Edge Geometry for making Knives

Custom hand made knife geometry

“Sharp” is a term used to tell how well our tools perform. But is a sharp knife the same as a sharp axe? Or how about a sharp razor, or a sharp hand plane blade. When we make a blade, it needs to be understood what the primary use of the blade will be. Why? Because sharp can mean many different things.

In general terms of Custom hand made knife geometry, the thinner the blade the better the blade will cut. The thicker the blade the stronger it will be. So we have an immediate trade off scenario. Each type of knife you make will have an of these edge that can be modified to be stronger by making the edge thicker or cut through the material easier by making the edge and the blade thinner. Again, a tradeoff is made.

A weaker edge means it gets dull quicker, need to be sharpened more often and will wear out quicker if cutting materials harder than it was designed to cut.

There are three places the angle and the thickness of the blade influences the strength and cut ability. One: the angle of the sharpening, 2: the angle of the grind (or the sides of the blade and 3: the length of the angle on the sides of the blade.

Here we see three possible configurations below. The image on the left would be strong. Most likely used on a hatchet or large chopper type knife.

The image in the middle would be the weakest. This would be used on a fillet knife or a knife that would typically cut soft material.

The image on the right is most often used for hunting and every day carry knives. It’s a balanced trade off of cutability and strength.

Each of these could be incorporated with the hollow grind, flat grind (as shown) or Scandi grind.

The red line in the image on the left shows what is typically thought of as the width of the edge. Beyond that would be the angle of the sharpening. For instance to use a typical commercial knife sharpening system the recommended agles are typically something like the following. These are somewhat typical to most recommended sharping jigs. The sharpening angles that are suggested for their knife’s intended use.

17° Angle – A severe angle recommended for razor blades, fillet knives or similar tools. An extremely sharp but delicate edge.

20° Angle – A commonly used angle for higher quality blades and provides an excellent edge for kitchen cutlery and slicing knives.

25° Angle – The recommended angle for most knives that need a durable, sharp edge. Ideal for hunting and outdoor knives.

30° Angle – An outstanding angle for knives that cut cardboard, rope or carpets. Best for heavy duty use.

These angles typically start at the red line shown.

To complicate matters even more, how the edge will cut is a factor in how it should be sharpened. For instance, a razor will cut by being forced through the material. This type of action requires a blade that is refined, polished and comes to a point that is consistent. A skinning knife however is used in a slicing action, and very fine cutting teeth will aid in its ability to “cut”. These “teeth” can only be observed under a microscope, but you can feel them by “VERY GENTLY” pressing your fingers against the blade. AGAIN, “VERY GENTLY PRESSING” not sliding!

You must determine what the design of your blade geometry will be. It’s important to understand the trade offs required and what they mean to the sharpness of your knife.

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