just plane flat?

[estimated reading time 7 minutes]

while i’m on the subject of frequent woodworking questions, the first one that always comes to mind is this. i’ll give you a little background. if you’ve ever used a jointer (and you’ve probably used a jointer if you’re reading woodworking articles online), you know the infeed and outfeed tables are slightly offset. this allows the workpiece to be registered flat against the infeed side before hitting the cutter and flat against the outfeed side after it’s been cut without having to pivot in the middle. after realizing this, students nearly without exception get this glassy-eyed look for about thirty seconds then one says “but doesn’t that mean a plane needs to have a split offset sole?” or, usually, some unintelligible version of this question that eventually starts to make sense when they get up and draw it on the board to illustrate their question. why is it ok for a plane to have a flat sole? how is it possible it can cut with the back (on a western-style plane) inline with the front, not inline with the edge of the blade and not end up cutting a taper.

the answer is actually, as most things in woodworking are, extremely simple. and this transfers to a huge debate about sole-flattening that isn’t just the domain of pure beginners but experts all across the craft in many countries and i suspect many of them won’t like what i’m about to tell you, though it is true and you are welcome to experiment if you don’t believe me. it’s not that the back of the plane needs to be inline with the front. or that it needs to be flat. it’s that it doesn’t make the slightest difference because the back of the plane sole is meaningless to the cut and simply there to hold the plane together and get out of the way.

let’s take a quick look at how a plane works. we’re going to do this for a modern western plane but the same mechanical rules apply to all planes — if it’s an asian plane, though, it may be moving in the other direction and “front” and “back” may have different meanings. i am a predominantly-japanese woodworker but i use metal western planes for almost all my work simply because they are far less involved to maintain. and yes i pull them. and it works just fine.

you put the plane on the wood and press down on the front as you move it forward until the iron digs into the workpiece and starts taking a very thin slice of wood (often called a shaving or curl — as in “happy little wood-curls” or “smiling shavings”). what you need to realize about this is that the front of the plane, the part of the sole riding on the wood between the front edge and the iron, is the reference surface. that is what guides the iron into the wood and keeps the depth consistent (if you’re doing it right when you’re smoothing, consistently-shallow or you’re going to take a huge bit out the wood and be filled with self-loathing and regret). once the iron has passed over the wood, this process continues. you’re never referencing against the back of the plane. even as you shift your weight to the back at the end of the cut to avoid diving off the end and sending the plane soaring (gliding?) to the floor. the back of the plane never becomes flush with the workpiece. it will feel like it does. but i promise it doesn’t. there will always be a fractional gap — the height of your iron if the sole is actually flat.

if you don’t believe me, that’s ok. take a thin piece of paper and fold it until it’s the same thickness as your iron’s penetration from the mouth. stick it to the sole just behind the iron. use the plane as normal. you won’t notice a difference in angle or sensation. this is what’s already been happening but the gap is no longer air — it’s paper. you can also take a high-resolution video as you plane with a bright light shining behind the plane so you can see the gap and watch for it. it’ll be there. the sole of the plane doesn’t warp or bend — for fuck’s sake, most of them are cast iron or bronze and if they’re bending from the weight of your body it might be time to start calculating your own gravitational pull. now this isn’t to get silly about it. if there’s a visible gap rather than just a tiny, perhaps five-hundredths of a millimeter space for light to penetrate (.05mm is approximately 2/1000”, absolutely the thickest shaving you’d want to be taking with a smoothing plane), you’re not just breaking the laws of physics. you’re breaking the laws of sensible woodworking. but this hair-thickness gap is the reason a plane with a stiff, flat sole works but a jointer with coplanar beds at the same height cuts a taper — it’s a subtle taper but it’s there and it’ll drive you crazy. i speak from experience. far too much experience, in fact. my first time using a jointer was long before the internet was anything other than an academic pursuit — i promise it was a happier place then before the rise of memes and hate and fake news but that’s a hugely-divergent tangent and mostly includes nostalgia about the jackson five and pickup games of basketball that usually ended up with me wondering if i should get stitches — and there simply wasn’t a community of people to ask basic questions. the answers were available and i was a fervent reader of fine woodworking but most of those articles don’t tell you how to set up your jointer tables. they tell you how to build furniture. and i was getting pretty good at that before i actually got a jointer. oh, the good-old days…

anyway, this is where we get to the controversial part of the article. actually, it shouldn’t be controversial at all. now, having understood the entire purpose of the back of the sole is simply to keep the whole plane rigid, not to guide the thing through the wood, you are well-aware it doesn’t need to be flat. remember that shop teacher all those years ago who told you to take the ruler from a 300mm starrett combo-square and use it to check your plane sole before you use it? and if it’s not flat, take it to a piece of granite with sandpaper glued to it and spend a half-hour rubbing the thing and trying not to breathe in the dust-of-death coming off that cast-iron? ridiculous. there are two places on the plane sole that need to touch the wood and only two — the 1cm at the front of the plane (the toe) and the 1cm directly in front of the cutting edge (or in front of the mouth, as you can think of it). the rest of the sole can be wavy as all hell and it will have no impact on your cut. with one exception. it can’t stick up beyond those two pieces. so if you flip your plane over and look at it like the silhouette of a mountain range, the two flat-topped peaks have to be the front 1cm and the 1cm in front of the blade. nothing else can be any higher than that, though all at the same height is fine. if you find something higher, file it down. sandpaper on granite is fine, file is fine, sanding block is fine, diamond stone is fine. don’t worry if it’s not exactly inline with those two. just equal-or-lower. it has to do what most good helpers do in woodworking — if it’s not going to help, get out the way and let things happen properly. because, realistically, they will.

why, you ask, if that’s the case, is this so controversial? two reasons. the first is that a dead-flat sole is a source of pride for woodworkers who like to keep their tools in perfect condition. most of mine are either flat or pretty close. it’s pretty. it feels smooth and nice. and if it’s flat you know there are no peaks sticking up so it’s pretty easy to be sure it won’t go weird that way. that’s not a problem. you just have to remember it’s aesthetic, not necessary for the functionality of the tool. the second reason is a bit more insidious. there has been a boatload (perhaps even a shitload, though i suspect not quite that many) of people in magazines but particularly on the internet (youtube, i’m looking at you!) who say things like “get yourself an old stanley and get the sole dead-flat”. and, while that might be nice for show, what people hear is “the sole has to be flat end-to-end” — mostly because they don’t actually explain how the mechanics of the plane work.

a little historical background, by the way. when stanley was making a lot more planes (because they weren’t just for hobbyists back then — or for christmas), they offered a “c” version of their most popular models. you’ll often see an old stanley jack called a “5c”, for example. the c is for corrugated — like the wavy cardboard in packing boxes that reminds you your father used to dance around the house at christmas with an accordion. ok, maybe yours didn’t. but mine did and he’s an awesome accordionist — older memories than the jackson five? most certainly. it had deep ridges in the sole. they talked about this as making the plane slide easier (it doesn’t) and removing mass (which it absolutely does but i can’t imagine why anyone would want a lighter jack-plane because the more mass the thing has the easier it is to keep moving when roughly-flattening the stock and i’d be pretty happy if the whole thing was made of uranium, except for … you know … the other reasons we don’t use uranium for things we hold in our hands). but the one thing they didn’t talk about — because nobody was stupid enough to worry about it when they used planes every day for their work and had grown up with their mechanics as firmly-etched in their minds as their multiplication-tables — was that this non-flat sole might have stopped the plane from working.

in our contemporary, planes-are-for-fun-times woodworking culture, we’ve started to ask more questions and that’s not always a bad thing. but in this case it’s become rather confusing to a lot of people. maybe this will help. perhaps it won’t. if you’re still confused, let me know. i’m always happy to help those who reach out, students at any level of expertise — i’m sure there are people who’ve been woodworking since before i was born (which was, admittedly, a long time ago, though, so probably not that many of you left who had planes and chisels before i got out the crib) — who want to ask. either way, that’s all for now, a little shorter than usual but it’s a relatively-simple thought for the morning. thanks for reading!

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thank you for reading. your eyes have done me a great honor today.