Straightening blades

[R.C.Reichert]

So I just heat treated some blades and out the quench a couple of them had a bit of a warp. I tried the whole clamp-shim-temper method, but was not having any success. That's when I stumbled upon this video. He just taps with a hammer and the blade straightens out. It worked perfectly my first time trying this method. My blades are edge quenched, so closer towards the spine where I tap is a little softer. I also gave another 1 hour temper cycle after doing the straightening treatment. There is also another video on Youtube of some German bladesmiths demonstrating the exact same thing to straighten blades. My guess is this is a very old technique...

A regular ball peen hammer worked just fine.

 

[Matt Rochester]

I use a bead blaster which works off the same principle. It stretches the steel which pushes the tip around. I mainly only do it on thin kitchen knives because that’s usually all I have that warp.

 

[John mc c]

I've seen that video and tried it on a fully hardened kitchen knife with no success.
Is it just for edge hardened blades or my bad technique?

 

[HSC ///]

I've seen that video and tried it on a fully hardened kitchen knife with no success.
Is it just for edge hardened blades or my bad technique?

I’ve also tried this peening method a few times with no success. I made a special hammer like the German video shows.

The Brazilian peens on the high side. The German indicates to hit on the inside of the banana...

Inability to straighten efficiently is one of the reasons I prefer laminated steels, which are considerably easier to straighten for me

 

[R.C.Reichert]

I was skeptical when I first tried this, so I initially tested it out on some of my failures. I suggest that anyone wanting to try this get out some of your scraps just to get the hang of it. Maybe even pre-bend some steel just so you get an idea which way it will move.

The process...

The blade needs to be hardened and tempered. I tempered once for an hour and a half.

Lay the blade on the anvil so the high spot of the bow is up. You neeed to tap on the outside or top of the curve (outside of the bananna) Tapping the other way will only make the bend worse.

I I started out by just tapping several times with the ball end of a ball peen hammer fairly firm blows about a half inch below the spine. Gradually work up and down where the bend is tapping and checking for straightness because it does go fast. There is no need to strike anywhere even near the edge of the blade.

After the blade is straight - it probably took anywhere from 50 or 60 taps (depending on the length of the blade) with the hammer to get it straight, I put the blade in the oven for another tempering cycle. That's it! You should have nicely straightened blade. Only downside it there will inevitably be some tiny divots on the side you just hammered that will need to be dealt with.

 

[R.C.Reichert]

I've seen that video and tried it on a fully hardened kitchen knife with no success.
Is it just for edge hardened blades or my bad technique?

I think it should work, as long as you tempered the blade first, but it most definitely works on an edge quenched blade.

 

[john april]

i am curious why the shim method was not working for you. how were you measuring your bow and shimming it ? tell me about your procedure ? the hammer idea is interesting but HSC's comments say each guy hammers on a different side so that is confusing. i just figure its better to get to the bottom of why the shimming was not working because according to the comments, the hammer method leaves behind marks ( more work, and the center of your edge will have to be moved over because you just removed some of one side of your knife getting rid of the hammer marks.).

 

[R.C.Reichert]

i am curious why the shim method was not working for you. how were you measuring your bow and shimming it ? tell me about your procedure ? the hammer idea is interesting but HSC's comments say each guy hammers on a different side so that is confusing. i just figure its better to get to the bottom of why the shimming was not working because according to the comments, the hammer method leaves behind marks ( more work, and the center of your edge will have to be moved over because you just removed some of one side of your knife getting rid of the hammer marks.).

Personally, the next time I do it, I will probably use the flat side of the hammer. Nevertheless, the marks are very minor/superficial and not something I am concerned at all about. The hard steel doesn't let the marks go very deep anyways.

With the clamping method I have had varied success. I just clamped the blades to a large file and shimmed up the middle with a washer or two to overbend the opposite way in which the bend was originally going. I tempered like that for an hour and a half, but the blades still had a bend in them. There is a tutorial on here somewhere that shows how to do this and I followd it step by step. It seemed to work for my last batch of knives, but it didn't work for this one. The hammering method worked perfectly and very quickly. What I like about the hammering process is that you can look at the straightening in progress as it happens, not first find out an hour later.

I followed this tutorial for the clamping method...

https://www.bladeforums.com/threads/straighten-during-the-temper.838681/

 

[R.C.Reichert]

Btw, I just tested this hammer-straightening out on a fully hardened and tempered (not edge-quenched blade) and just used the flat side of a ball peen hammer instead of the ball end. It worked just as well and there was only very, very minor marks left on the blade. I think the reason why I had some deeper marks before is simply because my previous blades were edge quenched and therefore softer along the top. I would suggest taking your time with this and just use lighter but more hammer blows to begin with. I was doing this at 5:00 am and was just getting impatient and wanted to go to bed already. I had been up all night tempering and was probably hitting the steel a lot harder than I really needed to, lol.

The method works. Hammer on the top of the bananna, not on the inside. Think of how you would straighten a bend in if you were just forging a hot knife blade...your hammer blows would be on top of the bend, but make sure there is contact on the anvil directly underneath where your tapping.

 

[R.C.Reichert]

I think what happens when you hammer the steel is that the side that you hammer on gets compressed/compacted a tiny bit and this forces the opposite side to stretch and curve.

 

[flatblackcapo]

I’ve also tried this peening method a few times with no success. I made a special hammer like the German video shows.

The Brazilian peens on the high side. The German indicates to hit on the inside of the banana...

Inability to straighten efficiently is one of the reasons I prefer laminated steels, which are considerably easier to straighten for me

i am curious why the shim method was not working for you. how were you measuring your bow and shimming it ? tell me about your procedure ? the hammer idea is interesting but HSC's comments say each guy hammers on a different side so that is confusing. i just figure its better to get to the bottom of why the shimming was not working because according to the comments, the hammer method leaves behind marks ( more work, and the center of your edge will have to be moved over because you just removed some of one side of your knife getting rid of the hammer marks.).

The way I saw it they both hammer on the concave side, which makes sense. I have never tried this method but I will give it a shot if I still have a bend after temper.

 

[R.C.Reichert]

The way I saw it they both hammer on the concave side, which makes sense. I have never tried this method but I will give it a shot if I still have a bend after temper.

Practice first. I know its sideways, but I'll make a little diagram with just a few symbols here... ■)● Pretend that the black square is your anvil, the curved bracket is the knife laying curve down on top of the anvil. The black dot on top is where you hit the blade...at the top of the convex/curve. That's what worked for me. If you hammer with the blade situated like this... ■(● it will just concave even more. Try it.

 

[R.C.Reichert]

This seemed to work just fine on the 1/8" thick blades I did. I don't know how well it will work or if at all on really thick blades.

 

[Matt Rochester]

If you’ve got a sand blaster, or access to one, then give it a try. .100” or thinner works great. Grind your bevels til you’re ready for hand sanding. Start in the middle of the bow a blast a line from edge to spine. Check and repeat as necessary. Work your way to the tip. I was very skeptical until I tried it. I’ve tried peening like the video you showed but never had much success. I probably was not hitting it hard enough.

 

[R.C.Reichert]

If you’ve got a sand blaster, or access to one, then give it a try. .100” or thinner works great. Grind your bevels til you’re ready for hand sanding. Start in the middle of the bow a blast a line from edge to spine. Check and repeat as necessary. Work your way to the tip. I was very skeptical until I tried it. I’ve tried peening like the video you showed but never had much success. I probably was not hitting it hard enough.

Makes me wish I had a sandblaster.

 

[flatblackcapo]

Practice first. I know its sideways, but I'll make a little diagram with just a few symbols here... ■)● Pretend that the black square is your anvil, the curved bracket is the knife laying curve down on top of the anvil. The black dot on top is where you hit the blade...at the top of the convex/curve. That's what worked for me. If you hammer with the blade situated like this... ■(● it will just concave even more. Try it.

Thanks for the reply R C , Are you sure that your diagram is correct, because that is the opposite from what both of the knifemakers in the videos show? The way I understand this working is when you hammer on the concave side you are spreading the steel thus pushing the bend out. I don't believe you can compact steel that doesn't have voids.
Please don't take this post the wrong way, I am just trying to clarify to clear up any confusion.

 

[R.C.Reichert]

Thanks for the reply R C , Are you sure that your diagram is correct, because that is the opposite from what both of the knifemakers in the videos show? The way I understand this working is when you hammer on the concave side you are spreading the steel thus pushing the bend out. I don't believe you can compact steel that doesn't have voids.
Please don't take this post the wrong way, I am just trying to clarify to clear up any confusion.

You'll have to try it. The bend will only get worse or better depending on which side you strike and you'll see very quickly which way you need to be going. There could be variables involved with this too...thickness of the steel, hardness, how it was quenched... My 1/4" thick bowie blade does not want to move at all, but it's mainly thin blades that have the warping problems for me anyways.

 

[flatblackcapo]

You'll have to try it. The bend will only get worse or better depending on which side you strike and you'll see very quickly which way you need to be going. There could be variables involved with this too...thickness of the steel, hardness, how it was quenched... My 1/4" thick bowie blade does not want to move at all, but it's mainly thin blades that have the warping problems for me anyways.

I will give it a try.
Thanks for making the post it is a interesting way straighten.
It is always good to know more than one way to do something.

 

[daizee]

My intuition tells me that the steel will stretch more on the side where the blow is more concentrated, hence the pointy or round-peen hammer should strike on the inside of the curve. A flat blow will have a different effect. Does that make sense?

 

[R.C.Reichert]

My intuition tells me that the steel will stretch more on the side where the blow is more concentrated, hence the pointy or round-peen hammer should strike on the inside of the curve. A flat blow will have a different effect. Does that make sense?

That's not how it worked for me.