculations for LMC and RFS cases as well. In the RFS case (Fig. 7-24), the straightness tolerance on feature D actually does affect its boundary!

I have not been able to make sense of this.



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

geenimi - I agree, there is no virtual condition for this case.

 

[chez311]

You have only the two points size and not the AME/ UAME/ MMC envelope

Overriding rule #1 does not override the swept spheres interpretation of size to two point measurement. The limits of size are still dictated by the swept spheres interpretation, see Y14.5.1-1994 section 2.3.1 - if rule #1 applies an addition requirement of conformance is outlined in 2.3.2

For an external FOS at MMC (or internal FOS at LMC) one can always calculate a RAME/UAME (RAMME/UAMME for internal FOS at LMC) even without rule#1.

For an internal FOS (or external FOS at LMC) one can almost always calculate a RAME/UAME (RAMME/UAMME for external FOS at LMC) unless there is excessive form or orientation variation allowed. This is unlikely in the case of MMC/LMC as your VC would limit such form or orientation error unless the allowed tolerance was extremely large relative to the size of the feature. This is possible with RFS in which case a form control I would say is not only recommended but required, otherwise form is uncontrolled.

A negative VC is possible in which case per Y14.5-2018 section 5.9.4.1 (I don't think theres any mention of what to do in 2009) the surface interpretation no longer applies - however if one cannot calculate a UAME (or UAMME where applicable) then I would say position is either not the correct control, or being used in a situation which is improper. What is the meaning of an axis interpretation position control for which one cannot calculate a UAME/UAMME?

 

[3DDave]

axym - any idea what real world problem they were trying to solve or is this just to add some more questions to the tests and more pages to consultant presentations? I assume the new version still gets the (c) part incorrect.

 

[chez311]

3DDave,

Are you taking issue with describing such a boundary as virtual condition, or are you saying that no such equivalent boundary (say the calculated 4.3 boundary in greenimi's example 6 Mar 20 13:28) which may not be violated exists and the surface interpretation no longer applies if rule#1 is overridden in conjunction with MMC?

 

[Burunduk]

Evan,
What you mention originates already at the 2009 standard, 5.4.1.2:

"Where the straightness tolerance
is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value."

This is probably because the Y14.5 generally tries to prevent the user from defining tolerances that can not be fully utilized. If the DML straightness & size worst-case boundary of a hole for which perfect form at MMC is not required is smaller than the virtual condition boundary applied by position at MMC for the same hole, the hole will not be able to deform enough to utilize the entire derived median line tolerance (Otherwise it will interfere with the duty of the position at MMC requirement).

 

[3DDave]

chez - what is the maximum interior envelope if the hole is so bent that there is no clear view through it as would allowed to be the case if there is no form limit?

 

[chez311]

3DDave,

I already answered that in my previous response, see below. It cannot be calculated in that case.

For an internal FOS (or external FOS at LMC) one can almost always calculate a RAME/UAME (RAMME/UAMME for external FOS at LMC) unless there is excessive form or orientation variation allowed.

I am of the opinion that such form variation would be limited by a MMC position tolerance at VC. I thought this has been answered in the thread (

https://www.eng-tips.com/viewthread.cfm?qid=454109)

however when I reread it, it seems that my specific question on this very topic actually went mostly unanswered (6 Jun 19 15:33 and 14 Jun 19 15:24).

In the case that your VC is negative, or if you are of the opinion that only the axis interpretation should apply in the case of independency from rule#1 then I also addressed that:

A negative VC is possible in which case per Y14.5-2018 section 5.9.4.1 (I don't think theres any mention of what to do in 2009) the surface interpretation no longer applies - however if one cannot calculate a UAME (or UAMME where applicable) then I would say position is either not the correct control, or being used in a situation which is improper. What is the meaning of an axis interpretation position control for which one cannot calculate a UAME/UAMME?

 

[3DDave]

chez, Since you answered your own question what exactly were you asking of me? I apologize for not memorizing every previous answer to see if I could glean your intention in this one.

I'm of the opinion that unrealizable geometry questions are generally a waste of time. I'm also of the opinion that "back-driving" a limit on a feature from some arcane combination of other requirements is a sign of an unrealizable geometry question.

The point of the short-cut descriptions in the standard is to be explicit, not to make a Sudoku puzzle.

 

[axym]

3DDave,

As far as I can tell, the MMB calculations in the new figure are identical to what they were in 2009.

Burunduk,

You're right - the statement about the straightness not being greater than the position/orientation tolerance was there in 2009. For some reason I'd forgotten that, or never fully realized it. In 2009 it didn't spell out that the straightness tolerance doesn't contribute to the boundary if a position/orientation tolerance is present, or provide an example which illustrated that.

It's been a while since I really thought about this issue, but it seems that there are deep issues with regards to surface interpretation versus resolved geometry interpretation when a straightness tolerance is present. I think that 3DDave is right that this is an unrealizable geometry question.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

3DDave,

I only repasted my response for clarity in hindsight that was probably not necessary. I did not mean to be confrontational - I respect your opinion immensely and was inquiring with the genuine intent for discussion, not with any ulterior motive or trying to get a specific answer.

My questions was essentially - do you believe that no VC or equivalent boundary which may not be violated exists for an MMC position control in combination with overriding rule #1 (with Independency or the the like)?

I always thought of MMC with Independency (or similar overriding of rule #1) as similar to the default behavior of MMC controls in ISO - the feature cannot violate the calculated MMVC and limits of size, otherwise form is uncontrolled.

 

[chez311]

It's been a while since I really thought about this issue, but it seems that there are deep issues with regards to surface interpretation versus resolved geometry interpretation when a straightness tolerance is present. I think that 3DDave is right that this is an unrealizable geometry question.

Evan,

Do you believe there to be also issues when considering an MMC tolerance combined with the Independency symbol? I would be interested in your answer to the same question I asked 3DDave (6 Mar 20 18:31).