Recognizing Sharpness

We all know that it is important to have sharp edges when working.  They cut more cleanly and require less force to use, either saving us effort or strain on the motor.  Knowing how to sharpen is certainly an important skill, but just as important is recognizing when a tool requires sharpening.

With enough experience, you will be able to tell by feel when a tool needs to be sharpened.  It will take more force to use and may yield a poor-quality cut.  Another way to tell is by looking at the edge and observing how the light reflects off it.  Good lighting is important, and it helps to angle the cutting edge back and forth until you see the light reflecting cleanly off the edge.

Take this chisel, for example.  We can see light reflecting off one bevel, closest to my thumb and another bevel almost as wide closer to the tip.  Then, there is a third bevel of light, quite fine.  However, there is one point along the edge that reflects light differently.  That is a dull spot on the edge.

Here is the back of the same chisel.  Most of it appears to reflect light consistently, but there is one area right at the tip that is different.  It appears that the cutting edge has folded over a slight bit, resulting in a dull edge.

I sharpened the bevel of the chisel, but it still won’t cut well.  That’s because of the wire burr that was raised on the back of the tool.

Here is a close-up of the wire burr on the edge.  After sharpening the bevel, I carefully lap off the wire burr using my finest stone.

Also, regardless how much you polish the bevel of a tool, the back must also be polished to the same level for maximum sharpness.  Here’s the polish on the back of the blade of my favourite plane, the #4 smooth.