Cambering blades, serves the following two main purposes:
- With a noticeable amount of camber, hogging wood off a wood surface, goes faster, than if no camber was put on the blade.
- With just a small amount of camber placed on the blade, say somewhere around 1/32″ or 1/64″, noticeable ridges from handplanes are eliminated.
The sketch below shows a cambered blade, along with the following:
- L = Your blade’s width
- H = The arc height, measured as shown. This arc height is what determines the amount of camber on your blade. As the arc height increases, the amount of camber on your blade increases, and vice versa.
- R = The camber radius
- The equation shown, R = (L^2 + 4*H^2) / (8*H), enables you to place your values for “L” & “H” into the equation, and solve for “R” the camber radius req’d.
- Points “A” & “B”, should be even with the sole of the plane, or above the sole of the plane. If this is not the case, then you will still have ridges, with a trough between the ridges, which is cambered.
The sketch below, shows an enlarged, not to scale view of the blade below the handplane sole. This enlarged view, is a visual aid, for discussing the following:
- The 45 degree angle shown, will be equal to the angle of the frog. In this case a 45 degree angle (a common angle for frogs) has been assumed, for illustration purposes.
- Line “A” is drawn parallel to the sole of the handplane.
- Line “B” represents the thickness of shavings.
- Line “C” represents the arc height, represented by “H” in the sketch above.
- Looking at Line “B” & Line “C”, its now obvious, that the arc height (“H”) is greater than the shaving thickness ( C > B ).
- The Sin of 45 = B / C, and Sin(45) = .707
An example of using the above information:
- Assume blade width = 2″
- Assume you want to plane 0.0625″ ( 1/16″ ) thick shavings.
- Assume your frog angle = 45 degrees.
- I like to think of Line “B” as apparent arc height ( H ) and of Line “C” as the true arc height ( H ), req’d to achieve the desired shaving thickness.
- The req’d value for “C” which equals arc height, is calculated as follows:
C = (B) / (SIN45) = (.0625) / (.707) = 0.08840″
H = C = 0.08840″
Now use equation for radius, shown in first sketch, of this post, to calculate the req’d radius.
R = (L^2 + 4xH^2) / (8xH)
R = (2^2 + 4×0.08840^2) / (8×0.08840)
R = (4.0313) / (.7072)
R = 5.700″
I have included the following table, which enables you to go to the table, and fine the req’d radius for a desired shaving thickness. The table data is based on a 45 degree bed angle (angle of your frog). Click on the table with your mouse, to enlarge the table, plus you can click on the enlarged table a 2nd time to enlarge the table even further.
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