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TOPIC: Stroke Direction

Stroke Direction 1 year 2 months ago #13443

  • PhilipPasteur
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razoredgeknives wrote:
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 ?
Phil

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Last Edit: 1 year 2 months ago by PhilipPasteur.
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Stroke Direction 1 year 2 months ago #13445

  • razoredgeknives
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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.
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Stroke Direction 1 year 2 months ago #13450

  • Mikedoh
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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.
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Stroke Direction 1 year 2 months ago #13461

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wickededge wrote:
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:
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Stroke Direction 1 year 2 months ago #13462

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cbwx34 wrote:
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.
Last Edit: 1 year 2 months ago by JameyHoward.
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Stroke Direction 1 year 2 months ago #13466

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cbwx34 wrote:
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

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I miss you Buddy!
Last Edit: 1 year 2 months ago by PhilipPasteur.
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Stroke Direction 1 year 2 months ago #13468

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JameyHoward wrote:
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...
Phil

MAX 2001-2013
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I miss you Buddy!
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Stroke Direction 1 year 2 months ago #13469

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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.
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Stroke Direction 1 year 2 months ago #13473

  • wickededge
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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 Allison
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Stroke Direction 1 year 2 months ago #13475

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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. 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 Allison
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