Straightening a chisel

I was using my chisel today with a mallet and noticed a curve in the blade of it. It is a 1” Ashley isles, which has a pretty thin blade, so I’m pretty sure I just overdid it when removing material. It is a fairly continuous curve and not very much - slight enough that I’d have trouble photographing it.

Any advice/experience with straightening a curved chisel blade?

I held it with about 1” on the edge of the bench and tapped the back with a mallet. That might have made a small improvement. Looking online I’ve also seen recommendations to clamp the blade and bend it by hand.

I am not a blacksmith but it sure is satisfying to watch a good one. I would go with tapping on the back, cold, starting with a small hammer and working up to a bigger one if needed. I do not have a proper anvil. I have some salvaged rail road track and other big hunks of iron and steel that function fairly well as anvils.

Yeah, this is fine, don’t worry about it. As long as first inch or two is “straight” — you’re fine. I’ll post pictures of some of my chisels, some of which were lapped like that over time, ‘cos it’s inevitable, but some were forged like that.

You could have also bent a tang, so figure where the extremity is. Tangs are soft, if you were mortising with (which you shouldn’t) and pried too much - that could happen. This can be fixed in vises.

But I think you just noticed what was there since ever. Check other chisels, see if they have a very slight curve to them. Also, if you have trouble snapping a picture, perhaps you could do a sketch with dimensions?

In The Complete Guide to Sharpening, Leonard Lee notes that some timber framing chisels and slicks were manufactured with a “belly”. Apparently no one knows the reason why (although it is assumed that greater leverage was achieved when hogging out large timbers), but his advice is to leave it be as long as the first inch or so from the edge is flat (essential for sharpening the edge). I discovered this because I have a very stout Buck Brothers framing chisel that has a noticeable belly and wanted to flatten the back. On the other hand, Lee indicates that the backs of most bench and all paring chisels should be dead flat or it will compromise and/or defeat their purpose.

A full flat or rather "straight" face is important when the whole face is being engaged, like with long paring chisels. But it's hard to mass manufacture absolutely straight chisels, so paring chisels are flexible enough to be straightened with just fingers pressure. The old chisels with a deliberate belly were probably an attempt of getting an extra inch of reach or so by letting a socket or a handle to clear off the work. This extra inch of reach is important in framing with its deep mortises, large joints areas, etc. In cabinetmaking rarely more than an inch or two of reach is required, and when it's required we have tools like routers. So I think that bellies on bench chisels are inevitably created over time because we hone a face only for 1" or 2" from the edge. Honing a whole face would be way more daunting task and probably impossible to do right with a typical woodworking setup.

Also I think a "belly" should be distinguished from a back bevel. Looks like it was quite a common thing to do at some time, because I have at least a dozen of chisels where a first eighth or quarter inch of the face was at almost 15˚ to the rest of the face, probably somebody trying to turn a burr the other way faster. Impossible to lap out, it would require removing maybe 25% of the overall blade thickness over the whole length, maybe this is why old books warn against it. There's this nice 1/4" Fulton socket chisel, long, slender but stout, very elegant, very pleasant looking, really great for mortising, but this back bevel dubbed over about 3/8". And I just can't bring myself to grinding this much of steel off. Now it just sits there and taunts me, because it's impossible to chop plumb or pare level with it.

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Originally Posted by Ben Ellenberger

I was using my chisel today with a mallet and noticed a curve in the blade of it. It is a 1” Ashley isles, which has a pretty thin blade, so I’m pretty sure I just overdid it when removing material. It is a fairly continuous curve and not very much - slight enough that I’d have trouble photographing it.

Any advice/experience with straightening a curved chisel blade?

I held it with about 1” on the edge of the bench and tapped the back with a mallet. That might have made a small improvement. Looking online I’ve also seen recommendations to clamp the blade and bend it by hand.

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The AI chisels are pretty straight, if they are hollow, it's an inperceptible amount and that's intentional.

If you can see the curve when you hold the back of the chisel to a ruler, then there's something wrong with the chisel. If you got it from a store like TFWW, you could contact them for a warranty claim.

The chisel should be hardened for most of its length, malleting would not have caused it to bend if the chisel is hardened all the way to the bolster.

It 's also possible the chisel was not hardened properly, that could explain it bending.

I don’t recall a curve this pronounced before, and I’ve had it a while. it is straight for the first 1/2 or so of the blade, then has a concave curve with about a 64” gap.

I tried bending it by clamping the blade to the bench and just using hand pressure. I think I made a slight improvement. I’m planning to call it good enough and use it as-is.

If the bottom half is straight, then it's likely the upper half is softer and bendable. If it bends again, you should be able to re-straighten it. Not a pleasant prospect, to be watching the chisel every time you mallet it. If it keeps doing it, then reserving it for paring or retiring it would be one way to deal with it.

Try heating in the oven and see it that helps.

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Originally Posted by lowell holmes

Try heating in the oven and see it that helps.

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That could soften the steel. Why would you do that?