Multi Axis Bowl
Tuesday 16 September 2008
Len Grantham shows us how to turn a three leaf clover bowl
After having some success with multi-axis turning of spindle work, I thought I would use the same or similar techniques on faceplate and bowl turning.
Obviously work cannot be mounted between centres and holding work off centre in a four-jaw self-centring chuck is not recommended. The answer has to be a screw chuck but there cannot be holes left in the bottom of the piece.
Therefore, my solution was to glue a block of mahogany (Meliaceae) to the base of the bowl to take the holes for the screw chuck. My screw chuck is one that is held in the four-jaw chuck and is only 50mm (2in) diameter. I was doubtful as to whether there would be sufficient surface contact when offsetting so to ease my fears, I fitted a piece of sycamore (Platanaceae) over the screw chuck the same diameter as the chuck body. I made the glue block 150mm (6in) diameter x 50mm (2in) thick. This could probably be smaller but I erred on the side of caution.
For this project, I chose a piece of olive ash (Fraxinus excelsior) 250mm (9 3/4in) diameter x 75mm (3in). I mounted the piece centrally on the screw chuck and then turned the outside to the finished size and shape. The size of the base of the bowl
was set at about 70mm-80mm
(2 3/4in-3 1/8in) and made perfectly flat to accept the glue block.
With the glue block securely attached to the base of the bowl and accurately marked out - as well as drilled for the fixing points - it was time to start the turning process.
The first move was to set the bowl up centrally and clean up the face, then mount it onto one of the offset points. Now, the fun really started because it was out of balance so make sure the lathe is set at a low speed and then gradually increase it (this is where a variable speed lathe comes into its own). On my Nova lathe, I got to around 250rpm before there was any vibration.
With the first leaf shape formed, move on to the next offset point. Things start to get really exciting now. Not only will you experience out of balance but there are also interrupted cuts to contend with. Fortunately, with some of the weight removed, I was able to increase speed to 300rpm.
For the final offset point, there was more out of balance and even more interrupted cut. However, with still more weight out of the bowl, I was able to get up to 390rpm.
Having finished the bowl, I realised I had perhaps made the wrong choice in using olive ash for this project as the grain pattern was overpowering the detail of the three-leaf clover design. Therefore, I repeated the exercise using sycamore (Platanaceae) and purpleheart (Peltogyne porphyrocardia). This combination gave a much more pleasing result.