Fig 4-43 ASME 2009 1

[AM Engineer]

Hello all.

Find the attached figure 4-43 of ASME Y14.5M-2009.

Consider the profile of the surface applied to datum feature E. The question is

If the actual measured value of the profile of surface tolerance is found to be non conformed then should we proceed to measure the position tolerance of the hole dia 3.1 whose FCF is revering datum feature E as a tertiary datum feature reference at RMB?

Thank you in advance.

 

[Belanger]

Recall that datum E is not the surface itself, but a theoretically perfect plane derived from datum feature E. So I guess the blunt answer is yes -- you could still go on to measure the position tolerance from the true datum E. But the part is to be rejected based on the fact that your profile tolerance was not met.

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

 

[AM Engineer]

But the datum plane derived from datum feature simulator will not be at the location where it should be.

The datum feature simulator will start its journey from MMB 13.5 and will keep on moving though the tolerance zone while being parallel to datum plane X towards LMC 13.3 until it makes maximum possible contact with actual datum feature. If lets say that actual datum feature surface is bit up so datum feature simulator will keep on moving and will cross LMC.

Am i right in this?

 

[Belanger]

Yes, I agree with your statement. But think of it this way: Each tolerance callout must be met on its own terms, but they are intertwined. As I mentioned, the position tolerance would be measured and potentially pass, but it would be passed based on a faulty datum E. So it's what I would call a Catch-22.

The part is bad because of the one profile tolerance, but you can measure it all day long if you wish.

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

 

[AM Engineer]

Thanx Belanger.

How the things will change if datum feature E is modified at MMB?
Will it be fixed in size or there will be same case as in Fig 4.31 illustration "c" ?

 

[AM Engineer]

Here is 4.31

 

[axym]

AM Engineer,

I think I see the point that you were making. The situation is similar to that in Fig. 4-31 (a), with datum feature E referenced RMB and controlling the last rotational degree of freedom. Y14.5 states that the datum feature simulator progresses through the profile tolerance zone from the MMB to LMB, until is makes maximum possible contact with the datum feature. If datum feature E is nonconforming and has exceeded its LMB, maximum possible contact would not be achieved within this range.

What does the simulator do in this situation? We don't know. Y14.5's definition only includes datum features that are conforming, and does not address what happens when one or more datum features is nonconforming. So we are left to guess at what should be done, and there are major disagreements on this in industry. Some people would say that we just stop and say that the part is bad because the datum feature is out of tolerance. Others (myself included) would say that we should be able to calculate a measured value even if a datum feature is nonconforming. But we would have to use an extension of principle in order to do this.

I would say that this is a gaping hole in Y14.5's definitions.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
Isn't it the OP question like Monty's door problem/game? Depends by what information you have during the evaluation and to re-evaluate your decision based on the new info emerged.

Had you measured the position of the holes before the profile and the holes would appear good then it is a piece of information that would be helpful for the process.

Is it something in Y14.5 that prevents you from doing it? ( measuring the holes before the tertiary datum feature E or D)

 

[AM Engineer]

greenimi

Interesting question.

 

[axym]

This problem is not just applicable to secondary and tertiary datum features applying rotational constraint. Even a single cylindrical datum feature referenced RMB would have a similar issue. Y14.5 states in 4.11.4 that the simulator geometry originates at the MMB and progresses proportionally through the tolerance zone. What happens if the datum feature is oversize or undersize? Can we extend the size range that the simulator covers? It's not clear. There is no mention of what happens if the datum feature is out of tolerance, and the figures that are referenced (4-3, 4-11, 4-12) just have a datum feature symbol and don't even include the size tolerance.

In other paragraphs such as 4.11.4. (a), the datum feature simulator is defined as the smallest circumscribed (for an external feature) or largest inscribed (for an internal feature) perfect cylinder that makes maximum possible contact with the datum feature (with nothing about there being limits at the MMB and LMB). I think that this makes sense - the requirement for the simulator to only progress through the datum feature's tolerance zone should not be there. I would say that Y14.5 needs to revisit this.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca