How did people resharpen convexed edges in the old days?
- Thread starter lsstaipei
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Cliff Stamp
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pendentive :
This is a poor analogy for cutting most materials. As you push a knife through ropes, woods and so on, in carving and slicing, the force is *NOT* constant along the contact point, it decreases rapidly in distance from the edge making a massive difference, specifically the deep hollow grind is ahead many times to one over the full convex grind for a lot of cutting as it has to induce far less wedging action.
In regards to friction/drag as compared to wedging, it doesn't take much force to move water apart as it isn't very rigid, most energy is lost as drag however when chopping an axe into wood (for example) most of the force lost in mashing the wood to the side to make room for the axe. Lubricating the axe makes little difference as the effect of friction is minor. In contrast, having a highly polished surface makes large changes in the efficiency of moving through wind, water and so on as drag forces are very high and wedging forces insignificant.
Take an opinel (full convex grind on a very thin blade) and cut through a piece of hemp rope (say 3/8") and record the amount of force required. Now take a full flat ground blade which is much thicker (1/4" at spine), but fully flat ground right to the edge so the edge angles are similar. This blade will cut the rope with a near identical amount of force, the extra thickness has little effect as rope opens up during the cut and thus all the rope "sees" is the little bit of edge which it actually exerts a force on (a mm or so high). This is why changing edge angles has a *massive* influence compared to edge thickness. You can notice the effect on cutting ability of just a few degrees, but large changes in edge thickness have no where near the same change in efficiency - again as the force most mediums apply on a edge isn't constant (or even linear, it falls off really fast like exponential for a lot of material).
This is the traditional way that parangs (and most other large heavy machete type blades - goloks, bolos, etc. ) are ground, there are two dual convex arcs that form the knife. This is one of the cases where the water idea holds as you are cutting very thick material which is capable of exerting high binding forces over the entire length of contact. My Battle Mistress is ground this way mainly accidently as I just pressed it flat against the belt and it hit along the spine as well as the edge. The spine grind has no effect on any cutting I do with it as I don't cut anything more than two inches deep which is rigid for that depth. It just did it that way to get the maximum relief for the edge.
-Cliff
This is a poor analogy for cutting most materials. As you push a knife through ropes, woods and so on, in carving and slicing, the force is *NOT* constant along the contact point, it decreases rapidly in distance from the edge making a massive difference, specifically the deep hollow grind is ahead many times to one over the full convex grind for a lot of cutting as it has to induce far less wedging action.
In regards to friction/drag as compared to wedging, it doesn't take much force to move water apart as it isn't very rigid, most energy is lost as drag however when chopping an axe into wood (for example) most of the force lost in mashing the wood to the side to make room for the axe. Lubricating the axe makes little difference as the effect of friction is minor. In contrast, having a highly polished surface makes large changes in the efficiency of moving through wind, water and so on as drag forces are very high and wedging forces insignificant.
Take an opinel (full convex grind on a very thin blade) and cut through a piece of hemp rope (say 3/8") and record the amount of force required. Now take a full flat ground blade which is much thicker (1/4" at spine), but fully flat ground right to the edge so the edge angles are similar. This blade will cut the rope with a near identical amount of force, the extra thickness has little effect as rope opens up during the cut and thus all the rope "sees" is the little bit of edge which it actually exerts a force on (a mm or so high). This is why changing edge angles has a *massive* influence compared to edge thickness. You can notice the effect on cutting ability of just a few degrees, but large changes in edge thickness have no where near the same change in efficiency - again as the force most mediums apply on a edge isn't constant (or even linear, it falls off really fast like exponential for a lot of material).
This is the traditional way that parangs (and most other large heavy machete type blades - goloks, bolos, etc. ) are ground, there are two dual convex arcs that form the knife. This is one of the cases where the water idea holds as you are cutting very thick material which is capable of exerting high binding forces over the entire length of contact. My Battle Mistress is ground this way mainly accidently as I just pressed it flat against the belt and it hit along the spine as well as the edge. The spine grind has no effect on any cutting I do with it as I don't cut anything more than two inches deep which is rigid for that depth. It just did it that way to get the maximum relief for the edge.
-Cliff
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Cliff,
These ideas are not my own. I'm presenting here what Jerry Fisk taught in his class, which happens to coincide with what I have gathered from other cutting competition winners.
True, that scientifically it may not seem to add up, but you have to look at the end product - the results of the cutting competitions and ask yourself why that particular knife won, etc. My consensus (in line with what the ABS teaches) is that a Flat/Convex grind displaces material better than any other. That grind could be modified to look like the #6 in the illustration above (which, as noted, is simliar to a traditional/ethnic grind) and be even better, IMHO.
However, I take issue with the idea that a hollow grind displaces better. Perhaps when slicing thin materials, but when push-cutting deep materials (which we all do a lot - to cut cardboard boxes, open packages, skin animals, chop wood, slice paper
, etc.) you have to be concerned about the material passing over the entire blade.
In this example, rope cutting is not a good comparative test for a convex edge. It will cut rope just fine, but does not add anything to it.
Woodcarving/whittling and convex edges are like peanut butter & jelly. A hollow grind will "bind" in your cut causing you to either fracture (cut out) your cut, or worse, damage your blade. A flat grind does much better than a hollow grind - hence the widespread use of puukkos for carving. However, a slight convex edge makes woodcarving a true joy because it cuts easy and is easy to keep sharp.
Don't get me wrong, I don't think the convex edge is the best in all cases, I just think that if you want a tough grind that will take a beating and not chip or break, you'll want a convex edge.
Consider this:
I had a 22" long khukuri that with a flat/slightly hollow 2nd bevel chipped out large pieces of the blade when I used it on 1.5" branches.
I took that exact knife and without changing the heat treat or anything, reground a new convex edge on it, and was able to go through 5" logs without even so much as rolling the edge.
I might be dubious too if I hadn't done it myself! The before and after of this test was posted here on bladeforums. There is no question in my mind that for all-out toughness, the convex edge is the best.
I try to just report it as I see it. My terminology is probably flawed, but the results cannot be misconstrued. If it cuts better, it cuts better.
These ideas are not my own. I'm presenting here what Jerry Fisk taught in his class, which happens to coincide with what I have gathered from other cutting competition winners.
True, that scientifically it may not seem to add up, but you have to look at the end product - the results of the cutting competitions and ask yourself why that particular knife won, etc. My consensus (in line with what the ABS teaches) is that a Flat/Convex grind displaces material better than any other. That grind could be modified to look like the #6 in the illustration above (which, as noted, is simliar to a traditional/ethnic grind) and be even better, IMHO.
However, I take issue with the idea that a hollow grind displaces better. Perhaps when slicing thin materials, but when push-cutting deep materials (which we all do a lot - to cut cardboard boxes, open packages, skin animals, chop wood, slice paper
, etc.) you have to be concerned about the material passing over the entire blade. In this example, rope cutting is not a good comparative test for a convex edge. It will cut rope just fine, but does not add anything to it.
Woodcarving/whittling and convex edges are like peanut butter & jelly. A hollow grind will "bind" in your cut causing you to either fracture (cut out) your cut, or worse, damage your blade. A flat grind does much better than a hollow grind - hence the widespread use of puukkos for carving. However, a slight convex edge makes woodcarving a true joy because it cuts easy and is easy to keep sharp.
Don't get me wrong, I don't think the convex edge is the best in all cases, I just think that if you want a tough grind that will take a beating and not chip or break, you'll want a convex edge.
Consider this:
I had a 22" long khukuri that with a flat/slightly hollow 2nd bevel chipped out large pieces of the blade when I used it on 1.5" branches.
I took that exact knife and without changing the heat treat or anything, reground a new convex edge on it, and was able to go through 5" logs without even so much as rolling the edge.
I might be dubious too if I hadn't done it myself! The before and after of this test was posted here on bladeforums. There is no question in my mind that for all-out toughness, the convex edge is the best.
I try to just report it as I see it. My terminology is probably flawed, but the results cannot be misconstrued. If it cuts better, it cuts better.
I think this is a really good thread, However, I am having a little trouble understanding it. Does convex edge = just the edge bevel, witch can be put on any blade grind, Hollow, flat, and convex? Just seems like some are talking blade grinds and others are talking edge grinds and some are mixing both togather.
Cliff Stamp
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Dan :
Not better, less. It can be ground to move less material out of the way. When pushing a knife through stiff material the thicker the cross section of the blade the more force required becuase you have to smash most material out of the way, thick of it like compacting a spring. This is why it is easier to cut cardboard with a 1/8" blade vs a 1/4" thick one, as you have to make tracks 1/8" and 1/4" wide respectively.
Yes, but as noted the force that the material exerts on the blade isn't constant in height from the edge to the spine for a lot of materials, and can be only significant near the edge. Yes there are some really binding materials like turnips and thick cardboard, but ropes, fabrics, flesh, most vegetation, isn't stiff at that depth as it opens up readily when cut. Even on those materials convex vs flat will defer to thickness in regards to performance and edge geometry can still make a massive difference as the force will be higner near the edge.
There is a further complication in that many of these materials have a sweet spot of sorts as if you go too thin (either convex or flat) the performance actually drops as the blade doesn't induce enough splitting. Some rigid materials (like vegetables) will crack and thus open up before the edge at a certain point, thus you need the minimal blade thickness which will induce this splitting or you will get too heavy a binding on the blade and the start of a lot of friction.
You can't make such broad generalizations. Consider the following :
http://www.physics.mun.ca/~sstamp/images/grinds.xfig.gif
specifically compare the flat and convex geometries (first and third respectively), the flat bevel is essentially the convex bevel with all the curvature ground away and thus it is thinner overall. It will go through any wedging material like cardboard much easier as it has a more narrow cross section - this doesn't mean flat out cuts convex, the difference is due to the cross section not the curvature. Now you may have a problem with edge durability and thus would add a slightly steeper secondary edge bevel.
As for the hollow grind, you have to consider how it is applied. The second and fourth grinds are both hollow but very differnt. The second one is readily prone to edge damage and can wedge badly in stiff materials and is optomized for materials which readily open up when cut. The fourth one is essentially the flat grind with two scoops missing from the sides (note if you make the scoops bigger you eventually end up with the second full hollow grind).
These howllow relief grinds remove contact points and thereby increase penetration and reduce sticking, this is how traditional felling axes were ground as well as a lot of japanese blades (and chisels, hollows on the back also aiding in sharpening/lapping). The current competition racing axes are also hollow ground in this manner to increase penetration and reduce binding. They also run small flat ground v-bevels on the axes required to cut the harder Australian woods.
Alvin Johnson also grinds hollow ground blades in this way for professional meat cutters and farmers (trimming ears and the like).
First consider that the edge mave have been overhardened. As Jim Aston has noted working with many production blades, performance can increase massively with extended sharpening with no shape change of the edge bevel just a removal of "bad" steel. Now yes you generally don't want hollow grinds near the edge as the cross section will be too small. After using a few golok from Valiant for example I would see dents primarily in the regions that had forging hollows so I ground them all out. Again though this is a matter of cross section not curvature. You can easily grind a convex (or flat) edge which is too thin and therefore weak and is easily damaged. The profile of an Opinel for example on a large blade would easily ripple on heavy wood as it is too thin regardless of the fact that it is convex. As noted in the above any edge rolling would be prevented with a slightly v-ground secondary edge bevel as was commonly used on felling axes.
db :
Yes, a convex blade generally means the whole thing, spine to edge in one smooth curve.
-Cliff
Not better, less. It can be ground to move less material out of the way. When pushing a knife through stiff material the thicker the cross section of the blade the more force required becuase you have to smash most material out of the way, thick of it like compacting a spring. This is why it is easier to cut cardboard with a 1/8" blade vs a 1/4" thick one, as you have to make tracks 1/8" and 1/4" wide respectively.
Yes, but as noted the force that the material exerts on the blade isn't constant in height from the edge to the spine for a lot of materials, and can be only significant near the edge. Yes there are some really binding materials like turnips and thick cardboard, but ropes, fabrics, flesh, most vegetation, isn't stiff at that depth as it opens up readily when cut. Even on those materials convex vs flat will defer to thickness in regards to performance and edge geometry can still make a massive difference as the force will be higner near the edge.
There is a further complication in that many of these materials have a sweet spot of sorts as if you go too thin (either convex or flat) the performance actually drops as the blade doesn't induce enough splitting. Some rigid materials (like vegetables) will crack and thus open up before the edge at a certain point, thus you need the minimal blade thickness which will induce this splitting or you will get too heavy a binding on the blade and the start of a lot of friction.
You can't make such broad generalizations. Consider the following :
http://www.physics.mun.ca/~sstamp/images/grinds.xfig.gif
specifically compare the flat and convex geometries (first and third respectively), the flat bevel is essentially the convex bevel with all the curvature ground away and thus it is thinner overall. It will go through any wedging material like cardboard much easier as it has a more narrow cross section - this doesn't mean flat out cuts convex, the difference is due to the cross section not the curvature. Now you may have a problem with edge durability and thus would add a slightly steeper secondary edge bevel.
As for the hollow grind, you have to consider how it is applied. The second and fourth grinds are both hollow but very differnt. The second one is readily prone to edge damage and can wedge badly in stiff materials and is optomized for materials which readily open up when cut. The fourth one is essentially the flat grind with two scoops missing from the sides (note if you make the scoops bigger you eventually end up with the second full hollow grind).
These howllow relief grinds remove contact points and thereby increase penetration and reduce sticking, this is how traditional felling axes were ground as well as a lot of japanese blades (and chisels, hollows on the back also aiding in sharpening/lapping). The current competition racing axes are also hollow ground in this manner to increase penetration and reduce binding. They also run small flat ground v-bevels on the axes required to cut the harder Australian woods.
Alvin Johnson also grinds hollow ground blades in this way for professional meat cutters and farmers (trimming ears and the like).
First consider that the edge mave have been overhardened. As Jim Aston has noted working with many production blades, performance can increase massively with extended sharpening with no shape change of the edge bevel just a removal of "bad" steel. Now yes you generally don't want hollow grinds near the edge as the cross section will be too small. After using a few golok from Valiant for example I would see dents primarily in the regions that had forging hollows so I ground them all out. Again though this is a matter of cross section not curvature. You can easily grind a convex (or flat) edge which is too thin and therefore weak and is easily damaged. The profile of an Opinel for example on a large blade would easily ripple on heavy wood as it is too thin regardless of the fact that it is convex. As noted in the above any edge rolling would be prevented with a slightly v-ground secondary edge bevel as was commonly used on felling axes.
db :
Yes, a convex blade generally means the whole thing, spine to edge in one smooth curve.
-Cliff
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Cliff,
I agree with much of what you have written...on paper. Indeed, even in controlled tests. I think your examination is good, but too focused on the individual parts...instead of the big picture.
On the whole, a convex edge is tougher, regardless of the thickness of the steel, compared to a hollow or flat grind.
Sharpening a convex edge is easier.
Overall, there are less tasks that a convex grind can't do best, than the other grinds.
It is true that many tools have hollow grinds or flat grinds to help them be more efficient, but this discussion is about knives, not chisels, adzes, etc. True, they fall under the umbrella of "cutting instruments" but I would be misappropriating my data to say that a convex edge belongs on every cutting instrument.
I think that if we all were to pick up a sheet of paper and write down the cutting tasks we do over the next month, we'd find out that a convex edge would handle most all of the non-kitchen duties better. How can I claim this? Because I have put a convex edge on nearly every knife in my household (which were previously kept shaving sharp on bench stones) and without fail, they all cut better. Even my $1 ebay-special cuts better. Were my knives just dull before I convexed 'em? Nope. They were all shaving sharp. And since then, it has taken less to keep them shaving sharp. This is regardless of whether the final edge was toothy or polished. The AK Bowie in the first video I posted is heavy and thick and yet it cuts material better than other, thinner bowies because of the convex edge. I was able to half a 2x4 in 5 hits (on average) with it - never needing more than 8.
The only knives I have not "convexed" are my paring knives and vegetable slicers - tasks not suited to the convex egde (as explained in Cliff's post).
I think sometimes we forget that knives are not tools. They are more than tools. Tools are usually "task-specific" and narrow in usage (that's why our shelves are lined with hundreds of 'em). A knife is a multi-tool and should come equipped for multi-tasking.
That's why I say that specific tests are informative and educational, but have to be combined with other tests to develop an overall "big picture".
I agree with much of what you have written...on paper. Indeed, even in controlled tests. I think your examination is good, but too focused on the individual parts...instead of the big picture.
On the whole, a convex edge is tougher, regardless of the thickness of the steel, compared to a hollow or flat grind.
Sharpening a convex edge is easier.
Overall, there are less tasks that a convex grind can't do best, than the other grinds.
It is true that many tools have hollow grinds or flat grinds to help them be more efficient, but this discussion is about knives, not chisels, adzes, etc. True, they fall under the umbrella of "cutting instruments" but I would be misappropriating my data to say that a convex edge belongs on every cutting instrument.
I think that if we all were to pick up a sheet of paper and write down the cutting tasks we do over the next month, we'd find out that a convex edge would handle most all of the non-kitchen duties better. How can I claim this? Because I have put a convex edge on nearly every knife in my household (which were previously kept shaving sharp on bench stones) and without fail, they all cut better. Even my $1 ebay-special cuts better. Were my knives just dull before I convexed 'em? Nope. They were all shaving sharp. And since then, it has taken less to keep them shaving sharp. This is regardless of whether the final edge was toothy or polished. The AK Bowie in the first video I posted is heavy and thick and yet it cuts material better than other, thinner bowies because of the convex edge. I was able to half a 2x4 in 5 hits (on average) with it - never needing more than 8.
The only knives I have not "convexed" are my paring knives and vegetable slicers - tasks not suited to the convex egde (as explained in Cliff's post).
I think sometimes we forget that knives are not tools. They are more than tools. Tools are usually "task-specific" and narrow in usage (that's why our shelves are lined with hundreds of 'em). A knife is a multi-tool and should come equipped for multi-tasking.
That's why I say that specific tests are informative and educational, but have to be combined with other tests to develop an overall "big picture".
My Campanion chipped,before my convexing,but also after the convexing.
It was much easier to remove the damage when the edge was still V-edge.According to my experience,the convex edge is not necessarily easier to sharpen,especially the edge is damaged.My campanion performs
much better after convexing,but to be honest,I don't know why.Maybe the convexing?Maybe the edge is thinned a lot?Maybe both.
There are other variables have to be considered.
It was much easier to remove the damage when the edge was still V-edge.According to my experience,the convex edge is not necessarily easier to sharpen,especially the edge is damaged.My campanion performs
much better after convexing,but to be honest,I don't know why.Maybe the convexing?Maybe the edge is thinned a lot?Maybe both.
There are other variables have to be considered.
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It seems to me that there needs to be a redefining of the term convex grind. Are we talking edge geometry or blade geometry? To give an example an axe with a convex grind can have a v-edge or dual v-edge. Further there is no reason a hollow-ground knife cannot have convex edge.
Let's get the terminology straight. You can have a flat grind, sabre grind, hollow grind and a convex grind as far as blade geometry goes but when it comes to edge geometry you basically have either a convex ground edge or a v-ground edge (whether it be single, dual or triple) which raises the point (excuse the pun) can you have a true hollow ground edge?
Let's get the terminology straight. You can have a flat grind, sabre grind, hollow grind and a convex grind as far as blade geometry goes but when it comes to edge geometry you basically have either a convex ground edge or a v-ground edge (whether it be single, dual or triple) which raises the point (excuse the pun) can you have a true hollow ground edge?
I have already had the priviledge of using the two test blades from Dozier that Buzz mentioned.
I reported my findings on the dozier forum, but to summarize, I gave the hollow grind top marks for general utility work around tanneries and the convex blade scored higher on more specialized chores such as skinning and slicing thick fatty type materials. Both grinds performed fantastically across the board, and any preferrences were by a very small margin.
Both edges got their final finish from the Dpzier shop with the same grit paper, and the only sharpening needed during the two weeks in my possession was accomplished with a steel.
Chuck B
I reported my findings on the dozier forum, but to summarize, I gave the hollow grind top marks for general utility work around tanneries and the convex blade scored higher on more specialized chores such as skinning and slicing thick fatty type materials. Both grinds performed fantastically across the board, and any preferrences were by a very small margin.
Both edges got their final finish from the Dpzier shop with the same grit paper, and the only sharpening needed during the two weeks in my possession was accomplished with a steel.
Chuck B
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A convex edge and a convex grind are 2 different things. See the diagram I posted earlier. 3 of them have convex edges, only one has a convex grind.
I try to specify in my post whether I'm talking about a grind or an edge, but I'm sure I make mistakes. Generally speaking, you can talk of a convex edge whether you have a flat, convex, or hollow grind. You can only talk about a convex grind if you have a convex grind.
Hope this makes sense.
Dan
I try to specify in my post whether I'm talking about a grind or an edge, but I'm sure I make mistakes. Generally speaking, you can talk of a convex edge whether you have a flat, convex, or hollow grind. You can only talk about a convex grind if you have a convex grind.
Hope this makes sense.
Dan
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So what any discussion re edge geometry comes down to is: Convex vs V-grind which in effect the Dozier test blades were about whilst one was a convex ground blade the other was a hollow-ground blade with a v-grind edge. What is the best grind for a knife? What is the best edge for a knife? How can we combine the best of both worlds? Or, are we becomming too complex. I have always maintained for the average knife user that the flat ground knife was best but having had the opportunity to test a Dozier hollow-grind I now lean towards the hollow-grind but I am reluctant to take it one further step and have a convex grind on a hollow-ground knife. I don't believe that the test results have shown up that much difference between the 2 different grinds or is it the 2 different edges.
JDB,
Not too much talk so far about the grind/edge comparison of the two Dozier blades. Having had the privikedge to use them, let me describe.
Both came sharpened with the same grit medium. The Convex blade had no secondary bevel, as one would expect. The hollow grind had the typical scary sharp Dozier edge.
To be as honest as I can be, and ignoring all of the Scientific data, there isn't a hill of beans difference. Yes, after using each blade for several days in tanneries where I work, I did develop some preferrence for specific tasks.
I sent the test blades back having really only confirmed my original bias: good steel, good heat treat, good design, and it will cut regardless of grind type.
Not too much talk so far about the grind/edge comparison of the two Dozier blades. Having had the privikedge to use them, let me describe.
Both came sharpened with the same grit medium. The Convex blade had no secondary bevel, as one would expect. The hollow grind had the typical scary sharp Dozier edge.
To be as honest as I can be, and ignoring all of the Scientific data, there isn't a hill of beans difference. Yes, after using each blade for several days in tanneries where I work, I did develop some preferrence for specific tasks.
I sent the test blades back having really only confirmed my original bias: good steel, good heat treat, good design, and it will cut regardless of grind type.
Cliff Stamp
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pendentive :
This is so false it is absurd. As a dramatic example, compare the toughness of an Opinel (convex), to an old style Strider WB (hollow) (or one of the Busse fat varients if you want a really lopsided comparison).
This is only true if you are sharpening v-bevels freehand and can't hold a tight angle tolerance, in which case you use a sharpmaker or something similar. V-ground edges with secondary edge bevels are in general much easier to sharpen than convex bevels. With a dual v-ground bevel you grind the primary edge quickly on a rough stone, no need for tight angle tolerance or to refine the polish. The secondary edge bevel is then applied with a sharpamaker (or something similar, or even freehand if you have the skill) which is near instant due to very narrow bevel (a few passes per side).
With a x-coarse primary stone, you can remove visible edge damage (0.1 mm deep) in 1-2 minutes (depending on the machinability of the steel), to bevel a few mm wide. The secondary edge bevel is then applied pretty much instantly as noted so total sharpening time is 1-2 minutes. This is a complete honing, not a minor touchup, and includes removal of visible damage. Try the same (without power equipment) on a convex bevel and see if you can come close to the time with similar abrasives.
With a convex bevel you are limited by a smaller contact area (if you are using flat stones) and by the fact that you must raise the polish of the entire bevel to the desired edge grit and thus the time increase is dramatic, many times to one. For full convex bevels (spine to edge), the increase is simply massive as now you have to do a full blade polish for every sharpening. Which is why such grinds are only practical on simple steels (such steels are generally optimal for such blades anyway - heavy choppers usually).
The principles of cutting are the same in all the tools. The difference is generally in gross blade shape or handle construction to allow various use. An axe for example has a long handle to allow power to come from the hip, however its movement in wood follows the same conditions as a bowie knife. The forces a chisel or drawknife see are the same as a carving knife (or even fillet blade, just with different elastic constants and coefficients of friction for the various media)
And a splitting axe (full convex grind) chops no where near as well as a felling axe (hollow grind). What is the relative edge thickness and angle (and heft) of the other bowies. Of course if the convex bowie has a very high shoulder resulting in an effect a much thinner edge then it cuts better.
To be specific lets assuming you take a Trailmaster from Cold Steel and put in on a slack belt and apply a wide convex edge. It now cuts much better, considering the initial edge is ~25 degrees, you can cut the shoulder down to 8-10 and thus pretty much raise the cutting ability to double on shallow cutting and the chopping ability would go up to 50%+ or more. Now does this mean convex grinds are superior. Of course not. The same thing would have happened if you applied a wide flat relief grind.
lsstaipei :
Convexing as commonly described (sandpaper + leather) adds a high relief grind to the edge, this will greatly increase the cutting ability but does nothing for durability. To increase the durability you want to increase the edge angle at the very extreme of the edge. In general the secondary edge bevel with should be at least half the width of the preexisting damage to prevent it from happening again.
JDEEBLADE :
Sure, it would be tremendously weak and in general due to the odd shape difficult to sharpen. Straight razors come close, they have v-ground edges at 3-5 degrees per side. Essentially full hollow grind and sharpen flat to the stone.
-Cliff
This is so false it is absurd. As a dramatic example, compare the toughness of an Opinel (convex), to an old style Strider WB (hollow) (or one of the Busse fat varients if you want a really lopsided comparison).
This is only true if you are sharpening v-bevels freehand and can't hold a tight angle tolerance, in which case you use a sharpmaker or something similar. V-ground edges with secondary edge bevels are in general much easier to sharpen than convex bevels. With a dual v-ground bevel you grind the primary edge quickly on a rough stone, no need for tight angle tolerance or to refine the polish. The secondary edge bevel is then applied with a sharpamaker (or something similar, or even freehand if you have the skill) which is near instant due to very narrow bevel (a few passes per side).
With a x-coarse primary stone, you can remove visible edge damage (0.1 mm deep) in 1-2 minutes (depending on the machinability of the steel), to bevel a few mm wide. The secondary edge bevel is then applied pretty much instantly as noted so total sharpening time is 1-2 minutes. This is a complete honing, not a minor touchup, and includes removal of visible damage. Try the same (without power equipment) on a convex bevel and see if you can come close to the time with similar abrasives.
With a convex bevel you are limited by a smaller contact area (if you are using flat stones) and by the fact that you must raise the polish of the entire bevel to the desired edge grit and thus the time increase is dramatic, many times to one. For full convex bevels (spine to edge), the increase is simply massive as now you have to do a full blade polish for every sharpening. Which is why such grinds are only practical on simple steels (such steels are generally optimal for such blades anyway - heavy choppers usually).
The principles of cutting are the same in all the tools. The difference is generally in gross blade shape or handle construction to allow various use. An axe for example has a long handle to allow power to come from the hip, however its movement in wood follows the same conditions as a bowie knife. The forces a chisel or drawknife see are the same as a carving knife (or even fillet blade, just with different elastic constants and coefficients of friction for the various media)
And a splitting axe (full convex grind) chops no where near as well as a felling axe (hollow grind). What is the relative edge thickness and angle (and heft) of the other bowies. Of course if the convex bowie has a very high shoulder resulting in an effect a much thinner edge then it cuts better.
To be specific lets assuming you take a Trailmaster from Cold Steel and put in on a slack belt and apply a wide convex edge. It now cuts much better, considering the initial edge is ~25 degrees, you can cut the shoulder down to 8-10 and thus pretty much raise the cutting ability to double on shallow cutting and the chopping ability would go up to 50%+ or more. Now does this mean convex grinds are superior. Of course not. The same thing would have happened if you applied a wide flat relief grind.
lsstaipei :
Convexing as commonly described (sandpaper + leather) adds a high relief grind to the edge, this will greatly increase the cutting ability but does nothing for durability. To increase the durability you want to increase the edge angle at the very extreme of the edge. In general the secondary edge bevel with should be at least half the width of the preexisting damage to prevent it from happening again.
JDEEBLADE :
Sure, it would be tremendously weak and in general due to the odd shape difficult to sharpen. Straight razors come close, they have v-ground edges at 3-5 degrees per side. Essentially full hollow grind and sharpen flat to the stone.
-Cliff
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Being realistic it comes down to the fact that as far as edge geometry is concerned we have 2 types:
1. V-grind (whether it be single, double or triple); and
2. Convex grind.
So! What is the best? As far as I am concerned it comes down to 'horses for courses' - the convex edge is a specialist edge and should only be used by those with some expertise in the sharpening techniques required. I would not recommend a convex edge to a novice. Further I believe the convex edge should be associated with the convex ground blade. I may be biased in my view somewhat as convex grinds whether it be edge or blade do not appear to be popular here 'down-under' and the decision left to the knife-buyer is basically whether to have a flat ground or hollow ground blade both with V-ground edges. Which is best really comes down to what the knife is going to be used for and personal preference or experience.
In my dealings with workers in the meat industry the flat ground blade oustrips the hollow-ground by at least 10 to 1. But, I leave you with the following thought - workers in the chicken butchering area have made requests for a 5" curved boner with a half flat ground and half hollow ground blade with the flat grind being to the point. Maybe I should ask them whether they would like to try a half convex edge and half v-ground edge - now that would be an interesting knife,
1. V-grind (whether it be single, double or triple); and
2. Convex grind.
So! What is the best? As far as I am concerned it comes down to 'horses for courses' - the convex edge is a specialist edge and should only be used by those with some expertise in the sharpening techniques required. I would not recommend a convex edge to a novice. Further I believe the convex edge should be associated with the convex ground blade. I may be biased in my view somewhat as convex grinds whether it be edge or blade do not appear to be popular here 'down-under' and the decision left to the knife-buyer is basically whether to have a flat ground or hollow ground blade both with V-ground edges. Which is best really comes down to what the knife is going to be used for and personal preference or experience.
In my dealings with workers in the meat industry the flat ground blade oustrips the hollow-ground by at least 10 to 1. But, I leave you with the following thought - workers in the chicken butchering area have made requests for a 5" curved boner with a half flat ground and half hollow ground blade with the flat grind being to the point. Maybe I should ask them whether they would like to try a half convex edge and half v-ground edge - now that would be an interesting knife,
Cliff Stamp
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JDEEBLADE :
Is this an advantage in processing ability, or popularity?
-Cliff
Is this an advantage in processing ability, or popularity?
-Cliff
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I would suggest there are a number of factors probably in the following order:
1. Processing ability
2. Ease of sharpening (this could be more of a perception than an actual fact)
3. Availability and cost
4. Tradition
There a few hollowgrounds on the market here - the main one is probably the Russells of which their skinner is popular. Most meatworkers would use a version of the double v-grind to sharpen their knives usually using synthetic bench stones. Many of my customers are chicken-boners and they would go through a 5" semi-flex curved boner on average every 6 weeks.
1. Processing ability
2. Ease of sharpening (this could be more of a perception than an actual fact)
3. Availability and cost
4. Tradition
There a few hollowgrounds on the market here - the main one is probably the Russells of which their skinner is popular. Most meatworkers would use a version of the double v-grind to sharpen their knives usually using synthetic bench stones. Many of my customers are chicken-boners and they would go through a 5" semi-flex curved boner on average every 6 weeks.
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This is more about steel type and thickness than about edge geometry...
I did this very thing (without power tools). Took the same amount of time. It's on my CD in "realtime" (the best I could given the camera I used). In fact, I think it took about that much time to sharpen it and make the videos. (the sharpening was done after scraping the edge on concrete and banging it on a metal rail - there were visible dings/marks - no lupe needed)
This is why convex edges on benchstones are not recommended. The key is a conforming material backing various grits of sandpaper.
Many felling axes come with a final convex egde put on after the hollow grind.
As stated before, if you strop, you are "convexing" the edge whether you want to or not. Simple as that. Sure, it may be slight, but the principle is the same - a conforming material backing an abrasive. Same goes for other fabled techniques such as carboard, newsprint, denim jeans...
The thickness isn't the point. The point is that even with a spine that is a hair under 1/2" thick, you can still slice fine paper, chope wood, cut leather, slice carboard - all with a convex edge. I did all these tasks after sharpening. It can handle "finer" cutting needs despite the large size. I anybody needs a second opinion, ask it's new owner "MrBadExample".
Yes, in theory I agree. However, consider that an octagon is not far removed from a circle, yet a circle is clearly "smoother, more efficient" by design. Point? => a multi-bevel edge can do nearly (probably imperceptibly) everything a convex edge can do - except that it's harder to put on and maintain, and doesn't look as good, etc. A convex edge is natural and intuitive.
"Toughness" in this discussion is being defined as "the ability to withstand heavy use without edge fracture" - or something along those lines....not: "Edge-Holding Ability" - which has more to do with the steel, than the grind, IMHO.
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I think Cliff's point here is important. When sharpening the Convex edge, unless one is prepared to take allot of time, there will be a natural tendency to increase the actual edge angle. How many users will actually take the time to do the job right.
At least with flat bevels, it is easy to maintain the optimum angles again and again, presuming of course that the sharpening system has angle setting ability.
Cliff Stamp
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JDEEBLADE :
Is this how long it lasts before it gets sharpened? Or until it has sharpened so much it has to be replaced? What steel is being used?
pendentive :
[toughness : opinel vs Strider]
The steel type does have an effect, the Opinel is slightly softer but this is indeed swamped out by the thickness, even if an Opinel was made from INFI it would have no where near the strength of a Battle Mistress, how can you even argue that point. The thickness is critical in the blade toughness and strength, and obviously is has the same role in regards to the *same* properties of the edge.
As an obvious recent example :
http://www.bladeforums.com/forums/showthread.php?s=&threadid=280313
The edge, even though convex, was too weak and readily damaged. The solution was to apply a heavier secondary edge bevel which had a more obtuse angle. This clearly showed the obvious point that you can not ignore thickness and angle and just generalize to convex is stronger than flat.
Yes, this does allow them to be sharpened with less skill, as Mel Sorg pointed out years ago on Knifeforms before Bladeforums existed, which came out during a discussion we had on how to maintain the edge on the D2 small utility blade I had from him and at which point I suggested to Joe that he update the FAQ which at that time still had convex edges only being sharpened by power tools. This of course means that convex edges require specialized equipment to allow honing to similar speeds as flat grinds, this makes them less versatile and of course much slower when the *same abrasives* are used for both compared to secondary bevels on flat grinds. It is always easier to grind or cut flats as it is curves.
Actually, they had v-ground secondary edge bevels for durability, see the Axe Book for example. Again it is angle and thickness which are the critical factors in durability not simply curvature.
This is insignificant, both in terms of the curvature unduced as well as the effect it has. If you want to be really anal about such distinctions then all edges are convex unless you use a jig on a *optically* flat diamond plate because otherwise all edges will have some degree of curvature. This of course makes no effect at all on functionality unless the curvature starts to be degrees in size and of sufficient magnitude in depth to exert an influence.
I have compared Opinels with full convex grinds to those with small secondary edge bevels (v-rod at 22 degrees) and you can not see any difference in cutting ability due to the more obtuse secondary edge bevel, but you do gain massive increases in durability and even greater in ease of sharpening. This is the primary reason as to why I switched away from convex bevels on most knives (except for heavy choppers which still usually get the secondary flat micro-bevel commonly used on felling axes, 0.1 mm or so deep, beyond visible)
Yes and it would do all that with a v ground edge as well. Of course the spine thickness has no effect on cutting paper as I noted in the above you need to be thinking about the binding forces, does the paper press hard against the spine, no, thus it is irrelevant. Paper cutting performance is just influenced by edge sharpness.
As to the other media, since leather, woods and cardboard all exert binding forces to various extents, the thickness of both edge and spine of the knife come into play. That knife will be *vastly* outcut by a flat ground SAK on all those types of materials (assuming you are cutting and not chopping), as the SAK will wedge less since it is much thinner. This is why Stanly knives (and other such blades used by tradesemen and for craft work on exactly those types of materials) are very thin, and flat ground for ease of sharpening.
As explained in the above a multi-bevel edge requires less skill and is more versatile in required equipment. The main bevel on a multi-bevel edge has little to no angle tolerance, it is just grind to shape, it can also be left fully rough, both of these vastly reduce sharpening time you can not be as sloppy on a convex bevel and the whole bevel has to be finish polished. The final secondary edge bevel of the multi-bevel can be applied with a v-rod system (which you can set up by eye with flat stones in minutes, no equipment required), in seconds. This is the most efficient way to sharpen edges (outside of power equipment as it doesn't really matter then).
To bring out the origional points, much of the benefits attributed to convex edges are not due to them being convex at all but due to other properties, specifically :
1) the cutting increase and durability gains are from an increased shoulder and final edge angle, the same would be gained from a secondary bevel system first popularized by John Juranich of Razor Edge
2) the ease of sharpening is simply due to the lower angle tolerance when using a soft media, this can also be achieved using a secondary edge bevel and a v-rod (the latter can be setup by eye with benchstones, no equipment required).
-Cliff
Is this how long it lasts before it gets sharpened? Or until it has sharpened so much it has to be replaced? What steel is being used?
pendentive :
[toughness : opinel vs Strider]
The steel type does have an effect, the Opinel is slightly softer but this is indeed swamped out by the thickness, even if an Opinel was made from INFI it would have no where near the strength of a Battle Mistress, how can you even argue that point. The thickness is critical in the blade toughness and strength, and obviously is has the same role in regards to the *same* properties of the edge.
As an obvious recent example :
http://www.bladeforums.com/forums/showthread.php?s=&threadid=280313
The edge, even though convex, was too weak and readily damaged. The solution was to apply a heavier secondary edge bevel which had a more obtuse angle. This clearly showed the obvious point that you can not ignore thickness and angle and just generalize to convex is stronger than flat.
Yes, this does allow them to be sharpened with less skill, as Mel Sorg pointed out years ago on Knifeforms before Bladeforums existed, which came out during a discussion we had on how to maintain the edge on the D2 small utility blade I had from him and at which point I suggested to Joe that he update the FAQ which at that time still had convex edges only being sharpened by power tools. This of course means that convex edges require specialized equipment to allow honing to similar speeds as flat grinds, this makes them less versatile and of course much slower when the *same abrasives* are used for both compared to secondary bevels on flat grinds. It is always easier to grind or cut flats as it is curves.
Actually, they had v-ground secondary edge bevels for durability, see the Axe Book for example. Again it is angle and thickness which are the critical factors in durability not simply curvature.
This is insignificant, both in terms of the curvature unduced as well as the effect it has. If you want to be really anal about such distinctions then all edges are convex unless you use a jig on a *optically* flat diamond plate because otherwise all edges will have some degree of curvature. This of course makes no effect at all on functionality unless the curvature starts to be degrees in size and of sufficient magnitude in depth to exert an influence.
I have compared Opinels with full convex grinds to those with small secondary edge bevels (v-rod at 22 degrees) and you can not see any difference in cutting ability due to the more obtuse secondary edge bevel, but you do gain massive increases in durability and even greater in ease of sharpening. This is the primary reason as to why I switched away from convex bevels on most knives (except for heavy choppers which still usually get the secondary flat micro-bevel commonly used on felling axes, 0.1 mm or so deep, beyond visible)
Yes and it would do all that with a v ground edge as well. Of course the spine thickness has no effect on cutting paper as I noted in the above you need to be thinking about the binding forces, does the paper press hard against the spine, no, thus it is irrelevant. Paper cutting performance is just influenced by edge sharpness.
As to the other media, since leather, woods and cardboard all exert binding forces to various extents, the thickness of both edge and spine of the knife come into play. That knife will be *vastly* outcut by a flat ground SAK on all those types of materials (assuming you are cutting and not chopping), as the SAK will wedge less since it is much thinner. This is why Stanly knives (and other such blades used by tradesemen and for craft work on exactly those types of materials) are very thin, and flat ground for ease of sharpening.
As explained in the above a multi-bevel edge requires less skill and is more versatile in required equipment. The main bevel on a multi-bevel edge has little to no angle tolerance, it is just grind to shape, it can also be left fully rough, both of these vastly reduce sharpening time you can not be as sloppy on a convex bevel and the whole bevel has to be finish polished. The final secondary edge bevel of the multi-bevel can be applied with a v-rod system (which you can set up by eye with flat stones in minutes, no equipment required), in seconds. This is the most efficient way to sharpen edges (outside of power equipment as it doesn't really matter then).
To bring out the origional points, much of the benefits attributed to convex edges are not due to them being convex at all but due to other properties, specifically :
1) the cutting increase and durability gains are from an increased shoulder and final edge angle, the same would be gained from a secondary bevel system first popularized by John Juranich of Razor Edge
2) the ease of sharpening is simply due to the lower angle tolerance when using a soft media, this can also be achieved using a secondary edge bevel and a v-rod (the latter can be setup by eye with benchstones, no equipment required).
-Cliff
- Joined
- Oct 18, 2001
- Messages
- 20,978
I agree and I hope I clarified that very thing in that thread.
This sounds like a pretty rough generalization to me...The reason I switched to convex edges (not full grinds) is two-fold:
1 - it is painfully simple to put on and maintain razor sharp
2 - requires no specialized equipment. Just a belt sander ~$30 at Harbor Freight - which everybody should have anyway for general sharpening, clean-up, etc.
Hey...you said it...not me...
1 - Agreed. However, the flip side of this is that thickness of bevel/knife does not hinder the cut.
2 - I somewhat agree. If you have a full flat grind, then yes, binding is an issue. But if you have a convex grind, then binding is not an issue.
3 - This is absurd. Just last week, I stacked 6 layers of carboard together and used my thickly convexed knife to slice right through it (push cut, no draw). 24 vertical inches, no binding whatsoever. An edge that displaces material efficiently will not bind. A full flat or even flat/2nd bevel will tend to bind.
Just for kicks I got out the carboard and had another run attempting the same thing with my fancy SAK. There is no comparison. The SAK took a lot more effort, was harder to control the cut, and I had to resort to draw-cutting because of the binding. Mind you, this SAK is not dull. It is my EDC. I use it everyday, and keep it sharp. It's great for cuts on thinner material, but can't handle the "tough stuff". Why? Because it's not tough.
4 - They are ground thin to be cheap and replacable. Have you seen the latest Stanley knives with thicker blades now? Seems they finally caught on that folks got tired of wasting time replacing blades...
This is still a comparison based on using bench stones, not on slack belt grinding.
The reason it's easier to slack-belt sharpen an edge is that you don't have to "eyeball" anything. There's no checking to see if you have the right angle on the stone. You just lay the knife up against the belt, and let the sander do its job. It will create the bevel for you.
1 - I agree with this for the most part. However, it is not true to the exclusion of a convex edge, but rather, supports what I have said about "toughness".
2 - running a piece of sandpaper up and down the edge of knife doesn't even require "eyeballing it"...
You have to ask yourself: What if I could just pick up this benchstone or v-rod or whatever and run it up and down the knife in my hand and it would practically sharpen the knife itself? That would be great, eh?
But most folks who try this would inevitably dull their knives 9 out 10 times (including me). But that's exactly what you can do to sharpen and maintain your convex edge. Honing a flat grind requires more precision - that's why there are folks complaining about it here on an almost daily basis.
Once you have established a convex edge, it's just a matter of stropping it to keep it sharp. I don't ever take my knives back to a stone or abrasive, etc. I only strop them now to keep them sharp. What could be easier than stropping?
The convex edge is an advantage over any other grind in these situations and for these reasons:
1 - Ease of sharpening/maintaining
2 - Wherever toughness and durability are a big factor (steel type and thickness aside)
3 - Push cuts (but it has to be a polished bevel, read: stropped)
4 - Chopping (really just an extreme push-cut)
The convex edge is not for everybody, nor for every situation.