Hand Tools Archive

studying edge failure...somewhat obscure

David Weaver
(long message below - the gist of it is that I think I can manipulate the edge of a tool a little bit - like the last half thousandth - to work better than just sharpening a final bevel off to a point - in regard to knives and chisels - plane blades, I haven't experimented with this - using a buffer to finish the very edge and mute the thinnest part)

one of the things that I'm interested in from a toolmaking standpoint is figuring out where an edge fails, and then seeing if I can come up with a way to stop it without compromising something else.

I get that most people don't really think of something like this beyond just changing edge angles or sharpening equipment or tools, but I'm looking at this a level deeper and have been off and on for years.

Back story here - years ago, I was looking for that kind of one-media setup that works like the washita does, but across some harder steels, so I ordered some cheap wax bars from mcmaster carr that are 5 micron particle size alumina. These turned out to be too much when used on MDF or anything else - the edge just isn't very good - 5 micron is too harsh and a wire edge is created that's pretty toothy once removed. It's not damage, just deep scratches, so I set that bar aside other than to use it to remove deburring wheel scratches. It works well for that on a hard buff.

But over time with the little thin knives that I make, I've learned that the failure when it occurs is right at the edge. In knife sharpening, there's a lot of static from people who don't really know what they're talking about and then there are a few people who actually experiment with purpose. One of them is cliff stamp. Some of what he does has nothing to do with woodworking, but I appreciate that he's always curious to see something, work with it, and be able to describe what's going on - whether it's making the point that uniform wear is a desirable thing, especially combined with sharpening things, or that really inexpensive knives with better geometry outperform chunky very expensive boutique knives in actual cutting tests.....

..at any rate, he put a video up at one point where he briefly discussed moving the failure point in an edge (and without changing the cutting characteristics of a knife much, because he was using a fairly thin bevel knife). This equates to having a knife fail only due to wear, and not due to deflection or chipping.

This is a nice thing to have in chisels and shop knives, too. In planes, I always chase something steeper than others seem to (measured my plane iron finished edges around 33, and I've never shipped something to someone and had them complain about short edge life - rather the opposite happens sometimes, where I'll get a follow-up email about an edge not holding up as well once it's resharpened. It's not the steel, but the geometry of what's going on at the edge).

This morning, I was sharpening my cheap pocket knife with the usual regimen (deburring wheel, then buff) and I thought about it - what I've come to over the years is shifting the failure point of knife steels away from the very edge, because deflection and chipping end things quickly, but they also create a lot of extra follow up work. The strange part of this is that as long as I make the knife itself thin, the buffed over edge and its performance in cardboard is always *sharper* and easier through the cut than edges I sharpen by hand. This is with that 5 micron wax buff stick - it's fantastic in a buff even though it's too harsh on a hard surface.

So, I figured I'd try this on a paring chisel vs. 1 micron diamonds on steel. What the edge looks like under the microscope with the buff (judicious use is key - to only change geometry where it's desirable) is more refined than the diamond edge, and ease through wood is better. The failure point of the very edge has been modified to slight rounding, just like a strop does to a razor, but at steeper angles, and the pairing resistance through end grain cherry isn't any higher, but the surface left behind is even better than off of the fine abrasive. The crux is this - while the chisel is no worse paring in a relatively hard (for cherry) piece of cherry, it never sheds anything and leaves visible lines whereas this will eventually happen with a hand sharpened edge.

Under the scope, this tiny rounding is less than a thousandth of an inch long, and only on the bevel side, but it's eliminated the thing that I really hate - edge failure that's due to anything other than wear, seemingly without compromising anything like a full steeper bevel would. And with the buffer, it's easier to apply and with better refinement than I can do by hand.

I think my chisel sharpening regimen is about to change from washita to finish stone to washita to buff.

I've long done this when mortising plane bodies - round over the last bit of an edge on a chisel so that the corners and the edge are protected a little bit. It improves the ability to mortise quickly by a factor of ten without driving the wedging resistance of the chisel through the roof.

With the little knives, in the event that the geometry ever gets a little wonky, I just walk the knife over to the tri hone and reset everything in a minute or two, but I cannot deny that in that case (and probably with chisels now) this may be functionally better as a finishing routine as long as one can manage to use it properly (a beginner would just blunt a whole chisel).

There is some version of this that applies to cap irons in a heavy cut, too - maybe something on that at some point another time, but the short version of that is that the very low angle cap irons that ward and matheison made always work better in a brisk cut than a more modern emmerich type that's highly sprung and more blunt.

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