Controlling "sharp" edges through geometrical controls 1

[semiond]

Searching the forum for information regarding application of GD&T on edges, i found thread1103-310372, where it seems that most participants agreed that edges (including those that connect surfaces at an angle smaller than 90 deg.) can legally be used as datums (recommended or not - is a separate question).

In relation to ASME Y14.5 2009, my question is: could the two edges which the OP of that thread intended to assign as datums, be controlled for orientation between each other by the use of perpendicularity control? If i was asked this question my answer would be "probably not" because the definitions in chapter 6 seem to only mention a surface, center plane, or an axis as the subjects for orientation controls. On the other hand, maybe the fact that it's not mentioned in the definitions, doesn't mean it is not allowed? As for inspection - i don't think it would be too difficult with an optical comperator or a microscope (simulating a tangent line to the datum feature edge and then finding the size of the zone within which the other edge lies).

For approaching this issue, please assume that the uncertainty related to deburring/ rounding size removed from the equation, since the rounding/deburring are small enough and tightly controlled.

 

[drawoh]

...

For approaching this issue, please assume that the uncertainty related to deburring/ rounding size removed from the equation, since the rounding/deburring are small enough and tightly controlled.

No way!

There will be a radius. There will be more of a radius if the part is being handled for any length of time. Try modelling it. A radius on an otherwise sharp, acute angle affects your fixturing point. You can specify each intersecting face as a datum. This defines a sharp intersection line. You can specify an accurate radius or chamfer on the edge.

The whole point of datums is that they are attached to fixturable features for fabrication and inspection. The features must be reliable. If you have a requirement for a sharp blade, fixturing to the sharp edge is just about the last thing you want them to do.

--
JHG

 

[mkcski]

I totally agree wiht drawoh. Datums are the orgin of measuerment and MUST be accurate and repeateble (reliable). Consider that the datum features on the part, when the part is resetup for machining or inspectoin again and again, must estabish the same reference frame. Many times a reinspection report will show the location of a hole (for example) changing. But the hole dose not move in the part - only the origin of measeurment moves.

Certified Sr. GD&T Professional

 

[semiond]

I agree with both of you in 100% about the lack of reliability of edges for fixturing.
However if two edges have to maintain an accurate geometrical relationship between each other from whatever reason, one edge could be used as a measurement reference for the other, and the inspection (including tangent line simulation for the datum edge) could be done without contact or fixturing. I mentioned microscopes and optical comperators, but there are also image processing measurement machines, laser scanning CMMs etc.

But let's say edges shouldn't be used as datums. Can they still be legally controlled for orientation in relation to planes or axes derived from adequate datum features?

 

[drawoh]

semiond,

Let's look at ASME Y14.5-1994 Fig.4-4 (ASME Y14.5-2009 Fig.4-7).

The primary datum[ ]A feature is the bottom face. The secondary datum[ ]B feature is the long edge, and the tertiary datum[ ]C feature is the angled edge. All three faces can be inspected for flatness. The secondary datum feature can be inspected for perpendicularity from the primary datum feature. The tertiary datum feature can be inspected for perpendicularity from the primary, and for angle from the secondary datum feature. That's it! Everything else is correct, by definition.

You can put a radius on the intersecting edge, and specify a dimension tolerance or a profile. If you need an actual sharp edge, you can write a note specifying that something like 95% of the edge must be intact.

--
JHG

 

[semiond]

The secondary datum B feature is the long edge, and the tertiary datum C feature is the angled edge

drawoh,
In fig 4-7 in 2009 all 3 datum features are faces (surfaces). Not edges. Just to make sure i understand you correctly - I think you meant to say that the surfaces that intersect at the edges, are the ones that should be selected as datums and controlled, is that right?

If a sharp edge is needed, and is inspected for chipping, and it is a functional element in the part (not for assembly), can one still specify an orientation control directly on it?

Edit: example
Link
If i was dimensioning this, i would want to specify straightness, and angularity (edit: in reference) to the handle axis, directly on the cutting edge.
The edge of course would also need to be of sufficient quality.

 

[drawoh]

semiond,

It is legal to select a sharp edge as a datum, but is it a good idea? Sharp mechanical edges are fragile, as are the people who come in contact with the sharp edges. Probably, you don't want them touched or fixtured to. When you specify datum features, you are specifying fixturing points.

If you are going to touch and fixture to the edge, you need to specify a radius or chamfer. If you specify a chamfer perpendicular to the primary datum, you have a manageable and conventional drawing. You need to apply controls to the angled face. With any other approach, the size of the radius or chamfer is critical, and must be made very accurate. The figures in the standard totally make sense to me.

--
JHG

 

[drawoh]

semiond,

If that blade were my drawing, the datums would be...

[ol A]
[li]One face[/li]
[li]The end that faces down into the handle[/li]
[li]The width of the tang that extends into the handle[/li]
[/ol]

The edges that extend out of the handle are not dimensionally critical. I would apply fairly sloppy profiles. I would then find some way to specify the sharpness of the blade. Note how the profile of the sharp edge is very much less critical than the actual sharpness.

--
JHG

 

[TheTick]

recommended or not - is a separate question

No question about it. Bad idea.

 

[semiond]

drawoh, I know it is very silly to quote my own messages, but... June 8:

But let's say edges shouldn't be used as datums. Can they still be legally controlled for orientation in relation to planes or axes derived from adequate datum features?

The main subject of my post was never specification of edges as datums - that was the subject of the old thread i referred to. It is more about the use of geometrical tolerance controls (feature control frames) directly on edges, as opposed to applying them on surfaces, like always shown in the standard. I'm sorry for not making it clearer earlier.
I appreciate your explanations and i'm totally convinced that edges shouldn't be used as datums (at least not in a sense of physical fixturing elements. Non-touch measurement reference is another story, but i'm no longer trying to lead the discussion in that direction...)

About that blade, i was raising the question if it is legal according to the ASME standard to use angularity to control the cutting edge (not the handle protrusions) with reference to any datum that you find appropriate: for example- one of those 3 that you suggested, or the axis of the handle at assembly (or if it was a one piece part) as i initially meant.

The reason why i find this question relevant, is because chapter 6 in ASME Y14.5 2009 includes definitions that only mention a surface, a center plane, or an axis as the subjects to be controlled for orientation. If we look in other chapters as well - not one single paragraph or a figure deals with application of feature control frames to something other than:
1. Surfaces
2. Center planes
3. Axes
and finally:
4. Line elements - only the ones that constitute separate "samples" of a clearly visible surface (and as a side note- yes, i'm fully aware that if zoomed in enough, any physical "sharp" edge appears as a curved surface, but they are not presented that way in drawings, and they can definitely be identified as "lines" that can fall into a given tolerance zone when inspected with adequate measurement equipment).

Sorry again for not forming my questions clearly enough earlier, and thank you for the input.

 

[drawoh]

semiond,

You need geometrical controls to define an acceptable outline for your sharp edge. You need some control to define the sharpness of your edge. If it is a cutting blade, you may have to define a cutting test. Otherwise, you are defining some maximum radius, inspectable with a microscope(??).

In the case of a small radius, there is a significant difference between the outline of the radius and the intersection line of the two planes. The geometry of a part like this is not simple.

--
JHG

 

[semiond]

I know that i need them (geometrical controls) to define the edge geometry, but i'm not sure the standard allows me to use them, from the reasons i mentioned.

Yes, usually for cutting edges, the "edge treatment" defines a range for the radius to be produced on the edge, that can be inspected with microscope, or more advanced scanning equipment that doesn't drag with it the human eye error factor.

 

[pylfrm]

semiond,

ASME Y14.5-2009 does not appear to define the meaning of a geometric tolerance applied to a sharp edge on a drawing. If you wish to do such a thing, it would be up to you to define the meaning.

I'd be interested to hear how you'd do that.


pylfrm

 

[semiond]

pylfrm,
Good question. Hope my answer will be good as well. The edge must fall into a tolerance zone that is limited by two parallel planes (if it's needed to explain why not lines- ask and i will) the distance between which is x (mm/") apart, and that have the exact geometrical relation (orientation, location, or both) with the datums specified in the FCF, or in case of form tolerance (straightness) no relation to any datums.

So far everything is in the spirit of the standard, but there are 2 nuances:

1. Unlike in the case of control of surfaces, it is mandatory to add that the 3D tolerance zone always applies to the drawing view on which it is shown (the limiting planes must be parallel to the viewing direction). Edit: this is true in the case of straightness control or orientation/location when the datum is an axis.

2. Even in the case of the use of profile of a line control or straightness control applied to an edge, the tolerance zone must be 3 dimensional (between planes) rather than two dimensional (between lines).

I suppose that i kind of answered my own question by listing the above difference from, and addition to the standard definitions. I guess that since it's not there, unfortunately it is not possible to apply feature control frames to edges (unless i'm missing something, and i wish i am...)

 

[pylfrm]

semiond,

To have a really robust definition, I think you would need to define the set of points that constitute the edge, and thus are subject to the tolerance.

I'd also note that view-dependent tolerances have some drawbacks. It might be better to come up with a scheme based entirely on datum features.


pylfrm

 

[semiond]

pylfrm,
Thank you for the input. I'm gald this thread is still alive

What is the advantage of using points if the entire edge should be controlled?
Wouldn't it make the sections between the points less controlled?

Also, could you elaborate a bit more about the datum features scheme that you are suggesting?

 

[pylfrm]

semiond,

I had in mind an infinite set of points, so I don't mean to suggest that the entire edge shouldn't be controlled.

I haven't given it enough thought to suggest anything in particular, but I'm imagining a scheme based on an idealization of an optical comparator. The projection direction would be defined with respect to the datum reference frame of the tolerance. This might be getting a bit too complicated for a reasonable drawing note though.


pylfrm

 

[semiond]

pylfrm,
I think i understand where you getting at regarding the projection direction related to the datums, and i agree.

Regarding the infinite amount of points, this is probably something more related to inspection methods than drawing definitions (am i right?).

I wish i could find some example in literature or standards where something similar is practiced. So weird that the subject of sharp edge geometry is totally ignored (as it seems). Even if the general opinion is that surfaces are more important than edges, it would be fair to leave the option for the designer to make his choice whether to apply controls directly on an edge or not, according to each unique application demands.

 

[pylfrm]

Regarding the infinite amount of points, this is probably something more related to inspection methods than drawing definitions (am i right?).

It's related to tolerance definition. For a similar example from ASME Y14.5-2009, consider a straightness tolerance applied to a derived median line. The following definition is provided:

derived median line: an imperfect (abstract) line formed by the center points of all cross sections of the feature.

Without this definition, the meaning of the tolerance would be completely unclear. Unfortunately, "center point of cross section" is not defined, so some ambiguity remains.

If you want to define a tolerance that requires an edge to fall within some tolerance zone, it would be good to define exactly what 'edge' means. Not everything is a sharp as a razor blade.

Even if the general opinion is that surfaces are more important than edges, it would be fair to leave the option for the designer to make his choice whether to apply controls directly on an edge or not, according to each unique application demands.

Many of the existing tolerances are defined in terms of surfaces. It could be quite an undertaking to add a separate set of definitions for use when those same tolerance symbols are attached to edges. It's not so much a matter of leaving the option open, as creating the option in the first place. I'm not terribly surprised it hasn't happened yet.


pylfrm

 

[semiond]

pylfrm,
A straightness tolerance for example has two different meanings - one when applied to a surface, and the other when applied to a median line of a surface of revolution.

Similary, other tolerances could have separate meanings depending on application to a surface or an edge.

But i understand what you are saying. Perhaps for that to happen there should be a sufficient demand from the relevant industries.