Here is how I determine if my plane or chisel edge is sharp. I have a high degree of confidence that this method is accurate and easy to replicate.
In my left hand, I hold one edge of a regular piece of copier or laser printer paper and let it it hang down. In my right hand, I hold the blade. I bring the blade to the top edge of the paper so the blade is pointing down and is perpendicular to the edge of the paper. Now I gently push the blade into the edge of the paper. Based on the cut the blade made, I can determine how sharp the blade is. Here’s a scale of comparison from sharp to dull.
- Cuts cleanly with no tearing (sharpest).
- Cuts with some tearing.
- Cuts, but only if I saw the blade back and forth.
- Will not cut, simply folds over the paper as I push (dullest).
Generally if I’m preparing a plane of chisel, I aim for #1 or #2. If it’s #3 I usually go back to honing, depending on what I’m doing. If it’s #4 I definitely analyze the blade and go back to honing for repair.
You should be aware that there are a few additional factors that impact the scale above.
The thinner the paper, the more difficult it is to cut cleanly and without folding. For consistent tests, use the same type of paper.
As I test blades, the edge of the paper ends up with a series of little cuts (or folds). Those cuts weaken the edge of the paper. Trial and error tells me how closely I can space the cuts and still get an accurate test.
The distance from my left hand (holding the paper) to the blade impacts the test. If it is too far away, the paper won’t have enough support and will fold more easily. The blade may not have the same sharpness all the way across its width. Especially when I am working on a new sharpening process, I check the edge at several different points.
A very sharp blade is not always required. However, I’m still acquiring hand tool skills and it is hard for me to judge when a sharp edge is essential and when it is not. Therefore, I tend to always aim for my blades to be at #1 or #2 above. I have also found that the sharper I make an edge, the longer it lasts.
I have tried other methods, including shaving my arm, pushing the blade into my fingernail, running my fingernail across the edge, shaving the words off a ballpoint pin, and simply feeling the edge. The method above is more accurate and does not use up a valuable resource like hair.