Edge grinds and sharpening

[James Y]

This may sound like a silly question, but how do the various blade grinds affect the long-term sharpenability of knives?

For example, do knives with thinner, flat or hollow-ground edges resharpen more consistently over time than, say, saber or chisel-ground blades, which have more abrupt angles?

This isn't a question of better or worse, just a curiosity. I'm under the impression the saber-ground blades would require more work to touch up, all else being equal.
Jim

 

[Corduroy]

It's a simple matter of geometry. Take out a piece of paper and draw the cross section of any grind and you will have a good idea how it will sharpen over time.

Simply put, each sharpening removes some metal. Quite a bit more than will ever be lost in use. This means the bevel (the final cutting edge, not the "grind") must get deeper (width, in an edge-to-spine direction) if it is to be replaced at the same angle. How much deeper depends upon the blade's cross-section. Flat-ground knives will require a constant increase in the depth of the bevel as they are worn down. Hollow-ground knives "cheat" slightly by presenting an edge that initially increases only slightly in thickness, so the bevel deepens little. Convex-ground knives do the opposite.

A saber-ground knife has a much steeper bevel than a fully-ground knife of the same width, because part of its width is unground (or ground in the opposite direction to make a false edge). To see how it will sharped, imagine the blade is only as wide as the grind.

Personally, I see little functional reason to saber-grind a knife, unless it is double-edged. I'd rather make it thicker and fully grind it if increased strength is required. But complex grinds appeal to the eye and give makers a place to show their talents in keeping grind lines crisp and straight, so they are more common.

That's just a small start at this question. I'm eager to see how others reply.

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-Corduroy
(Why else would a bear want a pocket?)

 

[george tichbourne]

Edge grinds do affect sharpenability. The steeper the grind the more difficult it is to repeatably resharpen the blade.

Edge grinds are not chosen however with a view to resharpening. The grind is chosen based on the type of work that the knife is expected to do. I use four different types of grind depending on the intended use of the knife.

Convex grinds are used in severe duty knives such as my moose hunter where the knife is subjected to a great deal of abuse, notably prying and twisting.

Saber or partial flat grinds are used on my Bowie knives because these will occasionally be used to chop with and need weight that the full blade thickness gives and the saber grind gives the edge enough rigidity to resist most damage.

Full tapers are used on light duty knives where slicing ability is rrequired but edge stiffness is also desirable.

Hollow grinds are used where slicing is of paramount importance and the hollow grind reduces slicing resistance. I will not use a hollow grind on any knife that might be used to pry or twist, the edge is just too weak.

The steel used and it's heat treatment is more important to the user than the grind configuration because a knife should hold an edge and not require frequent sharpenings thus leading to your question.

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george
www.tichbourneknives.com
sales@tichbourneknives.com

 

[Cliff Stamp]

George :

The steel used and it's heat treatment is more important to the user than the grind configuration because a knife should hold an edge and not require frequent sharpenings thus leading to your question.

On a related question to the above comment - how long should a good knife last? For some its practically forever. If you have say a dedicated skinner then how much work does it actually get in a year - very little. I would expect it to last a lifetime and change very little. However what about knives that are meant for every day utility and that have a wide range of uses?

For example Joe T. often comments that his Deerhunter (or similar) is used while camping, in the kitchen and generally for most utility work on a daily basis. Lets assume that you only have the one knife and don't rotate through an almost endless collection - how would even the best of knives hold up under constant use and how long would they last before the edge bevel started to get too thick from repeated sharpenings?

For you custom makers out there how would you approach this if a customer asked you to look at his knife that was suffering from this affliction? Would you say "Get a new knife that one is worn out" - or would you attempt to reprofile it? The only comment I have seen in this regard was made by Bill Martino regarding khukuris used in Nepal that have been handed down in families. Eventually the edge that has been differentially tempered has worn past the hardened part and they need to be rehardened - which they are. They seen to really wear knives out over there, but it does take them awhile.

I have seen this on kitchen knives that are really old (like 20+ years) usually as the profile changes so does their task. They might start out with the wide profile of a chefs knife suitable for chopping/dicing, but the gradually become a very narrow boning / fillet knife.

-Cliff

 

[Joe Talmadge]

James,

Each time I sharpen, before I actually do the edge bevels, I grind in what I call "thinning bevels" first. One of the roles the thinning bevels play is to continue thinning out the blade as I sharpen up, so that I put off the time when the edge is so thick that it's not worth sharpening anymore.

 

[george tichbourne]

Cliff
Reprofiling is definitely the way to go if the knife is badly worn if the customer will allow it, some would never change the profile preferring to show off the wear with pride.

What I was trying to get at is the fact that a properly treated blade of quality steel will seldom become dull enough to require a great deal of metal to be removed to restore the edge. Usually about 6 or 10 licks of a diamond stone will restore the dullest blade if the steel is up to snuff, if this is followed by a couple of strokes on a leather strop to remove the toothy edge the edge will hold for a long time.

When I started making knives it was because I was tired of resharpening my knife 5 or 6 times every time we dressed a moose. My target therefore was to make a knife that would hold an edge long enough to dress a moose without needing resharpening. Last fall two people called and said that they had field dressed, skinned, and quartered two moose in the first case and two elk in the other without resharpening their knives.
As a matter of fact both individuals said that their knives were still sharp but not sharp enough to shave with after their jobs were over. This type of performance is not unusual for custom knives.

With this litle wear on the knives after what I assure you is a great deal of work I don't expect that much metal will have to be removed during a knife's lifetime to resharpen it. Butter soft blades will have to be reshaped several times during their lives and probably will be assigned new roles to play as their shape changes.

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george
www.tichbourneknives.com
sales@tichbourneknives.com

 

[Cougar Allen]

Sheesh -- a knife isn't worn out just because the edge is getting thick; it only needs some grinding behind the edge. Adding a third bevel like Joe says is one way, or you can grind at the primary bevel of a saber grind instead. I like a slightly convex grind and most of my knives end up that way eventually. Probably the easiest way to do it is to add a third bevel and then round off the extra corners. When I feel like sharpening and don't have any dull knives I look for one that could use some edge thinning. I like to keep them thin enough so they're easy to touch up, thinning them when I feel like it so it doesn't take long to touch them up (because a dull knife has to be touched up now even if I don't feel like prolonged grinding right now).

-Cougar Allen :{)

 

[Cliff Stamp]

Cougar, I don't throw away knives that have been used so much that the edge has moved back really far on the grind. However they have certainly been worn out to the point to which they no longer have the geometry suitable for their original purpose. Its not just because the bevel is on a thick part of the blade as you can thin that out as you and Joe have described. The main problem is that you will have lost substantial blade width and a bit of length as well. Like George said before, they generally end up doing something else more suitable. Of course if the temper is differential then there are bigger problems to deal with.

-Cliff