Thin Datum Surfaces 5

[AMontembeault]

I have a sheet metal part, which is more or less square, with a bilaterally symmetric, sinusoidal contour cut on one side. I set my primary datum "A" at the large planar surface of the part, secondary datum "B" as the midplane of the part, and tertiary datum "C" as the edge opposite my contour. The contour is controlled by line profile with respect to A|B|C.

The problem we're running into pertains to ASME Y14.5-2018 section 7.9, specifically that all datum features must be controlled by appropriate geometric tolerances and/or size dimensions. Putting flatness on A, and perpendicularity on B with respect to A, and perpendicularity on C with respect to A and B makes sense in theory, but in practicality its proving to be very difficult to measure, given how thin the part is and how little real estate is available to probe with a CMM.

The only way I can think of to inspect this part is through the use of fixtures/functional gauges, but was wondering if perhaps anyone had run into something similar and might have better ideas. A change in the GD&T, or a different method of inspection?

 

[axym]

J-P,

I agree that it would be the RAME that applies for the secondary and tertiary datum features, when they are acting as datum features. But part of what Burunduk is saying is that the UAME is needed for the tolerance applied to the datum feature, if it's a Perpendicularity or Position tolerance RFS.

This goes back to what I mentioned in that long and rambling post from a few days ago. For a hole that acts as a secondary datum feature, the unrelated AME is not functionally relevant. The related AME is what matters. That's why the feature can still function properly, even though we have all sorts of trouble establishing its unrelated size and unrelated AME. So the traditional GD&T approach of controlling Size (unrelated Rule #1 boundary and unrelated actual local sizes) and Perpendicularity (of the unrelated AME) just doesn't fit the bill here. With thin parts, the issues involved in establishing the UAME become untenable. I suppose what is really needed is some kind of "related size" characteristic, but there currently isn't any way of directly specifying that. We can use Perpendicularity at MMC (with the Surface Method) to specify that the hole must fit over a related boundary of a certain size, to make sure that the fit of the hole is not too tight. Than we rely on the Size tolerance to make sure that the fit is not too loose. I suppose we could always use Profile ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Burunduk,
Yes, I suppose that's true; I guess I was caught up in the datum aspect of the discussion. Evan nailed the precise point. But alas, I guess we won't be able to eliminate sheet metal parts from industry!

 

[Burunduk]

Belanger, Evan,
So what we know so far about sheet metal parts:

* Can't apply perpendicularity with reference to the primary datum to control the secondary datum feature and control the  tertiary with reference to the primary and secondary.

* Can apply profile of a surface all-around the edges that will do only part of it's job by controlling size and form of a single cross-section, as a "psychological" compromise. The orientation of the edge surfaces to the primary datum won't be controlled.

* Can't apply position or perpendicularity at RFS to holes or slots without bending the interpretation of it. 

* Can't control actual local sizes at cross sections across opposed points.

I still say that sheet metal parts should be canceled. Too much trouble with them.

 

[greenimi]

Was an old standard only for sheet metal parts ANSI Y14.10, withdrawn in the meantime.
The standard committee needs a chair and can be revived, brought to life and improved, I am sure.

 

[metrologic]

This is a really good thread.

How is size defined for 2D and 1D features that exist in 3D space under ASME's Y14.5?

 

[3DDave]

Every so often someone proposes adding a datum reference frame callout to size dimension(s) so that the measurement will be taken in a specific orientation, eliminating the majority of the problems. If one has a wire then the measurement for length would be tied to the cylindrical surface preventing pathological outcomes. In other words, there would never be an unrelated actual mating envelope.

Practically this is what most people would expect would happen rather than turning the part some random amount to make a point. They look at some feature they can nominally align with and use that as a basis. This would codify it.

 

[metrologic]

@3DDave

So are you in favor of this concept? In the OP's example, it does seem strange to me that feature C's tolerance zone is defined with respect to datum B's related actual mating envelope feature simulator. But feature B itself, rather than being controlled with a datum A related size of some sort, is instead controlled relative to A via a tolerance zone on a derived center plane from an unrelated mating envelope. The perpendicularity tolerance on feature B seems more roundabout than a related mating envelope based tolerance.

 

[3DDave]

I have little doubt that it will be added. If it isn't then what will a new version of the standard be based on and with it, the new software, training courses, books, and charts to be marketed and sold? If they put all of them in at once, they would have nothing left for later.