Significance of Bevel Angle

[Mark Roderick]

It's been a while since I read Leonard Lee's book on sharpening, and I've been driving myself crazy with the following question:

Why do we care about the angle of the bevel on a bevel-down plane?

I know very well from experience that it matters - for example, when I sharpen my #4 to 25 degrees it works great on pine but poorly on walnut, and when I sharpen it to 35 degrees it works well on walnut but poorly on pine.

But why?

[Sam Yerardi]

One reason for the lesser angle for softwoods like pine is that they tend to compress and mash rather than cut. The lower angle helps with that.

[Jason Beam]

Bevel support is one reason, i know. If you have an needlessly accute angle, you'll actually weaken the edge and may get some chatter because there isn't enough steel left behind the forces at place to support it. Things will flex.

That's about the only reason I know of, though.

[Mark Roderick]

I can see how that would be affected by the angle of the frog on a down-facing bevel, but not by the angle of the bevel itself. Do you see what I mean?

[Pedro Reyes]

Bevel support.

In theory an angle <45 (or whatever your bed angle may be) would be the same assuming we could get them equally sharp. Achieving sharpness is easier at lower angles (up to a point I assume, not beyond when the edge becomes too weak). So the 20-35 range is a good compromise between sharpness and durability of the edge.

/p

[Wiley Horne]

An opinion.......You want the most acute included angle that the steel will take, given the wood. If the steel is good, the acute bevel will dull more slowly and penetrate better for longer than the less acute bevel.

Dulling is caused by radiusing of the leading cutting edge, along with formation of a wear bevel on the lower trailing edge. Two workers have found that when the lower wear bevel exceeds approx. '4 tenths'--0.0004"--we experience the blade as getting dull. It's not penetrating easily and pulling itself into the work any more; we're having to push down to keep it in the work. This formation of the lower wear bevel is identical with loss of clearance--when the wear bevel gets long enough, you can scarcely hold the plane in the cut.

This can be generalized in terms of clearance angle, which incorporates both the included angle of the blade, and the bedding angle of the plane. Based on geometry, the plane/blade combination with more clearance angle to begin with--will stand more metal loss before the blade gets radiused enough and 'wear beveled' enough to lose penetration and be considered dull. Conversely, the less clearance angle there is to begin with--fewer micrograms of metal loss will be required before the blade reaches the 'dull' state. This is geometry. So when you start with more clearance, you get a longer time span of the plane pulling itself into the work. This is also an argument for a bevel-up plane bedded at, say, 20 degrees rather than 12 degrees. Holtey's is bedded at 22.5 degrees, and similarly the olde-tyme British and German miter planes were typically bedded in the 20's.

Now of course, there is a tradeoff that has been mentioned--if the steel won't take the load, then the acute angled blade will fail faster than the blade with a fatter included angle. So there are limits to the argument given above. That's why you want to work with the most acute angle the steel will stand, and this is why people sometimes upgrade blades. The better steel will operate at a smaller included angle, provide greater clearance, and therefore pull itself into the work for a longer time before dulling.

Wiley

[Mark Roderick]

Just checked back with Leonard Lee's book. To paraphrase:

--- If the bevel angle is too low, the cutting edge of the metal will fail.

--- If the bevel angle is too high, there won't be enough room for chip clearance.

--- To provide sufficient strength at the edge and at the same time allow sufficient room for chip clearance, the bevel angle should be between 30 and 35 degrees.

Again, this is for a bevel-down plane.