Multi Axis Bowl archive

Tuesday 16 September 2008

Len Grantham shows us how to turn a three leaf clover bowl

Gallery

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.


David Preece

Tagged In:

Bowl , Len Grantham , Multi Axis

"The first move was to set the bowl up centrally and clean up the face"

Using Glue Blocks

- Use sound timber for the block
- Make sure that the face of the block and the area of the work piece to which it is to be fixed are perfectly flat
- Use good quality glue such as cascamite or PVA glue
- Clamp the block and the work piece together and leave overnight
- When the piece is finished, always turn the glue block away. NEVER try to prise or knock the glue block from the work piece

Top Tips

Off-centre (or "eccentric") turning:
- When turning off centre, only take light cuts
- Do not apply any forward pressure on the tool. Remember that you will be cutting fresh air at times so any forward pressure on the tool will cause it to jump into the space
- Always start with a low speed and gradually increase the speed to allows for smooth cutting without any vibration
Sanding:
- Only use good quality abrasive. It is a false economy to use cheap ones. I use a looped back variety available in 100mm x 1000mm lengths in all grades. I am able to cut this to suit my Velcro pads for power sanding
- Abrasives lose their cutting edge so never use abrasive that is past its sell by date
- Always work through the grades in sequence. Do not miss out a grade. If having reached the final grade - 400grit perhaps - and you are not satisfied, go back to 240grit, for instance, but remember to go through all the grades again
- Speed for sanding should be less than that used for turning
- I have found that applying a coat of cellulose sanding sealer between each grade of abrasive often helps with the end result

Diagrams Click an image to enlarge