Clearance angles
Neanderthal
You've created a wonderful site, but your bevel angles confuse me.
So some questions on bench planes (not a challenge):
My understanding is that the plane's cutting angle is the angle
between the plane of the blade and the plane of the wood. In using
the plane, you must push down as well as forward, and the downward
pressure compresses the wood. So, under the cutting angle, the
clearance angle created by the primary bevel permits the wood to
spring back.
From that, I inferred that the bevel angle could be as large as I
wished so long as it resulted in an adequate clearance angle. And, on
the "bevel up" page, you state that 8 degrees of clearance is enough
for performance and durability.
My confusion: Assume that my "polishing" smoothing plane has a 45
degree bedding angle and that I've sharpened with a primary bevel and
3 honing bevels of 2 degrees each. Then 45 - 8 - 2*3 = 31 degrees as
the max primary bevel I should use.
Why do you recommend regrinding to 25 degrees?
In an application where I'm cutting light passes in a soft, knot-free
wood, I see that the relevant damage to the cutting edge will be wear
be on honed edge. But if I'm cutting a hard plank or one with knots,
then I'm concerned about the edge's failure rather than wear -- that
is, I'd be concerned about the blade chipping. My understanding has
been that a greater included angle in the cutting edge gives more
structure to support the edge and, so, is less likely to fail. If I
only used a single, secondary bevel on my Jack plane, then I could use
a 35 degree primary bevel and still have an 8 degree clearance angle.
Do you recommend different primary angles for different applications?
I've always assumed that end grain compressed relatively little. So,
when I sharpened a blade for a shooting plane, I've reduced the
clearance angle to about 5 degrees to maximize the included angle for
durability.
Is my logic wrong?
Have you measured wear rate as a function of clearance angle?.
Thank you for any comments or suggestions that you can offer.
Very respectfully,
Larry
Brent Beach
As the clearance angle decreases, the time between sharpening decreases
as well. The change is not linear with clearance angle. I did some
testing a long time ago - it could well be somewhere on the web site still.
With a bevel down plane and clearance of around 13 degrees, perhaps a
little less, I get pretty good time between sharpening. With an 8 degree
clearance the time between sharpening is shorter. You can do it, it is
more work.
If you try this - 8 degree clearance, 37 degree final front bevel - let
us know your experience.
This leads to my problem with bevel up planes bedded at 12 degrees. If
you sharpen without a back bevel and remove all the back wear each time
you sharpen, you can get a pretty good result. This means removing a lot
of metal each time you sharpen - you probably have to grind each time
since honing with fine abrasives really cannot remove enough metal. If
they bedded the blade at 16 degrees, you could use a 4 degree back bevel
and still have a 12 degree clearance angle.
You can get a stronger edge - if you are having problems with the edge
chipping - by increasing the back bevel rather than the front bevel.
Make a jig with a slightly thicker back jaw - use the extension
calculator with various back jaw thicknesses until you get the final
back bevel angle you want.
I don't plane a lot of knotty wood but aside from problems shown in the
blade testing pages, have had no problems with chipping of the edge.
Typical north american hard woods will not chip the edge from hardness
along. Some Australian and south american hardwoods - particularly those
with included silica - can wear blades quickly. Increasing the included
angle can help, but blunter blades wear more quickly as well.
If you are getting wear faster than you want, you could try a tougher
steel - like M2. These are harder to sharpen, but do last much longer.
Brent
--
Victoria, B.C., Canada
Neanderthal
VERY respectfully,
Larry