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]

Sem,
Pardon my cryptic feature names and the lack of a drawing. Feature D does not mate with anything in my example, only the edge between the drafted feature "D" and the close to perpendicular feature "ND". Because the face of feature D is not functional, the preference would likely be to control the edge directly.

I think we may be on the same page on this. I have talked to others who agree, but it seems that there's some differing opinions on this forum.

Dean

 

[Sem_D220]

Dean,
So actually the edge of the molded plastic part is the mating/locating feature in an assembly (I now reread your description and I have absolutely no idea how I missed it the first time).
This serves the purpose even better than I thought - I don't think there are any proper alternatives to the PROJECTED EDGE scheme in this case.
I'm glad that there are others who agree, and I can only express my support and encourage you to promote the stanadardization of this method.

 

[3DDave]

As I mentioned before - cutting of metals is primarily a shear operation. The edge does not slice the metal; the removal is entirely done with with cutting face.

From Purdue Engineering, Published on Jul 18, 2018

https://www.youtube.com/watch?v=gjwPAgFAQUE

The video shows how intentional surface contamination can significantly lower cutting forces, surface contamination which never contacts the cutting 'edge.'

 

[Sem_D220]

3DDave,
I'm sorry to disappoint you, but what forms the cut surface on the workpiece is the cutting edge, not the face. The chips disposed during the machining process are what slides over the face. You can see it clearly at 1:55 at the video you linked to. We care a lot about the chips and about the exact way they are removed, but we care not less about the cut surface which remains on the machined part. That's why we want the cutting edge geometry to be precise and directly controlled in the production of the cutting tool. As for the shear zone, it starts at the cutting edge, and ends at the external surface of the workpiece. The shear zone is the area where the uncut material starts to deform and become a chip on it's way to be disposed, after being approached by the cutting edge.

Edit: since you are such a suspicious guy and obviously will not take my words for it, for the sake of putting an end to this argument I present you the "proofs" you always demand:

These are excerpts from ISO 3002/1 - "Basic quantities in cutting and grinding, part 1", chapter 3, general terms: