TURNING THIN WALLED BOWLS - by Graeme McIntyre

When any cross grain bowl is hollowed out the stresses locked up in the remaining wall thickness are relieved and the bowl goes oval. This becomes more apparent as the wall thickness is turned progressively thinner. In order to eliminate the top edge from appearing thick on the end grain and thin on the side grain, a special technique is required to turn thin walls.

 Firstly when the outside shape has been turned using a faceplate or between centres and the vessel is then fitted into a chuck for hollowing, the outside must be turned again to ensure it is running exactly true to the chuck. Fitting a wood spigot or socket into a steel chuck invariably results in uneven crushing of the wood fibres. This causes misalignment of the bowl and hence the need to skim the outside surface to suit the mounting in the chuck. Wear in the chuck scroll, slides and jaws, also contributes to misalignment. The outside surface should be sanded to a finished surface at this stage as once the inside is removed the outer surface will once again be out of round and it is easier to do now while it is running true.

 Secondly the inside needs to be removed in 4 or 5 steps with each step going out to the final wall thickness. The first step should be about 15mm down the wall and this will ensure that the top edge is of uniform thickness as it is being supported by the thicker wall further down the bowl. Each following step can be slightly longer. Each step is taken out to the final wall thickness, with frequent measuring to verify how much wall remains, until the bowl is fully hollowed. As each step is finished it will become apparent that the top edge of the bowl is moving more and more out of round. Some small ridges may be apparent at the junction of each step but these are easily blended by power sanding.

 The reasons for making thin walls are varied but two of the prime ones are when preparing pieces for piercing, where a thin wall is much easier to cut, and also when finish turning wet wood with a natural edge top.

 Graeme McIntyre

 Returning the outside profile               Holowing, the first step                    Hollowing, the second step            Hollowing , finished

                                                                                     Lace  9  the finished  product