Wednesday, December 14, 2016

Edge Geometry

There has been a lot of confusion as of late on one of the knife making forums that I frequent in regards to knife edge geometry.  There has been a lot of discussion about primary and secondary bevel angles on blades and a general fogginess around what edge geometry is all about and what geometry is best for knife use.  I thought that it would make a great subject for a post here on the Robinson Edge.  So, without further ado, let's have a little discussion about knife edge geometry.

For the sake of argument, we will not get into how blade profiles effect how a knife performs; that will be reserved for a future discussion should it need to be addressed.  For this post, we will focus in on the cross sectional shape, or edge geometry, and how different grinds effect edge performance.  To level the playing field, we will look at blades that are 1/8" thick and 3/4" high, just for comparison purposes.  As far as the actual cutting edge, also referred to as secondary bevel, we will keep that a constant of 25 degrees across the board.  Most knife edge angles fall within 20 to 30 degrees of included angle, so 25 is a good middle-of-the-road angle.

Although there are many different candidates for knife geometry, we will only look at a few of the most popular.  Most other geometries are spin offs from these basic designs anyway.  Let's first look at some of the flat grind varieties.

Represented in the graphic above is the common Saber or Scandi grind.  The sides of the blades are left parallel and a single bevel is ground which comprises the edge.  As far as edge strength and edge retention, this is probably the best grind available.  There is a lot of "meat" behind that edge, making this choice very sturdy.  The actual width of the bevel is much longer than the rest of the geometries that we will examine.  The edge is in contact with the material being cut, causing friction or cutting resistance between the two objects.  Since the edge is fairly wide, the cutting resistance is relatively high, making this choice a poor candidate for a slicing knife.  It is, however, a great choice for a camp chopper or axe blade.

The next example of blade geometry is the full flat grind.  This cross section resembles a simple wedge where both sides of the blade are ground flat from the spine to the edge with a single bevel. This creates a very keen, low angle edge which makes for an extremely sharp blade.  Since the edge angle is very low, this produces an extreme slicing edge.  The downside of this geometry is the thinness of the edge leaves it prone to chipping and rolling, making the edge retention relatively low. For blades that are wider, or blades that are shorter in height, the 9 degree angle increases respectively, which strengthens the edge, lessening the edge problems, but there is still significant cutting resistance due to the very wide bevel.  This geometry is best reserved for wedge-type straight razors, or very specific use blades.

The third example is a combination of the previous two.  The blade is flat ground at a slightly lower angle, and a secondary bevel is ground at the very edge of the blade.  This type of grind increases the strength of the edge significantly over the full flat grind, but it does gives up a little on the keenness of the full flat ground edge.  With the short secondary bevel, cutting resistance is greatly reduced, and, even though the edge angle is increased, it is still a great slicing geometry.  This geometry is very common in kitchen and hunting knives.  I've also seen this geometry in many folders.  It's a great all-around geometry.

The full hollow grind is very recognizable in straight razors.  You won't find a finer edge on any other geometry.  You also won't find a more fragile edge out there either.  The edge is extremely thin and weak, but the slicing ability of this geometry is second to none.  Although the weakness of the edge makes this a poor candidate to general knives, straight razors just wouldn't be the same without it.

The final geometry that I wanted to explore is the hollow grind with a secondary bevel.  This is my grind of choice for almost all of the knives that I make.  The secondary bevel strengthens the edge significantly and the short width of the edge gives little cutting resistance, making for a great slicing blade.  Another benefit that the primary hollow grind offers is that as the knife is sharpened repeatedly over its life, the secondary edge width stays fairly short due to the "undercutting" of the hollow, which keeps it slicing and performing well for a very long time, more than the flat grind with secondary bevel.  The hollow grind is a little more difficult to achieve than a flat grind, which is likely why many makers choose go with the flat grind geometry.  I believe the benefits make it worth the effort to hollow grind a blade, not to mention it looks really cool too.

While this list is certainly not exclusive, I think it will give you a much better understanding of the importance of edge geometry.  Each of the various geometries has it's place in the blade world where it is the best choice for a specific application.  It is up to the knife maker to choose the best geometry for a knife's intended use.

-  Brandant Robinson

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