GD&T Q&A session - Interpreting FCF (Feature Control Frame)

I was inspecting this part last night and I was confused on how to interpret the Feature Control Frames for the rectangle and obrounds. It goes against all known GD&T conventions governed by the ASME/ANSI Y14.5 standard. Would someone be willing to go through a Question and Answer session with me?

This thread will probably go on for a few days, so please bear with me if I put the details in, little by little.

Re: GD&T Q&A session - Need assistance interpreting FCF

What are you useing to check the part?

And what exactly do you need help understanding?

Re: GD&T Q&A session - Need assistance interpreting FCF

MysterHK said:

I was inspecting this part last night and I was having trouble intrepreting the Feature Control Frame. Would someone be willing to go through a Question and Answer session with me?

This thread will probably go on for a few days, so please bear with me if I put the details little by little.

Click to expand...

Hello,

I printed your drawing.

There's a few issues with the GD&T callouts.

The FCF that applies to the 6-hole pattern is correct.
It says that the axis of each hole must be located within a .040" cyclindrical tolerance zone at MMC relative to datums A, B, and C.

The FCF that applies to the rectangular hole is incorrect.
You can not have a cylindrical tolerance zone on this feature.
A positional boundary control or a bi-directional positional control would be more appropriate.

The FCF that applies to the two slots is incorrect.
You can not have a cylindrical tolerance zone on this feature.
A positional boundary control or a bi-directional positional control would be more appropriate.

I do understand what the design engineer is trying to accomplish, but it was not put on the drawing correctly.

Stijloor.

Re: GD&T Q&A session - Need assistance interpreting FCF

Umm, the type of feature does not matter one bit, and I have never heard anything like that before. Since I do not have the ASTM manual or any of my books I can not look it up. But there is nothing wrong with the callouts, they are legite. Also they are very easy to check on a CMM or a surface plate. The only thing I see that could raise a question is that the bolt pattern is not controlled to itself, so from a engineering standpoint the bolt pattern is not a bolt pattern but 6 ø's that have no relatinship to one another.

Re: GD&T Q&A session - Need assistance interpreting FCF

justncredible said:

Umm, the type of feature does not matter one bit, and I have never heard anything like that before. Since I do not have the ASTM manual or any of my books I can not look it up. But there is nothing wrong with the callouts, they are legite. Also they are very easy to check on a CMM or a surface plate. The only thing I see that could raise a question is that the bolt pattern is not controlled to itself, so from a engineering standpoint the bolt pattern is not a bolt pattern but 6 ø's that have no relatinship to one another.

Click to expand...

With all due respect, the ASTM Standard you are referring to is incorrect.
The applicable Standard is ANSI/ASME Y14.5M-1994.

The fact that you could possibly check "intent" does not make the callouts correct.

Check it out with the appropriate Standard in hand, and you'll see.

Stijloor (GD&T Instructor).....

Re: GD&T Q&A session - Need assistance interpreting FCF

Stijloor said:

Hello,

I printed your drawing.

There's a few issues with the GD&T callouts.

The FCF that applies to the 6-hole pattern is correct.
It says that the axis of each hole must be located within a .040" cyclindrical tolerance zone at MMC relative to datums A, B, and C.

The FCF that applies to the rectangular hole is incorrect.
You can not have a cylindrical tolerance zone on this feature.
A positional boundary control or a bi-directional positional control would be more appropriate.

The FCF that applies to the two slots is incorrect.
You can not have a cylindrical tolerance zone on this feature.
A positional boundary control or a bi-directional positional control would be more appropriate.

I do understand what the design engineer is trying to accomplish, but it was not put on the drawing correctly.

Stijloor.

Click to expand...

The tolerance zones are circular, not cylindrical. As far as using the callouts in question for geometric shapes other than circles (round holes), the point in question is the geometric center of the features, and the shapes of the features themselves is irrelevant. The geometric center point has to fall within the .040 dia. tolerance zone.

Re: GD&T Q&A session - Need assistance interpreting FCF

Jim Wynne said:

The tolerance zones are circular, not cylindrical. As far as using the callouts in question for geometric shapes other than circles (round holes), the point in question is the geometric center of the features, and the shapes of the features themselves is irrelevant. The geometric center point has to fall within the .040 dia. tolerance zone.

Click to expand...

I give up!!! :mg:

Stijloor.

Re: GD&T Q&A session - Need assistance interpreting FCF

Stijloor said:

The FCF that applies to the rectangular hole is incorrect.
You can not have a cylindrical tolerance zone on this feature.
A positional boundary control or a bi-directional positional control would be more appropriate.

The FCF that applies to the two slots is incorrect.
You can not have a cylindrical tolerance zone on this feature.
A positional boundary control or a bi-directional positional control would be more appropriate.

I do understand what the design engineer is trying to accomplish, but it was not put on the drawing correctly.

Click to expand...

Stij,

That's what I initially thought, but I just needed a real expert like you, to help me verify this. I was so frustrated and was banging my head on my desk. I was looking at those 2 FCFs and comparing them to the examples in James Meadows' book, GD&T. And I kept saying to myself, that it's not possible.

Re: GD&T Q&A session - Need assistance interpreting FCF

MysterHK said:

Stij,

That's what I initially thought, but I just needed a real expert like you, to help me verify this. I was so frustrated and was banging my head on my desk. I was looking at those 2 FCFs and comparing them to the examples in James Meadows' book, GD&T. And I kept saying to myself, that it's not possible.

Click to expand...

Thank you very much!

Send a PM if need further clarification.

Do you have the GD&T Standard handy?

Stijloor.

Re: GD&T Q&A session - Need assistance interpreting FCF

Jim Wynne said:

The tolerance zones are circular, not cylindrical. As far as using the callouts in question for geometric shapes other than circles (round holes), the point in question is the geometric center of the features, and the shapes of the features themselves is irrelevant. The geometric center point has to fall within the .040 dia. tolerance zone.

Click to expand...

Jim,

If this is the case, how do I apply the Allowed Positional Tolerance I would need to subtract in my equation to get the Equal Mating Boundary on the FCFs for the rectangle and the obround?

Thanks, in advance!