Why Use Aluminum for a TT Platter?

No Viridian it was not missed at all. As usual your need to be better and smarter than anyone else came radiating above all others. My apologies for insulting your imperical command of both language and knowledge.
Cheers

Aluminum (or aluminum alloy) were chosen many decades ago for a number of reasons depending on the company and its intentions. Aluminum is non-magnetic and easy to machine, shape, and balance. Copper or bronze is heavier, but is also more expensive, would take longer to spin up, and is more likely to bend out of shape.

And get this straight: The direct drive turntables were not designed for the DJ market; they were designed for the high end home audio market. Adjusted for inflation, the SL1200 was $300, equivalent to $1200 today. Most "nice" turntables at the time were $200-300. The SL1200 was designed around the SME 309 tonearm, and the SL120 version came with an SME armboard instead of a tonearm. DJs adopted SL1200s gradually because they're rugged and spin up to speed quickly. In the '70s, a 4-to-5 lb. platter was considered a heavy platter.

There are many things to commend an aluminum alloy platter, some from a design standpoint and several from a manufacturing standpoint. To control ringing Technics dampened the underside with a thick rubbery coating, and furnished a 17-oz 1/4" thick turntable mat to dampen the topside. This was later dropped to half the weight and thickness because the DJ market favored it. I have a heavy Oracle Groove Isolator sorbothane mat and it quieted my SL1210 M5G nicely.

Before belt drive really took off, aluminum was the platter of choice, inherited from the rim drive 'tables of Garrard and Dual. As mentioned before, it was easy to mill, balance and shape. It also has plenty of tensile strength for creating a thin rim to accommodate the pressure of the motor's idler wheel. Acrylic, MDF, and glass would not make a good platter material for an inner rim drive as implemented by Dual and Garrard.

So most of the reasons for aluminum are historical. If Technics had continued making turntables for the high end home audio market, they might have come up with an acrylic platter and better damping and isolation, but they didn't, so economy of scale dictated that they stay with the original design and materials and shift their market to the dance clubs.

As a former club dj and as current employee for one of the worlds largest aluminum companies here is my two cents worth.

First regarding DJs and direct drive tables

when you are a club dj you want to be able to cue up the record and have it accelerate to speed as quickly as possible on the break.

This is not true unless you are a bad dj and don't know how to mix. DJ's are attracted to the SL1200 for its ability to accurately adjust and maintain a steady speed with the pitch adjust. As a dj you want to mix the music such that the BPM's of the next track are in synch with the one playing so the change from one to the next does not disrupt the dancing of the crowd.

Second, upon purchase of a new table by a dj, the rubber mat that is provided is thrown out along with the rest of the useless packaging, then a felt mat is used to allow the platter to spin beneath the record such that the racord can be held in place or pulled back during the cueing process without disrupting the movement of the platter (it act as sort of a coupling). If you are scratch DJ you may even chose to add teflon record sleeve cut into a round shape between the felt and platter just to be sure there is no sticking.

Now that this is cleared up, high inertia could be benefitial for DJs provided the performance of the slip mat "felt coupling" can prevent the fly-wheel from slowing. So far the direct-drive quartz speed adjust is more reliable.

Aluminum or Aluminium, not much to add other then to confirm what has already been mentioned, it is relatively inexpensive, non-magnetic, easy to machine, cast, or forge and readily available in wide variety of sizes and formats. Copper has become extremely expensive. Steel is cheap but difficult to machine, magnetic and will it rust unless protected, bronze contains copper so it is also relatively expensive. Finally lead, is toxic. There also magnesium, berylium and titanium, these are expensive, difficult to machine (specialy for Mg, which the chips are flammable and potentially explosive) and toxic in the case of beryllium. So in terms of metals Al is pretty easy choice.

As for other materials such a plastic, carbon fiber or wood, I can't comment, but I would image they could be good choices for the given application.

02-09-11: Nick_sr
As a former club dj and as current employee for one of the worlds largest aluminum companies here is my two cents worth.

First regarding DJs and direct drive tables
when you are a club dj you want to be able to cue up the record and have it accelerate to speed as quickly as possible on the break.

This is not true unless you are a bad dj and don't know how to mix. DJ's are attracted to the SL1200 for its ability to accurately adjust and maintain a steady speed with the pitch adjust. As a dj you want to mix the music such that the BPM's of the next track are in synch with the one playing so the change from one to the next does not disrupt the dancing of the crowd.

things have probably changed quite a bit since the days when i used to go to clubs. in the 1980's disco scratching, mixing &c were much the rage.

"In the '70s, a 4-to-5 lb. platter was considered a heavy platter. "

Considered heavy by who? Certainly not any of the dealers or audiophiles I hung out with.

"The SL1200 was designed around the SME 309 tonearm"

I'm thinkin' the SL1200 was designed and brought to market before the 309 existed. Anybody know? I have sent an inquiry to SME.

Y'all be cool,
Robert