Why Use Aluminum for a TT Platter?

Livemusic, Yes Cocobolo is still the best platter material I have heard, but Garolite sounds nearly identical and is far less expensive.

Tbone, Coreless, slotless motors have very low cogging, but it is not zero. A top quality DD table is remarkably sensitive to even extraordinarily small amounts of cogging. I have no first hand experience with the fine motors you cite, but suspect that the very low cogging they exhibit is still audible.

Higher platter mass generally translates into better sound. But as is often the case with audio there is no consensus. Heavy platters sound different than light platters. Most but not all prefer the sound of heavier platters. I have experimented with platters up to 75 lbs. For my tastes I found that heavier was always better and never found a point of diminishing returns. But anything beyond 75 lbs starts getting very impractical.

An interesting discovery was that both belt drive and DD implementations seemed to benefit from heavier platters. But with DD the benefit from a heavier platter was significantly less than with belt drive.

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!