Feature Control Frame - Interpretation - Two Tolerances?

Manix

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Hi All,

I am trying to interpret the attached control frame to define a measurement requirement using a gauge that has been specifically built for this purpose.

What do I know, well, I know it is the profile of the line (actually it's a surface, I think they got the symbol wrong!) that is important. I don't however know why there seems to be two tolerance bands. 0.3 and 0.6? They both relate to the same datum contstraints and material conditions!!

It's a gap condition, so the variation of the surface will create differing gaps (between that surface and another surface on application and indeed the gauge).

Can anyone help as to how to interpret this tolerance?
 

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David DeLong

You are probably correct stating that this symbol should be profile of a surface rather than profile of a line. I will now assume that it is profile of a surface.

You have a composite feature control frame with the top section relative and dimensioned from the datums shown. In other words, it reflects the location of the surface relative of datums A, B and C.

Although the datums must be holes (features of size) since they are shown at MMC and, apparently, it is legal (ASME Y14.5M-94) but it is not practical application. That probably would be another question and not relative to the question asked.

The bottom section of a composite feature control frame controls the surface itself. It will control this surface for flatness, perpendicular to datum A and oriented (angularity relationship) to datums B & C.

That is the difference.

An example of a composite feature control frame with profile of a surface is shown on pages 176 & 177 of the ASME Y14.5M-94 standard. Please note that the example does not reflect datums at MMC.

Hope this helps.
 

Manix

Get Involved!!!
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You are probably correct stating that this symbol should be profile of a surface rather than profile of a line. I will now assume that it is profile of a surface.

You have a composite feature control frame with the top section relative and dimensioned from the datums shown. In other words, it reflects the location of the surface relative of datums A, B and C.

Although the datums must be holes (features of size) since they are shown at MMC and, apparently, it is legal (ASME Y14.5M-94) but it is not practical application. That probably would be another question and not relative to the question asked.

The bottom section of a composite feature control frame controls the surface itself. It will control this surface for flatness, perpendicular to datum A and oriented (angularity relationship) to datums B & C.

That is the difference.

An example of a composite feature control frame with profile of a surface is shown on pages 176 & 177 of the ASME Y14.5M-94 standard. Please note that the example does not reflect datums at MMC.

Hope this helps.

Thanks David, that has helped in understanding what they mean, but now I don't know how to measure them. We have a gauge that this parts is inserted into and the gap between this surface and the surface of the gauge is measured. How do you devise the acceptable variation in the measured values (the tolerance)? Both variation in the location and the profile of the surface will effect the gap!!??

The gap nominal is 0.
 
D

David DeLong

Thanks David, that has helped in understanding what they mean, but now I don't know how to measure them. We have a gauge that this parts is inserted into and the gap between this surface and the surface of the gauge is measured. How do you devise the acceptable variation in the measured values (the tolerance)? Both variation in the location and the profile of the surface will effect the gap!!??

The gap nominal is 0.

One would require 2 gauges.

1st gauge - The top section of this feature control frame is 0.6 or +/- 0.3 from the theoretic surface. One would have a checking fixture with two (2) pins of MMC size for datums B & C. The part would be placed onto the two (2) pins in datum holes B & C and resting on datum A. Now I would have a broad rectangular stepped pin of +0.3 and - 0.3 the width of the surface oriented and dimensioned from B & C. Place the part on the fixture and then try to get both sections of the rectangular pin on the surface. This gauge is a floating fixture since one will accrue tolerances from holes datum B & C. Please note that this gauge does not and cannot measure flatness of the surface but its highest point on that surface.

2nd gauge - same as the first except the stepped gauge is only 0.3 and the part is allowed to move relative to datums B and C but must be oriented. This is a bad gauge since, again, it does not relate the flatness of the surface. I really don't like it but, it you want a gauge, here it is.

If you want to discuss this further, you can send me directly questions on this gauge.
 

Stijloor

Leader
Super Moderator
You are probably correct stating that this symbol should be profile of a surface rather than profile of a line. I will now assume that it is profile of a surface.

You have a composite feature control frame with the top section relative and dimensioned from the datums shown. In other words, it reflects the location of the surface relative of datums A, B and C.

Although the datums must be holes (features of size) since they are shown at MMC and, apparently, it is legal (ASME Y14.5M-94) but it is not practical application. That probably would be another question and not relative to the question asked.

The bottom section of a composite feature control frame controls the surface itself. It will control this surface for flatness, perpendicular to datum A and oriented (angularity relationship) to datums B & C.

That is the difference.

An example of a composite feature control frame with profile of a surface is shown on pages 176 & 177 of the ASME Y14.5M-94 standard. Please note that the example does not reflect datums at MMC.

Hope this helps.

David,

Thank you very much for your post.

The OP is from the UK. I believe that he should refer to the appropriate ISO standard (ISO 1660:1987). Without seeing the entire drawing, including the datum features (of size), it will be very difficult to interpret the drawing with "100%" confidence.

What do you think?

Stijloor.
 
D

David DeLong

David,

Thank you very much for your post.

The OP is from the UK. I believe that he should refer to the appropriate ISO standard (ISO 1660:1987). Without seeing the entire drawing, including the datum features (of size), it will be very difficult to interpret the drawing with "100%" confidence.

What do you think?

Stijloor.

Stijloor:

I did make some assumptions for sure but I do know that if datums B & C are at MMC, they must be features of size rather than surfaces. Datum A is assumed the mounting surface.

ISO has some strange methods for sure and I was reflecting the ASME standard rather than ISO so my confidence level certainly is not 100%.

Good point!
 
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CMMMonster

I recently encountered a composite positional tolerance applied to two hole locations that should be concentric. I was puzzled by this because the top line of the control frame was true position of .010" ADC. and the bottom line was true position of .005" ADC. wouldn't the bottom line override the top line? I have only worked in Aerospace for a Year and this is the first time i have encountered this. Normally i would see the second line referenced to one, two ,or no datum reference at all. Is it even possible for to reference the same three datums in both lines of the composite tolerance? Any help or opinion is greatly appreciated. Please let me know if this should be posted elsewhere this is the only thread I could find relating to this topic.I also could not figure out how to start a new thread before i had to leave for work. Thanks!
 
Q

QC Rick

My very late 2 cents. (Same for CMMMonster's post)

This is not a true composite FCF, it is the same structure with two tolerances and the lesser tolerance negates the greater. (Based on ASME Y14.5)

It should be: (See attached image)
 

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CMMMonster

Thanks alot Rick. That is what I though. When referenced to the same datum structure the tighter tolerance will prevail.
 
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