Profile Tolerance with Single Datum - Does it holds its position/orientation? 2

[mrletters]

Hi all,

I am unclear whether or not a profile tolerance with respect to a single datum can suffice to control the position and/or orientation of a profile
Attached is an specific case, a simple block with a pocket. The datums A is on the lower face. Question are the following:

1. Does the profile tolerance specified controls only the size of the feature?
2. Can the part end up having the pocket rotated? (like 30deg, 45deg, 90 deg)
3. Can the part end up with the pocket skewed towards one side? ( shifted 1", 2")
4. Does the basic dimensions constrain the position/orientation of the tolerance zone?

Thanks!

 

[axym]

mrletters,

1. The Profile tolerance controls the form and size of the pocket, and its orientation relative to A.
2. Yes. The pocket can be rotated relative to the sides of the part.
3. Yes. The pocket can be located anywhere relative to the sides of the part.
4. The location and orientation of a Profile zone is constrained relative to any datums. In this case, the zone is only oriented to datum A and not constrained to anything else. The basic dimensions just define the theoretically exact relationship - they don't do anything on their own.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Think of your datum framework as if it was coordinate system.

Can you control your profile using only "Z" coordinate?

 

[mrletters]

Thanks axym and CheckerHater!

Can I make another inquiry related to the previous example?
I am attaching another image with different profile tolerance notations.
( Disregard the file names from Windows/CATIA )

From these could we say the following?

1.- Top Left - The pocket could be shifted, and rotated around Z axis, and/or tilted into the part.
2.- Bottom Left - This is the example discussed above
3.- Top Right - The pocket holds form, size, position and orientation ( within the tolerance zone )
4.- Bottom Right - The pocket holds the form, size and position, but it may be rotated around axises.

 

[axym]

mrletters,

1. With no datum features referenced, the Profile tolerance controls only form and size. The pocket could be shifted or rotated in any direction relative to the rest of the part.

2. As above

3. With ABC referenced, the Profile tolerance controls form and size as well as orientation and location to the ABC datum reference frame.

4. The explanation for this one is not correct. The Profile tolerance controls form and size. When combined with Profile in this way, the Position tolerance should control the orientation and location by imposing a fixed boundary. But there is an error in the Position tolerance - there should not be a diameter symbol. This is only used for controlling the axis of a cylindrical feature - the pocket is not cylindrical and does not have an axis. See Figure 8-24 of ASME Y14.5-2009 for proper application of combined Profile and Position tolerances on an irregular feature.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Question to Evan:

Are you comfortable with Position in example 4 specified RFS?
Y14.5 goes to great lengths explaining how Profile works together with Position at MMC using idea of "boundary".
But boundary by definition only exists when you have virtual condition.

In your opinion, does specifying Profile together with Position at RFS make any sense?
What would be the interpretation?

 

[axym]

CH,

No, I'm not comfortable with the RFS - I hadn't even noticed that. It's incorrect. Good catch.

It would not make sense to specify Profile and Position RFS together. It has to be MMC (or LMC) to define a virtual condition, as you say. Actually, the Y14.5 term would be Positional Boundary.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
Why you said RFS is incorrect? What means incorrect? How do you know how this part works?
I would say is highly unlikely that the part functions like RFS, but I would not say is incorrect and for sure I would not say is illegal.

Y14.5 -2009 states:

2.11 BOUNDARY CONDITIONS
Depending upon its function, a feature of size is controlled
by its size and any applicable geometric tolerances.
Material condition (RFS, MMC, or LMC) may
also be applicable. Consideration must be given to the
collective effects of MMC and applicable tolerances in
determining the clearance between parts (fixed or floating
fastener formula) and in establishing gage feature
sizes. Consideration must be given to the collective
effects of LMC and applicable tolerances in determining
guaranteed area of contact, thin wall conservation,
and alignment hole location in establishing gage feature
sizes. Consideration must be given to the collective
effects of RFS and any applicable tolerances in determining
guaranteed control of the center point, feature axis,
or feature center plane. See Figs. 2-12 through 2-17.

 

[CheckerHater]

@greenimi:

We were talking about combining Position and Profile controls.

From Para.8.8 COMBINED CONTROLS:

“For an internal feature the boundary equals the MMC size of the profile minus the positional tolerance… For an external feature, the boundary equals the MMC size of the profile plus the positional tolerance…”

See also Fig. 8-24.

 

[greenimi]

CH,
So, you actually saying is "illegal" to use RFS?
What about if it IS functional?

My opinion: because is not shown in the standard does not mean cannot be used (or does not mean is not functional)

 

[CheckerHater]

I am not saying anything. I am asking, what is your personal interpretation of example 4 (lower right corner)?

 

[axym]

greenimi,

By "incorrect" I meant "not supported in the Y14.5 standard". It's true that I do not know how this part works.

The reason that I thought RFS was incorrect is that the pocket feature is an irregular feature of size. Y14.5 does not define actual mating envelopes or resolved geometry (e.g. axis, center plane, center point) for anything other than regular features of size. So if RFS were specified, we would not know what geometry is controlled or how to extract it.

There may be applications in which the pocket needs to be controlled in an RFS-type manner - if, for example, the pocket mates with a similarly-shaped expanding boss. Even if this were the case (which I agree is unlikely), I'm still having trouble seeing how Position RFS would be functional - the resolved geometry would be complicated. The pocket is an extruded shape, so I suppose that line-on-plane resolved geometry could be extracted from the "maximum inscribed boss". But then what shape of tolerance zone would this be controlled in? Sorry, but I'm not seeing the connection to functional reality here.

In general, I agree that if something is not shown in the standard this does not mean that it can't be used or is not functional. But in this case, it appears that there are good reasons why Y14.5 did not support RFS on irregular features of size.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca