Flattening Stock

[Bühlmann]

I have some 24" x 1.5" x 0.25" sticks of 5160 that came to me with a slight bow in them along their length. I cut a 9" section off for a full tang pattern, laid it out and roughed out the profile. The bow in the stock is still significant. What's the best/fastest/easiest way to eliminate this bow? I know I can grind it, pretty sure I can heat & bend it, but I'm kinda concerned about it showing back up as a warp after heat treat. Is one method better than the other? Is there a preferred "stress relief" temperature to shoot for if I heat it and clamp it? Thanks!

 

[DevinT]

Hammer and anvil cold.

Hoss

 

[Bühlmann]

Thank you for the reply. Maybe this is a dumb question, but why would cold forming be preferable with respect to retained structural stresses in the steel?

My goal is to not introduce or add stress to the steel that could rebound or release during quench and manifest a warp.

 

[A.McPherson]

Bühlmann , you're going to get some warp in heat treating.
Even if you start with precision ground stock, you're gonna get some warp. It's just the nature of the beast.
Knife making is straightening warped steel.
Normalizing cycles, quenching, then moving to plates, shimming during tempering. All techniques to help reduce or fix warped steel.
Sure you can try to reduce it, but it's just gonna happen. Better get used to it!

 

[DevinT]

I’ve made knives for 44 years now, most of that time has been spent waiting for things to heat up and straightening. Make knives long enough and you’ll figure it out.

Hoss

 

[Bühlmann]

Yeah, I get that. However, if I’m blessed enough to get 44 more years of knife-making experience to figure it all out I’ll be 99 and way too shakey & crabby for anyone to allow me to be around rotating equipment & sharp things. Plus I’m a sloooowww learner.

Maybe I’m not asking correctly since my question hasn’t been answered yet. Let me try this: Is grinding or heating/bending the least stress-inducing technique for straightening stock?

 

[Bühlmann]

Bühlmann , you're going to get some warp in heat treating.
Even if you start with precision ground stock, you're gonna get some warp. It's just the nature of the beast.
Knife making is straightening warped steel.
Normalizing cycles, quenching, then moving to plates, shimming during tempering. All techniques to help reduce or fix warped steel.
Sure you can try to reduce it, but it's just gonna happen. Better get used to it!

I’m fine with getting used to it. It’s the reducing part that my question is centered on. If warping is inevitable as you say, then starting with warped stock isn’t going to improve my post-HT product. I’m asking how best to straighten (grind or heat/bend) to minimize the potential for post-HT warpage.

DevinT mentioned cold forming, but didn’t answer the part where I asked why that would be advantageous to heating or grinding. I’m just trying to learn, not trying to be argumentative or anything else. I’m just a newbie hobbyist, so wait times don’t matter much to me. I’m interested in the metallurgy of it, and also in techniques that increase my odds of success. This is why I ask “why?.

I do appreciate your replies!

 

[A.McPherson]

It's just easier and faster to cold form it, the amount of movement that you'll be subjecting it to is pretty minimal.
The blade will be normalized at the beginning of heat treating, and that should take care of the little bit of stress that you get from cold shaping it.

 

[Richard338]

I'm a stock removal guy who sends out for heat treat. If there is a slight bow in a piece of stock I'm working with, then since it's in the annealed condition, I just pull it straight in a vice. If I feel like I have moved it significantly, I might tell the heat treater to add a stress relief step before hardening.

 

[DevinT]

I’m fine with getting used to it. It’s the reducing part that my question is centered on. If warping is inevitable as you say, then starting with warped stock isn’t going to improve my post-HT product. I’m asking how best to straighten (grind or heat/bend) to minimize the potential for post-HT warpage.

DevinT mentioned cold forming, but didn’t answer the part where I asked why that would be advantageous to heating or grinding. I’m just trying to learn, not trying to be argumentative or anything else. I’m just a newbie hobbyist, so wait times don’t matter much to me. I’m interested in the metallurgy of it, and also in techniques that increase my odds of success. This is why I ask “why?.

I do appreciate your replies!

The trick to straightening is to over bend it the opposite way and then re-straighten the steel. You will gain nothing by heating it up to straighten or grinding it flat. And at your age you don’t have time to mess with all of that.

Hoss

 

[Bühlmann]

The trick to straightening is to over bend it the opposite way and then re-straighten the steel. You will gain nothing by heating it up to straighten or grinding it flat. And at your age you don’t have time to mess with all of that.

Hoss

Perfect. I’m prone to overthinking things, and it sounds like maybe I’m assigning too much value to a problem that doesn’t deserve it. Thank you for the reality check!

 

[Stacy E. Apelt - Bladesmith]

Bending in a vise applies a concentrated stress at one spot. That can create a stress riser. Done on a hot blade it applies local9ized cooling, which may create a worse warp or other stress.
Bending over a long curve applies more even stress but may not straighten the bend at the exact warp. The three-point straightening in a vise is a smaller area version of this that works pretty well. However, it takes a little setup time and only works on cold blades.

One way to straighten blades very accurately is to make a Japanese straightening board and tool.
1) Take a 16" long by X6" wide by 1.5" thick piece of hardwood and cut slots 2" deep at 4" , 8", and 12". Cut the first two slots in an X shape cut. Make one slot 1/4" wide at the center and 1/2" wide at the edges and the other 3/4" wide at the center and 1.5" wide at the edges. Use a file to slightly hourglass the slots ... just enough to give it a little curve. The last slot is 1/4" wide slot and leave it straight sided. This board gets clamped in the vise or can be screwed to the workbench or a post near the forge.
2) Make the straightening lever tool by cutting a 3/8" slot in a 24" long 2X4" piece of hardwood. Put the slot 3" from the end. Shape the rest of the tool as you like into a handle comfortable to hold.
3) Place the blade in the straightening board with the center of the warp at the center of the board. Pick the slot that best fits the blade size and amount of warp. The straight slot is for very minor warp and twists. Put the slot of the tool over the blade and apply the bending force by torquing the handle. Overbend a little and move the blade as needed to get it straight.

The board and tool are great for straightening a blade right out of the quench. They do not suck the heat from the blade like a vise or anvil and make uneven cooling areas.

Someone will probably chime in about the use of a tagane - Japanese straightening hammer. This is the carbide tipped hammer that there are several threads in Shop Talk about. A trained smith can take warp and twist out of a blade using one. Because the stresses added are small and spread out in many blows, it does not create a stress riser. What it does is stretch the surface in a very localized spot. A few hundred taps and a warp is straightened.

 

[Bühlmann]

Thank you, Stacy. That’s excellent info.

 

[i4Marc]

I use a cheap Harbor Freight shop press. Using the ram with a couple of heavy mild steel bars as a 3 point jig I can straighten anything. Depending upon how far apart I place the heavy bars under the ram or if the ram is closer to one bar or the other etc. I can straighten tight curves and slight curves with ease. I can dial in the pressure very carefully and it doesn't take long. I check the steel on a surface plate to make sure it is completely flat. If the piece has a twist I will use a very large crescent wrench and a bench vise to remove the twist. We are by definition metal workers. Learn how to work metal.

 

[Stacy E. Apelt - Bladesmith]

I took some photos of the Blade board and tool:
You can see how easily and evenly it bends the wakizashi in the photo. Force can be applied from minute to a huge bend with one hand.

A blade right out of the quench is still austenite until it drops to 400°F. It can be straightened and untwisted with minimal effort with this fixture.

This fixture has a 2.5" thick board and a 2" thick (at the notch) tool. The board is maple, and the tool is ash. You can use anything you have on hand. Even pine framing lumber will work. Once you make one of these and get used to it you will never use another straightening method.

To straighten a gentle war4p., put the tool farther away from the board. To do an acute warp, place it close to the board. For a twist, place far/close as needed depending on the twist severity and location. You can isolate any bend/warp/twist between the tool and board with no other area being affected.

Bonus, the back of the tool can be used as a "swacker" if needed to reverse a banana bend in forging, or other wooden mallet forging tasks.

Sorry for the foggy shot of both tools. I took it one handed I set the timer on the others.

Board and tool

Tool on tang

Blade in narrow slot

Blade ready to straighter

Simple low effort to straighten

 

[A.McPherson]

I took some photos of the Blade board and tool:
You can see how easily and evenly it bends the wakizashi in the photo. Force can be applied from minute to a huge bend with one hand.

A blade right out of the quench is still austenite until it drops to 400°F. It can be straightened and untwisted with minimal effort with this fixture.

This fixture has a 2.5" thick board and a 2" thick (at the notch) tool. The board is maple, and the tool is ash. You can use anything you have on hand. Even pine framing lumber will work. Once you make one of these and get used to it you will never use another straightening method.

To straighten a gentle war4p., putb the tool farther away from the board. To do an acute warp, place it close to te board. For a twist, place far/close as needed depending on the twis seveerity and location. You can isolate any bend/warp/twist between the tool and board with no other area being affected.

Bonus, the back of the tool can be used as a "swacker" if needed to reverse a banana bend in forging, or other wooden mallet forging tasks.

Sorry for the foggy shot of both tools. I took it one handed I set the timer on the others.

Board and tool
View attachment 1816377
Tool on tang
View attachment 1816378
Blade in narrow slot
View attachment 1816380
Blade ready to straighter
View attachment 1816382
Simple low effort to straighten
View attachment 1816383

That's pretty dang cool Stacy. Thanks for the pics!