Weekend Projects – Freehand turned sphere

Monday 9 July 2018

In this article I am going to show you how to calculate and mark out a cylinder with turning guide lines to make it simple to turn a good shaped sphere rather than an egg shape. I will also show how to hold the turned sphere to be able to remove the spigots and finish it on the lathe. The materials used to turn a sphere are not limited to solid timber; I often laminate blocks using veneers between the layers to give new effects. Orientating the grain and mixing different timbers can give some stunning results. You can use materials like alternative ivory, banksia nuts, good quality plywood – even a piece of wood with knots can look good – just be sure that the knot is secure so it doesn't fly out when turning. With any turning, face and eye protection is essential. Once you have turned a few spheres and gained some experience, you can let your imagination take over and you can further experiment with different effects. I still have a few ideas to try out like joining spheres together in groups or fitting them into other pieces of work, as well as making them into hollow forms – the list really is endless. I do hope you will enjoy trying and succeeding with this project, and above all, have fun.

Tools used:

10mm beading & parting tool

10mm bowl gouge

10mm spindle gouge

32mm spindle roughing gouge

Diamond parting tool

Sphere calculations

At this point, I need to explain some of the calculations that can be used on any size of sphere. Fig 1 shows the marking out for the first set of cuts and it can be seen that the sphere is enclosed by an octagon, which has eight sides. Fig 2. shows the marking out for the second set of cuts and this diagram shows the sphere enclosed by an hexadecagon – 16 sides.

For a very large sphere you could go to 32 sides – triacontakaidigon.

Formula for dimensions of tangent cuts:

For the first set of cuts: multiply the diameter by 0.293 – 100 x 0.293 = 29.3mm – rounded to 29mm

For the second set of cuts: multiply the diameter by 0.108 – 100 x 0.108 = 10.8mm – rounded to 11mm

The exact numbers for the calculation are 0.29275 and 0.10767


Select your wood; a 100mm diameter sphere is a good starting point, so you will need a piece of wood that will allow you to make a 100mm cylinder. Laminate several pieces together to get the size you need and then add some pieces of scrap wood on each end for the spigots. Fix these with hot melt glue as this is a good method for uneven surfaces. This method will save wasting expensive timber. The laminated blocks should look like these


Next, mount your blank centrally on the lathe. Here, you can see that the block is about 20mm longer at each end than the 100mm; this will allow for the spigots


The next step is to rough down the blank to a cylinder which measures 100mm in diameter


I find it better to start with a bowl gouge to take off the corners and then change to a spindle roughing gouge, as shown here. Please note that it is important to use proper PPE equipment here


The next stage is to make the cylinder 100mm long; this needs to be as accurate as possible as the diameter will be the equator of the sphere and the length will be the north and south poles, and to get a good sphere, they need to be the same


I use the same set of callipers that were used to check the diameter to set and check the length. The marking out of the sphere is shown in the drawing


You can use the diameter of the spigot as one of the marked points to turn the tangent, too. Use a depth gauge set to 29mm – 100mm sphere – to check the size as you part down the spigot


To mark the lines on the face of the cylinder for the first cuts you need to use the same gauge. Again, try to be as accurate as possible


You are now ready to take the first tangent cuts using either a bowl gouge or a spindle gouge. Here you can see the first cuts completed – note how the cut goes from the line on the cylinder to the base of the spigot. As you are making these cuts, try to ensure that the distance left to cut at each end is the same. If not, try to make corrective cuts as soon as possible. The toolrest should be set in line with the cut – 45° to the work – which will help to keep control of the cut depths


You now need to mark out the next set of cuts, using a ruler to mark the lines on the remaining four corners and a depth gauge to take the spigots down by 11mm – the second set of calculations. Next, turn the remaining four tangents between the lines and spigots as with the first set of cuts, again setting the toolrest in line with the cut


When these cuts are completed, you are very close to a well formed sphere


Use a spindle gouge to carefully remove the remaining corners. Be careful not to remove any material from the centre point between the high spots, as this is the final size of the sphere. The very last cuts can be taken with a scraper, a tubular scraper made from old hole saw blades, or a piece of tube. Note how the rest is used to prevent the modified hole saw cutter from catching and being dragged down. I have found it is better to use as large a diameter as possible with the tubular scrapers. You can make a simple template from card, plastic or thin wood, which can be used to check the shape


You are now ready to work your way through the grits sanding the sphere; it is important to keep moving the paper


Now remove the spigots; turn these down to a smaller diameter and use a saw to cut the spigots off. Be sure to cut all the way through as snapping off the spigot could leave you with a hole in the end of the sphere


Trap the sphere between two cup chucks with the spigots at 90° to the axis of the lathe. Start at a low lathe speed to ensure all is running true and then as you increase the speed, you will be able to see on the top of the sphere the ghost image of the spigots to be removed. Carefully turn these away stopping to check on progress at regular intervals. The scraper can be used when you are near to the final size, and again checked with a template, if necessary


As before, sand to a good finish and move the sphere into different orientations between the cup chucks. The freehand sphere is now complete