How to Turn a Perfect Sphere
Turned wooden spheres hold a certain fascination. They demand to be picked up and rolled in the hand, they show off the figure of the timber and they also hold a mystery to turners and non-turners alike: just how do you turn this perfectly symmetrical shape without marking it? It’s actually not that difficult to do, but there are several stages involved. The method I will show in this article is the simplest way that I have found, but it is by no means the only way. I’ve seen lots of variations, some involve some very careful marking out, some involve some mathematical formulae or the use of jigs. The beauty of the way I do it is that it is pretty much foolproof!
The first stage with any turning is sorting out the timber. The spheres I show being turned in this article were for a customer who specified softwood spheres, 110mm in diameter with a 16mm hole drilled through the centre. The hole needed drilling first, but isn’t relevant to making a sphere, so I need not go in to any details about that, suffice it to say, the holes make no difference to the making of a sphere. To achieve 110mm, I laminated two pieces of 65mm European redwood (Pinus sylvestris) together, to give me a blank of around 130mm thick, and cut it around 150mm long. The beauty of this project is that you can make your sphere from literally any wood, to any size you like and the technique to create the spheres will be just the same as I show here.
The next step is to make a template. It is quite possible to make a sphere without a template, but to minimise the work later on, a template will make life much easier. I use an offcut of MDF and draw a curve with a radius to match the sphere, in this case 55mm. The beauty of a sphere is that it should be the same in every direction, so I only make about a quarter of a circle, but you can make as much of the curve as you like.
First stages of turning
With the blank held between centres and roughed to a cylinder of the correct diameter, the next stage is to mark out the sphere. In this case I drew two lines to mark the 110mm sphere and a centre line at 55mm, simply marking them with a pencil and ruler. Using my 10mm beading and parting tool, I then turned down the waste at each end, leaving enough solid timber to give a positive drive, but not so much as to get in the way when shaping the ball.
Marking out the sphere
Using my 12mm spindle gouge I then began to shape the block into a ball. I find it best to do this gradually, first rounding off the corners, then, with each cut, increasing the curve. It is important here to keep an eye on the overall shape; to do this I will tend to work on one side until it begins to resemble a sphere, then go to the other side and balance out the shape. It is important to realise when turning a sphere, that the curve begins on the central line – or the equator if you like – there are no flats on a sphere, just curves, so make sure you cut right from the line.
As the globe shape emerges from your blank you can bring the template into play. As long as you paid close attention to making the template and did it accurately, you can trust the template. If you are not going to believe what it’s telling you, there was no point in making it! By now your orb should be looking quite spherical, but the chances are your template will tell you it isn’t. Take the blank
out of the lathe and turn it on its end so the poles are top and bottom. Iit is often surprising how this will show you how spherical your ball isn’t… yet! Little by little, cut by cut, remove small amounts of timber as your template guides you and soon enough, you should have a nearly perfect sphere with a relatively small block of waste at each end. With the shaping process, the waste blocks will naturally reduce in size, to the point where they are greatly reduced but still securely driving the blank.
Removing the waste
There are several ways that you could remove the waste, but the safest way is to saw it off with a hand saw. Parting on the lathe would only remove the waste from one end so you would still need to saw off the other waste block anyway and using a bandsaw could potentially be dangerous – if the blade grabbed the wood and spun the sphere it could endanger your fingers. With the waste removed all that is left to do is to tidy up the cut marks, but potentially the sphere might not be 100% perfect, so it would be useful to be able to remount the ball on the lathe in such a way that it would allow the removal of the cut areas and to correct any imperfections in the first turning and shaping stage… well there is a way, and this is the beauty of this method of making spheres and the part that makes it foolproof. In addition, it is an easy method to follow as well as inexpensive to create the parts to make it work.
Wooden mounting cups
To achieve this secondary holding method we need to turn some wooden cups to hold the sphere securely without marking it. I chose two pieces of tulipwood (Liriodendron tulipifera), a relatively soft hardwood of around the same density as the redwood I used for the balls. One is simply held in the chuck and turned to a cup shape, the other needs to fit onto your live centre.
I am fortunate that I own a Oneway live centre which features a threaded portion, designed to hold various attachments and homemade holding devices such as the wooden cup needed here. These centres are acknowledged to be one of the best on the market, but have a price tag to match. There are now some very similar versions available. To make accessories for this style of live centre, you need to buy a tap that matches, in this case a ¾in x 10tpi UNC, but check your centre to find the correct size. Any wood can be tapped; evidence of this is the fact that I have used this soft tulipwood here, but you need to select a piece of side grain – like a bowl blank, rather than like a spindle – as this will hold the thread, whereas the threads cut into an end grain blank will just crumble, unless you use a very dense wood, but this wouldn’t be suitable in this case. I cut a square of timber, oversized in all directions, pilot drilled a hole and tapped
out the thread. I’m sure an engineer would wince at my tapping technique, but it does the job! With the thread cut, I fit the block onto the live centre. With a drive centre in the headstock, I bring the tailstock up and engage the centres until the block spins and I turn it to a cylinder. This can then be removed from the live centre, reversed, and held in the chuck to turn the cup shape into the face.
What if I don’t own a fancy live centre?
If you don’t have this sort of centre, it isn’t a problem because you can turn a similar version to sit over your live centre to do the same job. The process is similar to making a box, in that you hold the blank in the chuck and turn out the inside, deep enough to accommodate the live centre and to a diameter that is a tight push fit over the parallel section of the live centre. At this point you can mount it and drive it between centres as I described for the threaded version. The main difference here is the grain direction, because the threaded version is cross grain, which means turning it needs a little more care and the use of a spindle gouge to cut it cleanly, but the standard push fit version will be end grain and can be simply turned, as a spindle, with a roughing gouge.
Once the fitting is made and turned true, mount it in the chuck and turn a cup shape into the face. Again the grain direction is relevant here, as the cross grain version will need turning from the rim down to the bottom, like a bowl, where the end grain version will need drawing out from the centre to the rim with the wing of the tool like a box. The exact shape of the cup is important here; too deep and only the rim will drive the ball, which will certainly leave ring-shaped dents in the sphere. You need to turn the cup slightly too shallow, which means that a good proportion of the bottom of the cup will drive the sphere, but importantly the rim won’t come into contact, leaving the sphere blemish free. To achieve this, I cut out the same radius curve on the other side of the template and turn the cup to suit, ensuring the rim doesn’t come into contact with the template, and so the sphere.
With the cups turned the sphere can be remounted. It is important to understand the grain direction of the sphere before starting to turn it. You need to ensure the poles of the globe are directly up and down, or north and south, to ensure a clean cut. When turning the blank initially, it was a standard spindle blank, so the cuts ran from the equator, down to the poles. Now, with the blank mounted between cups, it becomes faceplate work, so the cuts will be drawn apparently uphill, but still from the equator to the poles, and so still working with the grain of the wood. What you can’t do at this stage, is to mount the sphere with the poles running diagonally because then you won’t be cutting consistently with the grain; at some point you will be working against the grain. I’m not one to say this will never work, but it will be making life difficult for yourself when it comes to achieving a good, even surface finish.
Turning between cup centres
With the sphere held at the equator by the cups, and the poles running directly vertically, you can finish turn the sphere. Light cuts with the wing of the tool in a shearing cut work really well here. This is when it becomes clear just how simple this all is, because as you turn in this new orientation, the waste marks will be removed and, because the lathe simply spins the sphere, you will automatically be removing anything that wasn’t perfectly spherical from the earlier operation. Because you are using a light shearing cut, working with the grain of the wood, it cuts cleanly and a sphere is formed almost effortlessly. Check the sphere regularly and reposition it in the cups occasionally, but only by rotating it around the equator, so a different portion of the sphere can be turned. Once it is all turned and the cut off marks are gone, the sphere is ready for sanding.
The completed spheres
The sanding process is even more simple. I worked from 120 grit to 240 grit on these softwood spheres, but you might choose to go finer. You can sand in any direction, so now you can feel free to mount the ball in whatever orientation you like between the cups; just try to ensure every surface is worked evenly. Very quickly, you will find you have a perfectly smooth and spherical ball.