Newsletter Article: How to Test for Sharp and Dull

The most important skill a traditional hand-tool woodworker must master is how to quickly and properly sharpen or  hone your cutting tools because woodworking with dull hand tools is a frustrating experience (Note: In this article I use the terms “sharpen” and “hone” interchangeably). Imagine the ease at which you can plane a piece of stock with a “razor-sharp” blade in your plane.  Now image trying to plane that same piece of wood with a dull blade in your plane.  The sharp tool is a pleasure to work and leaves a beautiful surface; the dull tool is difficult to use and leaves a rough and torn surface.  This concept holds true for chisels, saws, and any tool that cuts the wood.  The woodworker’s goal is to be able to quickly sharpen and to consistently produce razor-sharp cutting edges on any tool blade.

I say “quickly” because cutting wood dulls your blade and you must refresh the blade’s cutting edge often by honing to keep it razor-sharp.  This process should only take 2-3 minutes and you are back to work.  However, if your re-sharpening process takes longer, say 5-10 minutes or you are not proficient at sharpening, you will likely delay re-sharpening until the blade is so dull you just cannot use it and now instead of 2-3 minutes to re-sharpen it will take you 15-30 minutes to re-sharpen since you are starting with such a dull blade.

This leads to two age-old questions woodworkers struggle with: 1) When is it time to re-sharpen my blade? and 2) When sharpening, how do I know when my blade is sharp enough?  To answer these two questions, we must first understand the definition of sharp and dull.

First, what is sharp? A sharp cutting edge only exists where two planer surfaces (for example the back and the bevel surfaces of a plane iron or the two bevels of a knife) meet with zero radius at the meeting point. The line of intersection where these two planer surfaces meet is called the “cutting edge”.   So, a blade is perfectly sharp when the two planer surfaces of the blade intersect with each other exactly along the last atom of space.  To achieve this intersection, the two planer surfaces of your blade must be perfectly flat and polished where they meet at the edge.  This will result in the “razor-sharp” cutting edge that we want to effortlessly plane those wispy shavings or make our chisels slice through end grain like it was butter.

The term “zero-radius edge” to describes the point at which the two planer surfaces of a blade come together to form a razor-sharp cutting edge.  If one or both planer surfaces is not flat, they will not intersect with a zero radius; there will be a radius (even slightly).  The greater this radius or rounding is, the less sharp (or more dull) the cutting edge will become. 

A true zero-radius cutting edge is a purely theoretical idea, but it represents the goal of sharpening.  In practice, if the cutting edge is honed as close to a zero-radius as possible, that edge will be “razor-sharp.”  Ron Hock, in his book The Perfect Edge, explains it this way, “The reason a zero-radius goal is only a dream is because the blade you are sharpening is made from steel and that is made up of crystals, which are made of molecules, which are made of atoms; and all of these microscopic building blocks have size. However small these microscopic building blocks may be, there is still some “there” and it is that little bit of size which determines how close to the goal of zero radius the laws of physics allow you to get. The smallest radius possible is the diameter of the largest discrete particle in the metal’s mixture that can’t be abraded away.”

Most modern woodworkers sharpen their blades by making use of micro-bevels.  Micro bevels both reduce sharpening time and make for a stronger and longer-lasting cutting edge, but this is for a “how to sharpen” article.  For our purpose of understanding sharp and dull, micro-bevels do not matter.  A micro bevel is just another planer surface. 

The last consideration for sharp is the size of the abrasive or grit that is used to abrade the surface of the blade metal.  The abrasive, regardless of type (sandpaper, water-stone, oil-stone, etc…) and grit size, leaves scratches in the metal.  If you were to look at the surface of the metal under magnification you can see these scratches.

These scratches create a “jagged” cutting edge.  The idea in sharpening is to use finer and finer abrasives, to make smaller and smaller scratches and thus a less and less jagged cutting edge.  While the scratches never disappear completely, for the purposes of woodworking the scratches become so small we say the metal is “polished”.  When two polished, planer surfaces intersect you will have a “razor-sharp” cutting edge.  The two pictures below illustrate this.  The first picture shows a plane sharpened with about a 1000 grit stone magnified 500X.  The jagged edge is clearly visible.  The next picture shows the same blade after working it through progressively finer stones until achieving a polished edge.

So how do you tell when your blade is sharp or maybe we should say sharp enough! I use the term “sharp -enough” because you could keep improving the sharpness of your blade’s cutting edge, theoretically forever, but there is a point of marginal returns (anyone remember your economics class?).  Meaning, that by continuing to sharpen you could improve the blade sharpness, but for all practical purposes you cannot detect this improvement, so why expend more sharpening time and effort after the “sharp-enough” point is reached?

So how do you test for sharp?  The only way to truly know if your blade is sharp enough is to test how it cuts the wood you will be working.  A sharp enough cutting-edge should cut the wood fibers cleanly without leaving marks or crushing the wood fibers and it should leave the wood smooth to the touch.  The problem with this method is that it requires getting the tool and a piece of wood set-up for a test cut, which takes time and if the test fails you must take the set-up apart and continue to sharpen.  While a test cut definitively tells you if the cutting edge is sharp enough, most woodworkers want a quicker sharpness test before doing test cut.  Maybe I should call this a “pre-cut sharpness test.”   There are countless pre-cut sharpness tests, here are the ones I like:

1. The Fingernail Test: This is my favorite pre-cut sharpness test. Pinch your first finger and thumb together then turn your thumbnail to the cutting edge.  Using caution, lightly glide the edge of your thumbnail along the length of the cutting edge.  If the edge is sharp, your thumbnail will glide smoothly along the edge as if it were made of glass. 

2. The Loop Test: This is my second favorite pre-cut sharpness test. Using a 10X to 20X power jeweler’s loop (a small magnifying glass) examine the cutting edge. A sharp edge will appear even and smooth with no nicks and only very faint scratches.

3. The Paper Test: Hold a piece of paper, hold it vertically and try cutting the edge of the paper will holding the blade at an approximate 45-degree angle.   A sharp cutting edge will cut the paper cleanly with almost no effort. 

4. The Hair Test: Please use caution when using this method.  Try to shave the hair on your arms with the cutting edge.  A sharp edge will cut all or almost all the hairs in one pass.  A downside to this method is bald spots on your arms!

A freshly sharpened cutting-edge begins to wear the moment you put it to use.  As you cut wood the cutting edge starts to round, introducing a tiny radius.  As you continue cutting, the size of this radius increases on the blades way to becoming dull.   At first this degradation is imperceptible to feel or visual inspection, but the longer you use the blade the duller the cutting edge becomes and soon you can detect the decline in cutting edge sharpness.  The big question is when do you re-sharpen?  You want to stop and re-sharpen the cutting edge at the point where the cutting edge has rounded to dull, but not so dull that you cannot re-hone the edge quickly. 

So how do you know where this point is?  The answer is almost the reverse of what we talked about relative to sharp.  The best way to tell this point is by feel as you are cutting the wood.  The tool will not sever the wood as easily as it did when it was freshly sharpened and will require more effort to cut the wood.  Also, the surface of the wood will not be as smooth to the touch as it was when freshly sharpened.  The key here is not to extend the time you are using the tool.  Stop now and re-sharpen before you get to the point you must take a long time to re-sharpen.

You can also use the same tests we use for sharp to test for dull: