Sharp vs. cutting ability ( and the Rotary Platen)

I

itrade

Joined
Jul 3, 2002
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A customer just gave me a 10 minute rant about how great his latest knife is. It's a 5" kitchen utility knife I built he now uses for everything. He recently tried another knife I sold him - larger blade that he used to think was the sharpest thing in the world - but that one doesn't cut near as fast as the new knife.

Knife 1 is flat ground, edge thickness about .020
Knife 2 is convex ground, edge thickness about .015

Due to a thicker spine I believe Knife 1 has a 'thicker' cross section.

Issue is, what created the huge difference in cutting ability!?

Knife 2 was done on the rotary platen and is ever so slightly convex. Could that be it?

Steve

If it is the convex grind, then I'm gonna have to buy one - instead of borrowing.
 
Don't shoot me, but is it too much to imagine that Knife 2 is dull?

Sharpening is both simple and complex, lots of easy ways, as well as ways to outsmart oneself. Did you get a chance to look at both knives? For instance, a knife sharpened with the same number of strokes (rougly) can get larger bevels over time, so that the same amount of sharpening work doesn't yield good results. If poor stone or stick maintenance is also happening, the knife will get seriously out of condition.

It's the edge that does the cutting, two equaly well sharpened edges sharpened at the same angle in similarly good material should cut just as well regardless of what backs them up.
 
Steve
I hollow grind all my knives and I have played with differnt edge styles,But the one that really gets scary sharp is a convexed edge
I sharpen on my platen,there is a space between the top wheel
and the platen,it gives in the spot,so my edge takes a convexed edge
and is wicked sharp.I have heared a number off makers who only hollow grind also put a convexed edge on there hollow grinds
 
I agree with the convex edge being the difference. Whether it's partial, or full, I put them on just about everything now.

Now, that said, maybe you could tell us more about the steel used and the cutting task at hand (or even the actual test the customer did).

Do you do the HT? could that be it? #1 losing it's edge quicker?

Also, could be ergonomics. Sometimes certain knives are easier to wield than others...

Just shooting ideas here...

Dan
 
Don't get me wrong - the first knife (Knife #1) is a cutter. No problems - very sharp. S30V, Pacific Heat treat.

Knife #2 is also S30V, maybe a bit harder. It was built and treated at Michael Vagnino's. This one was ground on his rotary platen, so the edge bevel is convex.

Both knives have a flat ground, polished edge.

However the cross sections are very different. The first is thicker with a flat grind. Second is thinner with a convex.

Slicing onions and it's a WHOLE DIFFERENT experience. Hence the question.

I've read alot about cross-sections, but didn't think the difference was that great.

Steve
 
gotcha.

Jerry Fisk gave a class last weekend on sharpness and cutting ability. Something he said really caught my ear. He said cutting is really "material displacement". Whatever can displace material in the most efficient way will cut the best. A flat grind is a great wedge, but will displace material only so well because the material flows in a straight line along the edge. A convex edge is much better at displacing material because each mm of the edge is pushing the material away, hence, making more room for the cutting part of the edge to keep moving.

It's a pretty convincing argument. I wish I could replicate in a way that was more understandable (ie. pictures, etc.)...oh well.


Anyway, it's my experience that a well-polished convex edge is the best at push-cuts - which is exactly what you're doing when you chop the onion.

A flat grind "chokes" the cut because the material is not being displaced quickly enough. Make sense?

Dan
 
Yeah Dan - make sense.

Maybe you can descibe it as a drag (friction) issue. A flat grind the material is in contact with the steel longer.

One point tho. Both knives have a flat edge grind. It was the bevel that was different - and edge thickness.

Wow, you got to go to a Fisk class? Wish I was there.

I might have to buy a rotary platen. Actually I was hoping some rotaty platen guys would chime in.

Nathan - I'm gonna try a few convex edges just as you said. I've tried some, but not using that spot on the grinder. Sounds very interesting.

Steve
 
i have a rotery platen and its the best thing since electricity
sharpens knives VERY well too
yes the convex cuts best
i say that not w a question but as a statement
harley
 
Well, we lost track of my original question. I'm sure I didn't explain it well enough (I was interested in the bevel grinds, not the edge grind).

Be that as it may, you guys prompted me to try something new. I'm gonna re-sharpen some knives this weekend. I'll use that spot between the platen and the wheel - like you said Nathan.

BTW How do you remove the burr? On a new knife I go:

220
A45
A30
A16
A6
Green Chrome on a buff.

Steve
 
I've seen a few take it off at the buffer. I get I do that too, in a way.

But buffing is not the last step. After buffing, I go back to a fine micron belt and then strop like crazy. That seems to get a nice polished edge.

I should mention that I also acheive the same results by handrubbing with 220, 400, 600, 800, 1200, and 2000 grit paper, followed by a stropping. Just let my finger slide gently over the edge and sand along the edge, not across it.


Dan
 
Originally posted by itrade
Well, we lost track of my original question. I'm sure I didn't explain it well enough (I was interested in the bevel grinds, not the edge grind).

Be that as it may, you guys prompted me to try something new. I'm gonna re-sharpen some knives this weekend. I'll use that spot between the platen and the wheel - like you said Nathan.

BTW How do you remove the burr? On a new knife I go:

220
A45
A30
A16
A6
Green Chrome on a buff.
thats a good one
there r a bunch of combos
some cut some things better thna others
i grind and get an edge at the same time
its sharp
harley
Click to expand...
 
Convex is all very interesting, but it isn't anything but the edge that is doing the cutting. Onions are pretty stiff, if you have some part of the knife higher up and it binds, that's important too, but it isn't to do with cutting.

Razors are some of the cuttingest edges I've seen, significantly concave.

If you sharpen on a belt sander, there may be some knives that sharpen better on that tool, certain shapes, others might require more relief not to scratch or whatever, that's all circumstantial.

Almost all woodworking tools (just another vegetable medium) are either flat ground or concave. Some like drawknives and axes tend towards the convex. But overwelmingly, a lot of powerful cutting is done with flat and hollow edges. One thing though, is that an overall 25 deg edge, hollowed out, can cut significantly finer, but for the same angle at the edge, and the same degree of sharpness, it's basicaly the same.

Take two edges that are both ground to the same angle, but one is flat and the other hollowed. The hollowed edge will get more stone bite, and therefor have deeper scratches, all being equal. Just another example of how things that are the same can perform differently, but get the same edge on both at the same angle...
 
Originally posted by Protactical
Convex is all very interesting, but it isn't anything but the edge that is doing the cutting.

(u r forgetting about friction
BOTH HOLLOW and flat produce more friction)



Onions are pretty stiff, if you have some part of the knife higher up and it binds, that's important too, but it isn't to do with cutting.



(it has everything to do with cutting)

Razors are some of the cuttingest edges I've seen, significantly concave.

(i assume u r talking about old ones
they were hollow ground but theres a flot before it progresses to the top of the blade and they r definately convex sharp)




If you sharpen on a belt sander, there may be some knives that sharpen better on that tool, certain shapes, others might require more relief not to scratch or whatever, that's all circumstantial.


(the knife dont know what u sharepened it on
edge is edge)


Almost all woodworking tools (just another vegetable medium) are either flat ground or concave. Some like drawknives and axes tend towards the convex.


(name some wood working tools that ar not convex or flat
never seen a hollow ground adz)



But overwelmingly, a lot of powerful cutting is done with flat and hollow edges. One thing though, is that an overall 25 deg edge, hollowed out, can cut significantly finer, but for the same angle at the edge, and the same degree of sharpness, it's basicaly the same.

(wrong!!!!!!!!!!!!!!!!!!!)


Take two edges that are both ground to the same angle, but one is flat and the other hollowed. The hollowed edge will get more stone bite, and therefor have deeper scratches, all being equal. Just another example of how things that are the same can perform differently, but get the same edge on both at the same angle



(if one is truley flat and the other hollow then the flat one will have more stone contact)


those were all good
try again:)
harley
]
Click to expand...
 
Originally posted by Protactical

Razors are some of the cuttingest edges I've seen, significantly concave.
Click to expand...


...sorry, but I've got to interject on this one since I have an interest in straight razors... the extreme hollow grind of a "full hollow ground" razor has nothing to do with its cutting ability, it's all in the sharpening angle that sets the secondary bevel. Besides, at the edge, there's the "wall" that meets at the "belly" which then proceeds to the concave section. ...very different than the hollow grind for a knife...

-Darren
 
"(u r forgetting about friction
BOTH HOLLOW and flat produce more friction)"

Exactly--and onions aren't that stiff, especially cutting thin slices.

You want to see how important friction is, cut some cheese with anthing but a cheese-plane or a wire cheese-cutter.

Anbody ever seen a ship with a bow shape that corresponds to flat or hollow? OK, you say "hollow, sure!" Those hulls are designed to get the hull above the water, the opposite of cutting.

Razors, I don't cook with hair, except by mistake.:)
 
Larry
I seen you at the Moran Hammer in but did not get to speak with you,I really want to talk Jay Darsey,Hogs and Pit Bulls.
Nathan
 
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