Is a hollow grind on a bowl gouge ideal?

[Matt Meadows]

Good day team!

I have a question that arises out of my endless desire to experiment and improve.

Let's assume we're talking about a standard parabolic fluted bowl gouge sharpened using something like the Ellsworth or Wolverine jig. The usual "go-to grind" for most bowl turning.

We all know an 8" grinder wheel gives better profiles than a 6", at least in part because the grind is less hollow.

What if we were able to get a actual flat grind? Would that be a better grind, generally speaking, for most bowl turning situations?

I'm not really sure that a flat grind is possible using round wheels unless you are satisfied with the idea of the grinder contacting the gouge sideways (imagine a CBN wheel mounted in a drill press horizontally).

But if a flat grind were possible, would it likely be an improvement?

 

[hockenbery]

But if a flat grind were possible, would it likely be an improvement?

I use the Ellsworth grind mostly and I sharpen on an 8” wheel using an Ellsworth jig
On convex surfaces the the hollow grind is not noticeable.
On concave surfaces I grind off the heel creating a short (micro bevel) this is close to a flat grind.
I don’t turn many long flats - these would be much easier with a flat bevel.

I have 3/8 bar diameter gouge with Michelson grind. This is a convex bevel. Terrific for turning tighter radius concave surfaces or for hollowing balls. Much much better than a flat.

Spindle gouges are another animal. I hand sharpen my spindle gouges with a flat or very slight convex bevel. Easier to control than a concave bevel.

A micro bevel on any gouge will make it close in performance to a flat bevel.
There are belt and disc sharpening systems that produce flat bevels. They haven’t gotten many users.

 

[Matt Meadows]

You know Hockenberry you point out something that never occurred to me. The hollow grind is of little consequence on convex surfaces, it's of far more on concave. That makes perfect sense. It also explains (probably) why I'm so much more likely to get a nasty catch hollowing bowls.

I'm going to look into ways of grinding the heel on these. I think you just help me immensely with that one tip.

I always assumed the belt and disc systems failed to garner a following because our HSS and powder metal tools would wreck the sanding medium too quickly.

 

[William Rogers]

I always assumed the belt and disc systems failed to garner a following because our HSS and powder metal tools would wreck the sanding medium too quickly.

Not so fast. I don’t think that there is a concern anymore on the steels wrecking the belts. I have the Sorby Pro Edge and use it for all my skews and scrapers, HSS and some being V10 steel. I mainly use ceramic belts and they hold up fine. For my gouges I use the Hannes Vector Grind Jig with an 8” CBN wheel. I also grind the heel on my gouges.

 

[john lucas]

I use a very short primary grind. All the metal below that is for all practical purposes inconsequential. When I sharpen I do 3 grinds. The main grind. Then I move the wolverine jig forward and grind away all but 2mm or so. Then I pull the gouge out of the jig and free hand grind away the heel. This gives a grind d very similar to the Vector grind. The only difference is the vector grind rounds over the wings to make them slightly less acute.

 

[robo hippy]

On my bowl gouges, I grind away over half of the 'primary' bevel, and keep a tiny bit of the cutting bevel to freshen up more easily when I go back to the grinder. That cuts the hollow grind down to almost nothing. I do prefer to keep mine rounded rather than a second hollow grind. Pretty simple to do, and no jig is needed since that part is not really in contact with the wood.

robo hippy

 

[Rusty Nesmith]

For me the best grind is 55 degrees and then remove the bottom half of the bevel.

 

[Gord Cameron]

A micro bevel on any gouge will make it close in performance to a flat bevel.

For me, this is a critical point in any discussion of hollow-ground cutting tools, not only because the cutting apex is effectively flat-ground, but because the hollow grind makes it possible to hone this micro bevel (or create the micro bevel by honing) much faster and much more easily, given that the hone is only touching (and only needs to sharpen) a small amount of steel--at the cutting edge and the edge of the heel--given that the bulk of the surface is in the concavity of the hollow grind. It is the hollow grind that allows us to put a quick new edge on a tool with a little hone, while keeping the original grind angle. As I think many others do, I can sharpen with a quick honing about a half-dozen times before I go back to the grinder.

With that said, even if there is a micro bevel, I too sometimes find what might be called "macro" reasons for wanting a flat or convex grind, having to do with the presentation of the tool to the surface, as discussed, but I think of that and things like removing the heel more as shaping the tool rather than sharpening per se.

 

[hughie]

There are wheels that are designed/made for side grinding, I have one my conventional grinder it often used for my custom scraper blades. As for the concave or flat grinds, I think it's a matter of personal choice. I personally dont have any issue with using either.

 

[Glenn Lefley]

You know Hockenberry you point out something that never occurred to me. The hollow grind is of little consequence on convex surfaces, it's of far more on concave. That makes perfect sense. It also explains (probably) why I'm so much more likely to get a nasty catch hollowing bowls.

I'm going to look into ways of grinding the heel on these. I think you just help me immensely with that one tip.

I always assumed the belt and disc systems failed to garner a following because our HSS and powder metal tools would wreck the sanding medium too quickly.

I think they don't gather a following because a wheel is a one time purchase, a flat system requires belts. Endlessly buying consumables. Always where a company makes their money. A company supplying consumables will never offer one belt, always multiples so you buy them all!

 

[Michael Ogden]

For me the best grind is 55 degrees and then remove the bottom half of the bevel.

I agree with 55 as far as being a very comfortable "open road" grind. Steeper grinds and shallower grinds are great for various maneuvers, but (for me) require a little more concentration. Cheers!

 

[William Rogers]

I think they don't gather a following because a wheel is a one time purchase, a flat system requires belts. Endlessly buying consumables. Always where a company makes their money. A company supplying consumables will never offer one belt, always multiples so you buy them all!

Glenn, I have the Sorby ProEdge belt system. Belts are consumable, but I doubt I spend more than $40 a year on the belts. I do buy my belts from a third party and not Sorby, so I’m not going to the poor house buying belts. The ceramic belts last quite a while. I do prefer the non-hollow grind on my skews, wood chisels, and Dway beading tools. I normally use the 220 grit on my scrapers, parting tools, roughing gouges, and most of the time my skews. I use the 600 grit on my Dway beading tools, chisels and sometimes skews. I do not use it for any of my bowl or spindle gouges. It requires way too much protrusion and those tools I use the Hannes Vector grind jig.

 

[Larry Copas]

I use a 2 X 48" Kalamazoo belt grinder with the a Oneway Wolverine and Ellsworth jig to sharpen my bowl gouges. Takes less than 15 seconds and hardly any heat generated in the tool. With two of us turning regularly, we go through maybe two $3 aluminum oxide belts a year. I can switch between 36 grit belts up to 2,000 grit in maybe 20 seconds. We can see no difference between the flat grind and a hollow grind.

Rest of my tools normally get sharpened on a wheel as I much prefer hollow ground for skews as they are easier to hone. Spindle gouges are hand sharpened without a jig and I may either use the belt grinder or a regular wheel grinder.

 

[Adrian Wilson]

As mentioned, there is the Sorby Pro-Edge. I have a Viel 1" flat-belt grinder; I like it a lot, use it for all my regular woodworking chisel and plane irons, but wish it was wider sometimes, and I have a 4 x 36 belt sander. I also have an 8" grinder and Wolverine system, bought that because that is what most people are using, and there is a lot more info out there when on the learning curve, which I am. I use them all for different tasks. Skews, for example, I like a flat-grind. I like having the options.

 

[Owen Lowe]

What if we were able to get a actual flat grind? Would that be a better grind, generally speaking, for most bowl turning situations?
...
But if a flat grind were possible, would it likely be an improvement?

Along with the thread's title: "Is a hollow grind on a bowl gouge ideal?" I'd say that, no, for working a bowl's interior, a hollow grind for a bowl gouge is a detriment to achieve great results. As evidenced by the replies above, most everyone, it seems, grinds away the hollow by doing additional grinding on the heel of the tool. The hollow grind forces the cutting support for a concave surface to the two extreme contact points: the cutting edge and the heel. I feel that a non-modified hollow grind (mirroring the grinder wheel) leaves bruising of the wood fibers because of the rubbing heel contact to support the cut. These "bruised" fibers are a bear to eliminate by sanding and they reveal themselves with darker stripes in the end-grain finish.

An outside spindle (convex) surface does not have this situation because the wood's surface can be contacted by a continuum of the grind just behind the cutting edge. For exterior work, a bowl gouge and spindle gouge can have the same hollow grind with excellent results for both – I often use my spindle gouge on bowl exteriors.

 

[Doug Freeman]

Let's assume we're talking about a standard parabolic fluted bowl gouge sharpened using something like the Ellsworth or Wolverine jig. The usual "go-to grind" for most bowl turning.

But if a flat grind were possible, would it likely be an improvement

No, no difference. A flat grind will still make contact at the edge and bottom of the bevel on the bowl ID. As mentioned, reducing primary bevel width, hollow or flat grind, is the best solution. With a narrow bevel, there is much less length from leading to trailing edge, so with the same force applied to the leading edge (when going ‘round the corner), there is much less force applied to the trailing edge to bruise the wood - its physics. Removing the heel also allows using a larger tool to get into a tighter area. Holds true for SG’s as well. I start with ~1/16” width, when it grows to ~1/8” from resharpening, I take it back down.

For skews, seems to fans of hollow and flat - I prefer hollow for easier hand honing.

 

[Odie]

For me, this is a critical point in any discussion of hollow-ground cutting tools, not only because the cutting apex is effectively flat-ground, but because the hollow grind makes it possible to hone this micro bevel (or create the micro bevel by honing) much faster and much more easily, given that the hone is only touching (and only needs to sharpen) a small amount of steel--at the cutting edge and the edge of the heel--given that the bulk of the surface is in the concavity of the hollow grind. It is the hollow grind that allows us to put a quick new edge on a tool with a little hone, while keeping the original grind angle. As I think many others do, I can sharpen with a quick honing about a half-dozen times before I go back to the grinder.

I believe Gord "gets it"!

-o-

 

[hockenbery]

A convex bevel like the Michelson grind is great for hollowing a tight curve.
I use a 3/8 diameter bowl gouge to hollow a spherical area in a ball

The bevel profile .
Hollowing in action
Check the hollow a round foam