Stroke Direction

[Eamon Mc Gowan]

Very interesting, I would have thought that edge trailing would have given more teeth. For some reason my natural assumption was that edge leading would flatten the peaks (teeth) more but I guess it doesn’t.

But teeth only help with slicing, not push cutting. How does your sharpness testing Jig work? Does it press down flat (no slicing) or does it move the knife in a sawing motion? Sorry if that’s been answered elsewhere, I did look at the specs for the sharpness tester but to be honest it was a bit above my head so I gave up 🙂

I was assuming more teeth from edge trailing as well which is why the images surprised me. The sharpness testing jig works by applying downward force on the blade. There is no slicing motion. I’m thinking that the force is being concentrated on the tallest of the teeth, resulting in a much greater force per distance of blade on the peaks and rupturing the tape more easily. With the smoother blade from the trailing strokes, the force would be more evenly distributed requiring greater force to rupture the tape. [/quote]

Clay,
I am still waiting for the triple beam to be built for my jig. My collar is on a wood dowel that I apply the pressure to. To support your theory? All of a sudden the knife being tested was giving readings of about 10 grams and I was like what the heck? What I had one is slipped down into the serrated part of the blade. The serrations took almost no pressure to cut the tape? I think you are absolutely right that the peaks and valleys make it easier to cut the tape. Hence a “sharper blade”?
The pics are fantastic! I too was wondering what the edge leading vs edge trailing looked like? You do some great testing! Thank you,
Eamon

[wickededge]

I decided to check what a little stropping would do to the Edge Trailing blade so I completed 20 strokes with the 14um strops and ran a test series. There was a 12% performance decline after stropping so I took it to the scope:

14um Diamond on Cow Leather Strops after 100# Diamond Plates – Edge Trailing Strokes 2000x Magnification

I also tested it with both push cutting and slice cutting through copy paper – not much difference that I could tell. I then checked it for shaving and that’s where I saw a large difference. It shaved significantly more easily than either the Edge Trailing or Edge Leading blades.

For reference, here is a nice shot of a blade I stropped down to .25um on kangaroo leather:

.25um Diamond on Kangaroo Leather Strops – 2000x Magnification

Attachments:

• h4f4f8bf.jpg
• h85cae72.jpg

-Clay

[wickededge]

I decided to run a few tests on the .25 kangaroo stropped blade since we were looking at edge straightness vs. point sharpness. It shaved amazingly well, push cut copy paper like nobody’s business and would hardly slice cut the copy paper. In the sharpness jig, it required 27% more force to cut the tape than the Edge Trailing 100# blade and 30% more force than the Edge Leading blade.

-Clay

[Josh]

I decided to run a few tests on the .25 kangaroo stropped blade since we were looking at edge straightness vs. point sharpness. It shaved amazingly well, push cut copy paper like nobody’s business and would hardly slice cut the copy paper. In the sharpness jig, it required 27% more force to cut the tape than the Edge Trailing 100# blade and 30% more force than the Edge Leading blade.

Man… That is really strange, exactly opposite what I would have thought. Any ideas as to why?

[Phil Pasteur]

I have been following this thread…and the original on the sharpness tester. Just have had no time free to respond. Still slammed at work…

Anyway I am finding this very intriguing. I think we will get back at some point to discussing the whole concept of “what is sharp”. I think Clay’s idea of the concentration of force on the teeth making the readings different makes very good sense to me.

I, personally believe that I would prefer the Kangaroo .25 micron stropped blade if I was looking for something that I consider to be sharp. This does not mean that I don’t think that there is a set of circumstances where an edge with some tooth might be preferable, but if we are trying for the most refined edge possible … an edge that is as fine as the steel will support, a toothy edge is not it!

To me, this calls into question the general utility of the tester for determining what is sharp, at least using my definition. I wonder how an obsidian scalpel would test. Maybe not as well as a 100 grit leading stroke blade… but really, which is sharper??

Of course, I could see where it could help in judging the relative sharpness of blades sharpened using the same set of techniques and grits… perhaps answering the question “am I doing this correctly with the best technique for the grits I am using”. But it seems at this point that the absolute numbers do not necessarily agree with what I am looking for when I am trying for the “perfect” edge…

I am sure there is more to come… but that is what I am feeling right now.

[Phil Pasteur]

Man… That is really strange, exactly opposite what I would have thought. Any ideas as to why?

Clay said… a few posts above:

There is no slicing motion. I’m thinking that the force is being concentrated on the tallest of the teeth, resulting in a much greater force per distance of blade on the peaks and rupturing the tape more easily. With the smoother blade from the trailing strokes, the force would be more evenly distributed requiring greater force to rupture the tape.

I think it makes sense. Ever think about why an awl is pointed… or a needle, or a nail ?

[Josh]

Oh yeah, that definitely makes sense… I wasn’t referring to that part of what clay said, but rather where he commented:

“I decided to run a few tests on the .25 kangaroo stropped blade since we were looking at edge straightness vs. point sharpness. It shaved amazingly well, push cut copy paper like nobody’s business and would hardly slice cut the copy paper. In the sharpness jig, it required 27% more force to cut the tape than the Edge Trailing 100# blade and 30% more force than the Edge Leading blade.”

I am curious why the post stropped 100 grit edge requires more force to get through the tape than the pre stropped 100 grit blade… seems to me like the strops refined the edge and made it “thinner” or removed any burr, thereby both burnishing and slightly abrading both the peaks and valleys.

It looks like from the photos that the edge has actually been sharpened on a stone or two higher than 100 grit, which looks like it is more refined and not as wide.

[Mikedoh]

Wonder if because the less refined edge has more peaks and valleys . . . That for any given linear measurement, the material to be cut comes into contact with a greater surface area of blade. Would this then act as almost a moving/ slicing blade?

Not certain i got my thought across.

[cbwx34]

I decided to run a few tests on the .25 kangaroo stropped blade since we were looking at edge straightness vs. point sharpness. It shaved amazingly well, push cut copy paper like nobody’s business and would hardly slice cut the copy paper. In the sharpness jig, it required 27% more force to cut the tape than the Edge Trailing 100# blade and 30% more force than the Edge Leading blade.

Be interesting to see how long it is before someone (somewhere) spins this into… “See, coarse edges are sharper.” :S

I wonder if either… increasing the area the blade contacts, or increasing the amount of material being cut would help solve this?

A bit ironic that a device that essentially measures push cutting determines slicing ability. :silly:

[Jamey Howard]

A bit ironic that a device that essentially measures push cutting determines slicing ability. :silly:

Well that’s what I was thinking. The results from this test suggest that a coarse edge should push-cut better but we all know that’s not true. However I suppose part of the problem is that we’re not specifying how much material is being push-cut.

When you slice paper or shave arm hair you are using a portion of the blade that is tiny, only as wide as the width of a sheet of paper or the width of a few hairs. So any teeth are irrelevant because the paper (or hair) will go between them.

But if you were to push-cut material wide enough to contact the whole edge of the blade then maybe toothy edges really do cut better.

But normally you don’t push-cut wide objects, you slice them. And teeth are definitely useful for that.

[Phil Pasteur]

A bit ironic that a device that essentially measures push cutting determines slicing ability. :silly:

I would expect there to be a significant positive correlation between push cutting ability and slicing ability of a blade. But it pretty interesting to see more of a negative correlation between reading provided by the tester and slicing abilities.

But more than that, I am disappointed that results don’t seem to follow the “real” level of sharpness of a blade (real being my definition, of course:) ). I think that tests such as tree topping arm hair or the hanging hair test are really (the way I do them) push cutting tests. I can say that I have never gotten any 100 grit sharpened blade to pass either of these tests. The machine seems to be telling us that the 100 grit blade is sharper.

I am not buying into the tester’s definition of sharp at this point.

I am still tempted to build one to test knives sharpened with like grits using perhaps different techniques. If there is no large disparity regarding peaks and valleys, or tooth, it may be possible to measure to a useful extent, how the differences that techniques, such as applied force, angles, and number of strokes might relate to push cutting force. If we can then correlate those numbers with real world testing this device might prove to be of some actual value to me.

I would be interested to see Clay do some testing along these lines, as time permits.

[Phil Pasteur]

But normally you don’t push-cut wide objects, you slice them. And teeth are definitely useful for that.

This last is dependent on what you are slicing.

I wouldn’t want my surgeon cutting into me with a scalpel sharpened with 100 grit diamonds…

[Jamey Howard]

Well… You could argue that the blade might actually slice quite well, but because the object being sliced is capable of feeling pain and will still be alive after the cut, with the tissue needing to heal, optimum slicing is secondary to a very clean incision.

But I feel like that might be a bit of a tangent to the main discussion.

[wickededge]

I love how this machine has opened up a wide ranging conversation already and we’ve barely just dug into using it.

One thing I was toying with at the end of the day yesterday was using the y-axis to test slicing motion. Since I incorporated the compound table into the jig, I can easily do this. The tool head provides downward force, so one force vector is taken care of there. Advancing the blade along the y-axis (while the tool head is pushing down) takes care of the other force vector. I can apply a preset amount of force down onto the blade, advance it along its axis to see if it is able to cut the material. If the material isn’t cut, I can add more force and try again. I imagine we could also test different materials on the tool head. I know that real world slicing motions are more complicated, but this still may be very helpful I think.

-Clay

[wickededge]

Seems we’re always going round and round with the definitions which I think is why Dan was so specific with the definition for the kind of sharpness this jig tests for: Apex Point Sharpness. He goes into in some detail on the site he created for the jig and includes some good quotes and references. Here is the link: How Sharp is It – Sharpness Definition[/url]. Looking back to the teeth in the micrograph of the 100# Edge Leading blade, it’s easy to imagine how all the force being concentrated on one of those teeth would require a small total force to rupture the tape. As Phil pointed out (ha ha) in mentioning needles, awls etc… a needle will require much less force to rupture a surface than the flat cross section of a steel rod. Does that mean it push cuts better? In the couple of tests I’ve done with copy paper alongside using the jig, my results show that the toothier blades don’t push cut well, as we all probably expected. The .25um stropped blade shaved and push cut incredibly well. I was pretty surprised to find that it would barely slice cut the copy paper.

-Clay

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