Turning a Square Bowl
Richard Findley takes a look at an area of turning that he has never explored before
A few weeks ago I had a discussion with the Editor. We felt it was time to do something a little different with my articles. Mark suggested it could be interesting to take me ‘out of my comfort zone’ by looking at areas of turning that I don’t normally do.
The item I am going to look at is a square bowl. I made one, very early on in my turning, but it was an ugly thing. This time I plan to make a much better job of it. I know the theory of making them, but how will this stack up against the practice of actually doing it?
Turning a square bowl should be largely the same as turning a standard round bowl, but the corners may cause potential problems. In theory, a sharp gouge, presented correctly, combined with the correct lathe speed and a smooth movement should solve nearly all of the problems encountered in turning.
We shall see!
My first step into turning any project is design. Usually, with my production work, I will have a CAD drawing or a master part to work from. Here I can make whatever design I like – as long as it’s square!
It’s always tempting to jump in with both feet and make something highly decorative and detailed, but I strongly believe for the first attempt at any form of turning, keeping the design simple will make life much easier, allowing me to focus on the techniques involved in making it. Once I understand exactly what I’m doing, I can then start adding extra detail.
Flicking through some of my books, I’ve found a square ‘Sushi-style platter’ in Mark Baker’s book Woodturning Projects (p114) which appeals to me. It ticks my main boxes of being square and simple,
with an elegant curve and good overall balance to it. With this design in mind I am using the proportions as a basis for my bowl, just adding a beaded foot because I’m a sucker for a foot on a bowl.
Beware the corners!
Throughout this project, it is the corners of the bowl that will cause problems. I remembered from my early attempt at a square bowl that, unlike a round bowl with a diameter of 250mm; if you cut a 250mm square bowl, its diameter will actually be the measurement across the corners, which is roughly 1½ times more than the straight face. So if your lathe capacity is 250mm, don’t cut a 250mm square – it won’t fit! You need to cut about a 165mm bowl to fit a 250mm lathe.
Fortunately, my lathe has capacity for a 400mm bowl, so the ash (Fraxinus excelsior) blank that I’ve cut isn’t going to be a problem. Like the original, I’ve gone for a blank of 250mm square, 350mm across the corners.
I cut the blank on my table saw with a fine cutting blade. This gives a perfectly square cut and leaves a surface that needs very little cleaning up. It occurs to me that centring the bowl will be an important part of the initial mounting, unlike a round bowl where any slight misalignment can be turned away in the early stages. If a square bowl is slightly off-centre it could affect the appearance of the finished item, showing up as a variable thickness on the square edge. I carefully mark the centre of the blank and fit a faceplate ring. I find visualising the end result a little difficult at this point, so I have decided that the shorter multiple screws of a faceplate ring will be preferable to single larger fixing of a screw chuck. There’s nothing worse than your initial fixing showing on the finished bowl.
Those corners again!
It is good practice to check your work turns freely before pressing ‘start’, but with square bowls it is vitally important. Positioning the toolrest and banjo in just the right place is essential to avoid damaging the blank, before you have even picked up the bowl gouge. Check, and check again.
When you do press ‘start’, the spinning corners can be quite alarming, both in the sound they make and the amount of air they move. Start at a low speed and work up; once again best practice, but all the more important here. I am working at 960rpm throughout this project, slowing to 750rpm for the final reverse turning operation.
My blank is deliberately a little over thick, so I can practise the cuts needed throughout the process. It is easier to practise when you have spare wood than when you are down to your last few millimetres on the inside of a bowl. My intention is to have the cuts sorted by that stage. The cuts available are the push cut, the pull or draw cut and the shear cut. At various points on any bowl project I will use all of these cuts. On this project, I’m not entirely sure how the wings will affect these cuts.
Break out is my biggest concern on this bowl. It is possible to fix sacrificial blocks to the sides, essentially making it a standard round bowl, but I want the challenge of turning it square. I will just have to deal
with any break out, one way or another. I try each of the cuts to see which would be better to protect the edges, but early signs are good with each.
My preference for roughing the underside of any bowl is the pull cut. As I know I will need a push cut on the inside of the bowl, I decide to give this a try on the underside too. Bevel position is much more critical for the push cut, so I use the solid portion of the bowl to find where my bevel rubs and then use that tool position to cut in from the edge.
The wings initially bounce the bowl gouge around a lot; too much to make a smooth cut. After trying a few variations of the cut, I get much better results by taking a slightly bigger and more positive cut than from trying to be too gentle and overly cautious. The fact that the edge is properly engaged in the wood gives me control, much like cutting a bead with a skew. If you are too cautious, the chances of a catch increases significantly.
The next cut to try is a push cut to add shape to the underside of the bowl. A few different variations of presentation are needed to get it right, but it works. I am much more comfortable with the pull cut,
so I switch back to that for the majority of the shaping. I feel it is easier to allow the wood to come to the tool with the pull cut. With the push cut, the temptation is to apply too much pressure, which causes problems with bevel bounce.
As the shaping progresses, I keep stopping the lathe and checking the shape I am making and the important square edges, to ensure I’m not chipping out. Usually you can check the curve of a bowl while it spins, both visually and by touch, but the corners of the square bowl make this impossible, so the only answer is to stop the lathe. Two problems emerge as I continue, but as I am frequently stopping and checking my progress, I am able to head them off before they become real issues.
The first issue to sort out is a section of the square edge where the grain pattern is such that it is chipping out rather than cutting cleanly. This problem is exacerbated because of the grain pattern. My solution is
to use a joinery technique: a sharp block plane is used to put a slight chamfer on the edge in question, reducing the harshness of the gouge cut on the edge.
Perfecting the curve
The second problem I have found with the square bowl is the lack of tool support over the wings. As there is less wood beneath the tool, I find that if I apply a continuous amount of pressure to the wood throughout the cut, the gouge removes more wood from the wings and the curve becomes uneven. After a few experimental cuts I realise I am going to have to change my approach slightly. I continue with the pull cut, but apply slightly more pressure, tool to toolrest. I position myself so I can make the full cutting movement in one go, without having to adjust my footing. I try to visualise the shape continuing in a smooth curve as I work. By focusing on my movement, keeping it smooth and fluid and the extra stability of the increased tool-to-tool-rest pressure,
I am able to make the cut smoothly, without the change of shape I have been experiencing previously. A great technique to check the quality of a curve is to use a length of flexible material, in this case a steel ruler, and flex it across the surface of the bowl. Material such as the thin steel of a ruler will always flex into a perfect fluid curve, so by flexing it against the curve of the bowl you can see how good it looks.
With the curve looking good and the edges finished crisply, I need to take a couple of finishing cuts to leave a perfect surface ready for sanding. There are two main options here: a push cut or a shear cut. My preference is nearly always a shear cut. The body position needed for a shear cut allows great visibility of the overall shape of the bowl, unlike the push cut, which I find restrictive. As the tool is against my body throughout the shear cut, I have better tool control.
Having perfected my body movement to cut the curve with the pull cut, the slight change of presentation to the shear cut makes it easy to continue the same sweep and finish the underside of the bowl. A shear cut uses no bevel contact, just a razor sharp edge, presented at around 45° to the surface. It looks and feels a little strange to begin with because the flute is almost facing the wood, but the fine shavings
that are produced show how well this cut works.
Great care is needed for sanding; those wings are very unforgiving to anything put near them. My preference for sanding bowls is always to combine hand and power sanding, which I find gives the best control and surface finish. Here I am sanding the solid centre section with standard hand sanding techniques. Then with the lathe stopped, I use an orbital palm sander to smooth and blend the wings. I finish with abrasive on a cork block, sanding with the grain to 320 grit. I also take this opportunity to sand the edges of the bowl.
Turning the inside
With the bowl turned around in the chuck, I draw a pencil line of the curve I am aiming for. Mark’s original bowl was only 5mm thick; I decide that there is little point making my first square bowl any more difficult than it needs to be and I’m not keen on overly thin bowls. So I aim for a consistent measurement of around 8 to 10mm. I mark this on two faces and I’m pleased that I can see these marks quite clearly while turning.
I deliberately leave extra waste wood above the final surface for a few more practice cuts.
As before, positive cuts work best, moving forward steadily and smoothly. The technique I use here is particularly suited to thin walled, large or wet wood bowls. Starting at the rim, the cuts focus on the shape, thickness and finish of the first inch of the bowl. Once this is done satisfactorily, the cut can progress to the next inch and so on until the turning is complete. The main advantage of this technique is that the bulk of the bowl remains for as long as possible, helping to resist flex and movement before it is absolutely necessary. I also find that, because my focus is on a particular area, it gives me the best chance of achieving a consistent wall thickness without cutting through the bowl.
Regular checks are needed and once the cut is within the solid portion of the bowl, callipers are used to maintain an even wall thickness. The turning at this stage becomes just the same as for any other standard bowl.
As always, there is more sanding required and the process is similar to the outside. I power sand the solid portion, then switch to my cranked drill to power sand the wings with the lathe stationary. This cranked drill is easier to use single handed than a normal drill. The sanding is finished off with abrasive wrapped around a cork block, as before. I find the square edges are incredibly sharp, so soften them with 320 grit.
Reversing the bowl
This is done using the same technique I use for all bowls. I pad the chuck with folded paper towel and hold the bowl between this and the live centre, essentially between centres, turning away the holding spigot. The tiny nib that is left is carved and sanded away. I finish the bowl with several coats of hard wax oil.
I have enjoyed making this bowl which has thrown up some interesting challenges, all of which I have been able to solve using techniques that I already knew from various areas of turning and woodworking. I think this proves, if proof were needed, that having a good and wide knowledge of techniques is the best base for making any turned project. If I were to make this again, I would have a better understanding of how the shape is formed and the relationship between the initial blank and the end result. On this bowl I have lost a lot of the olive ash colour and figuring because of this lack of vision and understanding. Next time I would hope to maximise this timber better, but overall I am pleased with the outcome.