How to straighten bent blades?

W

wvdavidr

Joined
Mar 21, 2007
Messages
370
Hello all,
I have failed on a few attempts to straighten bent blades. I have tried using a Workmate like a vice. Even with bending carefully to straighten, I end up breaking the blade. What did I do wrong? I didn't heat the blade. How do pros do it? Is the technique different with harder, heat-treated steel vs. cheaper steel?

Thanks,
Dave
 
I bought a vintage Henkel med. size ebony handle, full drop forged tang 8 inch off flea bay..Seller neglected to show or mention the bent tip..I was drooling over the wood on that old girl...I took it to a pro..for str8ting & a sharpening...Takes one look & says "say a prayer"...He set it on his anvil & hit it w a ball peen hammer, twice..That less than 1/4 bend at the tip flattened out & he resharpened it for 5 bucks..It is, to this day one of my wife's favorite kitchen knifes..I would not try that trick myself but it worked...He told me worst case [if it snapped] he would regrind it shorter...Mike In Ct
 
That's a good question. I have successfully fixed a few and broke lots. I'm anxious to see answers.
 
Depends on a few things but if the steel has taken a set then it's not likely to come out.
 
I think if the bend (or warp) occurred before or during the blade's heat-treat & tempering, it's probably gonna stay bent, because the temper was set into the bent blade; trying to force it would more likely break it. If the bend happened after the blade was heat-treated & tempered, it might be straightened again (though severe bending probably weakened the steel, so it might still break off). Still a gamble though, and with most, I'd just leave it be.


David
 
If you're gonna try to straighten 'em you have to be prepared to break 'em.

That said, if you rig up some pieces of wood in a vise and clamp to straighten then leave overnight (or longer) that might work without overbending in the other direction as much.
 
Murray Carter demonstrates how to remove bends from blades in his videos. One way is simply to hold the blade down on a flat surface, like a table, and lever the handle up (or down) to remove the bend. You have to use the fulcrum of the table edge and your hand near the point of the bend. Sort of like a very loose, soft vise made of your hand and a table. :)

The other way he demonstrates is with a "straightening stick". It's simplest form is a 2x4 with slots cut into it in the short direction. Stick the blade into a slot to the site of the bend. Then, holding the knife handle in one hand, and the "stick" in your other hand, bend the blade in the direction that will straighten it. You always have to go past the point of straight in order to remove the bend. Metal is flexible, so you'll have to "over bend" it to straighten it.

As some have said in this thread: DO be prepared to break blades if you do more than just minor bending. I've only broken one so far, and I was cranking on it pretty hard (using the hand on the table method). This particular blade had a VERY severe bend that had been there for 10+ years. It snapped with no warning, but amazingly didn't cut me.

As with sharpening and other maintenance, start with blades you don't care much about, and learn how it all works.

Brian.
 
I'm re-animating this thread because I'm straightening a blade.

My latest thing to tinker with is making metal slabs for Svord mini-peasants. (at work) Bought a few play with and found that bent blades are a problem as well as the poor grinds. I attempted to straighten two blades at work and broke one. That's about the typical percentage just straightening in a padded vise by just overbending in the opposite direction with leather gloves on.

So, I get another blade to go with those slabs and it's bent too. OK, go to plan B. You don't have to bend in the opposite direction as far if you can leave it bent for a while (like overnight). This is as simple as something like this. A popsickle stick, plate, and C-clamp.



Here is a picture of the Svords with new slabs. I've cut my arm semi-badly and am off work for now. If there are any Svord tinkerers who would like slabs down the road a ways I could probably make more.
 
I usually use two rods in a vise near the ends and one in the middle of the bend, tighten the vise and pray. it either straightens or it breaks. if it is one I made then I simply reheat treat and temper it annealed it straightens on the anvil. all mine are carbon steel usually spring steel so annealing and re doing isn't a problem. if it is a bought blade then it is iffy at best but I get around 75% success on them.
 
Having your hand close to the point of the bend is a good point. I have a small anvil mounted on a work table in my van, bellow that is a brass hammer I use to straighten bent tips. Probably 20-30 a week. I've broken two that I can remember. The worst are the ones that someone tried to put in a small crack and created more problems.
bgentry said:
Murray Carter demonstrates how to remove bends from blades in his videos. One way is simply to hold the blade down on a flat surface, like a table, and lever the handle up (or down) to remove the bend. You have to use the fulcrum of the table edge and your hand near the point of the bend. Sort of like a very loose, soft vise made of your hand and a table. :)

The other way he demonstrates is with a "straightening stick". It's simplest form is a 2x4 with slots cut into it in the short direction. Stick the blade into a slot to the site of the bend. Then, holding the knife handle in one hand, and the "stick" in your other hand, bend the blade in the direction that will straighten it. You always have to go past the point of straight in order to remove the bend. Metal is flexible, so you'll have to "over bend" it to straighten it.

As some have said in this thread: DO be prepared to break blades if you do more than just minor bending. I've only broken one so far, and I was cranking on it pretty hard (using the hand on the table method). This particular blade had a VERY severe bend that had been there for 10+ years. It snapped with no warning, but amazingly didn't cut me.

As with sharpening and other maintenance, start with blades you don't care much about, and learn how it all works.

Brian.
Click to expand...
 
IF the blade can be removed from its handle it can be straightened in an oven. Carbon steel only. Clamp the blade against a piece of angle iron with a shim under the tip or at the plunge depending on where the bend is. Set the oven at the correct tempering temp and let it sit there for a couple of hours. This will maintain the hardness and the bend can be eliminated. Most knife makers use this technique to straighten blades that warp during quench.

Fred
 
Time at room temperature does nothing to steel, Dave 14k. You can leave it for a year and it won't make any difference. The bending happens immediately.
The blade has to be over-bent in the opposite direction.
Holding the blade with your hand while doing this is a VERY bad idea.
 
I actually got a bend to the edge of my BK9, not a roll but a nice curve at about a 2 inch portion of the edge...
I broke out the anvil, a ball peen hammer and a peace of metal with a suitable sized hole in it.
Placed the blade with the curve over the hole and started working my way from the sides of the bend, and towards the center.
Then back out.... Took some patience, and a lot more force than I had imagined, but now the King is back to true....

I will admit there was prayers involved in the process though, LOL.......
 
Bill DeShivs said:
Time at room temperature does nothing to steel, Dave 14k. You can leave it for a year and it won't make any difference. The bending happens immediately.
The blade has to be over-bent in the opposite direction.
Holding the blade with your hand while doing this is a VERY bad idea.
Click to expand...

I normally clamp the blade in a vise with soft jaws and hold the handle. I bend in such a way that if the blade breaks my hands go away from the set-up. I wear gloves as an extra precaution. Still probably not a good idea.

I cut my arm falling against a screen door on an icy morning. :)

Now... I've been playing with the setup pictured above and it seems to be working. I've got plenty of time to tinker slowly and get my function back in my hand. Bending in the direction opposite the bend in the blade for 24 hours (overbending?) for the thickness of the popsickle stick (which ain't much) seems to take a little bend out. I may stack sticks up, dunno.

I'll let you know, film at 11.
 
davek14 said:
Here is a picture of the Svords with new slabs. I've cut my arm semi-badly and am off work for now. If there are any Svord tinkerers who would like slabs down the road a ways I could probably make more.
Click to expand...


Great looking slabs. :thumbup: What's the extra hole on the one on the left? :confused:

Sorry to hear about your arm. I hope it heals up well and quickly. :(
 
G. Scott H. said:
Great looking slabs. :thumbup: What's the extra hole on the one on the left? :confused:

Sorry to hear about your arm. I hope it heals up well and quickly. :(
Click to expand...

Good eye.

The extra hole was going to be for an extra little button head bolt to help adjust tightness opening and closing.

It was
1. Too close to the edge
2. Not needed

So I just put some set screws in there for appearance.

I'm making those on a CNC machine so reverse engineering to get the program right is the challenge. For the two biggest holes you can just measure centers and the shape around them can be just close. The stop pin hole took trial and error. I mean Svord doesn't have it right, right?

That purty stock is too thin to countersink the Chicago screw holes. Still, from what I've seen on you tube they are better (and thicker) than the metal slabs Svord makes. I'll put you on the short list if you have a mini and want a set.

I could make them easily now given the spare time at work. When I get back, that may be scarce. Personal time work is frowned upon when unlimited overtime is requested. Still, it's no problem during slower periods. Someday over the rainbow.

Thanks for the well wishes, they are appreciated a lot.

There is (they think) no tendon damage, just some muscle cut. I'll do physical therapy, hopefully just for a few weeks to keep the tendons sliding through everything properly and be good to go. A process, but one with (likely) a good prognosis.



For those into such things, I found a nice video on reverse engineering knives. Make things easier next time. Maybe someday I can draw some nice slabs for the Green River patch knife like I've wanted to.

[video=youtube;C5aueTxuXrY]https://www.youtube.com/watch?v=C5aueTxuXrY[/video]
 
Jason B. said:
Depends on a few things but if the steel has taken a set then it's not likely to come out.
Click to expand...

Blade steel at operating hardness is not very ductile. IOW you can bend mild steel pretty far back and forth and eventually it will break because it is very ductile. But hardened steel will not have that ductility. Bend it a little bit one way, then bend it a little bit back the other way and it fractures. I was actually demonstrating the high strength of Post Tension reinforcing wire in my office by breaking pieces, while a similar diameter piece of coathanger wire would bend back and forth numerous times. But if the OP's blade has a bend in it, it has already taken a set. IOW it is reached its yield stress and undergone plastic deformation.


davek14 said:
I'm re-animating this thread because I'm straightening a blade.

My latest thing to tinker with is making metal slabs for Svord mini-peasants. (at work) Bought a few play with and found that bent blades are a problem as well as the poor grinds. I attempted to straighten two blades at work and broke one. That's about the typical percentage just straightening in a padded vise by just overbending in the opposite direction with leather gloves on.
Click to expand...

You are onto something here- spread the points of force out as much as you can either side of the bend. If you clamp a blade in a vise and pull on it, the bending moment concentrates right at the edge of the jaws and you have to deform the steel there quite a bit to counteract the bend. Hardened steel can undergo limited deformation before breaking. If you spread out the points of contact then a greater length of blade achieves yield stress and then bend is more gradual forcing the steel at any given point to deform less. Less deformation means less chance of fracture.

But anyway, just because it bent one way doesn't mean it can bend that much back the other way without fracturing. If you really wanted to straighten a blade and it was worth the trouble, you could anneal it, straighten it, then re heat treat it.
 
I have come across bent blades in my longer chef knives, I currently have a 300 mm blade that is bent at the tip, but it's a slight bent, only visible when placed on a flat glass surface. Still it hinders cutting performance and it will be a long process of slowly bending it with a hand towel and pressure. I was told to do it slowly over a long period to prevent fracture.
I'm almost thinking about buying a new one, but at 500 for an almost decent one... I'll try my hand at bending lol
 
bdmicarta said:
But anyway, just because it bent one way doesn't mean it can bend that much back the other way without fracturing. If you really wanted to straighten a blade and it was worth the trouble, you could anneal it, straighten it, then re heat treat it.
Click to expand...

There is really no need to anneal a blade with a bend in it in order to straighten it. It is easily accomplished during a re-heat @ tempering temperature in a standard kitchen oven. Just shim the blade, opposite the present bend against a small section of angle iron. Leave it in the oven for an hour. There is no change to the HRC of the blade; just make sure the wife knows what your doing.
Fred
 
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