Objectifying edge quality

[Doug Hepler]

Andrew,

OK, since you asked (or is it my inherent proclivity to teach, or perhaps it's pure vanity). Here is my go-no-go test apparatus based on Cliff Stamp and Steve Elliott. I run a piece of thread (Elliott says 40 Wt Coates & Clarke rayon) over a gap to a weight. Then I lower the edge being tested onto the thread over the gap. Slowly, straight down, no slicing. If the edge is sharp, it cuts the thread. If not sharp, it depresses the thread and raises the weight.

So, how sharp? According to Elliott, if 40 wt rayon thread is used, the weight should be 65 - 100 grams. Since I did not know the weight or composition of my thread I chose a weight by trial and error until I got to a weight that would not allow a fresh single-bladed razor blade to cut the thread. Then I added one more 1/4" hex nut (5 grams more or less). That came to about 75 grams as it turns out. So, a new razor blade cuts the thread. If the edge being tested also cuts the thread, it is as sharp as a new razor blade. And so forth. If my standard is less than that, I can use a heavier weight (another hex nut or so) I already have a 100 gram weight that I plan to use when I obtain some 40 wt rayon thread. Here are the pictures. I Photoshopped them to hide the worst of my execrable workmanship. The thread runs from the spool over a UHMW sheave, over the gap, through another UHMW sheave with a pin in it to act as a pulley, and down to the weight. Don't laugh at the wing nut. It makes a convenient thread clamp. So, this was an afternoon's pastime. But it works more or less and does not cost $150. BTW, I could just have made a loop of thread and hung the weight from it, supported by the edge being tested. But where's the fun in that?

DougSharp-Gauge-2.JPGSharp-Gauge-3.jpg