Masked Area Location using Geometric Controls 1

[jonathan8388]

Hello Everybody

Had a question on correctly locating masking on aluminum surfaces that a coworker asked me and didn't have a good answer to.

The part is a flat aluminum plate that has a bunch of M6 threaded inserts as attachpoints. The part has paint and primer all over the surface, except he does not want paint and primer in the areas around the bolt holes since these areas mate to other parts.

The masked area has two different geometries at different insert locations. One geometry being a simple circle at some inserts, and the other being a slightly more complex shape at other inserts to represent the mounting interface.

Regardless, he wants the masked boundaries at each insert located relative to their respective inserts, so the masked areas follow the location of the insert. So essentially making each insert a datum feature and locating the masked boundaries to each of the inserts individually.

Normally would seem simple to me, but where I get thrown off is that these masked areas essentially are 2D and have no real depth. With that said, is there any way to do this with geometric controls? For the simple circular masked areas..... I initially thought about position for those, but I don't think the standard supports that for 2D shapes so that may be out. I'm completely lost on how to do that with the more complex masking areas as I don't think any form of profile can be applied to a 2D masking boundary on a flat plate.

Does anyone have any suggestions? Is this even possible with geometric controls?

 

[CheckerHater]

Profile of a line?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[jonathan8388]

CheckerHater

I suppose that can work. The only real example Y14.5 gives still shows it in the context of a surface that has depth..... but from a quick glance of 8.2.1.2 in Y14.5-2009, I guess I don't see anything that suggests profile of a line won't work for this.

 

[Burunduk]

Profile of a line won't work unless it is used with a well documented non-standard definition.
At least in terms of ASME Y14.5 (and I think similarly according to ISO-GPS), profile of a line relies on the true profile of the feature, whereas the true profile is a three-dimensional entity. Specifically, the 2D tolerance zones need to be on planes at right angles to the true profile. What will set the orientation of the tolerance zones if the true profile is merely a 2D-contour? The contour needs to be extruded in some direction to define a usable true profile.  The standards are lacking tools for controlling 2D geometries.

 

[engAlright]

Why wouldn't a positional tolerance apply? Y14.5-2018 lists "a circular element" as a type of regular feature of size (3.35.2).

 

[jonathan8388]

EngAlright

That definition does make sense, but in the 2009 standard in 7.3.2, it explains RFS tolerancing in terms of the Actual Mating Envelope. I think it also comes down to if a 2D feature actually has an AME or not. I am not sure that it does.

 

[axym]

Jonathan8388,

For an application like this, I think that you will need to adapt Y14.5's concepts to apply in 2D instead of 3D.

I believe that there are vision systems that do something very similar to what you are describing. The software defines geometric entities in 2D, and I believe that geometric tolerances like Position and Profile are used. They're just applied to 2D lines and points, instead of 3D surfaces.

For tolerancing the masked area boundaries, I would recommend Profile. This would be the most direct method - just use the nominal geometry of the boundary and create a zone around that. If Position was used, then it would need to be what used to be called Boundary Position - the interpretation is that the Position tolerance defines a Virtual Condition boundary that the surface of the feature must not cross. This is still defined in Y14.5 as part of what is called the Surface Method for Position tolerances at MMC. But it's less well understood by most GD&T users, so I would say that Profile would be better.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

Evan, what type of Profile do you refer to? Surface or Line?

 

[chez311]

I've always thought profile was probably the best way to do it as Evan indicated, but I struggle with whether or not everyone would interpret it as I do, at least without a note or supporting documentation. Perhaps not an ideal solution, but maybe one that can be more easily interpreted - instead of defining a tolerance zone with profile it might be feasible to define a "keep out zone" with basic dimensions and a note(s).

Ie: specify a masked limited area with basic dimensions and a note stating "NO COATING/PAINT ALLOWED INSIDE THE ZONE INDICATED WITH BASIC DIMENSIONS WRT |A|B|C|" or similar. If you trust your supplier (which might be too big a risk, unless the part is internally produced) to not mask too large an area, or are less concerned about that, then you could stop there. If you wanted to take it a step further you could create two boundaries at your min and max acceptable size with a note like the previous at the min size, and a note "NO UNCOATED/RAW SUBSTRATE ALLOWED OUTSIDE THE ZONE INDICATED WITH BASIC DIMENSIONS WRT |A|B|C|" at the max size. If the boundaries are very close together as to be indistinguishable in the view shown, it might be necessary to either attach it a basic dimension or include the dimension(s) in the note.

Seems to me its a step up from +/- toleranced dimensions at the very least.

 

[drawoh]

jonathan8388,

A profile of a line is 2D, unlike a profile of a surface.

Does your paint shop understand GD&T?

--
JHG

 

[axym]

Burunduk,

I don't know which type of Profile would be better - both of them apply to 3D surfaces and we're inventing something different here. I suppose that describing the 2D "profile" zone as Profile of a Line would be more intuitive, if nothing else.

This really is an unusual application though, to have masked area boundaries that need to be so accurate that they must follow the as-produced location of threaded inserts.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

Evan,

Out of curiosity, what you think of my proposal? I was attempting to bridge the gap between +/- toleranced dimensions and the extension of concept applying GD&T controls to 2D elements. I don't think it accomplishes anything much functionally different than the latter that you suggested, I was just trying to come up with something that might be more palatable to a supplier or inspection department.

 

[3DDave]

They aren't 2D. All paint has thickness. Determine a minimum keep-out boundary and set an Unequally disposed profile to set a limit outside of that where excess removal is acceptable.

Anymore I'd look at using at setting up a laser cutter/etcher to obliterate the paint rather than spending time masking and then picking the masking off and having the paint be uneven.

 

[Burunduk]

Evan,
I agree that the OP's problem is unique, but it's not THAT unusual. There are many cases where features that are in reality 3D (under a great magnification) need to get treated as 2D both on the drawing and at inspection. A recent topic that was discussed here, as you remember, is sheet metal. In the past, I brought up the topic of very shallow spotfaces the size and location of which has to be controlled.