Hand Tools Archive
(applicability of the rabbit hole below in a second post)
This follows on the post I made a couple of weeks ago about exploring rounding off the area on chisels that fails and guessing (Without testing so far) that if it's done correctly, it doesn't have an effect on cut resistance.
Wiley and Bill and I had a discussion about it. I tried to measure the tiny rounding, but I'm not sure how accurately I measured it. Also measured the other angles on chisels that I thought I was grinding around 25 degrees (my grinder rest is always set to 90 degrees and I gauge refreshing a hollow with my fingers as a fence on the edge of it).
Turned out on a sorby chisel that I tried this method on to be 19 degrees primary
Secondary honed bevel was 25 (thought that would be steeper, too) and the final rounding was somewhere around 33 or something, but the very tip of it could be slightly steeper.
This picture shows about how long the rounding is on a small file knife (I taking a chisel picture at a later date will require elevating the microscope and making an alternate fixture to capture the bevel).
This method makes a sorby chisel tolerable - most would find them intolerable on anything other than the softest softwoods and mahogany. They are at least as soft as the modern blue chip chisels I've tried, but they have "proportional potential", and I could probably reharden them, but won't do that yet.
It also makes these little knives zip through cardboard for the recycle bin to the point that they might be dangerous for the uninitiated.
Here's the trade - the very edge is rounded over. This rounding is a matter of a few thousandths, not a big chunk of the bevel. If it's kept small, there's no perception of extra resistance. I started thinking about this after watching cliff stamp talk about better performance from cheap thin knives in the kitchen than overbuilt fat ones that cost 20 times as much. And realized that previously, I'd done this to stanley chisels mortising a couple of cocobolo blanks (they held up better than expensive chisels that were just set up with a flat bevel around 28 or 30 degrees....actually much better).
the rounding is the minus on resistance, but combined with the thin follow on bevels, the cut resistance isn't more. Sharpening goes to almost nothing unless you abuse tools.
The equalizer in this is using something that's very fine. In the case of this knife, the rounding is done with a hard buff and 5 micron abrasive. The edge is as good as anything I've ever used (the abrasive is a cheap yellow stick from mcmaster carr - it's terrible on the hard surface).
Note that the scratching that you see in this picture is from a deburring wheel. I smash files into these thin utility knives and then maintain them with a deburr and then a buff. you can see that the only part of the edge that's really steeply rounded is the very edge - the dark area where the scratches disappear completely is probably 2 or 3 thousandths. The darkness is a consequence of metallurgical scopes (without complex lighting solutions) being very sensitive to light reflecting from directly overhead. As the bevel rounds, this doesn't occur evenly since the bevel is no longer flat.
So, the focus is just then the evenness of the edge as it'll show you any scratches that the light doesn't catch (as notching).
Here's one of plane iron test edges from black arkasnas. Some of the deflection occurs due to stropping on bare leather (extremly lightly for this test to get the best looking edge I could get).
The evenness of this buff is similar to the way submicron abrasives finish an edge, but with a cheap 5 micron al-ox stick. we saw in the wear tests that the appearance of this black ark edge isn't that big of a deal on a plane as it leaves a nice surface and on O1, evidence of it is worn off evenly, anyway. But it does cause edge life to be less than 1 micron diamonds (Something steve elliot also found)
(I just got a new scope yesterday - the one I used last year doesn't work with win 10, which is a pain, because the scopes that mount on microscopes aren't any $15 type like you find with hand helds).