Camber on Plane Irons

A recent discussion over at the Rennaisance Woodworker got me thinking about the amount of camber I put in my plane irons. Those of us familiar with hand planes and planing typically understand each other when we say that our fore or jack plane iron has “significant” camber or that our try plane has “moderate” camber or that our smoother has “just a hint” of camber. But these terms are mostly subjective and to someone new to using hand planes, “significant”, “moderate” or “just a hint” doesn’t necessarily translate well. So although I don’t typically measure the amount of camber I put in my plane irons, I decided to try to quantitate it in order to better qualify the meanings of “significant”, “moderate” and “just a hint.”

camber01

So these are the three planes I am going to talk about in order of most amount camber to least amount of camber. The fore plane in front (also called a jack plane) has what I consider “significant” camber. In the middle, my try plane (also called a jointer) has “moderate” camber. Finally, in the rear, [one of] my smoothers have “just a hint” of camber. I took all the irons out since they needed honing anyway and tried to quantitate their amount of camber.

So after some trial and error using a piece of string, a magic marker and my awl, I got a close estimate of the amount of camber in two of the three irons (no matter how much I tried, I couldn’t quantitate the camber in the smoother, the radius was just way too big).

camber02

In hind sight, this would be a good way to mark your irons in order to grind a specified amount of camber. Simply use a dark colored permanent marker (I used black) to color the edge of the flat face of the iron. If you have it, machinist layout fluid would work as well, but it’s not necessary, the marker works just fine and is a lot less messy. Tie a small loop in the end of a long length of string. Place an awl in the loop, hold the opposite end of the string to the bench top a distance equal to the camber radius away from the edge of the iron and scribe the camber radius through the marker onto the iron. Then simply grind to this line and hone as demonstrated in Episode # 6 of the podcast.

camber03

So here are the results. They are in the same order as the planes above and you can see the resulting radii from my crude measurements. The radius of the fore plane (on the bottom) measured at about 10″. You can easily see the radius ground into this iron. The radius of the try plane was about twice the fore plane at about 20″ (middle). You can also easily see the radius in this edge, though it is not as distinct as the fore plane. Finally, at the top is the smoother. This iron basically looks straight, however, if you place a straight edge up to the edge, you will see a hint of light at each of the corners. The relief is definately less than 1/32″ but I couldn’t measure it.

When it comes to establishing the cambers, I grind the camber into the fore plane and try plane irons. However, when I grind the smooth plane, I grind the edge straight. Then, when I get to the honing, I simply do some extra honing of the outside corners in order to relieve them below the center ever so slightly. Again, the iron still appears straight, it’s only when a known straightedge is shown to the iron that you can notice the slight relief at the corners.

camber04

Here’s the finished result. This is a picture of the fore plane after the freshly ground and honed iron has been put back in and the depth of cut set. You can see the effect that the camber has. The center of the iron will take a relatively heavy cut (maybe between 1/32″ and 1/16″ thick) but the corners won’t dig in and leave tracks behind. You can see how this makes the plane capable of removing stock in a real hurry (about 1/4″ in about 8 strokes). No fluffy shavings here. They’re more like chips.

camber05

Similarly, the try plane will not leave tracks behind due to its camber. The camber is less than the fore plane’s by about half so the shaving thickness is similarly about half that of the fore plane. Again, these shavings aren’t fluffy. They’re thick enough to bring a board face true in fairly short order, but they are still thin enough to remove the scallops left by the fore plane and prepare the surface for final smoothing. The camber in this plane also helps in squaring edges by enabling the plane to take a wedge shaped shaving just by shifting the position of the plane on the board’s edge.

I didn’t take any pictures of the smoother as you really can’t see the iron above the sole like you can with the fore and try. The smoother is set up to take extremely thin shavings to put the finish ready surface on the show faces of the “money” boards. It leaves behind a tearout free, polished surface that is ever so slightly scalloped. The scalloping is so shallow that you don’t really see it, but if you run your hand over the surface, you can just barely feel it.

So there is my definition of “significant”, “moderate” and “just a hint” of camber as it applies to my personal planes. I’m curious now as to how my definitions of “significant”, “moderate” and “just a hint” compare to yours for your personal planes.

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6 thoughts on “Camber on Plane Irons

  1. You just poked at a hobby-horse of mine, so I’m climbing up on the soap-box. 🙂

    I know that the radius is often used as a measure for the camber of a plane iron. No arguments, there. However, I don’t personally think that the radius is an entirely useful value to use. The reason is because the proper camber for a given surfacing plane iron depends first on the depth of the shaving you’re taking with it, and secondly on the angle that iron is mounted. Since you have these two factors involved, the radius depends on the width of the iron and what kind of plane the iron belongs do.

    If the irons in question are all the same width and all mounted at the same angle then a radius value can be passed around and everyone gets the same results and all is lovely, but if you apply the same radius to a 2.5″ wide plane iron mounted at 55 degrees that you did to a 2″ wide plane iron that is mounted at 35 degrees then it will cut much deeper for a given width of cut.

    The whole point is that you want to take the widest shaving you can without digging in at the sides. If you have a smoothing plane that you take 2 thou shavings with then you want 3 thou of *effective* camber. The amount of camber you need to grind onto your iron to attain that effective camber depends on the angle.

    It’s been a long time since school, but if I remember correctly the effective camber will vary according to the sine of the mounting angle. So effective camber ‘E’ = sin(angle) x applied camber (C), so we can rearrange that to get the camber you need to apply by saying:
    C = E / sin(angle)

    So if this hypothetical plane iron is at York pitch (50 degrees) then to get our 3 thou of effective camber we need:
    C = 3 / sin(50) = 3.916 or basically 4 thou.

    So in this case the 50 degree bedding angle means we need an extra 25%, roughly of camber in order to get what we need. The lower the bedding angle the more extra camber you need. A 12 degree (low-angle) block plane would need almost five times the applied camber to get what you think you need. So if you want 10 thou of camber you need to grind 50 thou of camber onto the blade. Crazy stuff.

    Anyway, so I guess my point is that if Chris Schwarz tells us that an 8″ radius camber is ‘correct’ for a jack plane, and I grind an 8″ radius camber onto the iron of my Veritas Low-Angle Jack Plane, I will have less than 1/3 of the camber that he has with his Stanley #5, even though “I did what he said to do.”

    In closing (finally, eh?) let me just say that the amount of *effective* camber for any flattening plane iron should be just a hair more than the thickest shaving that you’re going to take with that plane. This allows you to take the widest shaving possible without leaving tracks or tearing the grain.

    I hope that makes sense…

    • Aw geeez Mike, it’s way too early for me to be thinking about trig :o. Here I am trying to come up with a simple explanation of how much camber I have in my three planes…

      All kidding aside though, you make an excellent point. Now I don’t know what the radius of Schwarz’s plane irons are (8″ did you say?), but ultimately I don’t think it really matters. I guess that the point that I was trying to make (not very well by the way, you did a much better job of it) was very similar to your last paragraph:

      “In closing (finally, eh?) let me just say that the amount of *effective* camber for any flattening plane iron should be just a hair more than the thickest shaving that you’re going to take with that plane. This allows you to take the widest shaving possible without leaving tracks or tearing the grain.”

      I used the crude trial and error with a string method to figure out about how much camber I have in my irons, however, as you noted, this may only apply to another user’s planes if they wanted to use the planes to take shavings as thick as I do, with planes bedded bevel down at 45 degrees (as are all three of those I discussed) with irons of the same width as mine. To further support your point, I figured out that my fore plane iron has about a 10″ radius while you mentioned that Schwarz uses 8″ on his #5. However, my fore plane iron is wider than a #5 so the “effective camber” as you say may be about the same. Both planes probably take about the same thickness shaving, only mine would be slightly wider.

      At any rate, as I mentioned early on in the post, I really don’t measure these things. I don’t measure camber, I don’t mic shaving thickness, and I don’t use feeler gauges to see how far out of flat my plane soles are. In all honesty, most of the time, I don’t care. These measurements aren’t really important in woodworking. Matter of fact, the most accurate measuring tool in my shop is a century old boxwood folding rule with graduations only down to 16ths of an inch. That’s plenty accurate enough for my furniture work because I don’t do a lot of measuring.

      I attempted a crude measurement in this case just to try to explain how I apply the terms “significant”, “moderate” and “just a hint”. However, as your excellent comment pointed out, using the radius of the camber is really not the best way, especially if one is using vastly different tools than I am(e.g. bevel up planes).

      Probably the best explanation would be based on iron projection, not radius, and can be best explained with just the last two pictures in the post rather than actual measurements. One could simply color the iron with the marker, put the iron in the plane, adjust the depth of cut to the desired thickness and use an awl to mark the outside edges of the iron where they meet the mouth of the plane. Then simply remove the iron, freehand draw a continuous curve from the two scribed points on the outer edges of the iron to the center of the iron, then grind away. You will end up with the perfect amount of camber for your particular setup every time, regardless of the type of plane (bevel up or down), bedding angle or iron width. And you won’t need to do any measuring or trig to get there. That’s more my kind of woodworking ;).

      • You know, I hadn’t realized how huge and -let’s face it – arrogant that comment of mine was until I looked at it today. There’s something about spending all of a stinking hot day on a roof and then crawling home to see somebody has just saddled one of my favourite hobby horses to get me all in a froth and typing these miserable little diatribes. So for that I apologize. Not for the content, per se, but for the tone. Mea culpa.

        That having been said, you make an excellent point that trial and error is going to be important, here. What a person might want to do to set the camber for a fore plane is get some white pine and then keep extending their iron until they basically can’t reasonably push the plane through the material. That will be their maximum shaving thickness. Mark the iron as you suggest, and when the iron is cambered to that point then the plane will be significantly easier to push. That might work for a rule of thumb. When dealing with harder wood you simply don’t extend the iron as far. For something truly ignorant like some of the hard exotics you might be taking just a whisper of a shaving, but for normal wood it will hog material pretty well.

        • No need to apologize. I didn’t think your post was arrogant and was glad you made it. It really shed a lot of light on the subject and made some very important points that were not made in the original post.

  2. A few months ago I discussed the issue of the “functional camber” in a post on my blog. It explains, with the help of diagrams, how the functional camber changes with the blade’s bed angle. Here is the link:

    http://www.rpwoodwork.com/blog/2009/05/21/how-much-camber-should-be-in-plane-irons/

    I’m pretty much in agreement with your ideas. I think your post will be helpful for many woodworkers since this is a subtle matter and takes some trial and error no matter how one approaches it. The bottom line is feedback from how the plane performs!

    Thanks for your informative blog.

    • Thanks for the comment and the link. I definitely agree that one pretty much needs to try the plane out and adjust until it does what you want.

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