A rotary table is an extremely useful accessory for a milling machine; it allows you to combine straight and curved elements in the same piece (such as making rounded corners), cut details at precise angles (such as when making gears) and a variety of other tasks. However, a rotary table by itself has limited utility as parts must be clamped directly to the table, which in my case is quite small. With this in mind I made a chuck adapter. This allows me to hold a wide variety of parts both round and square, and more importantly allows me to transfer a workpiece from the lathe to the mill without re-alignment.
I began by making a foam pattern of the part, slightly oversized to allow for shrinkage and turning operations. I then buried the foam pattern in sand and poured molten aluminum into it, using a steel can half-buried in the sand as a funnel. I then allowed the casting to cool, removed as much of the sand as possible, and cut off the excess aluminum with a hacksaw. The process from that point onward can be seen below. One detail to note is the center hole and tapered pin, which makes aligning the table and adapter trivial.
To improve this design I would probably add a flange at the bottom to widen the base of the adapter, and secure it using four T-nuts instead of two. I don't intend to do any heavy milling operations using this adapter though, so it should work acceptably.