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Edge Geometry - Ha Kika 刃幾何

This section discusses proper and improper edge geometry for tameshigiri.  I use the term edge geometry, but I'm 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.

Cutting Planes

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. 

Edge Geometry

Hard Targets

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.

Hard Target Edge

Medium 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.

Medium Target Edge

Soft Targets

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. 

Soft Target Edge

Knife Edge

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.

Knife Edge

Hollow Ground

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.

Hollow Ground

Asymmetrical Edge

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.

Asymmetrical Edge