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efore
you begin to sharpen
a dull cutting edge or grind a new one, it helps to understand just how a tool cuts wood.
It seems a simple thing, especially since you may do it hundreds of
times a day in your own workshop. But on close inspection,
cutting wood is a wonderfully involved
process. |
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A DEFINITION OF CUTTING
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Down at the level where the
edge meets individual
wood fibers,
cutting is
synonymous with breaking. The point of the tool presses against the wood
fibers with enough force that they break, separating into two pieces.
A cutting edge
concentrates all the force driving the tool at its point. And because a
sharp edge contacts only a small amount of the wood surface, the
resistance is confined to a tiny area. The keener the point, the smaller
the resistance, |
and the smaller the force required to cut. The wood fibers
separate along a narrow line described by the path of the tool, and the
cut surface appears smooth and even.
A blunt or dull tool
contacts a larger surface area. Consequently, there is more resistance
and it requires more force to cut. The fibers fail along a wider, poorly
defined line, and the cut is ragged.
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CUTTING EDGE GEOMETRY
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Sharpness isn’t the only attribute that
affects the cut. The angle at which the tool is sharpened, the angle at
which it attacks the wood, and the shape of the cutting edge also
determine how a tool cuts.
TOOL ANGLE
Every cutting edge has a leading face
and a trailing face. The angle between the two is the tool angle — the
smaller the angle, the less force required to cut. Small tool angle
reduce the "wedge effect' of the tools -- they displace less material as
they are driven into the wood and therefore require less force. But if the angle is
too small, there’s too little metal to buttress the cutting edge and it wears
quickly. It may even break or buckle.
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What tool angle is
best? That depends on the tool and how it’s used. Mallet-driven
tools have more obtuse angle than hand tools because they are driven
with more force. For the same reason, power tools have greater angles
than mallet-driven tools.
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TRUE
SHARPNESS – Tool angle is often
confused with sharpness. It’s not true that the smaller the angle, the
sharper the tool. Sharpness refers to the condition of the edge, not its
angle. A router bit, for example, has a high tool angle. But when the
point is keen, the edge is sharp. |
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CUTTING ANGLE
The angle at which the cutting edge
meets the wood — the cutting angle — is measured from an imaginary line
perpendicular to the wood surface. This, more than any other angle,
controls how the tool cuts. At a large cutting angle, it lifts the wood
fibers as it cuts them; at a small cutting angle, the tool compresses
the fibers, then shears them off.
Because it’s more
difficult to compress fibers than lift them, the force required to
remove a given amount of wood increases as the cutting angle decreases. Consequently, you must
either supply more force or take thinner shavings. |
CLEARANCE ANGLE
The angle between
the trailing face and the work is the clearance angle. The size of this
angle is not particularly important as long as there is one.
Without a clearance angle, the cutting edge will not contact the wood.
The absence of a
clearance angle is often the reason you cannot cut wood when you find
yourself working with an improperly sharpened tool or an extremely dull
tool.
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When you drive a
cutting tool through wood, it lifts and compresses the wood fibers. The
force required depends on the cutting angle and the amount of wood
removed. As the angle grows smaller, you must either use more force or
remove less wood.
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GRIND
These angles are affected by the shape
or grind of the tool. There are three ways to grind a cutting edge —
convex (cannel grind), concave (hollow grind) and flat. Chopping tools
such as axes often have cannel grinds. The tool angle is high and
requires much force to cut, but the cutting edge is very durable.
Light-duty tools such as carving chisels are sometimes hollow ground.
This reduces the tool angle, making it easier to cut, but the cutting
edge is less durable. A flat grind offers a good balance between cutting
ease and durability.
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TYPES OF
GRINDS
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GROWING DULL: CHANGING GEOMETRY
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As a cutting edge wears, the geometry
changes. The edge rounds over — more in the trailing face than the
leading face, since the trailing face is in contact with the wood. As this happens, the tool angle increases while the
cutting angle and clearance angles decrease. More and more force is
needed to drive the tool through the wood, and the cut surface becomes
rougher.
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Changing Tool Geometry
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SEEING
THE LIGHT – If you suspect a cutting edge is growing
dull, inspect it under a bright light. Because the surfaces of a sharp
tool come to a crisp point, the cutting edge reflects no light. But once
the edge wears, the rounded surface reflects a line of light. Nicks in
the edge show up as bright spots. |
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