Practicing Hizumi: Hammering saws flat

Most woodworkers that are interested in hand tools manage to collect a fair number of saws that have seen better days…That’s a polite way to say they were beaten the hell out of, bound in a cut and bent, kinked, or otherwise mangled. A bent saw will give no end of trouble when sawing, basically relegating it to a wall ornament. That is, unless a little love and care can straighten them out.

Rather counter intuitively the best way to remove distortion from a saw is beating on it with a hammer, the process that is known in Japanese as Hizumi. Apparently hizumi also refers to working the kinks out of peoples backs and body, should give you some idea that this is analogous to massage. Hizumi is used in almost all of the blacksmithing trades dealing with tools, from knives to chisels and kanna, all require a bit of straightening after rough forging and tempering before the final work can be done profitably at the grinding stone. With saws metate take the art of hizumi to a whole different level, as a saw often comes out of hardening shaped like a potato chip.

One of the biggest reasons I endeavored to make my way to Vermont and Mark Grable has been to study hizumi, as I have no shortage of saws that need straightening, and metate is close to becoming a forgotten skill in this age of power everything and disposable blade culture.  Of course, you know better, otherwise you wouldn’t be reading this blog. So where to start?

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How about cleaning up that rusticle you just found in grandpa’s old barn or on ebay? Mark’s approach is to use 40 grit sand paper and a tool made by Yataiki that lets you apply a lot of pressure and control as you work off the rust.

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It consists of a sen like handle with a piece of bicycle tire nailed on to grip the sand paper. Simple and effective. The point here is not to sand out all of the pitting and restore the tool to a bare metal fine finish, merely to render it free of rust and to a point where the saw plate can be viewed well in the correct light condition.

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So, take one potato chip…I mean saw blade and sit back in some indirect diffuse light and get the shape of the saw in your head. The whole shape of the saw, its not enough to merely notice a bump here or there. The thing about distortions in a saw is that they have a habit of relating to each other, a certain balance of forces and tension that is effected in the saw as a whole. Attempt to remove one problem and you might just make another worse.

Its better to start on thicker western style saws, and better yet to practice on stuff like the above photo of an old circular saw blade that’s been annealed (a bit over annealed, I’d say).

Flex the saw lengthwise between your hands and listen for the dreaded nuki, poka-poka, or oil-canning. That consists of a section of over tensioned metal that wants to pop back and forth in its unrequited state of inbalance with the rest of saw plate. If that’s apparent you need to put the saw away and not work on it until you have quite a bit more experience, but at least its a problem that can be dealt with. Just today somebody sent Mark a ryoba saw with the worst twist and nuki I’ve ever seen in a saw, and Mark got right to work hammering that shit out, not that I could tell exactly how he went about it.

The first tool you’ll employ looking at the saw’s condition is a straight edge, with the saw held up to the light in such a way that you can see the line of light between the saw and the straight edge. First look along the width of the saw on both sides, then both directions diagonally, and finally along the length of the saw. Remember, you’re trying to get in your minds eye the total shape of the saw, not merely a localized spot or two.

Next you look at the saw along its width and length at a low angle to the incoming light. Its hard to describe what the right angle is like, but you’ll know it when you start picking up the surface topography of the saw. A cold beer is appropriate at this stage as you contemplate the saw and your local universe, the birds and the mountains, and the wind rippling the pond. If its late in the evening you put the saw down and go enjoy the sunset, maybe practice some calligraphy, hew a beam, or get stoned and enjoy the company of your friends.

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Back in the shop the saw awaits and the anvils and hammers are positioned. In the Japanese tradition you work seated, maybe with the anvil tilted to return the light to your eye at the right angle to allow you to see your hammer strikes on the lightly oiled surface of the plate.

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Hammers for major distortion are any heavy round faced hammer at hand. More refined and focused work involves a cross-peen hammer.

The hammer strikes the high spots, with the cross peen edge perpendicular to the length of the bow. Try to connect areas of distortion, and notice how working on one spot effects the others. Work both sides of the saw, an absolute necessity with removing twist that consists of concave bows that oppose each other diagonally. Strike blows hard enough to remove half of the distortion, and then half again, and so forth.

Flex the saw constantly to check for the pop of oil-canning, which can happen from working one spot too intensively. If you catch it early its much easier to remove.

A word on nuki. I don’t know enough at this point to give any advice should you find yourself with a saw that has this problem, but from what I’ve seen it involves working the periphery of the nuki on both sides of the saw, evening the tension and pulling it into the rest of the saw with a large round faced hammer.

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At first, to put it bluntly, I was beating the hell out of this saw blade, and left a pretty crappy surface as a result.

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But it is a lot straighter, no? If you just read this whole blog post and realized I didn’t really tell you how to straighten your saw, good point. Its not so simple as a prescription for hit here and the problem will be solved. If I can say one thing it is this, something Mark told me when I first arrived in Vermont, know when to stop.

 

4 thoughts on “Practicing Hizumi: Hammering saws flat”

  1. I’ve been doing copper work. I’m finding it a bit like ‘practice’ until I find a lot of bent and messed up saws that aren’t valuable; learning how to flatten pieces of metal to remove all bumps and twists.

    In all raising or sinking techniques with copper, you hit where the metal isn’t completely touching the anvil; perhaps this is why the saw flattening anvil is curved?

    1. I’m jealous, I’ve wanted to try raising copper and silver vessels for a while now, just haven’t put out for the sheet metal. Maybe I’ll save up my old copper pennies, melt them down, and pound out the sheet, haha. But I’d rather make a still, so…

      You definitely hit the saw only when it is making good contact with the anvil, same for tapping out a kanna or setting teeth. The curve of the anvil makes it a bit easier to make contact. When the distortion is so bad that the radius of distortion is worse than the curve of the anvil I guess that applies, but then you’re hitting between two or more supported points. There are some interesting cases where this isn’t the case, like setting the teeth on a western two man cross cut by bending the tooth off the edge of the anvil, so who knows, maybe you’re on to something. The other thing I’m curious about is the use of the wooden anvil, which has a lot more give to the surface, but I don’t really know why you might use one and not the other. Seriously though, if you can form complex shapes in copper I think you’ll be a dab hand at hizumi, its all about understanding how the metal moves, but without being able to anneal the work, haha.

      1. I’m using copper pipes- very fortunately provided by my godfather, a plumber. Slit or saw one side open, then hammer it down, use two pairs of pliers and a vise to unwind the pipe. Gets a nice rectangle of copper! May melt cutoffs down to get larger material.

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