Learning to sharpen has little to do with your sharpening stones. It has a lot more to do with being able to see your progress and knowing when to stop.
Showing a class of woodworkers what a sharp blade looks like in the flesh (a real poor choice of words) has proven to be tricky for me. So I’ve resorted at times to line drawings, which helps.
Today a reader sent me some great photos he made using a scanning electron microscope, usually called an SEM in the business. Want to read about how the microscopes work? Brace yourself for some scary images of pollen.
The reader is a mechanical engineer working in research for a medical device company, so these are legit. What you are going to see here are images of a Veritas spokeshave blade. The images of the dull blade show what it looks like with the factory edge on it after being used to build four benches. The blade still feels pretty sharp, the reader reports.
The images of the sharp blade show the same tool in the same position after being sharpened with #1,000- and #8,000-grit Shapton stones.
All of the images were taken from the same angle , 12Ã?Â° off of vertical looking directly at the edge. As a result, you can see both the flat face, the secondary bevel and the primary bevel all in the same image. The primary bevel is at the top of each image. The secondary bevel is the stripe in the middle. The flat face (some people call it the “back”) is the bottom part of the image.
So let’s kick this off with photos of the dull and sharp blade at 30x magnification, which is about what I can see with my jeweler’s loupe.
30x Dull and Sharp
You can see the factory scratches in the bevel on the dull blade, and you can see them disappearing on the sharp blade. This is about all our naked eye gets to see in the shop. As we zoom in, it gets more interesting.
500x Dull and Sharp
At 500x the differences between the two blades becomes quite evident. The scratches in the dull blade stand out like canyons, and you can see them and how they fade on the sharp blade.
1,000x Dull and Sharp
On the dull image you can see a torn bit of metal right on the edge, which the reader reports as being typical of this edge. On the sharp edge, the little light-colored spots are debris, not metal. The dark spots on the right of the photo are also likely debris, not the wire edge of the blade.
2,000x Dull and Sharp
At 2,000x you can really see the rolled-over edge in the middle of the frame. That, I suspect, is what reflects light when you look at an edge and see a glint right at the tip.
The photo of the sharp blade at 2,000 power also has a dark line at the edge. This could be debris or it could be the “wire edge” remaining after sharpening. It would take more testing to determine exactly what we’re seeing here.
So what do I conclude? Seeing is indeed everything. And is there an SEM section on Craigslist.com? How many kidneys would it cost?
– Christopher Schwarz
Sharpening Stuff for Other Sharpening Nuts
– Ron Hock wrote the book on sharpening, and I think it’s worth buying. “The Perfect Edge.”
– Brent Beach wrote the web site on sharpening, and I think it’s worth exploring. Be wary. It is a vacuum. http://www3.telus.net/BrentBeach/
– You know you are a sharpening nerd if you’ve been to this page. It’s the rec.woodworking post that kicked off the latest sandpaper sharpening craze. Note, I found an old Boy Scout reference to sharpening your pocketknife with sandpaper.