sharpening very dull blades with recurves from wear

B

bgentry

Joined
Aug 3, 2009
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It seems like I'm getting 3 out of 4 blades to sharpen these days that are beyond dull. Blades that have either been used really hard and sharpened poorly or blades that have been used for literally years without a single sharpening. In most of these Used and Abused blades, I find some amount of recurve in the middle or rear portion of the blade.

Some are so pronounced that you can easily see it just looking at the blade. In others it shows up as just a thinner bevel in the recurved area.

So far my approach has simply been to sharpen as I always do. As the burr begins to form, I focus on the areas where the burr hasn't formed yet and it gradually creeps down or up the blade (or both). As it approaches the recurved section, I usually hit a point where I have to grind and grind and grind and grind some more with no burr forming for a long time. I realize that what I'm really doing is grinding down the surrounding area of the blade, so that the recurve can finally touch the stone properly and grind the bevel in that area.

In fact in the surrounding area, especially at the heel, the bevel gets wider. So I end up with a bevel that is normal to thin in the area that was very recurved and a wider than normal bevel at the heel and perhaps in the immediate area closer to the tip from the recurve. The overall blade shape flattens out as well, but it doesn't become perfectly flat; part of the recurve generally remains.

I've tried focusing on the recurve area by using pressure from my support hand right over that area. It helps for sure. I've tried grinding with the heel or the tip of the blade *not* touching the stone so that I'm almost sharpening the recurve on the edge of the stone. With diamond stones I don't like this because the point pressure can strip off the diamonds. It also feels like I'm cheating the blade and not reforming the edge like I should.

I've considered biting the bullet and grinding the edge down flat, edge first, into the stone until the recurve is gone. But that's going to leave me with a very, very blunt edge and I'm not even sure that I'd get the bevel even starting from that point.

Wow that was a long explanation. :)

Anyone have any wisdom to share? Maybe I just need power tools. That Ken Onion edition Work Sharp is on my radar.

Thanks,

Brian.
 
What kind of stone are you using, only diamond? Vitreous and waterstones can be shaped along one edge to do recurve edges very nicely and with a lot of control. I made a couple videos showing how I've done it on an India stone. Might be helpful - you aren't the first person to get tired of dealing with them, especially on diamond plates. Dealing with recurves is what finally put me off of them.

http://www.youtube.com/watch?v=viC7mybM6gk

http://www.youtube.com/watch?v=viC7mybM6gk

http://www.youtube.com/watch?v=FmR4nSTukmE
 
On the problem knives I'm describing, I've used Norton Crystolon, DMT XXC, and Nubatama 150. The experience has been similar on all, except that on the DMT, I'm more cautious. They all do the same job, just a different rates with different finishes.

But I'm not looking for "how to sharpen recurves". I'm looking for how to correct a recurve that's been introduced into a knife through extreme wear and use.

Thanks,

Brian.
 
I say bite the bullet. That's what I do every once in a while when I need to reprofile after adding microbevels cause I'm lazy or after heavy use to remove nicks and such. It isn't as bad when you do it every once in a while cause it helps avoid just grinding in one spoy and creating recurves or flattening bellies.
 
I fix this exact problem 50 times a day on everything from cheap Walmart knives to blazens to single bevel yanagi's that have been "steeled" on a diamond rod :eek:

A belt sanders would be your best bet as it makes the fix easy but it can be done on a stone too. Unfortunately you must grind the edge flat, no way around it. A Nubatama 150 should make short work of resetting the edge but to grind it flat is going to be tough work for most any stone. If a belt sanders is not a option then you should look to the Nubatama Ume 24 grit or 60 grit stones.
 
What Jason said!:thumbup:

Get the recurve out, and then sharpen normally.

If the recurve is very pronounced, or if the recurve was an original part of the knife design, I use a Lansky Crock Stick to sharpen the curved portion.

A belt sander would be better, but I don't own one. (YET:D)

I hate recurves. I own only one, and every time I sharpen it, it has little less recurve!:p
 
Ben Dover said:
What Jason said!:thumbup:

Get the recurve out, and then sharpen normally.
Click to expand...

Like the above posted, if the recurve is man/poor maintenance caused, then I will straighten out the blade and then sharpen accordingly. If it is a customer's knife, then I will point it out to the customer when they deliver the knife to me and suggest it be corrected. Most of the time, the customer shrugs and says "gee, I don't know how that happened, or I thought it came that way". I have yet to have someone not want it repaired, and was usually surprised when I said "no extra charge to straighten it out". All the time hoping they would say yes, as it made my job easier, and resulted in a much nicer looking and preforming knife when done.

Blessings,

Omar
 
A rare thread indeed where all of us cantankerous old codgers are in full agreement!:D:thumbup::D
 
Aratae 24 grit (per knifenut's suggestion) is a MONSTER, so be careful if you use one and definitely reprofile at a very high angle, or you'll risk scratching the bejeebus out of your secondary bevels. Don't ask how I know. :(
 
bgentry said:
On the problem knives I'm describing, I've used Norton Crystolon, DMT XXC, and Nubatama 150. The experience has been similar on all, except that on the DMT, I'm more cautious. They all do the same job, just a different rates with different finishes.

But I'm not looking for "how to sharpen recurves". I'm looking for how to correct a recurve that's been introduced into a knife through extreme wear and use.

Thanks,

Brian.
Click to expand...


Different story, my mistake. In that case I treat it like a ding or chip or correcting an obtuse inclusive angle. You can hog off a lot of good steel truing up an edge to straight, and plenty of knives come right from the factory with a recurve at the heel just from being initially ground on a belt by someone in a hurry. I make the recurve portion the basis for the new edge line and grind it down to a good inclusive angle, then work the belly down to straighten up the line. If there's a lot of recurve I don't try to correct it too much - it frequently ruins the look of the knife if the choil winds up disappearing or the edge needs to be ground too deep into the ricasso, the cure is worse than the disease. It also frequently requires the back bevel to be thinned way down and that can be a deal breaker too.
Basically with the above replies - grind it out if possible, if its a knife you're likely to see again, work it out over a couple of reconditionings.
 
Wasn't familiar with the term "recurve" and had to watch HH's very informative videos to fathom what this is all about. When I find a knife with that feature, I usually treat it like an intentionally curved peeling knife with a concave edge and sharpen it using a series of ceramic rods from very coarse to very fine. I thus leave it in the shape it had when I got it, only sharper. Grinding it back into its original straight profile involves the removal of a load of steel further up toward the point, thus possibly shortening its life and removing some of its history. Sorry, in these things I tend to romanticism.
 
knifenut1013 said:
A Nubatama 150 should make short work of resetting the edge but to grind it flat is going to be tough work for most any stone. If a belt sanders is not a option then you should look to the Nubatama Ume 24 grit or 60 grit stones.
Click to expand...

I've been working on the current blade, a Benchmade 156 (partially serrated) for about 3 hours of "on the stone" time. I'm going slowly and checking my progress with a loupe and a sharpie pretty frequently. Someone with more skill could probably have done this in half the time, maybe less. I think I've probably got another 30 to 60 minutes of work to do, as I *finally* have a burr almost full length on one side. Just an area about a finger's width remains. Right in the middle of the recurve. :)

I find the Nubatama 150 to be very coarse and it seems to be doing the job well. I think the DMT XXC is faster, but I haven't tried it on this blade and I've never worked with D2 steel before, which is what this blade is made of.

Overall this seems like 5x the effort I was expecting with this blade, but it's been beaten pretty hard. Some of it's serrated teeth are literally broken off. It's seen a stone before, but I don't think it's been properly sharp since it came from the factory.

I'm intrigued by the Ume 24 grit stone. Seems like a *beast*. But I think it's probably finally time for a power tool if I'm going to go to that level. Seems like it would still be hours of work to try to grind the blade shape flat and then re-establish a good bevel using a stone alone, even the Ume 24. For the same price I can get a WorkSharp, which seems like a good introduction to belt sanders. I'd guess straightening the recurve and establishing a new bevel would take maybe 15 to 20 minutes of careful work with the WS. Probably less for someone with experience and skill.

It's good to know that pretty much everyone agrees that my idea was the right one. It's just very costly in terms of steel and time.

Thanks,

Brian.
 
Brian, you will love the Work Sharp. I have the original and use it when there is a major re-profiling to be done. It requires some caution and careful attention to avoid overheating the blade, and especially the tip. It saves me a lot of time when heavy grinding is needed on a knife. I will use it and then finish up on my Paper wheels. I have converted some serrated knives to a straight edge knife on it with good success. It is easy to round off the tip if you do not lift the tip about 1/2 way across the belt. I use mine to sharpen axes and other heavy tools like machetes, etc. The new one coming out soon looks like real winner with some great new changes.

Blessings,

Omar
 
OHALLUM said:
Brian, you will love the Work Sharp. I have the original and use it when there is a major re-profiling to be done. It requires some caution and careful attention to avoid overheating the blade, and especially the tip. It saves me a lot of time when heavy grinding is needed on a knife. I will use it and then finish up on my Paper wheels. I have converted some serrated knives to a straight edge knife on it with good success. It is easy to round off the tip if you do not lift the tip about 1/2 way across the belt. I use mine to sharpen axes and other heavy tools like machetes, etc. The new one coming out soon looks like real winner with some great new changes.

Blessings,

Omar
Click to expand...

Keep using it on axes and machetes but I've seen it ruin plenty of good Knives. That said if I works for you fine but there is a big learning curve.
 
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