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Your thoughts on torch straightening

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by JTknives, Oct 14, 2020.

  1. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com

    Jun 11, 2006
    Wondering what everyone’s thoughts are on the common practice of Heat treaters using a torch to straighten bowed blades.
  2. HSC ///

    HSC ///

    Nov 7, 2012
    Put the blade in a tray wet sand and torch the spine. I’ve done it. I haven’t had a need to do it often. But I’ve done it. No problems

    edit- just reread your post. You said heat treaters... so for that I would say no. Not in favor of someone else to doing it to my blades. I would favor cold straightening with carbide hammer
    Last edited: Oct 14, 2020
  3. butcher_block


    Dec 6, 2004
    that will be a hard no for me
    Nathan the Machinist likes this.
  4. 12345678910


    Jul 13, 2009
    I'm of the shim to a hardback and temper practice.
    Backyard, weo and Fred.Rowe like this.
  5. DanF

    DanF KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Apr 17, 2017
    Not from a pro service for me. I can do that at home, if I’m paying good money for the service I want better.
    butcher_block and SS369 like this.
  6. Jason Fry

    Jason Fry KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 5, 2008
    Heating below the tempering temperature does seem to help reduce the risk of cracking when straightening warps. I see no risk to pro's doing it that way.
  7. Joshua Fisher

    Joshua Fisher KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Mar 27, 2018
    The issue can be that if gone over the temper then it can leave splotches in a etched or polished finish even if it doesn’t effect the integrity of the blade, it’s very hard to tell what temp the torch is getting the piece to and if you over shot the temper. For a pro service I have and would avoid it, it’s the biggest reason when I first started that I didn’t send any blades to a certain company in the heat treating industry.
  8. Spalted

    Spalted My name is Britt Askew I like making knives Knifemaker / Craftsman / Service Provider

    Dec 9, 2010
    Up until last christmas I thought it was no big deal because if peters does it and does as many as they do it has to be ok right?? well about a year ago I got a couple of stainless blades that Brock sold here on the forum that was heat treated by peters and in the process of making the knife I dropped it on my workbench from no higher than 6" and the tip curled like a fish hook :( I can think of no other reason it was that soft on the tip other than the heat from the torch softened it ? I ground the tip away and the knife has been good but no way the tip should have been that soft
  9. AVigil

    AVigil Adam Vigil knifemaker working the grind Platinum Member

    Feb 17, 2009
    A big no, people will go on and on how they want the best for their blades... then someone takes a torch to the spine of a stainless blade and all of a sudden "it is not a big deal"

    Can you imagine the stress in the blade where a soft spot bordered by harden steel?

    It is a time saving short cut and not done because it is the correct way.
  10. samuraistuart

    samuraistuart KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 21, 2006
    I won't do it. Either I'll straighten during the fall in temp during martensite formation, or it gets the 3 point clamp method during tempering. If I can't get it out either way, I start over. And the only time I've had to start over was just recently with AEBL chef's knives. I couldn't get them straight enough to my satisfactory.

    My only experience with torched spines for straightening was with Peter's HT. I had a few AEBL kitchen knives that came back with the spine/torch method. These were at final dimensions and polish. The affected area was clearly visible by the oxides left over, which were easily removed with a final 800 grit polish. And the affected area was about the size of a pinky nail...a small semi-circle that extended down from the spine about 3/8" or so. Honestly, on a chef's knife, I don't think anyone would ever notice any change of any kind. A stainless steel knife that was going to be used for more rigorous duty than a chef's knife, like a chopper for example, I would not want to use the torch on the spine at all.
  11. N.W. Gean

    N.W. Gean KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Oct 10, 2018
    I agree Dan. I have a torch and a straightening hammer myself. If I'm sending out, I'm paying for professional experience and skill.
    DanF likes this.
  12. Richard338

    Richard338 Gold Member Gold Member

    May 3, 2005
    I requested stress relief prior to hardening last time and there were no torch marks.
    The time it was most annoying was a piece of DT damascus and it was etching differently until I got the whole piece dark enough. I was worried that it would look like crap.
  13. Jason Fry

    Jason Fry KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 5, 2008
    When I say torch, I mean plumbing torch, not acetylene, and when I say heat, I mean no oxide colors, not blue, etc.
  14. Nathan the Machinist

    Nathan the Machinist KnifeMaker / Machinist / Evil Genius Moderator Knifemaker / Craftsman / Service Provider

    Feb 13, 2007
    Hell no.

    We do thousands of blades a year. Our heat treat process results in more distortion than average. There is a fair bit of straightening, it is all done with shims clamps and tempering. Never a torch, not on my stuff.
  15. john april

    john april KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Feb 27, 2006
    interesting. the place in PA seems to be the cats meow, according to many people on the web. i have even heard they do heat treating for NASA, so i figured it must be good ? where else should i get HT ?
  16. Cushing H.

    Cushing H. Gold Member Gold Member

    Jun 3, 2019
    LOL. A lifetime ago (1979) I made a first knife. I was working in a machine shop that had a HT oven and everything else I needed. I got to the point, per Boye’s book where you heat to anneal the spine .. only thing was that the only torch the shop had was an oxyacetylene rig. Talk about overheat ... I melted a spot on the spine :-( . THAT little knife never got a handle! (Though I did keep it around for a long time, and used it occasionally.
    razor-edge-knives and Drew Riley like this.
  17. i4Marc


    Oct 19, 2011
    The only time I sent a couple of blades out to be heat treated I was warned in advance to specifically request NO STRAIGHTENING. Every blade I do in-house that requires straightening gets corrected in the temper cycles with clamps and shims. The first temper cycle is warps and all to relive stresses. Corrections start with the second temper.
    Backyard and weo like this.
  18. weo

    weo KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Sep 21, 2014
    This is my current procedure as well. I added the stress relieving first temper after my second blade went 'tink' 15 minutes into the 1st tempering with shims.
  19. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com

    Jun 11, 2006
    I guess we made the right choice in not torch straightening our blades. We use surface peening to straighten all our blades. We found with aebl that clamping straight do very little in helping to correct any bowing. We do clamp blades between plates in the freezer and then in the oven if thy are thin AEBL. But rarely does a blade come out of the tempering cycle to my standard of straight. I usaly have and avarage of 15-30 knives to straighten a day. Yesterday the wife handed me a stack of 40 and said have fun lol. One issue we have is shear volume and size of ovens. To clamp and shim every blade during tempering you would have to have more paid help and a crap ton more ovens. We all ready have 3 dedicated tempering ovens.

    I guess what I’m saying is I understand why places torch straighten but I will never do it on my customers knives.
    Bigfattyt and 12345678910 like this.
  20. weo

    weo KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Sep 21, 2014
    I can only imagine. I ran into the space issue the last time I HT'd 3 chef's knives and all of them had a warp. I didn't have enough clamps, angle iron or room to straighten all three at the same time. The clamps and straightening jigs would be an easy fix, but I'm pretty sure getting a bigger oven is out of the question.

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