Why Use Aluminum for a TT Platter?

Cogging is measured as torque ripple. It is the amount of variation in the developed torque as the motor rotates. A single phase AC motor by definition has 100% torque ripple since at the zero crossing of the waveform no toque is generated. DC motors have much more constant torque and typically will have torque ripple values of 10 to 15%. The motors used in DD tables have multiple overlapping phases that dramatically reduce torque ripple. In theory a three phase motor will have less than 1% torque ripple. But that is only if they are perfectly constructed and driven with perfect waveforms. But even a crappy three phase motor will have relatively low cogging compared to a single phase AC or DC motor.

Typical motors will have windings wrapped around a laminated iron core. This focuses the magnetic field and makes the motor more efficient. Core-less motors (AC or DC) have copper windings that are formed without a core. They have less cogging because there is no attraction between the magnets in and the iron cores. Cogging can also be reduced by angling the cores so that they have equal attraction between the magnets and the cores as the motor is rotated. You can tell how much the cores affect cogging by turning the motor by hand without power. Core-less motors are less powerful, more expensive and usually have less cogging.

Chris,
I was wondering what physical phenomenon generates the torque ripple in a slotless, coreless motor. Any thoughts?

Copper is the best for platter top!

There are problems with any metal for being the top of a platter. The top of the platter should be available as a means of damping the vinyl as it is tracked by the tone arm. Metal is ill-suited to this task.

Copper is a special condition that likely should be avoided. In the presence of a magnetic field (perhaps the one in the cartridge itself), the spinning copper will form a sort of primitive generator, and could easily be responsible for extra noise in the playback.

I wonder if Cocobolo is such a good sounding wood if one can't invent a cheaper Cocobolo plywood. I guess the glue and compression would have an effect.
Other alternatives are the numerous sandwich type composites where the upper most layer being your favored material with the easily machined, cheap but invariably aircraft grade aluminum, that is bragged over is used as the center then something like another dense wood or composite like paka or tank wood.
I personally don't think that well encased lead does not pose danger once it is properly manufactured. How about concentric brass rings embedded in the bottom layer. Perhaps a contest with the few remaining lucrative TT companies sponsoring the best platter material yet.
I noted that no one suggested melamine. There is an asphaltic roofing tile that might be good if it could be made hard enough stabalised and used as a center layer. You get the idea. It's like any recipe contest.