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• Thursday, May 21st, 2009

Trick question. It depends. This post will discuss factors in the amount of camber in the edge of a plane iron with attention to an under-appreciated trigonometric quirk.

The slight convexity or “camber” in the edge of a smoothing plane iron should allow the production of airy shavings that are thickest in the middle, say .001″, and feather out to nothing at a little less than the width of the blade. This produces imperceptible scallops on the wood surface and avoids square-edged tracks or “gutters“.

A similarly small, or perhaps a bit more, camber in the edge of a jointer plane blade allows one to bring down the “high” side of an out-of-square edge without tilting and destabilizing the heavy plane. The camber should be positioned at the center of the blade projection so the plane can be shifted toward the high side of the board’s edge to remove a slightly thicker shaving there.

For jack planes, more camber lets this workhorse take thicker shavings without producing gutters. The more pronounced camber also makes it easier to direct the plane’s cut at the high spots on the surface of a board being dimensioned.

When grinding and honing a plane blade, I check the camber by setting the blade’s edge on a small aluminum straight edge and holding it up to the light to look for the tiny gaps that gradually enlarge from the center to the sides of the blade. (I never measure this amount so I cannot answer the question posed in the title of this post.)

Now some trig. Let’s say the camber – the depth of the convexity of the edge – is .005″. When this blade is installed on a 45 degree frog in a bevel-down plane, the actual functional convexity is reduced. Think of it this way: if the blade were laid flat and you viewed it toward the edge, there would appear to be no camber at all. The functional camber is reduced by the sine of the bed angle.

sin 45* x .005″ = .0035″

Look what happens in a bevel-up plane with a 12 degree bed:

sin 12* x .005″ = .001

Therefore, I sharpen more camber into a blade for a low angle bevel-up plane than for a bevel-down plane to achieve the same functional amount of camber. The camber that you observe sighting 90 degrees to the face of the blade will mostly disappear when you install the blade in a 12 degree-bed, bevel-up plane and sight down the sole to observe the camber. Compensate for this by being generous with camber in the sharpening process. A more direct approach during the sharpening process is to check the camber against a straightedge with the blade tilted at the bed angle.

Again, I do not measure these things but rely on my eye, experience, and especially feedback from the work. Of course, sometimes I’m off, usually by over-cambering. However, since the middle of the blade is thus destined to dull first, it is easy to reduce the camber on the next honing.

There are undoubtedly other factors affecting shaving thickness, such as blade sharpness, blade edge deflection, and wood grain, so it is most important to monitor the performance of the plane and make adjustments when you resharpen.

You can use trial and error or a set of leaf gauges to work this out to your liking. Like just about everything else, there’s more than one good way. Use the principles and find your way.

Category: Techniques
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9 Responses

  1. 1
    Ethan 

    I had a wonderful “A-HA!” moment at a handplane class with Chris Schwarz when I figured out squaring the edge of a board with a cambered blade.

    With a high spot on the right, I adjusted the plane’s position on the edge to put the center of the camber over the high spot.

    Two swipes later and the edge was dead square.

    What an amazing feeling it is when a technique “clicks” and you finally get it!

    Great entry, Rob.

  2. 2
    tico vogt 

    So, I guess the answer to the question you pose is; “if you have to ask, you’ll never know.”

    I’m always impressed by those, like yourself, who have an aptitude with trigonometry, and can apply that knowledge the way you have demonstrated above. When I try to go there, a physiological mental dulling
    response kicks in. That’s when you save the day by bringing in the direct approach.

    Have you ever checked out the lapping plate created in cooperation with Toshio Odate? It is expensive, and the camber, of course, fixed, but it would be endlessly repeatable. If it suited you needs, maybe a good investment.

    A scrub plane is a bench plane I don’t yet own. The cambering blades I’ve used so far are on #5 and #6 bench planes, and the subtle cambers over the 2″ and 2 5/8″ widths are fairly subtle. Do you have a general rule of thumb for the curvature of scrub plane irons?

    Thanks,

    Tico

  3. 3
    Rob 

    Thanks Ethan. I try to bank those “click” feelings to balance the next time I screw up something.

    Rob

  4. 4
    Rob 

    Hi Tico,

    I suggest go with your intuition and monitor the feedback from the plane’s performance. For reference, I just went back into the shop and measured (first time!) the blade in the picture which goes in my low-angle, bevel-up smoother. I could just fit a .005″ or .006″ leaf gauge under each side of the arc of the edge. It’s a new blade that I sharpened to a steep bevel for figured wood so I’m not sure if that camber will work out but I’ll soon find out.

    I have an old Ulmia wooden scrub plane and I just measured the blade’s camber – the radius is 1 1/4″. That huge camber allows the plane to take a deep bite diagonally across the grain to take down thickness fast.

    I use a little pressure on my Tormek bar to grind a camber and then refine it with waterstones. I have never used the Odate plates.

    Rob

  5. 5
    Stuart 

    Interesting post Rob – thanks. I have the camber roller accessory for the Veritas MkII, but have never used it ‘in anger’, as much because I’ve not researched when or why it is particularly beneficial to have a camber, combined with a concern that I wouldn’t want to wreck a good blade by doing it at the wrong time, for the wrong plane.

    Your article goes into the extra depth that I need to start to appreciate the justifications for a camber, (as well as how much!), so thank you for that!

    If I was to start with any plane, I’m guessing my Jack Plane would particularly benefit from a camber, and my smoothing plane less so (it is currently very straight, and very sharp, and I’d be hesitant to risk that blade, initially at least).

    Stu

  6. 6
    Rob 

    Hi Stuart,

    Thanks for the comment. Yea, I think a jack would be a good plane to start for learning to camber the blade. Hey, no way are you going to ruin the blade – if it doesn’t come out right, just bring it back to the sharpening station. If anything, there’s a tendency to over do it when creating camber, especially for the little bit required for a smoothing plane. Good luck and let me know if I can help.

    Rob

  7. 7
    Vlad 

    Hi!
    Realy interesting article!!!
    i have one short question: what kind of square do you hold in your hand on photo?
    Thank you!

  8. 8
    Rob 

    Vlad,

    That is a small aluminum square which is handy for checking edges for square or camber. It can be found at:
    http://www.woodcraft.com/Product/2020077/18891/Pinnacle-Precision-Mini-Square.aspx

    Rob

  1. […] And what is the desired effect? You want to take the widest shaving possible without the corners of the cutter digging in. There is math here. Having a .005″ arc-to-chord curve at 45Ã?° results in a curve of .0035″ being exposed out of the mouth. (If you have a bevel-up plane bedded at 12Ã?°, the same .005″ arc-to-chord curve will result in .001″ curve being exposed in the mouth , thanks to woodworker Rob Porcaro for the formula.) […]