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flat-ground VS. hollow ground ??????
- Thread starter joebe
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Few blades cut better than one that has been thinly flat ground. A hollow ground blade might be a little easier to get razor sharp, but does not have as durable of an edge. And if you take a thin flat ground blade and convex the edge on it, you cant really do much better, IMHO.
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One should never generalize... 
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"All other things being equal", adding to what Danbo said:
For shallow cutting, the hollow grind will have less metal just behind the edge, so you should expect better performance, provided the thinner, weaker edge doesn't roll or chip out.
For deeper cutting, the non-linear expansion of the hollow grind will significantly damper performance, while the liner expansion of the flat grind will keep things more even.
For shallow cutting, the hollow grind will have less metal just behind the edge, so you should expect better performance, provided the thinner, weaker edge doesn't roll or chip out.
For deeper cutting, the non-linear expansion of the hollow grind will significantly damper performance, while the liner expansion of the flat grind will keep things more even.
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I can't describe it in scientific, technical terms, but in my experience, flat grinds and convex edges outperform hollow grinds for me.
My two favorite "using" knives are a Calypso Jr. Lt. (flat ground), and a Marbles Fieldcraft (convex ground). I use the Calypso for general utility and the Fieldcraft for skinning and processing game, and other outdoor chores.
Subjective experience can be skewed by a lot of factors, but those are my preferences.
My two favorite "using" knives are a Calypso Jr. Lt. (flat ground), and a Marbles Fieldcraft (convex ground). I use the Calypso for general utility and the Fieldcraft for skinning and processing game, and other outdoor chores.
Subjective experience can be skewed by a lot of factors, but those are my preferences.
Having not had the pleasure of owning one of yours yet, I am probably missing something.is it the military model that is flat ground?? also anyone have feedback on the 440 steel from the military model..
i had a CRKT with a very high hollow grind and it was a good cutter also, but many of the knives have the bevel very low on the blade and i have noticed this reduces performance. i think i'm just going flat ground from now on.
i had a CRKT with a very high hollow grind and it was a good cutter also, but many of the knives have the bevel very low on the blade and i have noticed this reduces performance. i think i'm just going flat ground from now on.
Cliff Stamp
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Joe :
This only holds for hollow grinds of extreme curvature that basically are parallel to the edge at the start. If the hollow grind is basically just a relief of a flat (or convex) grind, this won't happen. This profile is the ultimate in cutting performance. The blade is sharpened at the angle that is produced when the spine is flat to the hone, and the hollow grind just gets the unnecessary metal along the body of the blade out of the way.
In general the properties always go hollow -> flat -> convex, or the opposite (cutting ability vs impact toughness), which is what you would expect as the three profiles are just -curvature, 0 curvature, +curvature, so you would expect a smooth variation in performance. They bleed into each other at the extremes, for example a very shallow hollow grind and a very shallow convex grind are both flat grinds.
-Cliff
This only holds for hollow grinds of extreme curvature that basically are parallel to the edge at the start. If the hollow grind is basically just a relief of a flat (or convex) grind, this won't happen. This profile is the ultimate in cutting performance. The blade is sharpened at the angle that is produced when the spine is flat to the hone, and the hollow grind just gets the unnecessary metal along the body of the blade out of the way.
In general the properties always go hollow -> flat -> convex, or the opposite (cutting ability vs impact toughness), which is what you would expect as the three profiles are just -curvature, 0 curvature, +curvature, so you would expect a smooth variation in performance. They bleed into each other at the extremes, for example a very shallow hollow grind and a very shallow convex grind are both flat grinds.
-Cliff
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A lot of this depends on what you are cutting and how you are cutting it, plus a number of other variables, not least of which is the edge geometry on the hollow ground blade. Here's a blade profile that has a hollow grind above a convex grind. The hollow removes weight while the thin convex portion on the blade makes for a great wood chopper. Were the convex edge to extend all the way to the spine, it would generate lots of drag against the material being chopped and impede penetration. The hollow grind therefore relieves that tendancy, while making the whole blade faster. I call this a Saber Reflex grind.
Variations on this idea can range from what you see here at one extreme for efficiently chopping wood to a very fine grain of wheat edge that will cut through softer materials as though they weren't there.
If all you're measuring is an edge for fine cutting, like rope, cardboard or body hair, a very fine bevel will cut best for awhile but it's not the most durable edge certainly. With the same spine thickness and blade width, there is no fundamental reason why the same fine bevel can't be put on both hollow and flat ground blades. All the hollow grind does is remove some of the extra steel between the spine and the edge bevel, and reduce the opportunity for the material being cut to drag against the side of the blade.
Most of this is just logic, but it's necessary to think of it in all three dimensions.
Variations on this idea can range from what you see here at one extreme for efficiently chopping wood to a very fine grain of wheat edge that will cut through softer materials as though they weren't there.
If all you're measuring is an edge for fine cutting, like rope, cardboard or body hair, a very fine bevel will cut best for awhile but it's not the most durable edge certainly. With the same spine thickness and blade width, there is no fundamental reason why the same fine bevel can't be put on both hollow and flat ground blades. All the hollow grind does is remove some of the extra steel between the spine and the edge bevel, and reduce the opportunity for the material being cut to drag against the side of the blade.
Most of this is just logic, but it's necessary to think of it in all three dimensions.
Cliff Stamp
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Jerry Hossom :
This is a very old geometry, it is what you will find on felling axes and most khukuris. The reason is pretty much exactly as you describe, to give the greatest penetration with the least binding, and highest mass / performance ratio. The only difference there is that after the geometry goes convex -> hollow it then goes back to convex to increase the durability from harsh lateral impacts, plus to prevent wedging at the height of the cut as those blades can sink in easily past the full width of their body on soft woods.
The hollow grind is not trivial in nature on these blades as if it is done incorrectly the blade will just bind in the wood, essentially it has to act as a relief from the two convex grinds. Jimbo has some comments made along these lines on his webpages that are worth reading. The idea has really been investigated as of late by high performance axe manufacturers who have moved past the convex -> hollow -> convex profile and made it much more complex by adding multiple "lips" as chip breakers, essentially raised sections along the blade which create maximal force against the wood chips with minimal drag, not trivial obviously.
There are wood cutting blades that approach the same goal (highest penetration, lowest sticking) from a very different path. On those blades (parangs for example) the geometry is dual convex. One taper forms the edge and the other goes in the opposite direction to form the spine. Here is a shot of one such blade. You can see a distinct convex curvature forming the edge and a much more shallow taper taking over at the top and narrowing the edge profile :
http://www.physics.mun.ca/~sstamp/images/al_blade_profile.gif
The main difference between these blades and the convex -> hollow -> convex ones is that the parangs are usually 1/4" thick and less, khukuris and axes are much thicker (1/2"+) and thus need the hollow grinds for reasons of weight. An axe with the dual convex profile for example is just a splitting wedge. Both approaches seem to be optimal for the respective blade thickness, which you would kind of expect given how old they are and they they are simply user driven.
Ray Kirk does something very similar on his large bowies (as do other ABS members) and obviously they cut very well. Then again full flat ground blades have also cut very well in competition. I am getting a dual convex and pure flat heavy bowies next year to explore the performance differences in some detail.
-Cliff
This is a very old geometry, it is what you will find on felling axes and most khukuris. The reason is pretty much exactly as you describe, to give the greatest penetration with the least binding, and highest mass / performance ratio. The only difference there is that after the geometry goes convex -> hollow it then goes back to convex to increase the durability from harsh lateral impacts, plus to prevent wedging at the height of the cut as those blades can sink in easily past the full width of their body on soft woods.
The hollow grind is not trivial in nature on these blades as if it is done incorrectly the blade will just bind in the wood, essentially it has to act as a relief from the two convex grinds. Jimbo has some comments made along these lines on his webpages that are worth reading. The idea has really been investigated as of late by high performance axe manufacturers who have moved past the convex -> hollow -> convex profile and made it much more complex by adding multiple "lips" as chip breakers, essentially raised sections along the blade which create maximal force against the wood chips with minimal drag, not trivial obviously.
There are wood cutting blades that approach the same goal (highest penetration, lowest sticking) from a very different path. On those blades (parangs for example) the geometry is dual convex. One taper forms the edge and the other goes in the opposite direction to form the spine. Here is a shot of one such blade. You can see a distinct convex curvature forming the edge and a much more shallow taper taking over at the top and narrowing the edge profile :
http://www.physics.mun.ca/~sstamp/images/al_blade_profile.gif
The main difference between these blades and the convex -> hollow -> convex ones is that the parangs are usually 1/4" thick and less, khukuris and axes are much thicker (1/2"+) and thus need the hollow grinds for reasons of weight. An axe with the dual convex profile for example is just a splitting wedge. Both approaches seem to be optimal for the respective blade thickness, which you would kind of expect given how old they are and they they are simply user driven.
Ray Kirk does something very similar on his large bowies (as do other ABS members) and obviously they cut very well. Then again full flat ground blades have also cut very well in competition. I am getting a dual convex and pure flat heavy bowies next year to explore the performance differences in some detail.
-Cliff
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Quote by Jonesy from another forum:
I have a Hossom sharpened Ontario machete and I have chopped up a lot of brush and trimmed trees and the edge geometry is what keeps on cutting. BTW it will still shave hair on most of the edge. And what gets me is there is no deformation to the edge either. It really is a tough edge to beat.
Cliff, if you know everything there is to know about knives, edges, steel, and cutting, there is really very little reason for the rest of us to participate here...
I have a Hossom sharpened Ontario machete and I have chopped up a lot of brush and trimmed trees and the edge geometry is what keeps on cutting. BTW it will still shave hair on most of the edge. And what gets me is there is no deformation to the edge either. It really is a tough edge to beat.
Cliff, if you know everything there is to know about knives, edges, steel, and cutting, there is really very little reason for the rest of us to participate here...
Cliff Stamp
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Even the soft blades like the Ontario machetes are far harder than most woods, even the knots in the wood and thus you can't get direct edge impaction. As an extreme example, you can easily cut up woods with mild steel, it is still far harder and stronger. Most wood cutting blades are actually very soft very low alloy steels, traditionally they are still often preferred this way because they can be sharpened with a file. Most of my relatives won't buy a saw with impulse hardened teeth for exactly this reason.
In regards to the Ontario machetes, even NIB without a modified edge profile they will still stay sharp for quite some time on woods as wood is not very abrasive at all thus it can't wear any metal away. Just ask a carpenter how often they have to sharpen their handsaws and consider how soft and low alloy those blades are and the huge amount of work they will see every day.
I think I had to touch up the edges on the Ontario machetes I had every week or so when I was loaning them out, I never modified the edge profile. Jonesy's comments are valuable as they reflect just how much a skilled user can get out of a very simple steel and profile as there is nothing more basic than the profile or materials of an Ontario machete. It should give pause for thought on all the "high performance" blades that rave about sharpness after cutting through a 2x4.
Of course this is only true for soft to medium woods like Pine, Fir, Birch, Oak and the like. There are woods that are very abrasive and will wear down saws quickly (particle board, but that is because of the non-wood elements). I don't think though that even any of the Ironwoods would be hard enough to impact steel though (45+ RC) at least. Bark is the real problem for field use because it contains grit due to the wind.
The real problem with blades and edge durability on wood is deformation under laterial impact, rippling. I blew up those two Ontario machetes limbing out some trees, dented in the edge on two Barteauxs that I had reprofiled, tore the edge on a khukuri, rippled a RCM from Livesay etc. . They didn't have the strength to absorb the lateral impacts.
However the problem with trying to infer behavior out of such information is that it is heavily dependent on the skill and strength of the user and thus it is impossible to predict from what one person says in regards to durability in that regard as to how it would perform for someone else unless you had very good knowledge of their respective skill and strength levels.
For example I have friends who are both harder and easier on blades than I am. If I give it to a friend that is lighter and he sees damage, then I know I will too. If I give it to a really heavy use friend and he doesn't see any damage I know I won't either. If the opposite behavior is seen then no prediction can be made about how the blade will react to me with any confidence.
The easiest way to get around this problem is to have many blades involved as then you can apply an independent quantitative ranking which is not very dependent on skill (or to a much lesser degree anyway) which anyone can directly use if they have experience with any of the blades involved.
-Cliff
In regards to the Ontario machetes, even NIB without a modified edge profile they will still stay sharp for quite some time on woods as wood is not very abrasive at all thus it can't wear any metal away. Just ask a carpenter how often they have to sharpen their handsaws and consider how soft and low alloy those blades are and the huge amount of work they will see every day.
I think I had to touch up the edges on the Ontario machetes I had every week or so when I was loaning them out, I never modified the edge profile. Jonesy's comments are valuable as they reflect just how much a skilled user can get out of a very simple steel and profile as there is nothing more basic than the profile or materials of an Ontario machete. It should give pause for thought on all the "high performance" blades that rave about sharpness after cutting through a 2x4.
Of course this is only true for soft to medium woods like Pine, Fir, Birch, Oak and the like. There are woods that are very abrasive and will wear down saws quickly (particle board, but that is because of the non-wood elements). I don't think though that even any of the Ironwoods would be hard enough to impact steel though (45+ RC) at least. Bark is the real problem for field use because it contains grit due to the wind.
The real problem with blades and edge durability on wood is deformation under laterial impact, rippling. I blew up those two Ontario machetes limbing out some trees, dented in the edge on two Barteauxs that I had reprofiled, tore the edge on a khukuri, rippled a RCM from Livesay etc. . They didn't have the strength to absorb the lateral impacts.
However the problem with trying to infer behavior out of such information is that it is heavily dependent on the skill and strength of the user and thus it is impossible to predict from what one person says in regards to durability in that regard as to how it would perform for someone else unless you had very good knowledge of their respective skill and strength levels.
For example I have friends who are both harder and easier on blades than I am. If I give it to a friend that is lighter and he sees damage, then I know I will too. If I give it to a really heavy use friend and he doesn't see any damage I know I won't either. If the opposite behavior is seen then no prediction can be made about how the blade will react to me with any confidence.
The easiest way to get around this problem is to have many blades involved as then you can apply an independent quantitative ranking which is not very dependent on skill (or to a much lesser degree anyway) which anyone can directly use if they have experience with any of the blades involved.
-Cliff
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Yes, that was my point exactly...
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What no one has noted is that you can make a flat ground blade much thinner at the spine (relative to over-all lateral strength) than is possible with a hollow-ground blade. This makes easier to create a good edge geometry just prior to the secondary bevel and still have some substantial lateral strength in the blade. Sure you can take a thin blade and do a shallow hollow grind too, but that then weakens the blade considerably.
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Cliff,
I honestly have never tested a hollow ground blade, of any quality grind, that will perform like a flat grind for deep cuts in all materials. In any material that binds, there is a difference, in my experience. On the other hand, maybe I haven't been testing the right hollow grinds. What have you tested, or would recommend to look at, to see this effect? Based on what you're saying, I'm expecting this type of knife to perform as well as a flat grind, even in things like binding food prep.
I've been thinking I need to order a Hossom 9" fighter of some sort, maybe that's just the ticket.
Joe
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Joe, a 9" Hossom fighter just sucks on wood.
Edit begins here:
What I find wrong with this whole question of "what is better" is that it is an incomplete question. It neglects the "for what" part of the equation. I've never built a knife to cut frozen vegetables. I make fighters and hunters for the most part, and for those purposes I find my hollow grinds works well, both to create the edge I want and to produce a knife that moves and handles in the manner I consider important. The camp knife above was purpose built to chop wood, using the most efficient grind I could come up with. I suppose it will also cut frozen veggies and other stuff, but it would make a lousy fighter and I doubt it would be much good for skinning deer. I don't make utility knives either, but I suppose one could use almost any grind on one and it would produce a useful knife.
I find nothing wrong with flat grinds, and until I did the camp knife a few months ago, I routinely referred buyers to other makers who did flat grinds for anything that was to be used on wood or similar hard substances. A flat grind works extremely well there for all the reasons given. I have trouble with the idea that there are no further answers to be found on how knives can be built or how blades might be shaped for more efficient cutting. If I believed that were true, I'd probably quit knifemaking, since the quest for a better knife is what makes this job interesting. There really are no pat answers in knifemaking, and that's good.
For the record, my hollow ground fighters have endured stress testing designed to test how a fighter should perform and have uniformly passed everytime out. The exact same series of tests, run in parallel on flat ground blades of 5160, A2, and 1084/15N20 Damascus failed in the same tests (chipped edges and broken points). In all cases, however, the makers of those flat ground blades passed the same tests on resubmitted blades produced once they had a better understanding of the demands. That's what the craft of knifemaking is all about, and that's also why broad generalizations are IMO inappropriate. There is no "best" without a much more refined definition of "for what".
Edit begins here:
What I find wrong with this whole question of "what is better" is that it is an incomplete question. It neglects the "for what" part of the equation. I've never built a knife to cut frozen vegetables. I make fighters and hunters for the most part, and for those purposes I find my hollow grinds works well, both to create the edge I want and to produce a knife that moves and handles in the manner I consider important. The camp knife above was purpose built to chop wood, using the most efficient grind I could come up with. I suppose it will also cut frozen veggies and other stuff, but it would make a lousy fighter and I doubt it would be much good for skinning deer. I don't make utility knives either, but I suppose one could use almost any grind on one and it would produce a useful knife.
I find nothing wrong with flat grinds, and until I did the camp knife a few months ago, I routinely referred buyers to other makers who did flat grinds for anything that was to be used on wood or similar hard substances. A flat grind works extremely well there for all the reasons given. I have trouble with the idea that there are no further answers to be found on how knives can be built or how blades might be shaped for more efficient cutting. If I believed that were true, I'd probably quit knifemaking, since the quest for a better knife is what makes this job interesting. There really are no pat answers in knifemaking, and that's good.
For the record, my hollow ground fighters have endured stress testing designed to test how a fighter should perform and have uniformly passed everytime out. The exact same series of tests, run in parallel on flat ground blades of 5160, A2, and 1084/15N20 Damascus failed in the same tests (chipped edges and broken points). In all cases, however, the makers of those flat ground blades passed the same tests on resubmitted blades produced once they had a better understanding of the demands. That's what the craft of knifemaking is all about, and that's also why broad generalizations are IMO inappropriate. There is no "best" without a much more refined definition of "for what".