Edge Geometry
When using the term edge geometry, we are referring to the entire surface of the
blade. Knifes are usually sharpened by beveling the edge and leaving the
main surface of the blade untouched. A katana should have continuous
polished surfaces right up to the edge. The entire surface of the blade
must be reworked to sharpen it. A katana might come with a badly shaped
edge from the maker or it might be a result of amateur sharpening. Most
katana come from the maker with an edge optimized for hard targets. They
don't know what you are going to do with it, and this is the most durable edge.
Check the geometry of your blade by seeing how light reflects from it. Try
turning the blade to different angles and watch the reflection of a single
light. Pay attention to the surface right at the edge. Using a
plastic straight edge might also be useful. The dojo store offers full
sharpening,
repair, and
customization
services to keep your katana in top working order.
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A katana has two cutting planes. It slices best through a target
following one of those two planes. The geometry of the sword will
determine the optimum angle the blade should be at. The best analogy is a
chisel cutting through wood. Each sword has a slightly different optimum
angle. If you have an understanding of the geometry of each blade, you can
determine the best way to use it successfully. The blade will try to
follow one of these cutting planes through a target. Resistance of the
target will torque the blade and cause the cut to cup or round unless the sword
is forced to keep the correct angle. If the blade goes through the
target at the same angle as the cut, it will bind up in the target.
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Bamboo and hardwood dowels should be considered hard targets. Edges
meant for hard targets must be durable. More extreme geometries optimized
for sharpness will roll over, chip, or flatten when used on hard targets.
This is the type of edge most appropriate for surviving on a battlefield where
armor would be encountered. This type of edge has a tendency to bind
in medium or soft targets.
Tatami and softwood dowels should be considered medium density targets.
Edges meant for medium targets need to balance durability and sharpness.
This type of edge would be most appropriate for everyday use against lightly or
unarmored opponents. Some edge damage could occur, but must be weighted
against the increased cutting ability. Cutting hard targets may cause some
damage especially if the sword is allowed to turn in the target.
Beach mats and thin rolled paper should be considered soft targets.
Edges meant for soft targets are totally optimized for sharpness.
This type of edge would be most appropriate for unarmed opponents. Edge
damage will inevitable occur, but the extreme sharpness would be very effective.
This type of edge should not be used for hard targets and may need frequent
sharpening when cutting medium density targets.
A katana should not have an edge geometry like a kitchen knife or machete.
It should not have flat surfaces with an edge that rolls over. The
geometry should form a continuous curve that slides through a target without
binding. This type of edge does not have cutting planes and will bind in a
target. Soft targets will cut but have ragged edges. Medium density
targets will bind the blade and make cutting very difficult.
A katana should not be hollow ground with concave surfaces. This will
create an edge and blade that is too fragile for any real use. All blades
should be convex without low spots. A hollow ground blade cuts soft
targets easily, but chips very easily and can easily snap in two.
Both sides of a katana should have the same geometry. It must make cuts
from either side equally well. The geometry should be the same for the
entire monouchi (optimum cutting area of katana). This type of problem is
common to swords sharpened by amateurs or simply polished for esthetics.
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