straightening blades

M

Mike Mossington

Joined
Jan 1, 2004
Messages
108
I have made a couple of O-1 blades for kitchen use, and when heat-treated they came back with a 'bend' or 'warp' in them. Anybody have a recommendation for heating and straightening without jeopardizing the heat treat or temper?
 
I used to moan, groan, cry, whimper and throw things around when that happens. Then I started grinding things back to shape - which was a pain. Nowadays, I straighten them back to true. Heres my method I learnt from another maker...

I have 3 brass rods which I attach to scraps of Micarta so that it forms a "T" shape. I place these in a vice so that the rods hand down inside the jaws and they are held in place by the Micarta piece above. I position the bent blade in the vice jaws with 2 rods on one side of the blade and one rod on the otherside of the blade, where the apex of the bend is, and start screwing up the vice slowly.

I hope you can picture what I am doing. The rods have to be positioned so they will bend the blade correctly. With a bit of practise, you will get the hang of where to place the rods, how far apart, how much to tighten the vice.

Hardened, tempered steel will have a great deal of spring in it, so obviously the blade will have to be bent quite a way PAST straight, so that it will end up straight when you let the vice go.

I often find that doing this alone will not get the blade to bend back to true. I use a heat gun ($20) to blow hot air onto the blade to raise the steel temperature so that it has some "molecullar activity" and will actually take the bend. The heat never gets to a tempering level, just high enough so the steel molecules will move.

I think of this as a controlled way to bend the blade (back to true). The crude way would be to put the tip in a vice and strong-arm the blade till its as close to straight as you'd like.

This is the most simple explanation I can give. With this method, I can get away with 3/4 of my bent blades completely fixed. The remainign 1/4 wraped blade, needs a quick regrind to true up the surfaces.

Hope this helps, or at least makes sense. Jason.
 
Jasons explanation is right on. I may add that the American Bladesmith Society (ABS) requires the smith to (slowly) bend their test blades to 90 degrees in a vice. A good blade will not break but will bend and stay bent somewhat. The harder edge may finally break about 75-90 deg. but not always. This should remove some of the fear about straightening your blades.

A good normalizing will usually eleminate a warp when quenching
 
You guys give a great explanation of how to remove a curve in a blade but I have another problem.
I have a couple blades that I hollow ground out of L6 and during heat treat some waves formed along the edges. The waves are too bad to simply be ground out so I just hung the blades on the wall figuring they were useless but then Mike started this thread so I figured why not ask.
Anything a maker can do to remove waves from the grinds on hollow ground blades?
Thanks up front!!!
 
Hey Michael,

I had a 1095 blade do that to me when I was experimenting with how thin I could leave the edge before heat treatment. If you cant grind them out there pretty much toast I think.

Mark
 
Good suggestion, Jason. I can see how that can work. And using a heat gun is something I hadn't thought of.
Bruce, I have seen the demos of forged knives being bent. As a stock removal guy, I have not tried testing one of my blades that way. Can't bring myself to deliberately try to wreck a blade after so much work. But I should do - just to see what happens.
Thanks again.
 
Wavy edge problems are indicative of too thin an edge, as you've figured out. But there may be another side to it. Please pardon my digression.

1 year ago, I did an experiment - I ground 4 blades from O-1 steel, which I usually HT myself. All 4 were flat ground ultra-thin with distal tapers, starting with 1/8inch thick stock. The edges were ground 1/2 the thickness I normally would go.

I sent 2 to a professional HT-er. I Ht-ed 2 myself. IN my forge, I get up to quenching temperature pretty quickly because the blades were so thin. Both the blades developed a bend as well as a wavy edge. ONe was salvagable, the other had to be junked.

The Professional HT-er uses a vacuum oven chamber costing into the 6-figure range. The guys explained to me that the temperatures rise very slowly and the oil-quench occurs in a vacuum as well. The entire process is heavily controlled, in short. ONe of the blades came back with a very slight (minimal) bend, neither blade had a wavy edge.

I deduced from this that going very thin is a hazard - thinning a blade should be done after HT, grinding cold. Also, the speed of the heat makes a difference. Thus you should not be in a hurry, allowing time for the blade to reach critical. The speed of heat is only one of many thermal cycles that affect what happens to a blade when quenched. I could be wrong on this and appreciate any ideas that others might have.

To answer the question, I haven't yet found a way to effectively cure a wavy-edge blade. I have tried grinding back the edge and regrinding the bevels, hopefully eliminating the affected areas.

Also, I think there is one thing worse than a wavy edged blade - its a twisted blade !!

I have found that a blade that bends with the edge bending the opposite direction from the spine is only good as a throwing knife !! :mad: :D

Oh, another idea - Bruce mentioned the ABS testing - bending a blade. When bending the blade, please bend SLOWLY. No matter how good the HT, if you simply wrench a blade to 90degrees in 1/2 second flat, you run a significant risk of snapping the hardened area.

Mind you, I've never met anyone who could bend a 1/4inch thick blade in 1/2 second. It applies to those who are testing much thinner blades as they progress to thicker blades.

Apologies for the rambling. Hope it helps some. Jason.
 
A pretty easy solution is to grind after heat treat. It might mean an extra belt or two but better that than to have to junk a blade. If that's not acceptable then just don't grind so close to finish. I leave at least .080 edge thickness before heat treat.

Michael
 
jason is right as always. Something else Ive ran into, You know how the Japanese katana bends when quenching and gives the nice arch? Metal grows when heated to quenching temps and the spine on our short blades is too thick to flex so the edge warps.

Yea also what Mike just said too.
 
Sometimes the wavy edge will go away, or at least be somewhat lessened if you anneal and requench.
 
You must log in or register to reply here.
Back
Top