Slight bow in chisel - a problem?

[Adriaan Schepel]

Hey there.

I recently bought an old US-made 1/2" socket chisel, which is very nice in many respects.

On closer examination, however, I notice that there is a slight bow in the blade; there is a bend somewhere in the middle of the chisel that causes the blade to rise up prob 1/16" over its entire length (which is prob about 4").

I've been able to flatten the underside of the blade (so it can be properly sharpened), but even with some stiff hammer blows, I can't get out the bow in it. So it is flat across its width, but bowed along its length - if you know what I mean.

My question is simple: will a slight bow in the blade cause problems? Should I keep whacking this puppy with a heavy hammer? Should I lock it in the vise and do this? Or will I bust it in half? Thoughts??

Cheers,
Adriaan

[Frank Desaulniers]

Hi Adriaan,
It's not much use whacking a heat-treated chisel. At around RC 60 it will break before it bends permanently (a good chisel is supposed to flex a little but bend back to its original shape, or else break). It might be possible to squish and spread the metal with well-placed taps but I'll let metal smiths comment on that. If the back is convex then it's certainly a handicap (you'll have to lift the chisel for it to cut, similar to using a chisel with the bevel down). One possibility would be to anneal, straighten, then harden and re-temper. Someone with more experience than me might be able to fill you in.

[Steve Beadle]

Adriaan, I would think that for some applications, that bow is not going to make a problem for you. Like chopping out dovetails in 4/4 wood or of less thickness. There might be a problem with sharpening, especially flattening the back--but you seem to have gotten around that okay. But there are some applications where the bow would be detrimental--like when you need to place the flat of the blade completely against a registering surface.

In any case, I wouldn't whack at it with a hammer, or try to unbend it with a vise. I think you'd probably end up breaking it. I'll be watching this post to see what someone with the expertise might have to say about the suggestion of annealing.

Good luck!
Steve

[Derek Cohen]

Hi Adriaan

If it were my chisel I'd send it back to the factory.... :)

OK, so you can't do that. Seriously, I would grind it flat on the back. It will be nigh on impossible to pare accurately otherwise as the back of the chisel acts as a fence.

Regards from Perth

Derek

[Mike Wenzloff]

Hi Adriaan,

Like Frank mentions, about the only way to remove the bow would be to draw the temper out of it--anneal it--straighten, harden and temper.

The bow you describe is considerable and I feel if you removed that much metal it would weaken the chisel for all but paring and light chopping.

Myself, I would consider obtaining another without the defects for daily use. And then if I wanted to keep it and use it, I would pop the handle off and have at annealing it etc. The worse that will happen is it will not work.

Take care, Mike

[Jake Darvall]

There's another way .... though it takes some effort, and patience . You may not think its worth it.

lap it out. you'll loose 1/16" of thickness around the middle though. But at least it'll be flat. 1/16" isn't too bad I think.

uno,,,,remove steel from the centre of bow. I do that by just presenting the blade to the grinder. Make your strokes longer gradually, checking plenty as you go, making each pass longer each side of centre, until you feel you've developed a flat broad enough around the centre to give support when lapping off glass and adhered sandpaper.

Then on the glass, with very coarse sandpaper lap it out, with your new flat slowly extending out towards the blades edge and the handle simultaneously. May go through a few sheets. Just depends on how hard the chisels steel is I suppose (hopefully it'll be fairly soft eh)

I've done it before. Won't do it again in a hurry. Took up too much time, and knocked my hands about a fair bit. :D

[Chuck Stanford]

IMO, defective without a doubt.

Send it back

[Adriaan Schepel]

Thanks for the tips, friends.

I don't have the heating gear (or the confidence) for annealing and retempering, so that's not really an option for me.

I have already done a heck of a lot of lapping on 80 grit paper, and I think for me that's the final solution. It's visually such a nice little chisel, and is paired with a 3/4" one - together they're a real nice pair.

I don't own a grinder, so it's back to the sandpaper ...

Cheers,
Adriaan

[harry strasil]

I have several long small chisels that have a bow towards the front, makes it easier to work into deep places, and Slicks are made that way to start with.

Try it before you do a lot of work with it.

OOps just remembered its only 4 inches long, I don't own any that short.

You could try flame straightening it tho, just heat the front side with a propane torch a bit and then use an ice cube to cool the back side. We used to do that a lot in the navy on structural members. As long as you don't get above 400° you won't damage the temper.

[Ben Werner]

OK Jr., here's my dumb question. How d'ya know when the chisel gets to 400 degrees?
Thanks,
Ken

[harry strasil]

Soot the whole thing with a candle where the soot burns off is 400° approx.

[Ken Werner]

Whoops, I was signed on under Ben, but it's me, Ken. Anyway, Jr., you never cease to amaze me with your knowledge.
Thanks,
Ken