Why Use Aluminum for a TT Platter?

"But compared to other 200-300 turntables of 1975, the platter of the SL1200 was pretty heavy."

That may be but I remember thinking something like a 12 pound platter was heavy.

"Sorry. I meant the SME 3009"

I should have thought of that. Response from SME:

Regarding the SME 300 Series we can confirm production began in 1988 as an addition to the SV ‘range’ which of course includes the SME SIV. The 309 arm remains much the same apart the introduction of magnesium arm tube approximately 11/12 years ago. This change was to bring the arm tube into standard production alongside the SV and IV but clearly is also an upgrade to the 309. The 309 today is one of our most popular arms and provides many of the sonic benefits so often referred to in HiFi press of the SV tone arm.

Chris/Teres,
My understanding of the technical term for cogging means either slots or
cores are required (though I could imagine where badly-designed slotless
coreless motors could have other iron present. I assume that the 'cogging'
issues you mention on slotless AND ironless motors comes from the fact
the stators are not infinitely small. Does it also come from coil material
saturation/resistance of some kind (i.e. Induction is not perfect because of
materials issues)? Is there something else I am missing?

I completely agree that in the end it comes down to execution.

Copper is the best for platter top!

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?