Question on straightening blades

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Doug Lester

Well-Known Member
Having spent the last two days straightening blades that warped during the post hardening grind, I've got a question. My method was to clamp the blades to angle iron with C clamps while shimming them with pennies and heating the blades at just below the tempering temperature for 90 minutes. The question is does the heat applied while clamping really reduce the stress? Does it allow the warp to be removed without as much counter bending? If the blade was straightened in a vice with three rods to flex the blades without any heat would it be better to reheat the blade to just below or at the tempering temperature after straightening? I generally only use the vice and rod method before heat treating.

Doug
 
Doug,

I want to be sure I'm understaning you correctly, your blades came out of the temper straight ? and then warped while finish grinding ?

-Josh
 
That's what I'm thinking. I'm remembering that I checked straightness before grinding. Regardless the question is whether it's better to heat the blade to near tempering temperature when straightening. Remember that the blades were actually counter bent while cold and then put into the oven. Does this give any advantage over counter bending with the three rods in a vice without any heating? Would it be better to heat the blade back up to near tempering temperatures and then clamping it to the angle iron with a shim to counter bend? One of the blades that I straightened wasn't too bad. The other I had to use three pennies to shim it to get enough counter bend. Both were 2.75" blades.

Doug
 
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Doug,

I could offer my opinion on your question about heat buy it would be only that, and not a real answer based in metallurgical fact.

It is my opinion from my own experience though that any warp induced after the blade is heat treated is a product of uneven symmetry in the grind.

IE, The warp is ground into the blade and the correction is to grind the warp out and not straighten it in a conventional way. I pretty much always experience this when making blades from thin stock and out towards the tip on heavily tapered blades.

If you wanted to chat more about that I'd be happy to share with you on this subject.

As to your actual question, We both know your metallurgical knowledge far supasses my own so I'll be paying attension for a technical answer to your question also :)

Take care Buddy !

-Josh
 
Thanks, it just got me thinking. (Ya, you don't have to tell me how dangerous that is.) Tightening the C clamp down to bend that one knife with a resistant warp over three pennies also got me thinking that maybe I should heat the blade back up to tempering temperatures first too. But it would probably cool back down to room temperature by the time I got through fumbling with it with gloves on. Logic seems to indicate that there would be a degree of stress relief. It might not do any real good but, as long as I don't exceed the tempering temperature, it shouldn't do any harm.

Doug
 
Hey Doug. When I clean up 01 on my flat sander, after HT, I get that too, but when I change sides it usually straightens back on it's own. I find it odd, but that's the way it works for me more often than not. You are the first I've seen mention that.
 
I've seen it before too but not as bad as with the blade that I had to shim with three pennies to get it to straighten. I was almost thinking that I had forgotten to straighten it before doing the final grind; maybe I did. I don't know who's comment I was reading when I started making blades but they said that if you made knives you would have to learn how to straighten them and they were sure right. Maybe I need to make a checklist an post it on the shop wall.

Doug
 
Doug,

I can't recall having one warp during post HT grinding, but I have had plenty over the years to take a turn South towards the tip out of the quench.

Lots of folks say you can straighten out of the quench if you can do it before it gets below 4 or 5 hundred degrees. Well, I have tried that crap and burned my hands every time (yes, I had on gloves). I once over straightened and it looked like a snake.

HERE is what I have had good luck with...... If my target tempering temp is say 425 and I have a warp..... I will do a snap temper at around 375 for at least an hour before attempting to straighten anything.

Then, like you, I counter bend using washers as shims and C-clamps with either angle iron or between two pieces of thicker flat bar. I put it through a tempering cycle (2 hours) at around 400 degrees, and let it air cool.

If that still doesn't straighten it quite enough, I will shim a little more and do it again at 400.

Once it is straight, I will do a final cycle at 425 with no clamps or shims. Never had one want to act ugly after the final cycle.

Although it is a pain to even have to straighten one, we sometimes have to do it, and even sometimes get lucky and make it work.

Robert
 
Different steels, different rules. With 01 and A-2 you can straighten below 400° with gloves, or I just use a ragged out old towel. With 01, you have a good 5 minutes, and even when the temp allows bare hands you may have another minute or 2. However, you must have a feel for when the blade is becoming rigid.
 
Personally, I tried over bending a cold A2 steel blade post heat treat and temper the other day (while cold) and it ended up breaking on me.

While the "overclamp and temper" method doesn't seem like it should be enough to do anything (to me anyway), I have had a high success rate with it, although it does take more than a couple tries sometimes.

I've never tried to straighted directly after quench though, for fear of burning the crap out of myself.
 
I just noticed that two of four small O1 blades I'm working on are quite warped now that I have them polished up to 600 grit. Thanks for the suggestions I was thinking they were simply gone. I'll bring them back up to temper and try the shim and clamp method. They are thin 1/8" blades.
 
An old trick used by Bill Moran, and I use it if I can't get one straightened fresh out of quench. Which is seldom. After temper, put the blade in the vise, determine where you think the apex of the bend is, heat the spine in that area with a torch to dark blue plus a little to each side of center, then over straighten, hold and pour water on it. Sounds drastic, but it really isn't, and it works. With 01 and A-2, it may take a few trys.
 
I usually straighten out of the quench, but sometimes still need to tweak remaining minor warps out of the temper.

For this to work, it needs to be over 400 degrees. So, the temper when applicable is run up to 425-450. Get it out of the temper and to this bending jig without wasting any time, while it’s still hot,… flex the blade in the opposite direction of the warp, over correcting, until it returns straight. I never have had one snap on me this way. However, I only use it for minor warps.

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After the blade completely cools, you can run another temper cycle to make sure it doesn’t re-warp. I never have had one do that, but if it were to, I’d just repeat the process until it sets up straight out of the temper. That should be about as stress free and sound as you can get it.
 
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The whole issue of straightening seems to come up a lot. It involves science, skill, experience and a bit of intuition,… which is one reason I consider heat treating more of an art, at least the way I approach it.

Straightening out of the quench is a lot like that. You have a small window of opportunity and not much time. But all in all, if you can get it fairly straight real quick, you have another shot at it in the tempering cycle/cycles.

I really don’t worry about warpage much any more. It’s best if it doesn’t warp and you should do everything possible to avoid that happening, but…

Straightening out of the quench can be done at about 450- 350 with many of the popular blade steels, including the 10xx series and W series. The smoke point of some of the vegetable oils can be used as good temperature indicators. If it’s much over 450 you can get too much auto temper, and if it’s below about 350, it get’s more risky. With very thin blades you have less time and may be better off just going out of the temper (or in the temper). So, I do what I can out of the quench, then out of the temper if needed. I haven’t snapped a blade (out of the quench) for many years and they always come out straight as an arrow in the long run.

Don Hanson also says he's had good luck straightening out of the quench with shallow hardening steels, but does it strickly by timing with Parks 50, instead of using the smoke point as with some of the vegetable oils.
 
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With longer blades, I assume they will warp. For me its hard to see with the smoke and fumes coming off the blade after they come out of the oil. I built a "press" to help in keeping blades straight.
I use this pattern makers vise to sandwich the blades in, after the quench. I made up several sizes of aluminum clad, planned wooden blocks that bolt to the metal vise jaws.
The jaws pivot with the blade bevel and can be shimmed to match the bevel grind from spine to edge. If I can get it in the vise at around 6 or 7 hundred it will straighten the blade along both dimensions.

DSC01310.JPG
 
Learning curve? I heated both blades up to 425 (they were tempered at 500) and tried to get them straight, just kept turning up the pressure until Eureka! I straightened one but was surprised on how much over bending I had to do and the force it took. So then I grab the second blade with a bend in the handle. Well, I learn a little bit everyday. One turn to many in the vise and when it cooled, ping. 50/50, I'll have to pay more attention right after the quench and have my vise ready. Thanks for the tips and I'll keep learning (not being sarcastic, I really appreciate the help). IMG_2486.jpg
 
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