Reverse curve

This reverse curve was made from annealed aluminium. The softened material was much easier to shape than that of the bowl. I suspect that the non-annealed blank was part of the reason why my first bowl split. 

The curve is very shallow because I didn’t shape the blank in a smart way. Here’s how.

I used a small-domed nylon mallet with the stump to shape the curve that runs along the workpiece, and attempted to create the transverse curve at the same time. I think this is where I stuffed up, because by shaping the longitudinal curve, I work hardened the material and created stiffness along the workpiece. This meant the transverse curve was difficult to create, as the workpiece gained a resistance to bending in that direction.

I should’ve created a deep transverse curve, forming the basis of the entire piece, then moved onto the longitudinal curve, which would’ve stiffened and set the overall shape in place. This is as Wray Schelin demonstrated in one of the recommended videos (below).

I didn’t realise this at the time, unfortunately, so I continued with shaping the piece. I used a nylon mallet with a steel pipe to smooth and even out the longitudinal curve, which cause some problems later with finishing. The pipe was slightly damaged, which left significant scratches and dents in the workpiece. I kept switching between the steel pipe and the stump to deepen the longitudinal curve.

I moved to the horseshoe dolly to create a nicer curve on the edges of the workpiece, which proved to be time-consuming. One issue was that the longitudinal curve was not completely centred. This meant that the corners of the workpiece were too wide on one side and too narrow on the other. I now had to dance between the steel pipe, the stump, and the horseshoe dolly to get the shape as I wanted.

(Process videos: A. Sudarsono)

I managed to get the ends of the longitudinal curve much closer to the centre, and noticed a new issue while trying to do so. The centre of the workpiece was flatter than the sides. Perhaps this originated in the previous stage, when I worked mainly on the ends. I flattened the ends out to compensate, and checked that the width of the workpiece was even the whole way across. I couldn’t get it to be perfectly even, but it got closer than before. When I did that, I had to re-centre the longitudinal curve as the edges wanted to go back to their original, uneven shape.

When I was satisfied with the shape (as much as I could be at that point), I went on to planishing. There was an unexpected effect that this gave to the surface, as the hammer marks were long oval shapes, and not round. This may have been because I used long forms to planish on, such as the steel tube and horseshoe dolly. While I put in a lot more marks than I took out, I like the effect that planishing creates in this instance, as it’s reminiscent of eucalyptus tree bark. 

Overall, this reverse curve had composite problems. As I was fixing one issue, it would cause another issue to appear. I have a feeling this is because I started it off in a bad way. I was also reluctant to use the shrinker/stretcher because it left marks in all the test pieces I tried it on. Later on, I looked up how to use it without marking the workpiece, and many forum posts suggested to simply not use the shrinker/stretcher on areas that will show. Others advised to rig up the jaws to mark the material less intensely, such as by covering the jaws with sandpaper, grit-side against the workpiece.