Paul's hammer trick for flattening irons / blades
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Michael Sterland.
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Paul described a method of hitting the back of a plane blade with a hammer to knock out a belly and speed up flattening. A couple people have reported cracked blades. I tried the technique today and ended up with a twisted blade. I’m down to 60-grit paper on a granite block and am not sure I will be able to rescue the blade, partly from the original issues, but most definitely from the twist, too. I’m glad Paul shares his tricks, but it’s worth hearing both good and bad outcomes. At this point, I don’t think I will try the hammer trick again.
A lot of variables at work there, so without testing there’s no way to be sure, but at first blush, both cracked blades and twisting suggest to me that the metal in those irons was not properly taken through the hardening process.
Hardening increases brittleness, so failed temper can leave the iron susceptible to cracking under stresses such as hammering. Also, a complete soak releases the stresses in the metal, so again, a failure to follow process could mean those stresses were not fully released, and hammering allowed distortion from those stresses.
Or, I suppose it’s also possible you were just going to town on the iron with a sledge hammer over a bed of gravel and the metal simply conformed to the surface beneath it, is that what happened? 😉 (of course I’m sure it was not what happened)
“A lot of variables” is exactly right, and unfortunately they are uncontrollable variables. In this case, the blade was relatively new, maybe 8 years, and was a Clifton blade. That’s not quite the same as saying Lie-Nielsen, but close. While it is possible that I got a bad one, it is unlikely to think of this as a manufacturing error in this case.
I think the uncontrollable variables are related more to the application of force than anything else. I didn’t go to town and was actually reserved in how hard I hit it. Honestly, why should this work anyway? A reasonably flat iron is placed onto a reasonably flat bench and is struck with a mallet. How likely is it for the mallet face to lead to distortion of the hardened steel in a way that cups the center? Before you answer, look at your blade and notice the milled slot where the cap iron bolt passes. The cutting end is supported by the two narrow legs defined by this slot. To me, it seems completely reasonable that, when you dump in some random force/vibration, the weakest link is likely to be where the changes occur. My twist is consistent with a relief about an axis going through this area and I think at least one of the breaks occurred here.
There are alternatives to this, like Hack’s advise to use a hand held grinder on a drill to hollow out the high spots. I’m going to take that approach next time. This is too much of a gamble.
Hello Ed,
In Paul’s blog, someone mentions hitting their iron, as per instructions, and having it shatter. The OP was more upset that it was one of his scarcer irons. Paul helped him out and offered a replacement iron.
Another instance; someone here mentions using a credit-card-thickness Spacer to lift the iron just a thou or two, before administering the initial blow. Iron in pieces!
We’re more-likely to hear the bad-news occurrences here, but I wonder whether this technique requires more skill and judgement than initially thought? Perhaps Paul is underestimating his ability in this regard?
2 April 2018 at 1:39 pm #516145Ive broken one, the main variable I noticed in my case (to try to figure this out) was backing material and repeated blows. Done on the face of a pine board In the middle of my bench in one blow it has worked a charm. I once did it on a different table by a leg on a different board, didn’t get enough result, tried a second hit – crack
I can’t say what exactly happened, but just offering up my observations in an attempt to figure this out.
2 April 2018 at 2:07 pm #516159That method is used on Japanese irons, which are typically thick soft steel welded to a thinner piece of tool steel. I wouldn’t hammer on a western style plane iron. I gave up on flattening the entire back of plane irons and now use the ruler trick. Yes, you are putting a small back bevel on the iron but it doesn’t impair performance and involves far less material you have to remove to get to a decent edge. By the way, don’t use the ruler trick on your bench chisels!
Honestly, why should this work anyway?
@ed — cold working steel has many applications and benefits. However, in properly hardened tool steel it is typically done after annealing of the hardened metal, to restore ductility. In an older thread, I touched on cold working cast iron, and most dismissed the idea, but it can, in theory, be done, and recent science has caught up with this lost art, and validated the theory. I’ll get back to that but the TL;DR is that if we accept the steel was good, and you are right, it probably was, then it’s just that this process is best done by skilled hands.A reasonably flat iron is placed onto a reasonably flat bench and is struck with a mallet. How likely is it for the mallet face to lead to distortion of the hardened steel in a way that cups the center?
Sure, but if we zoom out, you started down this path because your iron wasn’t sufficiently flat, was it? If we agree that your bench is unlikely to have a matching curvature, then it’s reasonable to say that at or near the apex of your blade’s curvature, there wasn’t much support from your bench. So in that area, you probably did ask your blade to flex a fair bit.
Then we factor in the high hardness of your plane blade. Very hard tool steels are so brittle that we sacrifice some of that hardness, via tempering, just so that we can dial down the extremely brittle nature of highly hardened tool steels.
To continue in that vein, shock resistance is one of the compromises that is juggled when making tool steel. There is the “S” type of tool steel which is optimized specifically for shock resistance.
In other types of tool steel, battering or impact tools are put in service at moderate hardness levels for improved toughness, and to avoid the brittleness of highly hardened tool steels. For the tool steels typically chosen for plane blades (O1, A2), shock resistance is not optimized, but rather wear resistance. A2 is notably more brittle than other types of tool steel, but manufacturers and users love its excellent wear resistance.OK, fine, we get it, hard tool steels are still somewhat brittle, and we asked a brittle steel to flex, but so did Paul — he hammered on his iron and it worked. Circling back to hammering on cast iron, a story — what little I know about metal was taught to me by a retired machinist. He’s in his 90’s, but still incredibly smart and sharp. He told me that when he was a kid, apprenticing at a foundry in the Rust Belt before WW2, it was not uncommon to see the more experienced workers at that foundry hammering on cast iron parts that failed QC, to see if they could salvage that part and re-shape it sufficiently so that they would work for their intended purpose (he related that those parts became, in service, the best performing parts that came from the foundry, more on that later).
Back in that day, he said they called this “shot blasting”, and he noted that by the time he came up, those very experienced workers had retired, and he was not able to learn this method. In fact, it is a lost art to this very day, but even the Wikipedia entry mentions that cast iron was re-shaped by hand-hammering by very skilled workers once. Now it’s called “shot peening”, and if you want to geek out on the metallurgy, modern fatigue science (https://www.sciencedirect.com/science/article/pii/S0142112313002375 et al) has shown that a very specific pattern of impacts at certain frequencies actually improves the fatigue resistance and other properties of cast iron and for that matter many steels, too. So there’s science behind why those parts that had been hand-hammered later became the best parts from the foundry.
Apologize for the caffeine-fueled story; the bottom line is that @alan141 is correct when he speculated about Paul’s skill — hammering on cold metals is best done by a very skilled set of hands. Not to say it can’t be done with success by the rest of us, but the harder the steel and the greater the curve the greater the risk.
Sure, but if we zoom out, you started down this path because your iron wasn’t sufficiently flat, was it? If we agree that your bench is unlikely to have a matching curvature, then it’s reasonable to say that at or near the apex of your blade’s curvature, there wasn’t much support from your bench. So in that area, you probably did ask your blade to flex a fair bit.
@etmo No, the blade could have a belly because of variations in thickness rather than from bowing, so it could be fully supported by the bench. What is perhaps more significant is that the metal work is dealing with tighter tolerances (few thousandths of an inch?) than the typical beaten, wooden bench. So, the actual support could be exactly opposite to what is needed. Note that one person tried to game the system by adding a spacer, they broke the iron. When the skilled people you are describing did their thing, I’ll bet they used a number of smaller blows to build up the desired change and carefully controlled their work surface. I could easily believe that the skill involves riding the line between “just hard enough to move things” and “easy enough to just give a nudge.” Paul gives one good whack at a random spot on his bench.Thanks for the interesting details. I don’t doubt that people can do the work you are describing in some cases and I enjoyed reading about it (tell me more if you like! I’ll read it), but as for having it work on a production plane iron on a bench top, at least for myself, I’m going with the statistics. I’m not trying to say it can or cannot be done, but I do feel it is worth noting the failures so that people have a notion of what their betting odds might be.
This is my lunch break…I better get back to work.
@etmo I just realized this blade is marked, “hand forged,” so I wonder if that supports your theory of stresses from manufacturing?
I finally have this beast flat. More properly, I should say I have the final 1/4″ flat and polished and you can support the final 1/2″ to work the final 1/4″. I used Garrett Hack’s trick of mounting a grinding wheel in a drill and then going after the high spots. So, the blade is now a bit like a Japanese blade and has a 1/2″ rectangular hollow across most of the width of the blade about 1/4″ back from the edge.
One thing that I noticed is that my strop is causing scratches. It’s not revealing scratches from early stages, but is causing them. I wonder if it’s the compound itself (Lee Valley), crud in the compound from sitting on the shelf, or crud in the strop from sitting on the shelf. I always thought I was revealing scratches missed by earlier stages of polishing, but now I’m sure it’s the strop. I’m not sure it affects the iron. I’ve always just ignored it (but been annoyed).
@ed — hand-forged? Very cool! It certainly doesn’t rule out an issue during manufacturing. If, as you mentioned, the blade has no true curve, but instead the thickness grows and wanes causing a belly, that would also be consistent with imperfections in the manufacturing process.
Flattening backs can be so much work that it drove me to buy LN / LV products, with their perfectly-lapped backs. It’s odd that a manufacturer with such a great reputation as Clifton would produce so flawed a blade, I wonder if you just got the one bad one that slipped by QC or if it’s been through some accident (previous owner dropped it from a great height?).
My strop will also cause scratches if I don’t take care of it. I use it so much that it never leaves my bench, so it gets sawdust on it, small bits of wood, whatever it picks up from the bench, and of course plenty of metal from the various tools I have stropped. I therefore clean it regularly. At the very least I’ll rub it vigorously with a clean cloth once every handful of uses, but once every several dozen uses or so I’ll scrape it (with the long side of a chisel, for example) to get rid of clumped-up bits of compound and then re-charge it with fresh compound.
@etmo I must confess to likely being the cause of the belly. I haven’t treated this blade well. If you looked at the blade new, it would like the Veritas blades.
In my comment about variations in thickness, I was trying to point out that there are multiple reasons for a belly. For example, if you are restoring an old “banana blade,” the belly may be because the blade was ground unevenly over the years, e.g., on a dished stone.
In the end, I think what happened with this blade was that it was relatively flat except I’d probably dubbed the final 1/4″ near the edge in the process of going back and forth between a heavy scrub-plane like camber and straight by being either rushed or careless. When I tried to use the “just whack it,” technique, this lead to a twist, for whatever reason. It wasn’t a big twist, certainly not enough to see with your eye, but certain enough to keep the end from being ground when on a plate.
My Clifton #3 is my single best plane. I’m hoping to get my Clifton #5, the source of this blade, to the same level of performance.
Ed, I think that you will find that your iron was at the hard end of the tool steel spectrum (ie tempered at the low temperature end). Paul’s blade may well have come from the softer end of the tool steel hardness spectrum (tempered at slightly higher temperature or for longer) – given Paul’s propensity to harden and temper his own irons, maybe he had tempered this iron. I would stay away from hammering hardened steel like that. Having said this, a guy in Queensland used to manufacture and sell specialty plane irons and he used to use a snecking hammer to flatten his irons but he was one very experienced operator! Your iron possibly suffered residual stresses from hardening, not fully relieved during the tempering process and which resolved itself during your attempt to hammer it. You would still be able to get a replacement Clifton iron from the new owners of Clifton, or from Ray Iles.
Edmund, You wouldn’t be able to hammer grey cast iron, which is the form of cast iron used in most toolmaking. That article that you referred to had one clue about why the material was capable of being hammered – it was nodular iron having spherical graphite “nodules” instead of flake graphite,(as is the case in grey cast iron). The nodules are produced by the addition of innoculants to the melt immediately prior to casting – much more expensive to produce that common grey iron. Grey cast iron with flake graphite is a brittle material. The machinist had forgotten that his foundry was dealing with a very different alloy! A much higher quality cast iron. Note that the LN iron hand planes are all produced with ductile cast iron – not grey iron. A much more expensive process.
MarkH
@markh Thanks for the interesting stuff. The bottom line for me is that I’m more likely to try Hack’s approach of grinding out a belly with a small wheel in a drill than to hammer steel of unknown history, especially thicker irons, like the Clifton.
I’ve found another person (Maguire) who hammers irons to deal with bellies, but he does it with many deliberate small blows on the anvil of a machinist vise and does it on the old, thin irons. Rather than whacking the belly as if to drive it into the bench, he supports the belly on the anvil and taps the high portion of the blade back down. It is clear, deliberate work using the edge of the anvil to control what happens plus many trips back to the stone to evaluate the developing results.
@ed I’ve dug out my plane blade manufactured by Paul Williams, who used to make them in NSW and Queensland under the trading name of Academy Saws, and photographed the pattern of indents made by his fine hammer used to flatten the blades. The hammer was almost used to redistribute the residual tension inside the blade by a series of very small blows. The hole which can be seen below the millimetre rule in the picture is the bottom of the adjustment slot on the blade – so this is close to the hardened part of the blade. Note the size and the direction of the hammer marks – each a minute adjustment of the blade’s flatness. The line across the blade bottom left in the photo is residue from some sticky tape – my blade has been in storage for a few years now. As you said above, a series of deliberate small blows – Maguire probably uses the same sort of technique. I’ll have to contact Paul about this and ask him how he did it and how he selected where to hit next during the whole process.
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