Convex vs. Concave Grind ?

Re: Convex vs. Concave Grind ?

JamieDonaldson

>I'm not sure what specific area of the tool you are addressing, but the tip bevel should be flat or slightly hollow ground. The side(wings) should never be concave in profile- the cutting edge profile should always be convex when viewed from the side, across the flute.

Re: Convex vs. Concave Grind ?

Richard Allen

>Hi Rodger

The Convex grind will put the bevel closer to the cutting edge. I find this makes for a tool that is a little more touchy. Easier to get in trouble. ALso easier to get into tight places. Often you will find that the heel of the bevel marks the wood as you cut. A convex bevel won't do that (many turners will grind off the heel of the bevel on the tip to reduce this effect).

The concave bevel is a LOT easier to grind (with fingernail jigs), is easier to control and delivers a clean predictable surface.

JoHannes Michelsen uses a 3/8" A11 Glaser bowl gouge with a concave grind. He says, that gives him better control.

In the grinds library there is a spindle gouge that David Ellsworth uses which has a convex grind.

There is a lot more to it than I understand. It is likely worth a try to make a convex bevel gouge to see how it works for you.

Thanks

Richard

Re: Convex vs. Concave Grind ?

Russ Fairfield

>When used on the inside of the bowl the normal concave bevel that we would get from the grinding wheel will be across the surface of the wood and ride on the cutting edge and the heal (bottom) of the bevel. Because of the opposite curvature of the wood, the rest of the bevel is not touching the wood.

There are four (4) problems that can result from this 2-point contact.

1 -

The sharp heel of the tool can cause the tool to rotate slightly as it goes across the grain of a wood with hard grain boundaries. This makes a washboard surface on the wood that is only accented with subsequent cuts.

2 -

The pressure from the sharp edge at the heel of the bevel can bruise or burnish the suface of some woods such as Box Elder, Maple, and Cherry. These burnished areas can be difficult to sand, and the difference in the surface can be transferred to the finished surface as a color change or a difference in surface gloss.

3 -

Because there is no pressure from the bevel immediately below the cutting edge to hold the wood as it is being sheared by the tool, there can be more tear-out on end grain. This can be a problem on a wood like Walnut, where the wood in between the harder grain boundaries is relatively weak; and it can be a particular problem with a natural edge bowl where we can be exposing a lot of this "in-between" wood.

4 -

The sharp edge of the heel can start a severe harmonic vibration, particularly in the rim area of dry wood, as it crosses the grain boundaries. The joints in a segmented assembly can be source for this vibration.

A convex profile on the bevel solves these problems by making the curve of the bevel more like that of the inside of the bowl.

The first two are solved by removing the hard edge at the heel (bottom) of the bevel. The 3rd by moving the support pressure for the tool up to the area just below the cutting edge where it can hold the wood in place as it is being sheared. The 4th problem can be solved or its effect reduced because this bevel spreads the tool pressure on the wood over a larger area, and can be used to approximate a skew-type cut with very little tool pressure on the wood as it slides across the surface.

An added advantage is that the pivot point to moved up to just below the cutting edge, and this makes for a shorter turning radius for the tool on an inside curve.

None of these four (4) problems are as visible on the OUTSIDE of the bowl because here the curvature of the bevel is similar to that of the wood.

Now, to answer the obvious question, "Why isn't everybody using the convex bevel on their bowl gouges".

There are several reasons why we may not embrase the convex bevel as the answers to a woodturners prayers.

It requires that we have two (2) bowl gouges. Although there are those who use nothing but a convex bevel, I see it as an INSIDE tool, a supplement, not a replacement of the normal bowl gouge with the concave bnevel. If the convex bevel were to be our only tool, we would be moving the same problems to the outside of the bowl whenever they did occurr.

This a tool that really shines when we are having any of the four (4) listed problems on ther INSIDE of a bowl. The rest of the time it doesn't really matter and we can get away with using the same tool on both IN and OUT sides of the bowl. And, when these problems occur, we either accept their result as "the inevitable", or use a lot more sandpaper with a coarser grit to remove them.

It is a more difficult tool to grind, it takes longer to grind, and it isn't something that can be done with a single pass across the wheel with a grinding jig. I grind it "free hand" because it is faster to do it than with a jig.

Some may see the shorter turning radius as a tool that is more difficult to control.

I use both bevels on gouge because I make a lot of natural edge and segmented bowls, and see more of the 3 and 4th problems than most woodturners.

Even though they may never use a convex bevel on a gouge, I recommend that all woodturners make an extra free-hand pass or two across their grinder to remove the sharp hard edge from their concave bevel. This will resolve some of the effects from the first two (2) problem for everything they turn.

If you are having any of the described problems, try a convex bevel on them. You may be enlightened by the results. Look at the last entries in the Grinds Library for a photo of the profile and how it is used.

If you are open to being convinced of the merits of the convex bevel, contact Dave Sweitzer from Shelton, Washington, and schedule a demonstration for your club. Dave is a dedicated advocate of the convex bevel and gives an informative and entertaining demonstration.

What a definitive reply!

Lyn J. Mangiameli

>

Not a simple topic........

Russ Fairfield

>Not an easy subject to discuss with a website message. I think Richard said the same thing with about 5000 fewer words. I wish I could do that.

JoHannes Michelsen uses a convex grind!

Richard Allen

>I used the WRONG term.

JoHannes Michelsen uses a 3/8" A11 Glaser bowl gouge with a convex grind.

Re: Convex vs. Concave Grind ?

Rick R, So Cal

>Russ,

Thanks for the clear explanation.

Won't a micro bevel solve the same problems? I haven't tried this myself yet, but I watched Jimmy Clewes put a small secondary bevel (.5mm) on his gouge just for the finishing cuts inside a bowl. With the micro bevel, he could pull the heel of the main bevel away from the wood to prevent the washboard, burnishing and vibration problems while keeping the small bevel in contact with the wood to prevent tearout problems. When he was done with the interior finishing cuts, he would remove the micro bevel so the gouge could be used elsewhere, and very little steel gets wasted.

Rick

Re: Convex vs. Concave Grind ?

Doug Leite

>Russ,

I love reading your posts as you explain things in the most understandable ways that form pictures in my mind as I read it.

Our club recently brought in a demonstrator and he advocated the convex grind. He either failed to explain, or I missed the fact that the bevel will now support the wood fibers being cut.

I recently turned my first 2 segmented bowls out of Koa, and I experienced some chipping at the joints on the inside.

I am now excited to try some more segmented bowls as I am sure you have addressed this problem for me.

Thanks for your excellent explanations,

Doug- Hawaii Turner

Re: Convex vs. Concave Grind ?

Russ Fairfield

>There are many ways to skin the same cat. The micro-bevel is another way to do the same thing for a finishing cut. Adding a micro-bevel and rounding the heel of the tool at the same time is getting pretty close to grinding a convex bevel.

The advantage of the full convex bevel is that there can be a wider bevel under the cutting edge, and it can be used to remove a lot more wood with heavier cuts. (My opinion)

Another disadvantage that I forgot to mention is that the tool can require a different nose angle for the three different parts of the curvature inside the bowl - the sides, through the transition, and across the bottom. This may require 2 or 3 tools to do the inside of a bowl. But, we have to ask ourselves if this is a bad thing if the quality of the inside of the turned bowl is the better, and there is less sanding required, for doing it??

Re: Convex vs. Concave Grind ?

Roger F.

>Boy I opened up a can of worms didn't I Russ? You sure have a way of keeping them under comtrol though. Thanks for the advice, I'll try to use it wisely. I'm a reasonably new turner and

appreciate any help I can get.

Roger F.

Re: Not a simple topic........

Keith Tompkins

>excellent topic.....thanks, Russ!

Re: Flat Bevel Compromise

Brad Vietje

>I'm very glad we have a more mature group here than at r.c.w., where I have been called all manner of insults for discussing this topic and tool sharpening. Many turners are unable to discuss a topic like this without getting pretty hot under the collar; I just can't see why people get so attached to turning dogma.

Some folks view a convex bevel as a specialty tool for finishing the bottoms of pieces, where it works well. I have long held that a convex bevel would be best inside a bowl, and a concave bevel best outside the bowl, for all the reasons Russ stated so well. Therefore, I grind most of my tools with a flat bevel, as a reasonable compromise, and I'm very pleased with the results. I also have one spindle gouge ground with a convex bevel for back-hollowing end grain, where it is much better than a hollow ground tool.

Flat bevels are often frowned upon by people who want to hone their tools a lot, and rightly so -- honing by hand will tend to produce a convex bevel, which many people seem to want to avoid (sometimes for entirely dogmatic reasons, however). The answer, I believe, is to use a sharpening system that produces a flat bevel with a very fine edge, removes very little metal, and is as quick, or quicker than honing, thus eliminating the need for most honing. Such a beast can be constructed simply and cheaply by using a 12" disk sander, and a "V" block that can be set to a number of positions for accuracy and repeatability.

Works slick as a trout, really. I built such a system that is cobbled onto the end of an old Shopsmith, sharpening on the "up" side of the disk for most applications, though both sides are used for a flat-bevel approximation of the Ellsworth grind, with the Ellsworth jig anchored in a hole at the correct distance from the spinning disk. I use a 150 grit disk, and I have another 80 grit for any rapid grinding or re-shaping I want to do. I can turn away from the lathe, face the old SS, step on a foot switch, sharpen the tool, and be back at my turning in about 7 seconds.

I hone the inside of the flute for finishing cuts, but not very often do I hone the bevel. This is absolutely the slickest way on earth to sharpen a skew, especially one with a curved bevel -- just set it in the V-block (at the proper location), and rock it from side to side on both sides -- takes about 3 seconds. I hand sharpen a few tools, but use the V-blocks as a jig for most, so a tool can be re-sharpened in 5-7 seconds, removing only the slightest bit of metal, and producing very little heat.

My position is that if you can get a real sharp tool without honing, and without grinding your expensive tools to dust, there would be a lot less honing, and people would be sharpening a lot more often, which is good for the end result.

I find that many people hone a tool 25 times or so to AVOID sharpening, because they either hate sharpening, or have a frustrating experience at it, or have never found a way to get a consistant bevel geometry, so when they finally get the tool to work correctly, they spend the next 3 months trying "not to screw it up", and many end up using dull or mis-shapen tools that hurt their turnings and their enjoyment of the sport. I usually sharpen the tool to maintain a sharp edge, and rarely wait until the tool starts performing poorly; I might re-sharpen 5 or 6 times finish turning a single bowl.

If you've never tried the flat bevel experience, try it -- you may like it.

Safe spinning,

Brad Vietje

Thetford, VT