I got a commission to do some paintings onto an elliptical enamel plaque. The major and minor axis of the ellipse are to be 150mm by 90mm, respectively. The paintings are to be of a house in each of the seasons, so four paintings in all, and each one is to be inset into the seat of handmade bar stool.

I think enamel-on steel is probably the most appropriate way to do this, the steel doesn't need to be formed to keep it's shape during firing, so this is most appropriate for insetting into the seat of a stool.

The steel I'm using is 0.8mm thick low-carbon cold rolled steel for enamelling (EN 10209). I'm cutting out eight ellipses to give me plenty of extra plaques in case things go wrong! (see picture above).

I'm going to mill the steel into the precise mathematical shape, so I'll need to clamp it well, for this I'm using some plywood above and below the steel:

Here I'm roughly cutting it to shape.

The steel in sandwiched between the two pieces of plywood and then clamped onto the table of the Sherline mill.

The mill can only cut a section of the ellipse due to the clamping, so I have to cut the back first and then move the clamping to do the front.

Once the back of the steel is cut I carefully change the clamping set-up and cut the front.

I've used a piece of brass to cover the plywood here because I've got a different clamp for the back. These clamps have a lower profile so the mill can move over them without crashing. The clamping was fixed into place whilst the previous clamping set-up was still in place, and then the previous clamping was removed.

I wrote a g-code programme to control the mill to cut out the ellipse. I wrote it so the mill makes tool paths that are progressively closer to the desired ellipse. For ease of use, when planning the tool path not to crash through the clamping, I made the programme so the cutting starts and finishes at chosen angles around the orbit of the ellipse.

I've only a small mill, but it does a good job!

The ellipse is perfect despite having to change the clamping position and do the cut in two goes.

Just need to deburr it...

OK that worked out alright. I repeated this three more times to iron-out any problems then did it again for four at once to complete the set of eight:

It was a bit more stress on the mill, but by changing the depth of cut and cutting speed it was able to cope

I did the same as before by cutting the pieces using two clamping positions, here's the front-cutting clamping position:

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This gave me eight ellipses in total ready for the next stage.