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Ed Caffrey's Workshop Talk to Ed Caffrey ... The Montana Bladesmith! Tips, tricks and more from an ABS Mastersmith.

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  #1  
Old 01-19-2002, 03:23 PM
Josh Blount
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Hollow grind vs. flat grind


I'm curious about what causes the difference in cutting between a hollow grind and flat grind. I personly prefer the flat grind for my knives, but I'd like to know the difference. As I understand it (I've never done one myself) you grind the hollow, h/t, and then put a flat or convex edge on the blade using the slack part of the belt. Is that right? If so this is basically the same edge as a flat ground blade, except for the cross-section of steel that's behind it. At least this is what it looks like on most of the hollow-ground knives I have. Any thoughts on this?

Josh Blount
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  #2  
Old 01-19-2002, 06:39 PM
Ed Caffrey
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Josh,
Your line of thinking is correct. The BIG difference between the two it that area just behind/above the edge. I true hollow grind actually has no edge bevels, or very minimal ones. The grind concaves in from the spine to the edge creating an extremely sharp edge..........however, it is also a delicate edge and will not tolerate any abuse. What most knifemakers call a hollow grind is really a semi-hollow grind. The hollows are ground to where they run out just before reaching the edge of the blade. This leaves a mass of material just behind where the edge will be. On most "hollow" ground knives the edge bevels are at a very steep angle, and even those who do creat a convex edge, still must contend with the "bulge" of material just behind the edge bevels. Nearly always these bevels are angular and they will create a wedge effect when cutting. Basically it increases cutting resistance by making the user apply more force to the knife. Here's a little experiment for you to try.......... get two blades, one with a "hollow" grind and the other with a flat grind. Put on some opti-visors, or watch under a magnifying glass and cut some smooth leather. 99.9% of the time the "hollow" ground knife will "wedge" up and you can actually see the edge being lifted away from the leather by the edge bevels. Do the same experiment with a flat ground blade that has a good convex edge on it. The leather will follow the blade until it gets to the spine and then some minor "wedging" will occur.
Now, for a real eye opener, take a finely ground convex blade, and do the same thing.............there will be no wedging anywhere along the cutting path, and once the leather reaches the spine, it will "flow" right back to the other side. If you pay attention during the cutting test, you will also notice that there is a great deal of difference in the amount of force necessary to cut between each type of grind.
If you haven't guessed by now, I'm not a big fan of "hollow" grinds. Don't get me wrong, they do have their uses, but not on a high perforance knife that is likely to see a wide variety of cutting chores.
There is one more consideration that forces some makers to use hollow grinds.... That is the fact that some steels will not hold and edge well, unless the Rc is very high. Along with that high Rc hardness comes brittlness. The extra "meat" left behind the edge helps to compensate for the brittleness by providing a thicker cross section for added strength.
As always in knifemaking, it is a give and take situation. Each maker must determine what qualities are important to them, and which are not so important. The only time I can remember rolling my eyes is when I asked a maker why he chose to hollow grind his blades, and the answer I got back was "because I can get that nice grind line up the blade." Just my thoughts, but that's the wrong reason.
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  #3  
Old 01-20-2002, 01:38 PM
Knifemaker01
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Holy Twilight Zone...is this possibly *GRINDER TALK*>>?? i KNEW i WOULD CONVERT YOU eD. lol

Steve
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  #4  
Old 01-20-2002, 02:56 PM
turkeyman8
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Ed, I agree with you about the flat and hollow grinds. The flat ground hunter you made for me out of 5160 is one of the finest cutting blades I've ever held in my hand.

Larry
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  #5  
Old 01-20-2002, 04:22 PM
Don Cowles
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Since I am a hollow ginder, I do have to make one observation. If a hollow-ground blade has a swell near the edge, it was improperly ground, which could be either poor technique, or too small a wheel. The idea is to have the two radii meet the edge while they are still converging, rather than having begun the trip away from each other.

This does indeed make for a thinner (and, thus, more fragile) edge, but if you are in doubt about its slicing capability, study the geometry of a straight razor.
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  #6  
Old 01-20-2002, 05:26 PM
Knifemaker01
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Amen Don.

Steve

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  #7  
Old 01-20-2002, 06:30 PM
MIKE KOLLER
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Each edge has its place,or so I feel.And each has its drawbacks and limits.
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  #8  
Old 01-20-2002, 09:29 PM
Ed Caffrey
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Don,
Your right! If the hollow grind is done CORRECTLY it will come out just as you said. Problems is that most folks don't take the time or effort to see that it does come out right. Again, just like you said, if it's done correctly there's not a better "slicer" around. I get to study numerous knives at shows and find that about one out of every 20 folks who hollow grind make the effort to get it a true hollow grind. I usually ask most makers why they hollow grind (in private of course), and am usually surprised at the answer(s). The folks, such as yourself, who know and understand why they are doing a hollow grind generally have "all their ducks in a row".
As for Steve............well, I think he just has "Pounder" envy! LOL!
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  #9  
Old 01-20-2002, 09:43 PM
Knifemaker01
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GRRRRRRRR!!!!

Steve
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  #10  
Old 01-21-2002, 08:37 AM
Terry Hearn
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:rollin: :rollin: :rollin: :rollin:
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  #11  
Old 01-21-2002, 09:21 AM
Cactusforge
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Ok Ed & Don I have a question, edge thickness before
it is sharpend, do the bevals or hollows meet at infinity or do thay stop at .005 .010 .015 or more. I currently stop at .010 on hollow ground small hunters, convex ground large Bowies at .020+ depending on the knife Gib
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  #12  
Old 01-21-2002, 10:37 AM
Don Cowles
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Gib, mine are at about .020". Any thinner, and the blade would be too fragile.
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  #13  
Old 01-21-2002, 07:41 PM
Cactusforge
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Thanks Don Gib
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  #14  
Old 01-21-2002, 08:17 PM
Ed Caffrey
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My intent when I'm doing a final grind is to have the blade sharp by the time I come off the 400 grit belt. I do my best have the smallest edge bevels I can when the blade is finished. Actually on about 75% of my knives I combine a flat grind (to remove excess weight), and a convex grind (to ensure I get the strength I need). The results are usually a blade that is sharp when done, but a minor "touch up" is needed before it goes to the customer. Realistically I don't think there is a way to have no edge bevels at all. The trick is to make them as tiny as possible (while building in a very slim taper from the area just above the edge....to the edge) all the while ensuring there is enough "meat" behind the edge to withstand abuse.
That is part of what I'm talking about when I write/speak about "The Overall Package".
The maker has to figure all the factors out to produce a blade, of the chosen steel, with a specific heat treat that will allow him/her to achieve the grinds/edges that make a blade "high performance".

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  #15  
Old 01-22-2002, 08:44 AM
Cactusforge
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Very informatvie this shedes a lot of light on this question.
Thanks Gib
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