Microbevel or not

[John Coloccia]

Wait, I'm dumb - are you being funny or serious here? I guess I can see poor quality leather being a problem (but if it's not tanned, doesn't it rot eventually?) But how does the wood behind the leather matter other than being a stable substrate?

Ha ha...of course I'm kidding a little. It's a piece of scrap leather glued to something somewhat flat. I guess in the instrument building world, there's so much myth, lore and worrying about minutia that you just have to poke fun at it sometimes

[David Weaver]

Other than derek's page articles, I can't recall someone else using the term microbevel for the honed surfaces on a hollow grind.

We could poll it, but I think almost universally, people think of a secondary bevel when the term microbevel is used.

[David Weaver]

Hi Joe-

I think it's a mistake to seek a consensus on issues like this one. My take is that for thick tools, honing an entire bevel is a waste of time and effort. Guys who do it seem to do it for reasons of expedience for their sharpening technique. The chisel doesn't need a fully polished bevel to cut (but read below carefully).

I'll defer to Derek on the naming. What I do is grind low- like 20 degrees, then hone a small area at the cutting edge to the angle I prefer.

Only thing I can add is that the area behind the edge is somewhat important. For those of you who hollow grind, the chisel thinks that rough ground area IS the bevel. From the chisel's point of view, you hollow grinders are creating secondary bevels (using Derek's terminology). And the hard line that foams between the bevel at the edge and the hollow is a terrible stress concentration. I know some of us hone the hollow completely away- which is fine.

I've seen various people talk about the weakness of the hollow grind, but if you are using a 20 degree primary bevel and then creating a secondary, and someone else is grinding a hollow at 25 degrees, no part of their edge will be less steep than a 20 degree primary bevel and the hollow ground edge will be the stronger of the two. There is no reason for the stresses to be any different at the break into the hollow than they would be at the back of a secondary bevel on a shallow primary.

This comes up on here, and someone has followed odate's book too closely about the hollow grind, or whoever else has talked about it without ever actually seeing a failure. Or maybe it's discussion about japanese chisels and very steep hollows. Whatever it is, I've never seen edge failures caused by a hollow grind when the hollow is just as steep as a common primary. Actually, I've never seen them at all. Edge failures on good chisels are usually small chipping, and on planes, the same type of chipout if the total angle is too shallow at the edge.

I think the optimal profile is without regard to the shape of the edge, it's with regard to the final angle for practicality. If A2 steel is less than 31 or 32 degrees, it often has chipout problems. It doesn't matter if the edge is convex, hollow ground or flat ground. The fact that it is difficult to be that precise freehanding a convex bevel suggests that if someone is changing their angle until they get good edge durability, they'll shoot past that and instead have an edge that is almost the exact shame shape as a worn bevel that is made of intersecting planes.

But in the end, it doesn't matter a lot. Anyone with hands and eyes to judge can figure out what works best under any given method. The condemnation of the hollow grind for anything other than super small radius (smaller than any of us would use) is not accurate, though, certainly not in practice.

[Chris Griggs]

What about this idea that side sharpening creates a weak edge, like a perforation in a paper towel? I had read that "in theory" a bevel honed side to side would be keener and stronger. In practice I'm not sure it makes any difference - what is easier for someone to hone consistently is, in my opinion, what works best in practice. I tend to be more consistent with the edge turned sideways or almost sideways anyway.

Could someone please elaborate or share further thoughts on this?

[Andrae Covington]

I was just wondering how many put a microbevel on their plane irons/chisels, or just finish the primary bevel to the sharpness level needed.

I used to microbevel nearly everything that wasn't nailed down. I am one of those lightweights who uses a honing jig, in my case the Veritas Mk.II, which makes it very easy to increase the angle a little for secondary and even tertiary bevels. Eventually they would get too big to even generously be considered "micro", and then I would have to re-establish the primary bevel. Without a grinder, I had to do this on my DMT stones, which meant grinding the entire flat bevel. I found that the steeper angles of the secondary (and sometimes tertiary micro) bevels required me to remove more and more metal to recreate a clean flat bevel at the primary angle. So I've mostly given up on steeper additional bevels (micro or otherwise).

I know that people are going to tell me that I'm not reducing the amount of work involved. So let me geek out here and run some numbers. Consider a blade which is 2" wide and 1/8" thick, with a single bevel at 25°. If I have to remove 0.001" (1 thou) to clean up the bevel, that is a volume of 0.00025 cubic inch. If that blade has a 27° secondary bevel which is 0.01563" (1/64"), then to take the entire bevel down to 25° requires removing 0.00092 cubic inch. That is over three times the material. If that secondary bevel has grown to 0.0625" (1/16"), that requires removing 0.00238 cubic inch, which is over nine times the material.

If I only use one bevel angle, and keep up with sharpening regularly, then I don't have to remove as much material each time, and I can get back to work. Out of laziness, some of my blades have a single bevel at the steeper 27°, because I don't feel like grinding them back to 25°. I just extended the secondary bevel until it became the only bevel.

My chisels are narrow enough to work on the hand-cranked grinder I eventually picked up. For some of those, I have rough ground down to ~20° and then honed what is really the secondary bevel at 25°. I'm not hollow grinding in the traditional sense, leaving flats at the front and back of the bevel to register on the stones. I'm just excavating the back arris of the bevel so I don't have to grind it away on my stones.

[Jim Koepke]

What about this idea that side sharpening creates a weak edge, like a perforation in a paper towel? I had read that "in theory" a bevel honed side to side would be keener and stronger. In practice I'm not sure it makes any difference - what is easier for someone to hone consistently is, in my opinion, what works best in practice. I tend to be more consistent with the edge turned sideways or almost sideways anyway.

Could someone please elaborate or share further thoughts on this?

It may be the scratches from side to side sharpening (parallel to the edge) are more susceptible to compression stresses than scratches from sharpening perpendicular to the edge. In most cases, this may not rear its ugly head unless one is working with extremely acute bevel angles or very deep scratches.

Hi Joe-

I think it's a mistake to seek a consensus on issues like this one.

My thought is the best consensus for a subject like this is that most of us can agree that whatever works best for anyone is what is best for them.

The single biggest thing I learned about sharpening in last 5 years I learned from my friend Warren Mickley- sharpening should be quick and easy- so easy that you can do it every 20 minutes if need be without much interruption. I used to look at it as something I did between projects. Now it's integral with my bench work. I think Warren can touch up an edge at his bench in under a minute.

Between work in the shop and SWMBO's "Honey Do List" there isn't time for me "between projects."

Stropping at the bench helps to save an edge between trips to the stones. Usually my blades are honed before getting dull enough to require major work on the stones.

Since there currently isn't a grinding wheel in my shop, my blades do not get hollow ground. One of these days a round tuit will come my way and a stand will be built for the large wheel my brother gave me.

The only blade of mine that gets a secondary bevel is a thick plane iron. It is difficult to hone the entire bevel since it sticks to the stone due to the large surface area of the bevel.

Occasionally a make shift blade holder is used. Most of the time my honing is done free hand.

jtk

[Adam Cherubini]

There is no reason for the stresses to be any different at the break into the hollow than they would be at the back of a secondary bevel on a shallow primary.

I'm not sure I disagree with what you're saying overall. And I agree that the science may contribute very little. But just for sake of discussion, that break we're talking about should be smooth. And that's the natural effect of certain styles of hand honing. Good to think of mortise chisels when we consider this topic.

This comes up on here, and someone has followed odate's book too closely about the hollow grind, or whoever else has talked about it without ever actually seeing a failure.

Are you sure that's true, David? Leave the invectives out. You are making good points without them.

I've never seen edge failures caused by a hollow grind when the hollow is just as steep as a common primary. Actually, I've never seen them at all. Edge failures on good chisels are usually small chipping, and on planes, the same type of chipout if the total angle is too shallow at the edge.

With respect, if you find your tools are chipping, you may be waiting too long to inspect or hone. My tools' edges all start to roll over long before chips occur. The rolling is the evidence of bending being the failure mode as opposed to compression since the edges tend to roll always to one side (tho the side does seem to vary based on usage). And analytically, chisels see great amounts of bending. Of course plane irons do.

The condemnation of the hollow grind for anything other than super small radius (smaller than any of us would use) is not accurate...

Ok if you don't agree or think in your experience it doesn't matter.. happy with that. But scientifically the hollow grind does matter. There are several technical issues- the shape, the discontinuity in tangency and or curvature, and the surface finish of the facets.

BTW, I never read Odate's book. But I do think it's curious to say the least that guys with great amounts of practical experience (I'm not including myself in this) all basically arrive at the same conclusion.

[Adam Cherubini]

What about this idea that side sharpening creates a weak edge, like a perforation in a paper towel?

When a chisel is malleted, pressure builds up on the beveled side due to the wedge effect. That force is often reacted not by wood, but by our hands holding the chisel. That means one side of the tool goes into tension and the other side goes into compression.

Now you can't take this too far but...
Imagine stress like running water in a stream. The scratch is like a large stone on the shore. The stone interrupts the flow and causes the water to have to travel faster to get around the stone and keep up with the water in the center of the stream. That increased speed is increased stress. Get too much at the end of that scratch and the scratch can become a crack.

In any case, scratches reduce the strength of a material to transmit load. When testing the strength of metal, we polish them to a mirror polish to get the best possible results. "Notched" or scratched specimens are subsequently made to test more real world conditions. The result is always much reduced capability.

There's something else at work here that I don't know much about. It's called edge "morphology". As I understand it, the act of honing front to back can encourage metal crystals to align themselves parallel to the scratches, creating a stronger edge. Maybe somebody else can help me with this one.

Again, I think you'll find that guys all over the world have been honing essentially the same way for 500 years. We question everything which is good sometimes and bad other times.

[David Weaver]

Are you sure that's true, David? Leave the invectives out. You are making good points without them.

That's no invective, it's something that's cited as a reason not to hollow grind (Odate's book) - not by you, and my response wasn't intended to read "adam says _____ from odate's book"

With respect, if you find your tools are chipping, you may be waiting too long to inspect or hone. My tools' edges all start to roll over long before chips occur. The rolling is the evidence of bending being the failure mode as opposed to compression since the edges tend to roll always to one side (tho the side does seem to vary based on usage). And analytically, chisels see great amounts of bending. Of course plane irons do.

None of my tools are chipping. The rolling indicates that your chisels are soft enough that they fail by rolling instead of chipping.

Ok if you don't agree or think in your experience it doesn't matter.. happy with that. But scientifically the hollow grind does matter. There are several technical issues- the shape, the discontinuity in tangency and or curvature, and the surface finish of the facets.

If there is no practical or material difference in edge durability at a given final bevel angle, then it doesn't matter. There is no legitimate argument that a hollow grind weakens a similar grind in woodworking tools. Long ago, I asked folks to speak up about the hollow bevel angle they had that had greater failure rates, or cracks at or up to the hollow when a flat bevel at the same grinding angle didn't have similar failures and nobody has ever identified any. Overanalyzing a conclusion won't make it different.

[Jack Curtis]

Microbevels? No. Secondary bevels? No. Hollow grind? No.

As to hollow grinding, I use a Makita horizontal grinder which doesn't produce hollows. And the normal vertical wheel grinder is not wide enough to grind most plane blades in one go, you have to move the blade back and forth, which makes it more difficult to make a hollow that goes straight across the blade. Granted, it's easy with most chisels, but from a purely visual standpoint, they seem weaker than a flat bevel to me.

I read Odate as confirmation. The rest is original logic. Overanalysis? I don't think so, but it's of no importance.

[Chris Fournier]

Other than derek's page articles, I can't recall someone else using the term microbevel for the honed surfaces on a hollow grind.

We could poll it, but I think almost universally, people think of a secondary bevel when the term microbevel is used.

A micro bevel is certainly a secondary bevel which naturally has it's own distinct geometry relative to the back of the balde and the primary bevel. A micro bevel is just that though -micro.

[Zach Dillinger]

Microbevel or not?

Not.

[Michael Ray Smith]

I used to add a small, secondary bevel to both plane irons and chisels, but I've pretty much stopped because I don't see a significant benefit. The only exception is when I'm sharpening something to sell. Then I add a secondary bevel just because there's so much stuff out there than says you're supposed to do it, I don't want the buyer to complain about the absence of one.

[Michael Ray Smith]

Oh, one other context. . . . I generally put a convex bevel on draw knives. A lot of them are factory ground that way. I sharpen them freehand, and I just got used to doing them like that. I also put a very slight crown on the back, mainly because the backs of old draw knives are seldom flat and they often have a lot of pitting, so it's less work to put a crown on them. I also like using them better if they have a bit of a crown.

[Tim Put]

... the hard line that foams between the bevel at the edge and the hollow is a terrible stress concentration. I know some of us hone the hollow completely away- which is fine.

I suspect one of the major failure modes of edges involves bending. Supporting the edge behind the honed tip is important. Reducing stress concentrations behind the edge can help the tool react the stress we apply. I think the best shape scientifically is a smoothly polished elliptical (convex) bevel....

The (relative) outcropping of material is not a stress concentrator, at least not in the sense used in engineering or materials science. It may well cause the compressive forces exerted on the bevel to be distributed over a smaller area, but the tensile forces induced by those compressive forces (the bending) will be unchanged. It is those tensile forces which count since it is very unusual for hardened steel to fail in compression when bending.