A Good Cut – 1

What goes into how well a blade cuts?

To my mind there are just a few things that  determine how a blade performs:

  1. The geometry of the blade
  2. The skill of the cutter
  3. The metal and heat treatment of the blade
  4. What you are cutting
  5. The sharpening process itself

The biggest contributor to a good cut is blade geometry.

Geometry of the Blade

The cross-section geometry is the shape of the blade if you band-sawed it in half and looked at it end-on: the shape from the spine down to the edge. This is sometimes called the profile, the relief, the grind, or the bevel. In contrast, the outline (side-view) geometry is the shape you see when you lay a blade down on the table – this is your normal “beauty shot” photo angle. Now watch out because the outline geometry may also be called the profile, which is confusing.

Outline geometry certainly relates to how a blade performs. Extreme examples are the straight line an épée or dagger, and the curved outline of a saber or scimitar. In the simplest terms, a straight outline excels at stabbing while a curved outline excels at slashing. But blades that are tools for daily use fall between these extremes.  And while outline geometry effects how a blade performs, it’s the cross-section geometry that is crucial to how well the blade cuts.

Cross-Section Geometry

Here are some classic cross section examples…
Blade Grinds

These grinds and countless variations have been in use since humans first made blades. The flat grind is a good general purpose geometry. The fatter convex grind gives more strength to the edge. Hollow grind makes for easier touch-up sharpening, but provides a less sturdy edge. And a geometry with distinct primary – secondary grind angles performs pretty much like the convex grind.

When someone says “primary” and “secondary” they are talking about the angle of the main body of the blade as opposed to the angle where the blade comes up to the cutting edge – and while I’ve seen some people call the grind near the cutting edge the “primary”, most people call the grind near the cutting edge the “secondary” grind.

If you really want to get picky, even on the flattest Scandinavian hunter has what could be called a secondary grind which is the exceptionally wide honed edge on those blades.

The “Apple Seed” edge popularized by the late great bladesmith Bill Moran is something of a cross between the flat and convex grinds.

Most of the gee-whiz tactical blades are some artful combination of distinct primary and secondary grinds with maybe some recurve outline and sawtooth back thrown in. So you may be getting my hint that the  limit for creative blade geometry is in the imagination of the bladesmith – which is a real treat!

But back to business: What makes a good cut?

The narrower the angle at the edge, the better the blade works for slicing through softer materials. The wider the angle at the edge, the better the blade holds up to tougher or harder materials.

The overall angle of the blade also determines how it will perform, for instance if I’m splitting firewood I want the fat blade (ax or maul) – probably with convex or distinct primary/secondary grind. If I’m filleting fish I want the thin flat geometry of a fillet knife.

The perfect blade geometry for trimming burs off an aluminum casting will make a total mess of dicing a tomato. Try to use a paring knife to trim those aluminum burs and you’ll dull or chip a perfectly good kitchen blade!

So my first “Rule of Cut” is: Match the geometry to the task.

For very fine work the edge should be sharpened at an angle of 15° or even a little less. For tougher work increase that angle up to 20° or more. For my general use knives I go for around 15° and with good steel I have not had problems with rolling the edge or chipping. I keep most of my kitchen knives a little thinner than that. My axes are closer to 20°. You want the most acute angle you can get that is still sturdy enough to protect the steel you’ve got from the punishment you are about to give  it!