about thirty years ago, i saw a trick for the first time to make flattening the back of a plane iron far easier if it was badly out of flat — take a thin coin and put it under the iron, tilting the back just a little off the surface of the stone, allowing it to register against a slightly different plane. this makes the edge sharp without having to take the whole back down to flat. i thought it was brilliant. the woodworker who showed me (it doesn’t really matter who it was and they’re long since departed this world) just said it was something he’d seen his teacher do and it was an old trick, probably dating back centuries. his teacher had done it with a coin, too, though i vaguely remember him mentioning long after that his teacher’s teacher did it with something else because coins back then were pretty thick and the angle was too steep. anyway, this trick has become popularized in the last decade or so as the ruler trick and a thin metal ruler is definitely a better object to use than a coin because of the larger bearing surface. i wish i’d thought of it but i was still doing it with coins. they definitely work, though. something you might not know is that japanese coins are mostly standardized by thickness so you don’t have to remember if it was a 1, 5 or 10 yen coin — they’re all 1.5mm thick. that’s pretty close to the 1.2mm of a starrett steel ruler (other brands vary quite a bit) so you can see why this works with whatever you have around. in a world of electronic money, though, i’m much more likely now to have a ruler in the shop than a coin, i must admit.
the narrative aside, though, what’s the point of the “ruler trick” (or, i suppose the “coin trick” but i think that means something else)? a good plane iron should be completely flat on the back. but we don’t always have good plane irons. get an old stanley with its original iron, for example, and you’ll discover decades of rust, pressure-warping and various other environmental impacts (combined with the fact that the quality of the steel was originally slightly short of uninspired excrement) have taken a blade that was euphemistically thought of as straight to something so bent it might not qualify for legal marriage in some countries. if you want to restore that iron (and you do cause there’s nothing wrong with it, despite the quality of the steel not being very good and the thing being so thin you want to teach it to lift weights), you have two choices — or at least you have two choices if you don’t want to break out the ruler. you can spend a lot of time flattening the back on your stones — this is the preferred option but it can take literal hours and the result might be an iron that is significantly thinner than it used to be to get through all the warping and twisting to make it flat. or you can just say “no, it’s flat enough” and get on with your day, knowing it might be truly brutal to try to sharpen. there’s a way to sharpen it without the whole back being flat if you want to do it. hang the plane off the edge of the stone and just flatten the last 10mm of the back, the part right at the edge. if that’s flat, the rest of the back doesn’t actually matter for the purposes of cutting. it’s twitchy and can mean the chip-breaker won’t seat properly and all kinds of other things. i don’t recommend this approach, even on a bevel-up plane where the only thing the back has to do is seat snugly against the frog, something it doesn’t have to be flat for, just willing to bend enough to sit without moving.
enter the almighty steel ruler, though. if you put the ruler under the back — between your stone and the non-cutting end of your blade — the ruler raises the angle and allows you to sharpen only the first few millimeters of the back. the side effect is that you’re actually putting a very small back-bevel on the iron. that’s not all that important but it means this procedure can only be done on plane irons, not chisels. we’ll get to why in a minute but you can assume it’s the case. what you’ve done is changed the total angle you’re getting on the blade. if you sharpen your plane blades to 35 degrees (if you have a standard-angle bevel-down plane, that’s probably what you should be sharpening to unless you’ve got a very good reason to — and i can’t think of a good reason that doesn’t start with “well, someone told me…”) and your back-bevel is 2 degrees, your effective angle against the wood is — no, not 37 degrees. and that’s exactly where everyone gets confused. remember, the effective cutting angle is the angle of the frog, not the angle of the blade, on a bevel-down plane. if you have a standard bench plane (stanley et al) this is 45 degrees. so put two degrees of back-bevel on your iron and it’s like having a 47-degree effective cutting angle.
ah, i hear the question in your mind before you can even ask it. doesn’t that make it harder to push the plane? well, yes. but a couple of degrees one way or another isn’t significant. here’s the other interesting thing about a higher angle on the plane. the higher the angle, the better performance you’ll get on difficult grain. so why stop at 2 or 3 degrees? why not do 10 or 15? actually, there’s no reason at all. of course, the higher the angle the harder the plane is to push. so you don’t want to just go to town on easy wood with a 60-degree effective cutting angle if you don’t have to. but when you bring home that beautifully-figured piece of maple crotch and don’t want it to have tearout from here to next week, ditch the ruler and shove the blade in your sharpening guide at 15 or 20 degrees and sharpen that back like it’s another iron and you’ll be blown away by the results.
of course, you might have a bevel-up plane — those have become very popular in the last decade or so in a way they’ve never really been until now. the old stanley 62 and its modern incarnations from veritas, lie nielsen, etc, have probably surpassed the standard 4s and 5s, though i think they might be a little overhyped. i don’t particularly like the low-angle jacks but it’s all personal choice and they do great work, even if they are a little more twitchy and prone to vibration from their lack of chip-breaker. they also have a little different geometry and that’s why we’re talking about them now. but let’s start with chisels because that’s where the confusion begins.
you can’t use the ruler trick on a chisel — actually, you can’t put a back-bevel on a chisel at all or you’ll screw that thing up so badly you’ll be back at the grinder before you can get a string of curses out your mouth. if you think about it, it makes sense. what’s the most important job a chisel can do? paring. it takes anything sticking up from a single plane and flattens it to that plane. sort of the way sliding a razor blade across a sheet of glass will clean anything off its surface, if that helps you to picture it. now picture putting a back-bevel on the chisel. you’ve just raised the cutting edge so it’s no longer in the same plane as the back. so if you want to pare something, you’re absolutely guaranteed it will never quite get there and you’ll be paring a shallow taper instead of a smooth surface. the moral of that story is don’t put back-bevels on chisels or they stop functioning as chisels. it’s the same reason a knife with a bevel on each face can’t be used to smooth a surface if you hold one face flat down. yes, in theory you can pare with a back-beveled chisel (or a double-bevel knife) if you want to. but this requires shifting the angle of attack — you have to move the chisel in the plane of the cut but hold the chisel slightly elevated from the reference plane, a combined movement that’s realistically unwieldy at best and that i’ve never seen successfully done in practice with any degree of reliability. so if you’ve got a chisel whose back is out of flat, what’s the trick? no trick. just flatten it. all the way. hey, it’s probably a lot narrower than a plane iron so it shouldn’t take nearly as long and that’s what your ultra-coarse stone is for, right?
back to bevel-up planes. ok, a bevel-up plane is realistically functioning like a chisel held in a jig moving along the work and slicing the wood. it’s not the same angle of attack as a bevel-down plane and the intersecting plane geometry is different. so, given it being more like a chisel used bevel-up, does that mean you can’t use the ruler trick on it? or put a back-bevel on it at all? no. and here’s the simple reason. on a plane (regardless of bevel direction or bed angle), the reference surface isn’t either face of the iron — it’s the bed of the plane. that’s what’s riding against the workpiece. when you’re paring with a chisel, this reference face is usually the back of the chisel (if you’re using it bevel-up, which you will be if you’re paring). so beveling that reference surface means shifting the angle of attack and ruining the chisel. but beveling that surface on a bevel-up plane has no real impact on its performance so you can feel free to go ahead and do it. practically speaking, it will actually slightly change the way the blade performs but this slight change in total angle is something you won’t notice — i know i certainly don’t.
something to note about the ruler trick, by the way. if you do it, you’ll have to keep doing it unless you want to grind off the first few millimeters of your iron. once that edge has a few degrees of back-bevel, you’re never going to get the thing completely flat without going to the depth of the bevel or eliminating the bevel completely at the grinder. i don’t see this being a problem but it’s just wise to keep it in mind in case you think you might not like it — you can change your mind but it means grinding the tip back to past the bevel, reflattening the back and creating and honing a new bevel before you can use it the next time.
this has been a frequent (more frequent than usual, in fact) question of late so i figured it was worth writing a short article about. if any of this wasn’t clear, please let me know. hopefully this trick, regardless of which version you want to use, will help you get the back of your iron flat and possibly, if you take it ten or fifteen degrees beyond flat, give you vastly better performance in your standard-angle bevel-down planes on difficult grain. anyway, have fun with it. thanks for reading!