47
General Sharpening Procedures
Assuming that you have the gouge sharpened at a 25°
bevel angle, you would fi nd that it just begins to cut the
wood at about a 30° angle of attack. There are times
when you might want such a high angle of attack to
clear surrounding work, but for most relief carving a
lower angle of attack would give better control.
To consider this for a moment, think of using a
carpenter’s chisel to make a paring cut on wood. Using
it bevel up, the applied force will be in line with the
direction of cut. But if you are forced to make the
cut with the bevel down (because of some adjacent
obstruction), you know that this carries with it greater
danger of a slip because you will be applying force at
an angle (equal to the bevel angle plus a few degrees) to
the direction of cut. You now not only have to overcome
the resistance of the wood being cut, but you have also
substantially increased friction between the chisel and
the wood because of the additional downward force.
And, as any physics teacher or your own experience
in winter
driving might tell you, friction is at its
greatest just before slipping occurs. These are the ideal
conditions for an uncontrolled cut. So, we try to keep
the shaft of the tool as closely aligned with the direction
of cut as possible.
As you decrease the bevel angle on a gouge, you bring
the shaft into closer alignment with the direction of cut.
You also weaken the edge of the tool, but this is easily
strengthened by putting some bevel on the inside of the
gouge. This "in-cannel" bevel not only strengthens the
edge by increasing the included angle, but it also makes
it much easier to use. To understand this, you have to put
yourself in the position of the wood. Consider a gouge
that has been sharpened at a bevel angle of 25°: viewed
head-on, it would look something like
Drawing 1
on
the next page; but as the gouge is lifted up just past the
25° angle so that it can cut into the wood, it would look
something like
Drawing 2
.
Notice that the gouge would be able to cut in this
position only if it partially compresses the wood at the
same time that it cuts, since, as presented, the gouge is
wider than the cut it is going to make (the cutting edge
is the smaller of the two semicircles). So to get any
signifi cant depth of cut, the gouge has to be tilted above
25° so that a reasonable portion of the gouge can enter
the wood before wedging action takes place. This angle
might be 30° or more (
Drawing 3
).
The problem with all of this is that only two points of
the gouge have the correct relief angle behind the edge.
Over the rest of the surface the relief angle is either too
great, as would be the case at the very deepest point of
cut, or too shallow, which would require compression
of the wood for the gouge to penetrate.
Compare this situation to one where we have part of the
bevel on the outside of the gouge and part of the bevel
on the inside (see
Drawing 4
). Of course, now we have
only to lift the gouge just above 15° in order to begin
the cut because there is only 15° of outside bevel. It is
interesting that at the cutting point (see
Drawing 5
), we
see immediately that such a design would have much
less side wedging than one where the bevel was entirely
on the outside of the gouge. Lifted up to get the same
amount of cut as in
Drawing 3
, we can see that the
compromises are not as pronounced (see
Drawing 6
).
There is still a section where the gouge is lifted past the
ideal relief angle, and the cut will still be a compromise
between some compression at the upper edges of the
cut and some tearout at the very bottom of the cut, both
combined with some control problems (though these
would be substantially less than would be the case when
all the bevel is on the outside of the gouge).
If transferring part of the bevel to the inside is a good
thing, why not transfer all the bevel to the inside?
Well, if you do that you will have a gouge that can cut
only straight grooves. Having no bevel on the outside,
you could not change direction with it. This is called
an in-cannel gouge, and, while it is much loved by
patternmakers, it is virtually useless for carving since
it can be used only on a fl at surface where the cut is
going to start at one edge and run some distance in a
straight line at consistent depth of cut. In addition, since
it would be used almost fl ush with the surface, it must
either have a cranked neck or be very fl exible, as shown
in the photo on page 49.
The repeated refrain of “the lowest bevel angle consistent
with edge retention” has a special meaning in carving.
Anyone who has carved knows that there is a constant
threat of a sudden slip that can ruin a piece. For this
reason, the carver wants to be able to cut using the least
amount of pressure on the tool because this makes it the
most controllable.
Well, the degree of sharpness is not the only thing that
determines how much pressure must be applied. The
carving tool is a wedge that is being driven into the wood.
The shape of the wedge
determines its mechanical
advantage, and mechanical
advantage determines the
force required for penetration.
Since mechanical advantage
equals D/W (depth/width), as
you lower the bevel angle on
a carving tool, you decrease
the force required to use it.
Mechanical Advantage
Tool
Wood
W
D
Summary of Contents for Mk.II
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