Thread: Flat Grind Versus Hollow Grind

All my new accursed skews have come with flat grinds and I hone them on a diamond pad or 220 grit sandpaper on the lathe bed to maintain the flat. Some of the used, butchered accursed skews I have ground on the side of the wheel and then maintained with sandpaper on a flat surface or flat diamond pad.

Everything else I grind on an alox 8 inch 120 grit wheel. I do mostly bowls, so use bowl gouges and big heavy scrapers at the end before sanding.

I would think that a concave grind, especially from a 6 inch wheel, would need to be ground stubbier than a flat grind to get the same cutting angle out at the tip.

If ground the same length as the flat grind, it might be more aggressive in cutting, just like a long Ellsworth grind vs a 45 degree grind on a bowl gouge.

ADD:

After reading Jon Siegel, I more or less agree with him. I have been turning about 7 years and have never measured the angle of a turning tool. I some times think there is a bit too much measuring and figuring and trying to be precise that just turning something and paying attention to what takes off timber and gives a smooth surface would provide the answer.

If your tool grind angle and tool rest height are approximately right, and you start by rubbing the bevel and raise the handle to start cutting and then move up or down and twist the bowl gouge to make a long stream of curlies all the way across the bowl, that is what counts. After a while you don't even think about it, just present tool, up, down, twist, nice clean cut.

If a person spends 100 hours turning with decent tools and a grinder at hand, they will be a pretty good turner.

Those two articles make good reading. Thanks Neil. (And finally I have the source for the proposition that a thou's wear at the tip = blunt ).

I don't yet have experience of using flat bevelled tools but soon!

Tormek provide a figure for the degree of concavity in a grind using their 10" (250mm) wheel and it's very small.

Clearly that from a 6" (150mm) wheel will be much greater. And with a 6" wheel and an acute grind it's possible to end up with the included angle at the tip some degrees lower than a simple angle setter indicates. And so run into more of the problem with cutting edge life and control that Siegal mentions.

I would expect that in principle a flat grind will provide more support for the cutting edge.

One matter that all this raises for me is what exactly is meant by 'rubbing the bevel' with a cutting tool. Specifically, how much of the bevel? Clearly some of the bevel is necessarily riding the wood surface that the edge has just created, and that pressure has to be roughly matched by the downward pressure on the top bevel or flute from the emerging shaving in order to get an even and controllable depth of cut (Darlow writes about this). In practice the length of bevel rubbing can be small and in some interactions with wood have to be very small (eg. a drumstick shaped by a skew). In which event bevel concavity may mean relatively little.

But it did to Siegal. And I can see his reasoning when applied to planing a cylinder with a skew but cutting curves on a bowl with a gouge?

(PS, sorry Paul, we were writing at the same time).

Originally Posted by rsser

One matter that all this raises for me is what exactly is meant by 'rubbing the bevel' with a cutting tool. Specifically, how much of the bevel? Clearly some of the bevel is necessarily riding the wood surface that the edge has just created, and that pressure has to be roughly matched by the downward pressure on the top bevel or flute from the emerging shaving in order to get an even and controllable depth of cut (Darlow writes about this). In practice the length of bevel rubbing can be small and in some interactions with wood have to be very small (eg. a drumstick shaped by a skew). In which event bevel concavity may mean relatively little.

But it did to Siegal. And I can see his reasoning when applied to planing a cylinder with a skew but cutting curves on a bowl with a gouge?

(PS, sorry Paul, we were writing at the same time).

As long as one is getting a nice clean cut, what does it matter how much is rubbing. I found the blunter sharpening angle less volatile than the fingernail grind. The first time I used my new-to-me Thompson with a long Tormek sharpened swept back grind, I nearly soiled myself.

It really removes the timber, and I learned to go by the book and have the tool right down on the rest and have a firm grip, because once it starts cutting, it is hang on and devil take the hindmost.

You have nothing to be sorry about, I am gratified to know my mind is running in the same ditch as that of a very competent and experienced turner.

I am not anywhere near an accomplished enough turner to really comment on this issue. I have read the Jon Siegal article, and he does make some interesting points. one comment that I do have though, is that he very vehemently makes some points in the article, where I have heard others state exactly the opposite opinion just as forcefully (some of the things about chisel shapes etc were what I noticed. He really doesn't like oval skews for example, where I have heard others state firmly that they are the best kind of skew to use).

I think there are too many variables to say one grind shape is better than another.
It can probably be proven in an isolated circumstance but that circumstance may be momentary.
The size and shape of the work piece is constantly changing so the tool presentation and edge support must change with it.

Does a flat bevel provide better support for the cutting edge than a hollow grind? No. In order for the edge to cut there must be clearance behind the edge. It seems more likely that a hollow grind can give clearance and then support the edge closer to the cut.

Does a hollow grind change the effective edge angle? Yes. It will make a more acute angle but a smaller angle penetrates a little better with the down side that it wears more quickly. The grinding angle is easily adjusted to allow for this.

Does a curved cutting edge take a smaller cut than a straight edge? Not necessarily. In a concave cut a curved edge will have more contact than a straight edge.

I can see a benefit in a flat bevel on a skew for some cuts especially if you like to hone the edge. Honing a concave bevel, unless done on a wheel the same diameter as the grinding wheel, introduces a secondary bevel that can effect tool control.
A smooth bevel, sharp edge and support in line with the forces of the cut will give the best cut with the most control.

Best Wishes

Steve.

To throw a cat amongst the pigeons, Israeli turner Eli Avisera has demonstrated a skew with a convex grind, one he developed for teaching his children how to use the skew. He says it is more forgiving for beginners. I firmly believe most skews are ground with far too sharp an angle (<25deg) and much prefer a blunt angle (around 45deg) myself for the work I do. The sharper bevel angle is fine for detail like deep vee cuts.

The flat & convex grinds on skews have one extra benefit, in that they don’t create as much bruising from the back edge of the bevel on concave cuts with the skew.

Originally Posted by Mobyturns

To throw a cat amongst the pigeons, Israeli turner Eli Avisera has demonstrated a skew with a convex grind, one he developed for teaching his children how to use the skew. He says it is more forgiving for beginners. I firmly believe most skews are ground with far too sharp an angle (<25deg) and much prefer a blunt angle (around 45deg) myself for the work I do. The sharper bevel angle is fine for detail like deep vee cuts.

The flat & convex grinds on skews have one extra benefit, in that they don’t create as much bruising from the back edge of the bevel on concave cuts with the skew.

You coming to Prossy?

The only advantage I can see to a flat bevel would be on a skew chisel if you are making long straight pieces, or ones with very gentle arcs. Mostly the flat would work like the sole of a plane and help you keep on a straight/flat line. With the Eli Avesera convex grind, I found it great for more rounded pieces with more coves and arch, but more difficult to make long straight cuts like on a rolling pin. Other than that, the closer the bevel rub/kiss/very gently toughing the wood point is, the easier it is to control the cut. On the outside of a bowl or any convex piece, bevel angle makes almost no difference because the curve will have the cutting edge right where the cut is happening. On the inside of a bowl, you will be on the heel of the bevel, and most of us grind away most of the heel to prevent heel marks/bruising, and bring the rub point closer to the cutting edge.

robo hippy

I deliberately hollow grind my skews then hone across the hollow to give me my working edge, a narrow bevel for the 'rubbing' and a clearance.

Originally Posted by Mobyturns

I firmly believe most skews are ground with far too sharp an angle (<25deg) and much prefer a blunt angle (around 45deg) myself for the work I do. The sharper bevel angle is fine for detail like deep vee cuts.

I also have a sharper angle than 25 degrees, I've never n=measured it, but new skews get an immediate regrind to give a longer profile. As mI said previously in another thread, this works for me, but it does mean that I have to hone very regularly to maintain my [preferred] edge.

Originally Posted by

You coming to Prossy?

Yes, been following your woes. Nothing wrong with Barky, just not the place to be for a long time when you are used to greener pastures.

Moby, I was given a set of marples (small chisels) when I was starting and the skew was and still is concave. I sharpen it by running a ceramic stone following the curve. It is forgiving and is still brought out for awkward grain. It appears to be the original grind.
Cheers,
Jim

I keep having trouble with Windows Explorer making and keepin g contact withe forua.

I posted on this yesterday and it obviously fell ver the edge.


To me it stands to reason that a flat grind - at the correct angle of course - will be sturdier and longer lasting.
After all is has more metal supporting it.

I would normally let this topic "go through to the keeper". But I couldn't resist. The arguments about grinding and honing turning tools comes and goes. I have been turning in various capacities for a while now, and still find some of the points raised interesting. The following is my own personal views based on about 50 odd years turning, and subscribe to the concept, "This is how I do it. Try it and if it doesn't work, change it to work for you". I have never measured the angles used, and was never interested enough. When teaching, rather than quote numbers, in general state, "Make the length of the grind about twice the thickness of the steel in the tool at the grind point". The tool is then honed on a whet stone to get a sharp edge, with both the front and back of the bevel rubbing on the stone. The grinding angle was determined to a degree by the work it was to do. Softer timbers, I may make the bevel a little longer to produce a sharper edge to produce a finer surface. For hard timbers I would shorten the grind angle to produce a stronger cutting edge that may not be as sharp. This flies in the face of the need to replicate the angle every time. It is apparent then that I prefer a concave grind as produced by a 200mm diameter wheel. I agree with Paul, about arguments about this or that method and about "the correct" angle, is irrelevant, and not enough time spent observing the interaction between wood and tool. In my opinion far too much time is spent on arguing that this or that method produces a sharper tool. How sharp is sharp enough? The ultimate test is the quality of the surface left behind and to some extent to the quality of the shaving produced.

As for the argument, flat grind verses hollow grind, I prefer the hollow grind, it has stood the test of time. I am also very aware of "modern" methods and have replaced my carbon steel tools with the latest Thompson tools, I also use a Tormek, and use a lathe with a "V" belt and the latest electronic variable speed control. I also firmly believe that many of the "new" ideas, are not new and many of the "new" tools are gimmicks.

Jim

Tongue is nearly missing.