Monday 9 July 2018
I am going to show you how to support the wings or natural edges on burrs so they can be turned down to 2-3Mm (5/64-1/8in) thick without the flex that occurs when the wood is un-supported.
This trick was brought about by the necessity to use up a lot of burr caps that were oblong in shape and didn't have a great deal of depth.
This is a challenging exercise, so it is a good idea to start using the rings on smaller work until you become familiar with the process.
Shaping the bowl
I usually mount natural edged burrs on a screw chuck or faceplate, shape the back just like any other shallow bowl and cut a spigot to fit my chuck – the tenon will usually be shaped to create three support feet later on. The biggest problem encountered with burrs is that often there are irregular shaped and lengthed wings, voids and fissures which means there is a loss of integrity in the shape and these faults can lead to the wall flexing, etc. when turning the inside, especially when you want to turn a thin wall. This will add a bit of support to stabilise the piece which will make it more rigid when turning, and this will help no end
Cutting a support ring
When the back is cut you need to study the wings and note the fissures and cracks. You can see where the piece will need supporting when you turn the inside and you can measure the widest diameter where you want a ring to fit. For the rings use 18mm (23/32in) ply – MDF isn't any good as it flexes – and mount the ply on a wide faceplate, or screw chuck in wide jaws to add support. Using a gouge, clean up the outer edge to your measured diameter then at about 30mm (1 1/8in) from this outer edge, part off a ring of plywood. Don't have the speed too high. 30mm (1 1/8in) provides a nice amount of stiffness to support the work, but experiment with the size as you see fit. Depending on the wood used – it doesn't have to be a burr to have fissures and gaps in it – you can have as many rings as necessary on a piece to provide adequate support
Where to place a ring
Align/sit the plywood ring on the back of the burl and place pencil marks where it will be located. Run a band of hot melt glue around the inner section of the ring. If your glue gun won't quickly flow then place hot glue around the entire circumference; you can apply the glue bit by bit and then heat all the glue with a hot air gun so the melt is consistent. Apply the ring to the turned wood at the pencil marks. Use as many rings as necessary to prevent flexing and movement during the cutting process
Firm fixing & support
You now have support around the inside of the ring. Around the outside diameter there are likely to be gaps so you need to fit wedges and glue to further support these areas. Just keep adding wedges and glue until you are satisfied all the wings are well supported. With these thin-winged bowls I want perfectly even wing thickness. It is very important that when I turn the bowl over to cut the inside that the wood runs perfectly true. If you release it from the first mounting method, turn it over, and remount it in a chuck, it may not be true
Initial cutting of the wings
Once the support ring(s) are in place you need to make cuts that cut as cleanly as possible, applying as little pressure onto the work as possible. The pressure should be enough to maintain bevel contact and no more. Too often people press too hard into the work rather than making sure the blade is stable on the rest, and this can cause the blade to deviate when running over the fissures, etc. Present the tool with handle well down so the edge is cutting at a shearing angle
Refining the wings & inside
To locate the outer wings, bring the chisel up to make contact between the wood and the heel of the chisel. Then adjust the tool so the bevel is rubbing and proceed to cut the wings. Check the wall thickness regularly. Some will use a pull instead of a push cut to shape the wings, but check to see which cut minimises the tear out on your timber
Cutting the internal section
The shape of the burr and the natural flaws will dictate where the deeper central hollow will be, although you may decide to make a piece that is one continuous, even-walled flowing curve. Whatever your choice here, continue to shape the internal section as necessary
Keep the ring in place when shaping
I mentioned that I usually create a tripod-foot from my tenons, but you can choose to do as you like and desire to create the effect you require. Whatever you do, ensure to keep the support rings in place as long as you can to provide support for shaping before you remove them from the burr.
I mark the three locations for the feet using the lathe index or, alternatively, take the work off the lathe and use a measuring jig. But, the feet need to be positioned to make sure that they will support the work properly.
By taking the work off the lathe you can study where the feet are best placed so the piece doesn't tip over. I then use a chainsaw-type or toothed cutter to remove the waste wood to start to shape the feet. Keep checking the curve as you go; I find a hot melt glue stick excellent for this step
Removing the ring
If you are taking the piece off the lathe to cut, sand, and finish the feet, then those uneven wings need a soft place to rest. I have an array of bean bags – actually filled with rice – to support the centre of the bowl and ensure that the wings don't touch anything.
After the feet are cut and sanded the glued-on ring can be removed by using a little heat. A heat gun, or a microwave can be used to soften the glue but use too much heat and you may crack your work. You can also try using a solvent such as mineral turps to soften the hot melt glue. Whatever method you choose, once the rings are removed, clean off any excess glue and sand the whole piece to blend the old and new-cut parts together
I tend to use an oil finish for this type of project. This provides a durable, easy to use finish that is easy to re-apply. It can resist marking well and can be either obtained or manipulated to create a matt, satin or gloss finish, as required
(PHOTOGRAPHS BY TERRY SCOTT)