Along The Bevel
Wednesday 16 December 2009
Michael O Donnell looks at the multitude of bevels available for turning
I get a real kick from looking at the bevel on my tools as they come off my 150mm (6in) high-speed grinder. It only takes a single grind from cutting edge to heel to give a superb finish and a real sense of achievement.
Having a set-up that allows me to do this does have many benefits. The tools are sharpened quickly and easily, with less metal removed and less heat generated during the process, ensuring a longer lifespan for the turning tool. My favourite tool is a deep fluted gouge, sharpened with a swept-back grind. It is very versatile and does over 90% of my turning.
Cutting across end-grain
One day I was squaring across the end-grain of a 100mm (4in) diameter piece of pine, which was held in a chuck; I would normally have picked up the deep fluted gouge, but this time I picked up the shallow fluted gouge, and made the cut. The finish was fine, but the surface shape was like the surface of a pond after a stone has been thrown in, ripples running evenly out from the centre. This showed up dramatically because the particular lighting I was using was for photography. Disappointed, I repeated the cut several times. Each time there was an identical pattern of ripples, totally repeatable. I made a quick cut with the deep fluted gouge, just to be sure there was no effect from the previous cut. I went back to the shallow fluted gouge and the ripple patterns repeated. I was sure it wasn't the machine causing the problem as my variable speed Vicmarc VM300 usually runs very smoothly.
Reducing the curvature of the bevel
On careful examination, I noticed that the pitch of the ripples was exactly the same as the bevel length on the shallow fluted gouge - quite a revelation. The bevel angle of 30 degrees on the shallow fluted gouge did make for a long bevel, and because it was sharpened on a 150mm (6in) diameter wheel (or slightly smaller as the wheel wears) the curvature becomes very noticeable. It seemed probable that this was the cause of the ripples. I went back to the grinder and put on a secondary bevel at about 15 degrees - enough to shorten the primary bevel by half - thereby greatly reducing the curvature of the bevel. I went back to the wood and made another cut - nice surface. I did it again and again, just to make sure it wasn't a fluke, and the results were exactly the same each time. Probably, if I made a closer examination, there could be a ripple equivalent to the new (but very small) curve of the bevel. So, reducing the curvature of the bevel really did make a big difference, showing the problem was with the curvature of the bevel all along.
So, what was happening? As the tool enters the wood, it starts by following the curved shape of the bevel, just behind the cutting edge. As the cut proceeds and goes into the surface, the rest of the bevel contacts the wood, and by the time the heel of the bevel reaches the wood, the first ripple has been made. Continuing the cut, the cutting edge again follows the bevel shape while the heel follows the cut surface, and the ripple shape is repeated. I found this showed up better on 125mm (5in) sycamore. Ultimately, would a long 'flat' bevel, achieved by sharpening on a belt sharpening system, completely eliminate the problem? I sharpened another gouge using the Robert Sorby belt sharpener - I also added a secondary bevel for good measure - then, made the cut which turned out to be absolutely perfect.
Making coves using a shallow fluted gouge
The next cut I made was a cove. I usually set myself a target of three cuts in soft wood, using the shallow fluted gouge, which is not usually a problem. But this time, using the shallow fluted gouge with the flat primary bevel and a secondary bevel, to my surprise, produced a perfect cove in two cuts. (Well, it needed a quick rub with a fine abrasive to give it a sparkle before it was perfect). Again, to check it wasn't a fluke I made a further four coves - all in two cuts each. I was amazed, so just to check what was going on, I picked up a shallow fluted gouge with a single concave bevel, and turned a cove. Three cuts was the best I could do to make the same cove. Watching the action of the tool, I realised that the heel of the long curved bevel fouled the edge of the cove as the cut progressed, preventing the cutting edge from following the cove shape correctly. So, was the problem the 'length' of the bevel or the 'curvature' of the bevel? I made it my mission to find out. Next, using a shallow fluted gouge with a long flat bevel, I made a series of coves, and guess what? There was still a problem with the heel catching the rim of the cove, although not as bad as with the curved bevel. So, a shortened flat bevel produced much better shape control while cutting various different coves.
Turning inside bowls
Then thinking back, I could remember getting a series of rings inside some bowls and it is likely the problem was the same - this thought sent me back to look at the effect on bowls. For comparison, I shortened the bevel on my deep fluted gouge and began to make a cut. I then made the same cut using a shallow fluted gouge with a long curved bevel. The difference was dramatic, and using the deep fluted gouge with the short flat bevel, not only greatly improved the finish, the tool felt easier to control than with the standard bevel. The only thing I can think of which would improved the control in this situation is for the fulcrum point to be closer to the cutting edge of the tool. So, a gouge with a short and flat bevel is easier to use and it gives better results on both spindle and bowl turning.
Creating short bevels
Creating short bevels starts with the selection of the tools, as the length of the bevel is related to the thickness of the metal at the centre of the gouge for a given bevel angle. On deep fluted gouges this can be relatively thin as the strength of the gouge is in the wings. This thickness (or thinness) is one of the factors I look at when selecting a deep fluted gouge.
On the shallow fluted gouge it is the thickness at the centre of the gouge that provides the strength, so it is usually thicker than a similar sized deep fluted gouge. Here, it becomes a matter of getting the right balance between strength to avoid tool chatter and creating a short bevel.
Of course the 'bevel angle' has a great effect on the 'bevel length,' but I would not alter the 'bevel angle' to reduce the bevel length as this would drastically change other important characteristics of the tool. Reduce any long bevels to less than 8mm (5/16in)) (or even down to 3mm (1/8in)) long by putting on a secondary bevel at an angle of 15-25°. I now have double bevels on both my deep fluted and shallow fluted gouges and I could even be convinced that making the bevel convex would make for a very nice tool.
This might look like sacrilege on the perfect single bevel, of which I am so proud, but if it is easier to use and does the job much better, then it is the way to go. Putting on an extra facet is much like cutting diamonds, so maybe we should call it the 'diamond grind.' Give it a go next time you are using a gouge. OK, so I can hear some of you saying "why didn't he use a skew chisel in the first place?" Well, that's another story, but I do now put a flat bevel on my skews.