Not a smart aleck question. When turning spindles, or bowls, a lot of shavings come off the same place on the cutting edge and the blade becomes dull. When pondering this, one thought is that the shavings are rubbing the bevel at the existing angle so the effect should be like using a hone on just one spot for quite a few minutes. But a hone leaves a sharp edge, why not the shavings? Is the impact similar to running a hone along the cutting edge, making a tiny flat surface? Is the impact of the shavings simply making the secondary bevel much more obtuse, so the cutting edge, while still sharp, doesn't pick up a shaving as well as a more acute bevel? The same question could be applied to the blade of a hand plane, but of course the volume of shavings past a plane blade is almost minimal compared to the volume of shavings made by a spindle or bowl gouge.
My understanding of this: If the edge was only rubbing the bevel at the "existing" angle (with a zero clearance angle) it wouldn't cut at all. To cut needs a (small) positive clearance angle, at least 2-deg or so, which tilts the cutting edge just the right amount into the wood to cut the fibers. IMO, "rubbing the bevel" is mostly a myth, but taught to beginners to keep them from suddenly digging the cutting edge into the wood with disastrous effect. I teach this to beginning skew students too but only as a way to apply the tool safely before adjusting it to make it cut.
If a rounded piece were turning in reverse, yes, honing may happen. But with cutting, the sharp edge is constantly being abraded by being violently jammed into the fibers (on a microscopic scale) to tear them apart and make the shaving. Since wood often contains silica and a variety of extractives it can be quite abrasive. I've looked at a freshly honed edge under the microscope and could see the effects of abrasion with just a short use on the lathe.
The best discussion of all this I've seen is in Mike Darlow's book "Fundamentals of Woodturning." It's chock full of diagrams and photos and explanations. For example, this photo on the first page of chapter 4, "Cutting and Tools" (I don't think Mike would mind me posting a sample!):
This is the first in a series of much higher magnification photos on the subsequent pages that well show the effects of various clearance angles, accompanied by excellent descriptions and diagrams.
I highly recommend this book for the reader and the thinking person. It may be a bit too technical for the show-me-a-quick-video generation! I basically learned woodturning from this book and from "Turning Wood" by Richard Raffan. I still refer to these books more than any of the others on my shelves.
(If you don't have it, do you want to borrow a copy these or at least Darlow's? I keep an extra copy of each here to loan to students. I could send it with John W. when I see him tomorrow if you are close enough to him to get it from him easily.)
It wears the metal off by abrasion and in combination with that wear, changes the geometry until you get something that won't enter the cut. First to go as the bevel gets scratched is the very apex of the steel, but over time, the whole area wears more and more and the angle becomes more blunt.
In order to test abrasion resistance of steels, there's a machine that effectively runs the blade "through a hone". In this case, the "hone" is impregnated cardstock (paper cards with abrasive embedded in them).
If you could manage to work only the bevel of a tool on wood without ever cutting into it, you could probably slowly hone it (it gets scratched a little and the edge would get beat up, but it wouldn't round the edge from abrasion).
If your hone was similar to what's going on when you're cutting wood (as in, if you cut through or cut layers off of the hone), you'd get the same profile you're getting with wood more or less, but much faster.
The machine that cuts through abrasive cards is called a CATRA tester.
(doesn't look very impressive, but with abrasive in the cards, a knife gets very dull very quickly. The data from this machine turns out to be a very good proxy - in terms of ratio of one steel to the next - of what I have found testing blades planing long grain in wood- which would be similar to cutting a ribbon of wood off of a billet uninterrupted with a skew).
There's a chart - if you're turning something abrasive or just turning long lengths of wood, these will give a good idea about the wear rate of a tool edge (can be slightly different if an edge is getting beaten by an interrupted cut).
CATRA results don't account for heat, so if you're turning dry wood with a lathe, you'd probably want to rely only on the HSS tools on this (e.g., the M2 cuts a length of 480mm through the cards, and 10v cuts about 800 at high hardness).
When results get much different than what's shown on this page, they're not due to abrasive wear (which makes the discussions of turning tools that last 5 to 10 times longer than this or that kind of suspicious), but could be due to drastic differences in hardness, or how one steel handles contaminants of a certain type vs. another.
Thanks for the detailed reply John. As a matter of fact, I have 4 of the Darlow books and a set of DVD's, the first books I purchased. Those, the Raffan books and DVD's, and a Raffan VHS tape are my reference library. It has been too long since I re-read the Darlow books, your post has told me I need to start re-reading Fundamentals of Woodturning again.
I took a lot of pictures of edges when testing plane irons. These are going to be a little different looking, because they're the backs of irons with the chipbreaker installed (what happens in that case is the original "triangle" that forms the bevel instead gets scoped on the top and rounded on the bottom, and the edge is worn back).
This is the scooped side, but the scoop is about 4 thousandths of an inch long (which is a fairly large amount). You can see the rounding at the very edge.
If there is no chipbreaker involved, both sides become rounded instead and the center wears back. I didn't test without a chipbreaker because it's not very useful to use most planes without one (unless you like doing things like walking in overlapping circles to get to a point straight ahead of you).
The bevel side is harder to photograph because it doesn't reflect light as evenly and getting it wasn't the point of this.
