Friday 29 May 2009
Bob Chapman turns a bowl using plywood for an interesting skeletal effect
I know of a skip, somewhere in the North of England, in which usable quantities of very good quality birch plywood regularly appear. Multi-laminated plywood is an expensive material to buy, but this source of offcuts - I did ask before plundering it - means that I can afford to experiment with plywood and how it can be used when considered for turning.Because plywood is a sheet material, making a block suitable for turning usually involves cutting and gluing pieces together (take a look at my website www.bobchapman.co.uk for examples).
However, in this instance, I'd had the idea of removing alternate pieces to create a skeleton effect in the final bowl, but obviously I couldn't do this if the pieces were glued together. Eventually I decided to bolt the loose pieces together for turning - this would allow me to remove the bits I didn't want after turning, and I could fasten the remaining parts in position with wooden dowels. As I didn't fancy making a large number of dowels on the lathe, I visited to my local DIY store instead. I found that I could purchase ready-made 9mm (11/32in) dowel in lengths of 2.4 metres, and this solved that particular problem.
ConsiderationsI planned to have the plywood layers vertical rather than horizontal as I found this a more pleasing arrangement on previous projects, and I wanted an odd number of layers so there would be a middle one to form the middle section of the bowl shape I was thinking of.
A second consideration was that I didn't want too many layers as they would be held by bolts and it didn't seem a good idea to make these bolts too long - I certainly didn't want to risk the thing falling apart on the lathe. These ideas led me to decide on thirteen layers of 15mm (9/16in) ply which would add up to a total thickness of 195mm (7 3/4in). Obviously, to accommodate a round bowl shape, each layer must then be 195mm (7 3/4in) wide. I intended the bowl to be quite deep in relation to its diameter as I thought it would be more striking than a shallow bowl, and I decided to use a catenary to give the bowl's shape.
Step-by-step1 The finished bowls 2 I borrowed a flexible chain from my wife, let it hang, and drew the curve it formed on a cardboard background. Adjusting the length of chain will allow you to arrive at a suitable depth for the bowl, and hence the final height of the plywood layers 3 Cut out the internal shape to use as a template when hollowing the bowl, and use the external shape to make a template for the dowel positions. The dowel hole positions are placed by drawing a matching curve approximately 10mm (3/8in) outside the catenary and dividing this into thirteen positions, six symmetrically down each side and one in the centre. Mark two extra dowel positions near the lower corners of the template 4 Cut thirteen pieces of 15mm (9/16in) birch plywood to 195 x 165mm (7 3/4 x 6 1/2 in) on the bandsaw 5 Each of these pieces was sanded by hand starting with 120 grit paper and working through 180 to 240 grit 6 Sand the edges with 120 grit paper on the disc sander 7 With the pieces cleaned up, use the template to mark out the dowel positions on only one of the pieces 8 If you do not have a 9mm drill bit long enough to drill through all thirteen layers in one go, try clamping three layers together with the marked out layer on top. Drill all three as carefully as possible. Retain the top one of these as a 'master template', and place the other two on one side 9 Using the same master template each time, clamp the layers in groups of three or four. By lowering the drill bit carefully into the holes in the template, the dowel holes will be drilled in line with those in the template. Follow this procedure very carefully to avoid getting the dowel holes misaligned in different layers 10 You may decide to ebonise the faces of each layer to emphasise the gaps intended to leave between the layers. I painted seven of the layers with Indian ink, which gives a beautiful indelible black and dries quickly - the remaining six layers would serve only as spacers for these seven 11 Bolt the thirteen layers together, with the seven ebonised ones arranged alternately, using lengths of 8mm (5/16in) threaded rod through the holes nearest the corners. As the rod is smaller than the holes, take care to line up the edges of the pieces accurately. Another waste square of plywood was fastened across the bottoms of the layers by screwing into the six spacer layers. Centre a faceplate on the block and screw to this waste layer
12 Mount the block on the lathe by the faceplate and the maximum diameter of the bowl and mark from the internal template13 Use a 13mm (1/2in) bowl gouge to hollow the bowl, and use the template to monitor its shape and size as work progresses. You will see chippings of plywood on your hand, in the flute of the gouge, and on top of the banjo 14 With hollowing completed, form a narrow bead around the top edge of the bowl. Don't overdo the decoration as this is a complicated enough piece already and you don't want it to look 'fussy' when the six spacers are removed. Sand the bowl down to 400 grit, seal, and polish with a mixture of beeswax and carnauba wax, applied from sticks of solid wax 15 Cut the dowel into 195mm (7 3/4in) lengths and sand the cut ends lightly on the disc sander 16 When you try the dowels in the holes in the plywood layers, they may be a tight fit. Don't risk hammering them into the holes because any slight misalignment of the holes would stop them without being able to be removed again. Sand each one until there is sufficient clearance to make them a good sliding fit 17 After making some small removable spacing blocks to keep the layers the correct distance apart, fasten the alternate layers together by inserting the dowels. Although they go in relatively easily with a little gentle tapping, ensure there is sufficient friction to stop the layers from moving - glue may not be necessary to hold them in place. It was as I was dismantling the block that my wife pointed to the six alternate spacer layers I'd removed and said, 'You've got another one there'. Of course! There are times when you need someone to point out the obvious. The idea simply hadn't occurred to me
ReplicationIt didn't take long to convert the six pieces into a second version, although this time I decided to ebonise the edges of the layers instead of the faces. This was partly to ring the changes on the first version but also to cover the filler I'd used in the screw holes.
Seeing the two final pieces side by side, another thought struck me. My background is in science and I know that a DNA molecule reproduces itself by splitting into two halves, each of which is then used as a template to recreate the missing half and so form two new identical molecules. Thus one molecule becomes two molecules, and so on. The process is called 'replication'.
In a similar manner, at least theoretically, each one of my skeletal bowls could be used as a template to reproduce an exact copy of the other one. Thus I called them Replicator No.1 and Replicator No.2, but I have absolutely no intention of testing out this theory!