Why Use Aluminum for a TT Platter?

I had a Technics SL-1100. I think it had the distinction of being the first DD machine made. It also had the heaviest platter of any DD machine, including the SP-10 and certainly heavier than any Dual, Garrard, Thorens, or the like.

The 'cogging' thing is not an issue on the better DD 'tables. However I should point out that this is a problem with any servo-controlled motor (often found on belt-drive machines) if the servo is not designed properly. The result can be that if you graph the overall speed of the table over time, the result will be something that looks like a sine wave, centered around 33.33 rpm. The slower the sine wave and the less amplitude it has the better.

Subtle speed variations like this are heard as a loss of focus in the sound stage due to variable skating forces on the arm. This is why a belt drive, using an AC synchronous motor, might be preferred over some of the cheaper DD machines. But to assume that all DD machines have this problem would be a mistake.

"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.