The truth about sharpening

not2sharp

not2sharp

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We seldom discuss this, but sharpening is only a temporary solution for keeping our knives at their best. Ultimately, our knives will wear beyond the original edge bevel and become very difficult or nearly impossible to sharpen. At this point we should send the knives back to the original manufacturer, or a local knife maker to have the blade reground to create a new edge.

The question is how do we know when its time to send a knife back?

We often struggle to put an edge back on a knife that simply has no edge left to sharpen. Eventually, we can get the thing to cut but only by inducing a great deal of blade wear.

Ultimately, the life span of the knife will be determined by how far from the edge the blade has been hardened. Once we get beyond this we will be left with a hunk of untreated steel. So it is important to keep blade wear in check, and professionally restoring the edge periodically might be the best way to do this.

Any thoughts?
 
not2sharp:

<font face="Verdana, Arial" size="2">The question is how do we know when its time to send a knife back?</font>
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For flat or convex ground blades you can periodically work a little on the primary grind and thus keep the same edge geometry. For hollow ground blades this is a little more difficult but still possible, and they are designed for a longer lifetime in that regards anyway.

If you don't feel comfortable doing this and want to have it redone by someone else, then when the blade stops performing at the level you want then you get it worked on. However, unless you are doing a lot of unnecessary sharpening or have a really cheap blade, a knife will last a long time before you have worn enough metal off of the edge by sharpening to change the width of the primary grind.

This assumes of course that you are not using the knife as your primary work tool. For example Mel Sorg's father used to work in a paper processing plant and would wear out a knife in a couple of years. He would work on both the primary and secondary grinds so it would remain a very high performance cutting blade but eventually it would get so thin it would not be stiff enough.

Of course there is nothing saying that blades can't change duties. I have seen very old Chef's knives that from 20+ years or steeling on a butchers steel were turned into fillet blades. Then again if they had used a smooth steel instead of a butchers steel that would not have happened.

-Cliff

[This message has been edited by Cliff Stamp (edited 06-13-2001).]
 
I'm not sure why you would want to send the knife to a professional for work on the edge, unless maybe each time you sharpen it you use a steeper angle to save time, and eventually the edge is way too thick. If you sharpen at roughly the same angle each time though, the edge would just travel up the blade without any problem.

At some point, you have to thin down the primary grind so the secondary (or edge) bevels don't get too thick. I've personally never had to do this, but I can sure see why you'd want a pro to do the job. If you don't have power tools it would take forever, and even if you do have power tools, there are a lot of ways to screw up.

Say, Cliff Stamp... didn't you have a pic of an old Calypso Jr. that you totally wore down? That might be a good illustration for this thread.

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Cerulean

"The hairy-armed person who figured out how to put an edge on a suitable rock made it possible for us to be recognizably human in the first place." - J.K.M.
 
Cerulean,

The question is somewhat academic for me. Life is too short, and I have too many knives to seriously worry about wear. But, we should be aware of the issue, and it is something that we need to be concerned about especially when we look at older knives. Then again there are knives out there that have very poor edge geometry to begin with.

We seldom see knife reviews that get into MTBF/lifecycle issues. Yes, the knife may cut well and it may even be easy to sharpen today, but the hard tempered edge might only be 1/4 inch deep.

Or, there may be other issues. If you have a heavily serrated knife, like one of the many Spyderco models, we know that we can use a Sharpmaker to sharpen the knife. But, with each sharpening we are going to distort the serrations. Perhaps the knife should be sent back and re-tuned after it's sharpened 10 or twenty times. Jerry Busse makes a terrific knife, and his custom series bears a special edge bevel that is convexed on one side and flat on the other. It cuts great, and we can restore it by sharpening the flat side of the edge. But, the convex side is less than an 1/8 inch deep, we will need to restort to a slack belt or give up some of the advantages provided by the initial edge bevel.

The knife may be built like a fine automobile, but even a fine automobile needs regular maintenance, and sometimes a knife needs a little more than a few passes on an abrasive surface.
 
This is one of the reasons that I like hollow ground blades, the blade will go about 10x as long before you would need to do a regrind of the secondary bevel (the hollow). At that point you'd probably consider junking the blade since it will be getting significantly smaller than it started out.

On full and saber ground blades I work the secondary bevel almost every time I sharpen them. I totally ignore the manufacturers intent and thin down the secondary bevel the first day I own the knife. I keep reworking it to my standards (thin!) whenever it needs sharpening. I often resharpen the knife and rework the bevel when it doesn't need sharpening. I'm still working on achieving nuclear fission using a knife edge.

For me life is short and I'd rather spend it sharpening a knife than using a knife.
 
Cerulean:

Sure the edge travels up, but the steel behing the edge gets way too thick. See the Razor's Edge sharpening book for a picxture of this. I'll try to explain it here:

Imagine a flat ground blade from 1/4" stock: A Busse Battle Mistress is a good example. Imagine you have sharpened the knife so many times that there is only 1/2" of blade width left. The thickness of the steel near the edge, if the Battle Mistress is only 1/2" wide, would be nearly 1/4". If the steel right behind the edge is 1/4" thick, you will have a hard time cutting through ANYTHING!!! Grinding the steel behind the edge so that it is thinner is called grinding n relief, and this is a very important step in sharpening.

What Cliff was talking about is that if you have a flat groudn blade, sometimes when sharpening you ideally should lay the blade flat on the stone and grind down the main bevel again a bit, to maintain the relief. This step will wreck the finish on your blade, however. A good compromise is to not lay the whole blade flat on the stone like that, but to grind in say a 10 degree bevel or so on both sides, and then set a 20 degree beveled edge. You'll have thin steel behind the edge and a nice sharp knife again.

Not2Sharp:

I've seen a pic of a loveless blade that was sharpenned a billion times. It looked liek a funny boning knife instead of a skinner! i think it just takes time to get to know your blade to know when it is tiem to have it reground. Not all things about knives are academic; some things are intuitive.

------------------
"Come What May..."
 
Pardon my ignorance...but is the whole blade made out of the same material with the same properties? Such that you can reprofile the blade and then get basically the same cutting edge it had when new, just higher up on the blade?

Or is only the metal near the cutting edge hardened and treated so it's a good edge?
confused.gif


Thanks in advance.
- Johnny
 
Most blades go through a uniform hardening and tempering process and therefore don't suffer a lot from reprofiling. This is an approximation since any rapid quenching process in the initial hardening will tend to reach the thinner edge region faster than the core of the blade. If you hardened the blade and then did several tempering cycles you will achieve greater hardness uniformity when you are done.

Some heat treating processes are less uniform either intentionally or as a side effect. When the quenching and temporing are not controlled precisely you may get an edge that is harder than the core, but in this case the edge may be harder than optimal and the core may be closer to the ideal. In this case the knife gets better as you wear it down. In custom knives made from alloys that harden a bit more slowly the maker will intentionally make the edge harder than the core. I would expect the hard edge to be 1/4" to 1/2" deep and allow some considerable reprofiling before you hit the soft zone. In this case the soft zone will be really soft (down to 50 RC).

 
Jeff,

1/4" sounds pretty frail. When you consider where this area is located there is very little harden mass behind the edge. Even worst, every time we sharpen the knife we loose material from the edge and both sides. By the time you loose 1/8" of edge depth you would have lost about 75% of your hardened edge mass.
 
Interesting post, I was sitting around pondering this very topic while reading the Razor Edge Book of Sharpening. It seems that on knives with a flat ground blade, there should never be a problem. The thickness behind the edge is the same as the thickness of the spine, so over time the edge would travel up the blade and no difference would be present in sharpness if the tempering and edge angle remain the same. For saber ground blades, it all depends on how acute the relief is. If you have a relatively thin relief, then you would simply be able to use the back bevel technique as opposed to completely regrinding the relief. But if you have a rather obtuse relief then the problem would arise quicker. It seems that Razor Edge Products does not really take into account that many expensive knives should not be tapered back EVERY time, this would severely compromise the life and appearance of the knife. They seem to not take into acccount either that many knives come with expensive coatings that would be scraped off quickly if you were to grind the relief as thin as they advise. i would use a strop or steel as much as possible before going to abrasive stones. Despite what Razor Edge Book of Sharpening says, you CAN touch up an edge with fine abrasives instead of completely resharpening the entire knife from scratch. I do it all the time with the Sharpmaker. The only time I would think that reprofiling of the blade would be necessary is if you have poor relief to begin with or if you have ground all the way up close to the shoulder of the original relief, which in some cases could mean that you have sharpened the non-tempered area or are fairly close and a new knife is needed. Hope this helps.
 
not2sharp, I don't request that edge-hardening only be 1/4-1/2 inch deep, I just kind of think thats what the methods are likely to produce. If your edge hardening goes too far back the knife becomes brittle and you've lost the advantage of differential hardening. There are a lot of funny ways people do these things--dipping just the edge in your oil bath, torching the back edge of the knife for an annealing effect, and pouring a light stream of water on the cutting edge of a normally oil-quenched alloy.

Here's a link to a picture of one of Maddog's differentially hardened blade that shows the hardness of different sections of the blade.

http://www.bladeforums.com/ubb/Forum32/HTML/002634.html
 
Soem further thoughts on this reprofiling bit, and a reply to Carl:

Indeed the Razor's Edge book ignores expensive finishes and coatings and such. In the book they even talk about this, and sharpen a knife with an engraving of a deer on it. John Juranich says he'd rather thave a sharp knife than a pretty, but dull, knife. His book is about 100% performance of the blade.

This is why the "modified" technique that I mentioned works well. You don't have to re-grind the whole knife each time. you don't even HAVE to grind in relief each time your knife doesn't shave hair anymore.

I have a Benchmade AFCK, which came with thick bevels. I clamped on the guide and ground in relief USING their guide, just so that it would be even and such. Grinding relief with that precision is not necessary at all, by the way. This relief "edge" is 1/8" wide!!! After I ground in the relief edge, I set the guide up properly and sharpened the knfie with a regular, double beveled edge. So in all, that makes the main grind, my relief grind, plus the 2 bevels of the double grind, which makes a total of 4 bevels per side. Note, though, that the double ground edge is tiny, tiny, tiny. But man could this knife cut after that!!!

Since then I have used my sharpmaker to touch up the double ground edge. Once sicne the initial relief grinding have I re-established the relief grind, though that time I used a water stone.

My points are:

1) Relief is IMPORTANT!!

2) Grinding in relief need not be done EVERY time

3) relief grinding won't take away your pretty ti ni black mamma jamma coating! Well, it does on the "small" relief section, but you don't need to grind it off all of the blade!

4) relief grinding takes only a rough stone, a shallow angle, and a good rough scrubbing: accuracy is not all that important

5) if you grind in good relief, putting an edge on a knife takes VERY little time to do.

6) you shoudl be maintaining your edges with strops, steels, ceramic rods, or whatever. Grinding in relief need not be done every sharpenign session. Maintaint he edge until it gets too thick



------------------
"Come What May..."
 
I've been a pro-meat cutter for many,many years and I'm sure I sharpen a knife more times than any body on here and I must say
you guys most be doing it wrong or something!? I've never wore a edge down like you guys are talking about! My good friend at my job has been a meat cutter for 30 yrs and is still using the same 10" breaking knife he learned to cut with and it only has about 10% blade wear. What are you guys sharpening on a brick?? Not tring to be funny
but it seems odd to me!
smile.gif

Todd

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"when in doubt,empty the magazine!"
 
Maybe we should be considering a less destructive way to keep our knives sharp.

I believe that it is a much better idea to first use a leather hone and then if that doesn't work a smooth steel and if that doesn't work then use the Sharpmaker (or similar device) then if all still fails reprofile on benchstones.
*Note-Remember don't forget the backbevel (or secondary edge when sharpening on stones.) Also remember that you will have to backwards through these steps to put the finishing touches back to your blade. That's why it pays to maintain rather than let the edge go.

This procedure will only work on a non-damaged blade. It is designed for regular touch-ups. If the blade must be reground/reprofiled then do all those steps above in the reverse order.

Using a hone or steel regularly is far less destructive and wear indusive for a blade edge.

Sounds logicle to me at least.
smile.gif


--The Raptor--

[This message has been edited by Raptor (edited 06-13-2001).]
 
Wow, good post. I also have the Razor Edge book, and am fuzzy about some areas.

Let's say that you are using a sharpening system such as an Edgepro. If you lay down a 30 degree (total) bevel on the edge, why worry about the secondary angle/bevel? 30 degrees is 30 degrees.

The book talks about having a max blade thickness of 0.02 inches about 1/4 of an inch behind the blade, but then talks about the grinding the secondary bevel all the way up the blade.

I think a GOOD picture here would be worth a thousand words...
 
The only problem I see with the "modified" technique described is the fact that the primary edge is very small, and you cannot "touch up" the edge if it goes dull. You need to regrind the whole edge, not only because it is difficult to mimick the exact angle you used last time due to the very small tolerances, but also because many people are not sure as to the angle used for the final primary edge. This happens if you use the Razor Edge system, the final angle is unknown, which makes it almost impossible to duplicate it when resharpening. Other than that I really see no problem with it, and it sounds effective. If only we lived in a perfect world where knife edges stayed crispy forever, and knife sharpeners of all kinds were put in museums for us to laugh at.
 
Jeff Clark...

<font face="Verdana, Arial" size="2">
For me life is short and I'd rather spend it sharpening a knife than using a knife.
</font>
Click to expand...

That's pretty radical! How long have you been doing this, and what have you learned about what kind of an edge can be given to different types of steels and geometries? How do forged vs stock removal blades perform when you try to give them that atom splitting sharpness? Do you have any standardized tests of your edges? What would you do differently on a chopping tool as compared to a small utility knife? How thin is too thin? Someone who takes an interest in sharpening to your extent would be great on a blade test committee.

Somewhere, someone is using terms reversed in this thread... My understanding is that the "primary bevel" is the one that starts at the spine, or maybe at some point below the spine if the upper part is "unground". The "secondary bevel" is the one that begins (typically) about 1/8 to 1/16 inch from the edge and forms the actual edge... Please tell me someone if I've got these reversed.

Todd
<font face="Verdana, Arial" size="2">
you guys are talking about! My good friend at my job has been a meat cutter for 30 yrs and is still using the same 10" breaking knife he learned to cut with and it only has about 10% blade wear.
</font>
Click to expand...

This sounds about right to me. I would not be suprised if a lot of people over-sharpened, or more commonly just didn't maintain the edge the easy way when it would have made a difference. I have to admit I have been guilty of this myself, but I have learned that at least initially, I find I like a secondary bevel of just under 20deg. for a knife up to the utility 3"-4" blade size, and even larger. I find that most decent modern steels (even the simple ones) are adequately tough to let the edge be this thin and not require resharpening over-often. Rather than monkey around with a third bevel (something between the primary and secondary) I achieve this secondary bevel thinning simply by letting the thickness of the secondary bevel grow, typically to about 1/8" on my medium sized survival and utility knives. Even my hollow ground Livesay Air Assault got a little wider secondary edge when I first thinned it. Its been on a couple of camping trips with only a couple of strokes on a fine (sometimes I've been known to use the coarse/fine combo) DMT hone to bring the edge back. At this rate, I won't notice edge wear any time during my lifetime.

 
TODD1:

I worked in a pork plant for 2 summers. We processed 800 hogs per day, 5 days a week. We also went through a lot of knives there. Though there was a lot of sharpening done there (i.e. maybe too much), I don't see how a meat cutter there could use the same boning knife for 30 years!

What is a breaking blade? What are the dimensions? How is it used? What is it made of? What kind of edge is on that blade?

All of these questions are VERY important when talking about edges. The knives in the plant I worked at were Victorinox boning knives. I sent 1 to Phil Wilson for hardness testing. I forget the exact number, but it came out to above 50 but below 52!!! Soft buggers. If someone had a 420V boning blade there, I bet it wouldn't need sharpening all day, where the Victorinox blades were touched up on stones by the meat cutters 2 times a day.

Take a look at chinese cleavers: I bet a lot of the cleavers I have seen people use in "China Town" (Sorry if that term offended anyone: e-mail me and I'll change it) in the big city near me are old and will last many many more years. Is a preaking blade for twisting apart bones? If so, that isn't really cutting.


Which brings me to exactly WHAT is being cut. You cut meat all day. Not2Sharp is probably thinking about his EDC or "bush" blade. My AFCK is used to cut a lot of card board, plastic straps, pop can six pack rings, letters, slice other paper, whittle on wood (made a walking staff the other day!), sharpen pencils, etc. This is much rougher on the edge than meat. I steel the edge to maintain it. I bet a lot of people don't steel or strop, and that causes too many trips to the sharpener. But if you are cutting things like I am every day, you need to go to the sharpener more often than if you are just cutting meat.

Also, it depends on the edge you want on your knife. If you like a hair shaving edge and you don't have a super CPM alloy blade, the hair shaving sharp edge will wear kinda quickly. Though a long lasting working edge still exists, it doesn't shave hair, which is often a signal to knife users to go to the sharpener. If you like a hair shaving edge on your EDC user, you'll have to sharpen more.

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"Come What May..."
 
Very interesting topic with a bunch of thoughtful, intelligent replies.

This may seem a bit of the "profound grasp of the obvious", but I think the real answer is to touch up your edges before they really get "dull", if this is practical under the circumstances. I use the white(fine) stones on the Sharpmaker. That way, very little metal is taken off and a very long blade life is assured.

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Dick

AKTI Member # A001365
 
Great thread, dudes!
smile.gif
<font face="Verdana, Arial" size="2">Somewhere, someone is using terms reversed in this thread... My understanding is that the "primary bevel" is the one that starts at the spine, or maybe at some point below the spine if the upper part is "unground". The "secondary bevel" is the one that begins (typically) about 1/8 to 1/16 inch from the edge and forms the actual edge... Please tell me someone if I've got these reversed.</font>
Click to expand...
That's my understanding too. I think things get really confusing in these threads because Juranitch talks about secondary and primary edge bevels.

I don't have the Razor Edge book, but from what I've extrapolated, the ideas in the book are similar to the one's he expounded on in his Popular Science article here: www.ameritech.net/users/knives/Juranitch1977Feb.htm .

In the article, Juranitch calls the relief bevel the "secondary edge bevel" and the bevels that meet to form the actual edge the "primary edge bevel". These terms frequently get confused with "primary bevel" (the main grind that starts at or just below the spine) and "secondary bevel" (the final bevel that forms the actual edge). You don't necessarilly have to have a secondary bevel though; the primary bevel could go all the way to edge, as in a Scandinavian grind.

The Scandinavian grind (saber grind where the primary bevel forms the edge) seems to cut very smoothly. That's probably because there aren't any angles to hang up on material as you push and draw the edge into and through a cut. That would also form the basis of a possible criticism of the Razor Edge method: For each relief bevel you put on (and some people may put 2 or 3) you're adding another angle that increases drag. If you're a fan of the Scandinavian grind (or Japansese swords, which have a similar design), you'd argue that even one edge bevel is a compromise for the sake of strength.

By the way, I found a page that might help illustrate what I was talking about when I wrote, "I'm not sure why you would want to send the knife to a professional for work on the edge, unless maybe each time you sharpen it you use a steeper angle to save time, and eventually the edge is way too thick." www.ameritech.net/users/knives/Tormek10.htm

Unfortunately, this illustration doesn't show why you need to do something to thin out the primary bevels after awhile, because the "bevels" in these pictures are not angled at all; they're totally flat. In this regard, the illustrations look much more like a Scandinavian grind if you just change the scale a bit. Or, it could be a hollow grind, but not any kind of flat grind.

Both of the links I made are to Steve's Sharpening Site, a really great site. http://www.ameritech.net/users/knives/index.htm

[This message has been edited by cerulean (edited 06-14-2001).]
 
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