EDGE RETENTION/ROLLING PART II

[grepper]

Meaningless babble post:

My experiments show still air has little effect, however hold the blade against a 15 mph breeze and a butter knife is the result.

Next up: A roller coaster ride to expose the blade to increased G’s.

That said, I know Mr. EOU has let a DE razor blade simply sit overnight exposed to air and the result was a less sharp blade the next day. I believe it was assumed this decreased sharpness was due to oxidation, but now we know that it was probably rolling due to air pressure. Thanks for clearing that up Mr. Jan.

[Bud]

Put enough PSI with it and i bet it would roll.

[Jan]

I have suggested to use for the refined rolling experiment the weight equivalent of three cubic feet of air because it is circa 100 grams.  

Jan

[grepper]

Does three cubic feet of compressed air weigh that same as three cubic feet of uncompressed air?   I was just thinking that a three cubic foot weight on top of the SET would be a bit bulky and that making it smaller might be more workable.

[Jan]

Yes, Mr. Grepper, the density of dry air is directly proportional to the absolute air pressure.

Jan

[EOU]

Third Section Results:

These tests were conducted with our HRC58 and HRC62 A2 tool steel. These samples are "chisel ground" at 30° with no grinding conducted on the back side of the blade. These hardened samples were sharpened with a 180 grit ceramic belt and then deburred. 

Each edge was tested at two different points yielding two sets of data for each sample tested. These tests will be of particular interest to our industrial customers who use "skives" for trimming operations. It has been our observation in the past, from customer samples received, that this style of blade usually shows rolling toward the backside of the blade. It was not clear whether this is due to to how the trimmed material is approached or if the geometry of the grind simply lends itself to this behavior. Here is a picture of the samples tested:

                                               

As you may recall, the SET tester roller is canted at 10°. This gives us the capability of forcing the roll in a predetermined direction. In this case each test section of the blade was forced in a different direction. In one section, toward the bevel and in the next test section, toward the flat or backside of the blade. We took initial sharpness readings at two locations on each edge and then conducted SET tests on both sections. This was done for both blades producing a total of (4) sets of test results.

                                               

Here then are the results of those Phase I tests:

                                              

It seems to be clear that skive blades are following the "path of least resistance" when rolling.

[grepper]

Well, that makes sense. There is less support on the flat side.

Once again, steel hardness was inconsequential.

[Bud]

It makes sense to me. I looked at a couple of my old wood plane blades a while ago at work under the scope. Both were rolled in places toward the bottom side of the blade. They weren't rolled all the way across though like a knife blade. Just mainly toward the corners. I wonder if first the corners roll under and then the middle doesn't touch much.

[Jan]

EOU, thanks for posting the results of the third section.  

It clearly supports the intuitive expectation that a blade with chisel grind will roll towards the less supported flat backside of the blade. 

Similarly to the results of the second section we see that the POST SET sharpness almost does not depend on steel hardness. It is hard to digest for me, and it brings us back to the question about SET weight suitable to resolve this riddle.

Jan

[EOU]

Yes, we think that the chisel grind results are intuitive, as well, Jan. That's a nice departure from some of the other indicators that we have uncovered. It also seems to indicate that bevel angle must matter. The hardness versus roll rate results are not intuitive in our estimation but that intuition was based in what we thought we knew about sharpened edges i.e. harder steels are more brittle so should therefore be less prone to edge rolling and, since there is no free lunch in the physical world, more prone to fracture and chipping. According to our results, other laws of physics have inserted themselves into our intuitive equation. Right now, your suggested theory of "foil effect" is the leading interloper. Speaking of foil, we offer the following strictly as food for thought;

If we think in reverse on this topic we know that very thin sheets of many metals, foils, even hardened versions, can be formed, creased and folded flat practically as if they were wet paper. We would not expect .125" thick versions of these same metals to have the same attributes. We might even expect the thicker .125" versions to split and fracture when forced into tight bends and radiuses. So should we really be all that surprised that very thin edge apexes might not act exactly as the thicker metal they emanate from?

We're somewhat on the fence regarding the amount of force that the SET unit exerts on the edge apex, Jan and for a couple of reasons. The current design seems to produce "real use" kinds of roll characteristics. If the current design accomplishes this then we ask "why be more gentle with our test edges than real use itself is?" Here's the second reason; it requires multiple passes over the same point with the current SET force used (150 grams) to produce significant rolls. In fact it may take a total of some 50 -70 passes over the same point until maximum roll has been achieved. This indicates to us that we are probably not setting thumb tacks with a sledge hammer at 150 grams of force. 

This all brings us to our next test. We think that Phase I SET results need to be given some relativity to ordinary use. In other words, how many carrots do we have to chop before the edge has been rolled 100 points? We think that this will be our next experiment and one that anyone with an edge tester and a carrot or onion can participate in.