Another lazy man’s blog post today. My friend Joel of Tools for Working Wood recently wrote about his method for determining if a blade is sharp. I am trying, with this blog, to add or highlight information about sharpening that is not in my book*. The subject of sharpening unfolded before me as I did the research for and wrote The Perfect Edge and as is so often the case with such projects the more I learned the more there was to learn. So I am using this blog to add to the general sharpening knowledge from time to time as new techniques and equipment come to my attention. So today’s post is just another that points you to something that I find interesting, in this case Joel’s “Feel the Burr.”
Joel’s finger-test for sharpness was not included in my Chapter 5, Fundamentals, which includes a couple pages on edge testing. Here’s my section (it looks much better in the book):
Low-Tech, Touch-feely Edge Testing
Your blade is dull when too much effort is required to push a blade through material, or your edge is leaving behind a rough surface. A dull blade can be an educational opportunity, however, and with a bit of study, perhaps a magnifying glass or loupe, you can learn a lot about how to test for sharpness.
Know anyone with self-inflicted bald patches on their forearms? They’re all woodworkers, right? This phenomenon relates directly to the most popular technique in determining sharpness: whether or not the edge shaves hair. The shaving test provides revealing data on how sharp an edge is by how it cuts hair and by how the blade feels to the skin underneath while the test is performed. If the edge presses the hairs against the skin, not cutting them at all, but trapping them and then scraping along until the hairs slowly give way to being cut – that blade is not so sharp. If the edge snaps off hair with little or no pressure applied, it’s sharp! Shaving hair off your forearm provides a relative scale based on empirical evidence and it is a traditionally straightforward, handy and useful test for many of today’s (and yesterday’s) woodworkers.
I find it easy and quick to gently apply the blade’s edge to the top of my thumbnail. I use just the lightest pressure; not trying to cut into the fingernail even a little. If the blade catches with no sliding at all, it’s sharp! An edge that’s dull will skid on the nail without that telling catch. If you’re squeamish, use a ballpoint pen barrel instead of a thumbnail. You can also test the edge for uniformity by sliding the edge of the blade along the edge of a thumbnail (or pen barrel) as if to cut it but, again, with only the lightest pressure. Even the slightest imperfection or roughness will telegraph clearly and tactilely. You’ll know without a doubt whether or not you have more work to do on that edge. If the edge is sharp along all of its length, it will glide smoothly on your nail’s edge, with no discernable vibration to indicate a rough spot.
Then there’s always the paper-cutting trick. A sharp edge will slice a sheet of paper easily and cleanly, without catching, pulling or tearing.
The fourth, handy, low-tech sharpness test is to simply look closely at the blade’s edge to see if it reflects light. The closer an edge is to zero-radius the smaller that edge is; therefore the less it reflects. When properly sharp, it won’t reflect at all. You’ll need to get the light at the proper angle – a magnifier helps – but it’s not at all difficult to do. A couple of tries and you will clearly see that a dull edge reflects light while a sharp edge does not. Try this method with that blade that isn’t cutting well. In fact, that dull blade offers a good opportunity to try all these simple tests and get a feel for each of them. Then, when the edge has been sharpened, try them again to feel and see the difference. I mostly use the thumbnail test because it’s so quick to do, but I have a few bald patches on my arm as well. At least with the latter two methods, you won’t need to wear a long sleeve shirt.
*BTW, the $10 Blade Bucks offer is still in effect.