How does the blade grind effect performance?

[jujawa]

Can somebody explain the specific strengths and weaknesses of different blade grinds (hollow, flat, convex, and all the others i hear you guys talking about). I read all about knife grinds, and i have become able to tell the difference between some of them by looking, but i dont really understand how they effect the performance of the knife. Thanks in advance.

 

[Anarcus]

Well there's an argument for buying at least one of each

 

[Cliff Stamp]

Can somebody explain the specific strengths and weaknesses of different blade grinds (hollow, flat, convex, and all the others i hear you guys talking about).

The first general rule to consider is that the more steel left on the blade the stronger it will be and the worse it will cut. This is often more important than the type of grind. Both of these are specific to where the steel is on the blade. You can have a knife with a really thick spine and thin edge for example which cuts well for shallow cutting but performs poorly for deep cuts and is very strong for prying with the blade but weak with the edge.

In terms of the grinds, *at the same height* on a blade, generally cutting ability goes convex -> flat -> hollow from worst to best, and strength and toughness runs the other way. Note the edge geometry plays a significant factor as well. You can take a blade with a really efficient primary hollow grind and put a horrible thick and obtuse edge on it and it won't cut well at all.

There are other facts as well such as the rate of edge thickening which can be a factor in long term sharpening is improved from convex -> flat -> hollow. Ease of low angle sharpening runs the same. However reducing wedging in binding materials can often run the opposite. A hollow ground blade on soft woods can bind so bad it has to be knocked out with a stick for example.

-Cliff

 

[jujawa]

Very interesting, i'm glad i asked. I had been thinking that a flat grind must be the best for cutting because my military, calypso jr, and manix seem to cut real well.

 

[Cliff Stamp]

Yeah, Spyderco uses a high flat grind and tapers the edge very thin and acute so the blades cut very well. People often give hollow grinds a reputation for low cutting ability after handling something like the hollow ground Strider WB's with the low hollow grind and the thick and obtuse edges. However there is no reason why hollow grinds have to be that way, they can be high and taper to fine edges.

-Cliff

 

[jamesraykenney]

Can somebody explain the specific strengths and weaknesses of different blade grinds (hollow, flat, convex, and all the others i hear you guys talking about). I read all about knife grinds, and i have become able to tell the difference between some of them by looking, but i dont really understand how they effect the performance of the knife. Thanks in advance.

Here is an article on it, but I do not agree with everything in it... But it is usefull anyway!

 

[WadeF]

However there is no reason why hollow grinds have to be that way, they can be high and taper to fine edges.

-Cliff

Like a Sebenza!

 

[Cliff Stamp]

Like a Sebenza!

Yeah that is a hollow grind geared towards high cutting ability, contrast it for example to the hollow grinds used by TOPS/Strider which aim for main blade strength.

-Cliff

 

[Jeff Clark]

Think of it this way, your cutting isn't done until the knife comes out of the other side of the material. If you were shaving hair that is .005 inch thick all that would matter is the first .005 inch of blade edge and bevel. You need that to come to a narrow edge and have an acute honing angle since hair is kind of hard and flexible. An extreme hollow ground blade like a straight razor is very convenient for this work since you can hone at a low angle and not need to remove much material in the process.

Usually we are cutting thicker material which pushes back or drags on more of the blade. Often this material pushes back a lot harder than hair and requires a blade with better support behind the edge than the few hundredths of an inch found on razor blades. You are looking for the optimum blade shape to get through the particular material you are cutting.

Rope is a lot like braided hair. It isn't sticky so it doesn't drag on the blade and the fibers bend and part individually so they don't usually pinch the blade to resist cutting. Rope responds well to a well honed hollow ground blade. Meat is made of muscle fibers in a very flexible binder and doesn't take much force for the blade to seperate. It can be sticky and drag on the sides of the blade as you work. I like a thin hollow ground edge for this since it offers less drag right behind the edge and is the easiest to hone to an extremely low angle.

Most other things you cut are wider than hair or rope fibers and stiffer than meat. A major factor in cutting these materials is pushing aside the material that you have already cut so that you can advance the edge through the materials. These materials can also have hard spots that will ding your edge if it is not well supported. You want a blade shape that will plow through material that you are cutting like a ship through the water. There is a lot to be said for the sleek lines of the bow of a destroyer. It is hydrodynamic and strong. A convex edge on a thin blade gives an excellent combination of thin edge with good lateral support. It avoids abrupt contour steps on the edge which can increase drag. If the blade is thick or real wide you may find that even a convex blade is hard to get through the middle of a stiff material. The other big advantage of a convex blade is that it is easier to get back out of your cut if you have to make another chop. If the convexing goes the full width of the blade (the blade is not a variant of a saber grind) you have a highly optimized blade for a wide range of materials.

Thin flat blades like a Calypso Junior are not as rugged as convex blades, but for most materials they are tough enough. You could think of them as approximating a convex blade with a slight angle step near the edge and a very large radius of curvature on the convexing. They allow you to whittle material off surfaces at very low angles. If the blade is pretty wide you gain some lateral strength at the sacrifice of a bit of extra drag. With a little artfull honing you can convex the last few hundredths of an inch of edge on a flat blade to give the edge a little greater strength and lower drag.

Whether you convex the bevel or not, you sacrifice some cutting performance and blade maneuverability if the primary blade grind does not extend the full width of the blade. Basically these variants of the "saber grind" are more obtuse and require ramping cut material up and out of the way at steeper angles. This requires more force. Even hollow ground blades suffer from drag as material digs into the shoulder where the grind meets the flat of the blade. If you are whittling material off the outside of a stick the main problem you may note is that you can't whittle at as low an angle with a thicker saber ground blade. If you take deeper cuts it will take you significantly more effort to push through the material. If your saber grind is flat like a scandinavian Mora blade you will have an acute edge, but you will still find those deep cuts harder than on a Calypso. If you have one of the thinner bladed Moras this may not be significant. Convexing a Mora would not make the edge cut better but it would make the edge stronger and might reduce the drag at the top of the bevel a little bit. You would probably see the performance as reduced in most instances. All of the disadvantages of a saber grind get worse as the blade gets thicker or the bevel gets narrower.

So in general thinner, narrower and smoother blades are more efficient on thick stiff material. A thin, narrow, pen knife blade that has been stropped convex is an incredible cutter. Everything else is overhead for the cutting process. There may be reasons that you need more blade material, but it isn't to improve "cutting". It may be to improve chopping or prying.

 

[phatch]

I must be a schizophrenic. My main EDC is one or more of the following.

BM RSK MKI-- high saber ground
AG Russell Featherlite-- High hollow grind
Bark River OMF -- Convex

A little of each I guess.

Phil