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.

 

[DeanD3W]

Hi Dave,
I'll try this again.

If the intersection between two surfaces on a CAD body is sharp, and it is desired to apply a tolerance zone to that sharp edge for which the boundaries have a particular orientation where they do not lie about the true profile of either surface, then Y14.5 does not support that tolerance. The fact that the tolerance is not a typical profile of a surface must be noted by some means. The orientation of the tolerance zone boundaries must be defined by some means. The proposal is that "PROJECTED EDGE" be placed adjacent to the feature control frame, and any additional notes to explain this non-supported tolerance will also be added. By doing this, the fact that the tolerance is not a normal profile of a surface is clearly indicated, and the projection angle can also be explicitly defined using a note, unless the user deems the projection direction to be clear without the addition of a note.

The fact that there is not the same discrete edge on a real part is not relevant to this discussion. This discussion is about tolerance zone boundaries and their orientation and location with respect to a datum reference frame. A discussion about the specifics of how to verify conformance may be a separate thing to talk about. The simple approach would be to use a vision measurement system to verify that the projected edge is within the tolerance zone.

Best Regards,
Dean

 

[semiond]

3DDave, first of all - I wasn't "making the claim that a change is required", at least not until other people pointed it out for me that the subject I was asking about is not covered by the standard. I was merely asking a question - about whether or not edges can be controlled similary to surfaces. After the interaction here, I am currently convinced that doing so without a note and a suplementary definition is indeed extension of the rules. "Edges" are not "surfaces" no matter how many microns their radii are. If they are to be treated like surfaces per the current definitions - they should also be shown as surfaces on a drawing. Since they are shown as lines that constitute in theory nothing but intersections of different features, they can't be used for establishing a fully defined tolerance zone limited by parallel planes or curved boundaries disposed about a true profile. It will also make no sense to show a magnified detail in a scale of a hundred to one, of each edge that needs a control, to show it as a radius. The "PROJECTED EDGE" modifier can can solve the problem and is perfectly corresponding to what's currently practised - adding legitimacy and making the definitions unambiguous.

As for your "interesting question" about my industry - it is interesting indeed. What's interesting is what makes you make the exaggereted and detached from reality conclusion that we don't use GD&T at all. I was only mentioning a fact that some drawing definitions, that use geometric tolerance controls and are applied to cutting edges, are not defined by the standard. To change that, nothing like the "conversion of convention" you talk about is needed. Only good advises from people like Dean and perhaps an addition of a modifier to the standard - which will be very welcomed.

Edit: Dean, sorry if it looks like I "recycled" some of the concepts from your last post. It was posted while I was writing mine.

 

[3DDave]

Dean,

The orientation and location of the zone is still set by the datum references and the view the profile is dimensioned in. This is generally no different than any other tolerance with a DRF**. The zone for the tiny edge is just the same as for a deep one. Do you envision a case where the profile of a surface tolerance is smaller than a half micron, such that the entire surface does not lie in the tolerance zone?

I believe that any dimensioning and tolerancing discussion that does not involve real part geometry is not suitable as a standard worthy item. Are you willing to reject parts where the surface at the edge of a part is not smaller than an atom? If not, why not?

Moreover, if I ask how wide a razor blade is, would you refuse to even try, claiming it's impossible? I'd say a razor blade width is a feature of size; certainly it also has a profile? What does a caliper or micrometer contact if one assumes there's no surface?

Anyway, it seems less like a 'projection' than 'silhouette,' the typical description of the grazing rays making the barest contact with the extreme surface of the part without regard to depth. The calculation of silhouette is a typical computer graphics operation in industrial design and mold manfacture - particularly to find a curve on a surface that is identical, when viewed from the same orientation, as the silhouette one obtains from a shadow. A profile of surface tolerance zone is already a projection of the tolerance zone for the full depth of the part so there is ambiguity in using "projection" for a specialized sub-context.

If you want to go all in I certainly want to move the entirety of 14.5.1 into 14.5. Everyone using the standard needs to understand spines. And I want all inspectors to learn how to use matrix multiplication to concatenate transforms, and the Jacobian of those transforms, in order to understand how changing the DRF basis for a feature changes its virtual condition. If a typical inspector doesn't know the difference between the dot product and cross product maybe they have no business in the business.

But unless that happens, let's just keep it simple and obvious and recognize that profile of surface applies to even really narrow surfaces already. and that real surfaces will always exist at acute approaches between other surfaces.

On a side note, did VSA incoporate all the Voelcker datum modification material from '2009? It's been a full decade since it was clearly going into the standard. I'm pretty sure it's not in a lot of CAD systems anyway. I liked using VSA more than the TI-Tol/Raytheon/AD-CATS hack, even though the latter evaluation is more in line with the Jacobian understanding. I prefer the Monte Carlo method that more closely duplicates the sort of variation one sees in discrete item production.

**semiond - here's your chance to find a counter example. I don't care. The statement is a reflection of the typical drawing practice, not a statement of exhaustive exploration of any misinterpretation possible. I certainly don't care for the MBD group and their desire to eliminate view-dependency problems - they have their own standard and can do as they please to make up for the shortcomings in the original STEP specifications.

 

[3DDave]

semiond - you clearly stated "Those are also specified in drawings, mostly without any care to "legality" and correspondance with the definitions of any standard." If they are used incorrectly they are not really using them.

That's the reality - they are, your words, being used outside any standard. Which means they are not standardized. Which means that to change that to correctly use a single interpretation from a standard requires re-education.

Maybe it's just your factory that has this problem? If so, you could just use the same descriptions that big, well known makers of cutting tools use, which you would be asking about in the Machines and Machining Engineering forum. That would be the most sensible as that's where potential customer issues would be discussed.

 

[semiond]

3DDave,
Since you expressed an opinion that the current definitions in the standard (such as Profile) can be used on edges, then everything is according to the rules, isn't it? Why a re-education is needed then?
I was the one expressing concern about legality - but if opinions are split regarding that, it's a good sign that no re-education or far-reaching changes are required (and I suppose that you now imply the opposite to indicate that my query is pointless). What is needed is only slight modifications of the existing practises, preferably accompanied by a corresponding minor update to the standard - like Dean suggested.

And believe me, I've seen enough drawings by the big names to know what they are doing.

 

[3DDave]

semiond - You leave unanswered your own statements that no one in your industry is following any rules.

Do they need training or not?

Surely you are proud enough to give a link to your company and its products. I'd like to see the documentation your company provides to its customers.

 

[semiond]

3DDave, I never made such a statement regarding the entire industry. I did say that some specifications (the ones related to sharp edges) which are practiced in the industry are not supported by the standard. You can keep quoting my previous posts and twist the meaning as you wish, but even if this is not the exact phrasing I used - no need to take my words out of their obvious context.

Do they need training or not?

I don't think that there is one single industry or company where they don't need to improve their skills. That is of course, unless all the employees of that company are 3DDave, who knows much better than the commitee members who write the definitions, let alone other users of the standard.

I'm not participating here as representative of my company, but from my own personal interest in GD&T, at my free time. Therefore posting the links you demand from me here is both pointless and inappropriate, and it's not a matter of pride.

 

[DeanD3W]

3DDave,
A profile tolerance zone is based on the true profile of the feature that it is applied to. There is absolutely no support in Y14.5 to have a profile tolerance be other than following the true profile of a feature.

The definition of a feature is vague, but definitely does not include the edge between intersecting surfaces.

To do what semiond would like to do, then the PROJECTED EDGE modifier, as I have used it for about the past 10 years, with no complaints from design or manufacturing or inspection, or some other similar method is needed.

You are welcome to disagree and continue to bring up points that I don't consider to be relevant. I don't intend to respond again.

Best Regards,
Dean

 

[drawoh]

semiond,

Can you show us a practical drawing of something with a controlled sharp edge? I have proposed a couple of examples, and noted how I would specify them. You can make something up. It doesn't have to be real.[ ]

--
JHG

 

[semiond]

drawoh,
You probably remember the example I showed you in this thread a while back (the sculpture knife) you told me how you would choose the datums but I'm still waiting for your suggestion on how to specify the feature control frames to define the cutting edge geometry (angle and location)
The link I posted is still here...

 

[3DDave]

Standards wise:

"A true profile is a profile defined by basic radii, basic angular dimensions, basic coordinate dimensions, basic size dimensions, undimensioned drawings, formulas, or mathematical data, including design models."

"A profile is an outline of a surface, a shape made up of one or more features, or a two-dimensional element of one or more features."

The argument that makes the most sense for new nomenclature is that there are people who think it doesn't apply to sharp edges and therefore a special term is required to placate them. Placating customers is a frequent problem to be overcome.

This is, no doubt, the source of confusion:

"Profile of a surface may be applied to parts of any shape, including parts having a constant cross section as in Fig. 8-5, parts having a surface of revolution as in Fig. 8-17, or parts having a profile tolerance applied all over as in Fig. 8-8."

where "may be applied to" is different from "is exclusively applied to" but people often read what they want to read, not what is there. Hanging onto the 'constant cross section", am I right?

Seems like controlling the extents of the top surface and bottom surface are what is desired, even when there is no theoretical side surface between them.

After that it's just a matter of defining the taper, for which there are a number of methods, and a limit to the acceptable transition between the top and bottom surfaces; most probably by specifying the maximum distance between the apparent intersection of the top and bottom surfaces and the furthest extent of material, similar to the way chamfers are specified.

Here is one source for cutting edges on metal machining tools:

https://pdfs.semanticscholar.org/1a00/9c4aacde89015ce11b31fe36246ff196bbd4.pdf

(This is going to be new for some - it would have been easy for experienced cutting tool designers in the industry to post such a document to this discussion. For some reason they chose not to.)


Here's how knife blade geometry is measured:

http://www.catra.org/pages/products/kniveslevel1/lglm.htm

Here's how knife blade sharpness is measured:

http://www.catra.org/pages/products/kniveslevel1/st.htm

 

[semiond]

3DDave,
With the dimensioning scheme you described, I suppose you were referring to the example I presented to drawoh (the sculpture knife).
I have to admit that my experience is not in dealing with this specific kind of products, but if I can rely on my knowledge from the metal cutting tools field to approach this, I have a few disagreements (how surprising) with your dimensioning scheme. Specifying controlls for the rake and flank surfaces (top and bottom) and the taper of the crosss section (even given that it's constant) without directly dimensioning the outline of the cutting edge itself , will not guarantee unambiguous and accurate definition of the cutting edge legth, "straightness" and angle relative to the axis of the handle or the feature that is used for mounting of the blade in the handle. Even if you could acheive reasonable control that way for this specific case, would you still dimension the same way if the edge was not designed straight, but was meant to be of a specific curved profile, or consisted of a specific radius tangent to a straight section of particular length and angle? I suppose that I don't have to tell you that if a feature is important and functional, it is necessary to dimension and tolerance it directly, rather than letting it's geometry be subject to tolerance accumulation. The same basic principle should be valid for sharp edges.

On the other hand, you also imply that controlling edges directly is not illigitimate standard-wise (It's not clear to me yet - is there any formulated position that you try to promote here?) profile/true profile definition may not be irrelvant for sharp edges, but in order to control them, you also need to know how a tolerance zone of a controlled edge is defined. If for example there is a sharp edge that should be parallel to a datum feature, can you tell based on the definition in paragraph 6.3.2, what will the parallelism control mean? Or based on para. 6.4.2, how will the tolerance zone look like?

It's nice to see that you actually googled for "Edge Preperation" after I referred you to that term, but can you tell me for once, how this, or the sharpness issue in general, is relevant here? And why was I expected to link for such sources? Has anyone in this thread requested this information?

 

[3DDave]

semiond, it was not my intention to elicit any response from you. Presently your problems are entirely of your own making, having nothing to do with my investigation into the topic. It was not you that brought any diagram of cutting geometry to the discussion. For all I know, a co-worker of yours mentioned 'edge preparation' to you yesterday and you passed it along. I've looked into this before in terms of finite element analysis of the shear driven flow during metal removal and the fact that the edge not only isn't really sharp, it is a disadvantageous to even try. Doesn't matter. It shows how experts define cutting tools and they use more than rake and flank to describe their (ready for it?) profile.

I do not imply anything. I clearly stated that the perimeter of the part can be controlled by profile and the edge characteristics by controlling the individual faces. The exact control depends on the exact problem at hand. In the hobby knife example one could show a cross-section and use angularity with a basic dimension controlling the angle and a notation about the acceptable geometry of the cutting surface. Or one could use profile. Or use a directly toleranced angle and flatness. One can add symmetry when appropriate if the cutter should be symmetric.

You keep looking for a magic wand solution but have not shown a single example of the exact geometry that concerns you.

I suppose that I don't have to tell you it's your own concern expressed in the topic title that makes "sharp" edges relevant.

ps - I just looked back and the Google search that found the article was "specifying sharp cutting edge design manufacturing" No edge prep search at all.

 

[semiond]

The reason why the word "sharp" is in apostrophes in the title, is that no real edges are completely sharp as in a theoretical intersection of two planes(not surfaces) - these things only exist in theory, and in drawings, where edges are depicted as lines and do not represent something with an area, chamfer or a radius. The word "sharp" included in the title, to make it clear that "edges" are meant as intersections of features, and not as I've seen this term used in this forum - to indicate end surfaces of a thin part.
Therefore, everything that has to do with "sharpness" or lack of it is irrelevant to this thread. You could understand it from the content of the posts here. For all I know the entire edge with it's radius/chamfer can be detected by inspection equipment and fit into a geometric toletance control (see para. 1.3.62) tolerance zone, which is not at the same scale as the parameters that are dealt with at edge preperation, just like it isn't at the same scale as the parameters that describe surface roughness.
If the sharpness/lack of it issue is the subject of your own investigation not related to my query (which is for the 100th time, about geometric tolerances), why do you find it approriate to bring it up in this discussion?

If I understand you correctly, you suggest the sharp edge could be specified with a Profile tolerance.

Here is what the standard says about Profile:

"8.2.2 Profile Specification

The profile tolerance zone specifies a uniform or nonuniform tolerance boundary along the true profile within which the surface or single elements of the
surface must lie."

Is an edge a surface? The standard doesn't define what a "surface" is, but it is reasonable to assume that it has to be something with significant area, certainly not something that is represented in a drawing as a line.

 

[3DDave]

An edge is the edge of a surface. The profile of surface controls the variation in the allowable extent of the surface(s) that comprise the faces, therefore profile determines where the edge of that surface is relative to the true profile. This is true whether there is a gap between the top and bottom faces or if they coincide at some location as they tend towards on a sharp edge.

There is no requirement that the faces be perpendicular to the true profile.

Sharpness is not irrelevant to the thread, even if you insist on dismissing it as such. It forms a limiting condition to the separation of the faces and seems to be the only conceptual hurdle you are unable to get across. Understand what sharpness is is also critical to describing the way the top and bottom faces are separated.

 

[semiond]

When you said that you would use profile to control the perimeter, I thought you were referring to a view where the edge is part of that "perimeter" (where the edge is viewed as a line). If by controlling a "perimeter" you mean controlling the outline of the cross section with profile, like I previously understood (where the edge is viewed as a point), than I already explained the disadvantages of an indirect specification of the OUTLINE of the EDGE.
If it is still not clear why this is not practiced, consider a geometry of a lathe form tool. The geometry of the cutting edge is copied to the workpiece when products such as ball bearing races are produced. If the edge OUTLINE will not be directly defined according to desired machined part dimensions, you are not likely to get the correct PROFILE of the surface of revolution finished workpiece.

 

[3DDave]

semiond, you can use two orthogonal profile tolerances at the same time on the same part.

Since the profile of the faces establishes the location of the sharp 'point' in the section view, there won't be a need for a conflicting locating dimension on the cross section profile. It will be aligned with the edge. Or use angularity, or flatness, or some other pleasing method. It was clear there were alternative constructions, but they are applicable to different edge requirements, so you can choose the one that works and ignore the ones you dislike.

Your 'ball bearing' lathe form tool example isn't very good; the entire working end can be a revolved section dimensioned and toleranced with a profile of surface geometric characteristic symbol in the FCF. It can have a basic radius and precisely define the full cutting geometry, including a chip-breaker if desired.

 

[semiond]

3DDave,
If you propose a dimensioning scheme where the rake and the flank surfaces will be defined, each one, in a view where it appears as a contoured surface with each line element along it parallel to the projection view, you will find that there is no such view available. Because of functional requirements regarding relief angles and also some mfg. technology limitations, often each feature of the contoured flank has a different inclination direction, and different inclination angle. In other words if you check different cross sections perpendicular to the flank, you will get different profiles with different sizes and forms. At the end of the day, what matters in order to get the desired geometry at the workpiece is only the cutting edge, because it's the one that makes the cut, not the surfaces it connects. The contour will always be measured and defined on the connecting edge of the flank and the rake, regardless if the standard supports application of geometrical controls on edges or not.
And I have no clue what you were talking about, saying the working end can be a "revolved section".

 

[3DDave]

semiond, I guess you would not use a round end cutter to cut a semi-circular groove. Sandvik sells a lot of them. Perhaps you know so much that you can't recognize anything else but some arcane phrasing, but is seems odd you are unable to name it.

Amazingly it's the face of the cutter which does the cutting by compression and shear displacement of the metal it is forced into. The edge treatment is useful to delay catastrophic failure by cratering of that face. A sharp transition between the rake and flank faces increases the active area and decreases cutting forces.

 

[semiond]

By round cutters you probably mean form milling cutters, not lathe tools. Those are not different in principle. The face under the cutting edge (flank) is gradually tilted away from the workpiece by the means of a relief/clearance angle ground on the tool/toolbit. It could be a flat face or a curved surface reliefed from the cut surface.
Rubbing of the flank face against the workpiece causes horrible vibrations and tool breakage.
If you got any tools from Sandvik that work by the principle you described, I strongly recommend you to return them and get your refund