Richard Findley takes a look at an area of turning that he has never explored before.
I decide to take myself 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? The process turned out to be rather extensive and detailed. Not wishing to miss anything out, I have turned this foray into a two-part series.
The theory
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!
Design
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. However, 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. I am working at 960rpm throughout this project, slowing to 750rpm for the final reverse turning operation.
Practice cuts
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.
Checking progress
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. However, 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.
Chipping out
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.
These initial processes have certainly been a learning experience. Turning a broad item with square corners can be daunting. That is why I urge you to proceed cautiously. It is likely, as is my case, that you haven’t attempted this before, so thinking about each process carefully is crucial to success – and avoiding a potential injury. My initial cautions where soon overcome by the satisfaction of achieving workable solutions. That is almost as gratifying, if not more so, then creating the item itself! In part two, I’ll continue to detail this exploration with further processes, including how to hollow out the inside and finishes.