Surface Interpretation of Position

[axym]

Hi All,

Here's a question.

Let's say you have a drawing of a simple part that is a rectangular plate with a 4-hole pattern. The feature control frame for the hole pattern has a Position tolerance referenced at MMC. There is a typical ABC datum structure, referencing one face and two sides.

If someone asked you to inspect the FCF using the surface interpretation of position, what would you do?

This is not meant as any kind of trick question, I'm just wondering what everyone's general knowledge is on the subject.


Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Not quite sure what you mean -- do you mean the boundary interpretation of position as opposed to the axis interpretation? Or do you mean the relationship of the hole to the surface that forms one of the datums?

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

 

[axym]

JP,

I mean the boundary interpretation of position as opposed to the axis interpretation.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

This can be veryfied by functional gaging or with CMM.

Lot of guys I work with think that only first option is possible when position at MMC is applied, and actually I am pretty sure that this kind of false belief is one of the major reasons why M modifier is not used in applications in which it is really required (at least from functional point of view). One thinks "I would not go for position at MMC because it would require hard gaging, so extra money for inspection and so on...".

They wat this can be obtained by CMM is quite nicely described on Tec-Ease website, so I will not rewrite it here.

 

[dingy2]

The boundary interpretation really simulates the actual assembly condition on the shop floor and a hard gauge (made to ASME Y14.43) simulates the mating condition. This is absolutely the best method of confirming the position of a pattern of holes (PLTZF) or individual holes (FRTZF) inside a pattern or a single hole.

Attempting to calculate the "bonus tolerance" and then finding the location of the axis using a CMM can be performed but does not cover all the simultaneous requirements and thus is not as good a method as hard gauging. Most CMM Operators realize that they must confirm the location of the axis at the top and bottom of the hole but do they confirm it in the center (straightness)?? There is a possibility that one might find the CMM accepts the positional tolerances but a checking fixture (made correctly) rejects it. Which would be correct.

Many companies that only have a production run of 1, do not use MMC.

Dave D.

http://www.qmsi.ca

 

[dtmbiz]

Axym,
After reading many of your in depth comments, I find it odd that you didn’t ask for clarification regarding the question. Well, maybe you did and this would be my response..
If someone asked me to inspect the “FCF” ….. First of all, I would clarify whether or not they understood that FCF is the Feature Control Frame that cannot be inspected, but rather states the tolerance of location.
I would say that the surface condition of the cylinder face (surface of the hole, presuming it is diametric) must lie within the size limits called out. The cylindrical face axis would then need to lie within the cylindrical tolerance zone specified. Finally, the tolerance zone could increase in size as the hole increased in size. This would allow the axis of the hole to move within the tolerance zone, however the cylindrical hole surface could only deviate within the bounds of 2 cylinders representing the min and max of the size limits.
In summary, the interpretation of the hole surface could only deviate within the size tolerance and the M refers to additional possible movement of the surface axis or location of that surface. It is my understanding that the axis location doesn’t affect the hole surface, however the hole surface can affect the axis. Therefore the M doesn’t impact the actual hole “surface”.
And then if the surface interpretation is relative to the surfaces that the hole passes thru, then maybe as mentioned earlier…. Boundaries..

The question actually begs the question… What the hay!?
And Happy T-day to all !

 

[Belanger]

Interesting comments, dtmbiz, and I agree. One question remains: what is the "axis of the hole"? I would clarify that it is the axis of the "actual mating envelope" of the hole -- this is why the boundary interpretation is often the best, because it doesn't have to mess around with any mathematical derivation of the actual precise axis of the hole.

So you're right... the axis location doesn't affect the hole surface, but the hole surface can affect the axis. My vote goes with boundary interpretation, but that doesn't mandate fixed gaging. It merely allows fixed gaging.

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