Bowls all turned green and unsanded.
I commend David Ellsworth’s article in the December 2019 American Woodturner on bowl turning. In it he advocates using a single bowl gouge sharpened as shown in figure 1 in different presentations. His method works. But is it the optimum method? I admit that I don’t know what the optimum method for turning bowls is, or even if there is one. But I’m certain about the following:
- Bowls are best turned outboard or using an equivalent. This allows maximum freedom of body positioning and movement, and tool presentation and traversing.
- The outboard facility must be quick to adjust and absolutely rigid. Figure 2 shows one I’ve recently constructed. Any flexing of the toolrest will be transmitted along the tool blade to the bowl surface.
- If you’re right-handed you should turn bowl outsides with the workpiece running anticlockwise, and should turn bowl insides with the workpiece rotating clockwise. If you’re left-handed, use the reverse rotations.
- Turning in both directions brings with it the problem of the chuck holding the workpiece unscrewing. You should use a lathe which has the facility to lock the chuck onto the spindle nose. One is shown in Photo. 3.
- When turning the outside of a bowl, the shavings’ momentum means they tear themselves free. When turning a bowl’s inside, the shavings’ momentum means they don’t tear themselves free. Therefore tear-out is more likely when turning bowl outsides.
- The smaller the sharpening angle of the active edge (that part of the edge actually cutting), the smaller the effort required to cut wood away at a specified rate.
- By far the most important variable in active edge presentation is side rake (figure 4). For finishing cuts on bowls an active edge’s side rake should be 70° or more. There are two reasons for this: the edge’s effective sharpening angle experienced by the oncoming wood decreases (figure 5), and the mode of cutting becomes more a slicing and sawing one.
- A selection of curved toolrests enable tool overhangs to be reduced (figure 6).
- You should traverse much more slowly during finishing cuts because the shavings taken can be thinner.
- As cuts approach the intended finished surface they should be thinner. A thin shaving is weaker, and therefore less able to tear wood from below the intended surface.
- When an active edge’s path of traverse is straight or convex, a hollow ground or flat active bevel is fine. When the active edge’s traverse is concave the active bevel should be convex to prevent bevel-heel crushing and enable zero-clearance-angle cutting.
There are many other factors which influence how you turn bowls. These include your particular bowl-turning objectives. Mine are:
- I use only gouges. I don’t use scrapers because I don’t need to, and because they leave a different quality of wood surface to gouges, and on soft woods a much inferior surface.
- I don’t sand. (After seasoning and during finishing the grain may raise, and I do then wipe the surfaces by hand with 320-grit abrasive paper.) Because I don’t sand or shear-scrape away imperfections, I turn so as to ensure that the finished turned surfaces are free from any tear-out and crushing. I also seek to eliminate any rippling, but am often not fully successful. The resulting bowl surfaces are then a memorial to my turning competence or incompetence. Interestingly, wheel thrown pottery with a rippled surface seems to be well accepted by the market. Why not therefore unsanded woodturnings with a rippled surface (figure 7)?
- I typically seek to achieve a constant, and usually thin, rim, wall and base thickness (figure 8).
Why have I adopted these three objectives? Because my equipment and techniques allow me to, because sanding is boring and dusty, and because I enjoy the challenge. I don’t ask that you adopt these objectives. They may be inappropriate; for example if, say, you want to minimize production durations. In the remainder of this article I’ll focus on the equipment and techniques which I use to enable me to achieve my three objectives.
Achieving thin rims, walls and bases
The conventional wisdom is that you hollow bowls using a series of arc-like cuts as shown in the top half of figure 9. However if you seek thin rims and walls (and ideally bases) you’ll need to hollow in layers typically about 20 mm thick as shown in the bottom half of figure 9, and advocated on page 172 in my 1997 book The Fundamentals of Woodturning and on page 65 Ernie Conover’s 2000 book Turn a Bowl. To monitor the wall thickness I use homemade plywood calipers (figure 10). Unlike metal calipers these can be used with the lathe running. Shining a light through the wall is also useful
To turn the bottom thin, if the rim is full the usual method is to grip the bowl by its rim. I usually use the chuck plate system which I invented in about 1980 (figure 11). This equipment grips the whole rim, and allows you to compress it, thus imparting increased rigidity to the bowl’s form and allowing, for example, tall hollow stems to be turned (figures 12 and 13).
Not often, but most commonly with green sapwood, if the flute is presented in line with the approaching wood’s velocity, tear-out of parallel grain may be very difficult to avoid. A more horizontal flute presentation probably allows the lower, upward-facing flute surface to act like a hand plane’s chip breaker.
If you want to turn the truest possible cylindrical spindle, and avoiding tear-out was of no importance, you should use a wide skew presented at zero side rake (figure 14). Similarly, to minimise rippling, the component of the shaving width parallel to the direction of tool traverse should be as long as possible (figure 15). This of course conflicts with the desirability of at least a 70° side rake for the active edge for finishing cuts which greatly shortens the horizontal component. How to achieve an optimum compromise? Increase the flute radius of the active edge. I therefore use big-fluted gouges for finishing cuts.
If you use a hollow-ground or effectively-straight-in-longitudinal-section bevel, to achieve a desirable near-zero clearance angle when traversing along a concave path the bevel heel must act like a fulcrum (figure 16). The likely result is a ring of paler crushed cells. In some woods this damage can descend to a considerable depth. By using a convex bevel whose convexity is tighter than that of the concave path, crushing is minimised. Also when hollowing in layers where the traverses of finishing cuts are short and overlap, it is easier to feather the start of a cut.
I first showed how to grind convex bevels on page 293 of the 1995 edition of The Practice of Woodturning, and later on page 50 of The Fundamentals of Woodturning. To date I am not aware that anyone else has adopted convex bevels despite their simple and obvious logic, although I note the increasing promotion of compromise two- and three-facetted bevels.
Grinding convex bevels is easy. Dress the periphery of a grinding wheel concave with a dressing stick (figure 17). I adopt a 35-mm-diameter recess because it is the tightest concave curvature I turn in bowl surfaces. You can vary the convexity of a bevel by the tilt of the gouge blade which I manipulate by hand (figures 18 and 19), although you could jig. I invariably hone all the tools I use for spindle turning, but because in bowl turning I present active edges with high side rake and the edges are therefore cutting largely by sawing, I don’t bother to hone.
A concave grinding wheel periphery can also be used to grind hollow-ground bevels on gouges (figure 20). Figure 21 shows a hollow-ground-and-honed gouge with a small sharpening angle I use to finish-turn bowl outsides.
The sharpening angle of the gouge advocated by Ellsworth is 55° to 60°. It cannot therefore be used in a zero-clearance-angle presentation in the bottom of bowls with steep walls. I also mentioned earlier in this article that the smaller the sharpening angle, the easier the edge cuts. I therefore use several different bowl gouges for hollowing bowls (figures 22 to 24). The first turner I saw using multiple gouges in sequence for bowl hollowing was Tasmanian turner Adrian Hunt in about 1980.
I also sometimes use a ring tool (figure 25) in bowl bottoms, especially with soft woods. A hook or ring tool is merely a gouge with its shaft perpendicular to the usual direction. Because it cannot be resharpened many times, I use it only for finishing cuts, mainly on bowl floors. And because it will be used mainly to cut bowl insides, it too should have a convex bevel which in most brands you will have to carefully grind and hone yourself.
Much of the bowl-turning advice one reads focusses on the tool(s) used. I believe that the focus should instead be on the active edge and its preferred geometry and presentation. After all, you can achieve similar active edge geometries and presentations with different tools.
The equipment and techniques that I currently use (because I continue to experiment I’m likely to modify them in the future) are somewhat more demanding than the norm. Even so I hope that this article will cause you to reassess and perhaps modify some of your current objectives, equipment and techniques.