
Hi John, I don't know if this is any help as it's copper, not aluminium, but a splendid chap from the Coventry MES told me how to do this when I admired a model of his at the Large Scale Model Rail show at Leamington last year.
I put an offcut of copper water pipe into the lathe and knurled it, and whilst it was still spinning I took the sharp edges off with emery. I then opened out the pipe along the long axis, and a very crisp, smart piece of chequer plate emerged. The pipes in the picture are 15mm and 22mm.
I hope this is of some help.
John.