Vintage DD turntables. Are we living dangerously?

Dover.
The figure I used for Aluminium is a generic number we use for calcs here. If we want to be more precise we take into account the method of forming the material and of course the specific alloy. I did say "around 0.04mm"
The point I was making, was that the dimensional change is a tiny fraction of the several millimetres per degree c that Halcro sighted.

When I made the decision to build my TT plinth out of acrylic the problem of stiffness was considered. My TT uses 2 x 30mm thick Acrylic sheets, separated and fused to a lead spacer. By separating the two structural plates a form of 'I' beam is produced since shear between the two plates is strongly resisted.
This is a similar idea to that used in the light weight wooden 'I' beam floor joists. Individually the three parts of the beam are quite flexible. Gluing them together in this form however makes a very stiff structure.
The same principle is at play in plywood construction, which my TT copies.

The triangular shape with the motor centrally mounted puts the bulk of the material around the motor, concentrating the strength in that region. It then tapers towards the edges progressively reducing in strength as it approaches the feet. A square plinth, however, will be more sensitive to the problem you sight. If in fact it is actually a problem.

My room is air conditioned 24/7. Temperature is tightly held

It is a 25 year old design and I am still happy with it. That said, if I was to build a ground up TT today, just for fun, I would likely use different materials and architecture.

Richard - to change the subject slightly, can you elaborate on the nature of the "Krebs" mod that you are offering for Technics DD tables? Not asking you to give away all of your secrets but since you have been so forthcoming on such matters, I would be interested in a better understanding of what I would be getting for my investment. I have a SP-25 in the rosewood Technics plinth and have already upgraded the bearing (Jim Howard). Does your mod do anything to the bearing?

05-13-15: Richardkrebs
When I made the decision to build my TT plinth out of acrylic the problem of stiffness was considered. My TT uses 2 x 30mm thick Acrylic sheets, separated and fused to a lead spacer. By separating the two structural plates a form of 'I' beam is produced since shear between the two plates is strongly resisted.
This is a similar idea to that used in the light weight wooden 'I' beam floor joists.

By no stretch of the imagination could you refer to your plinth structure as analogous to an I Beam. I have never seen a lead sandwich described as an I Beam in the time I studied Engineering at University or in the 5 years I spent working for NZ's largest timber company. The middle of an I Beam has high structural integrity in the vertical direction so that the beams do not sag. The top and bottom of the "I" provide the lateral stability. Your lead spacer has no structural stability, it is not possible for this structure to work like an I Beam. Plywood in of itself is not structural, it flexes a lot. Flex in plywood plinths is reduced by running multiple layers and a lot of glue, but they still flex. If you want stiffness from plywood you would cut the plywood into strips and glue them with the board vertical ( as is done in I beams ), like Albert Porters layer of panzerholz in his plinth. The panzerholz is not used in sheet form, it is cut into strips and glued sideways.

Pod Stability

05-11-15: Richardkrebs
Halcro.
Check my math if you like, the numbers are sound.

A perusal of your numbers shows they have failed the sound check, and here is why -

05-10-15: Richardkrebs
There, one TT is mentioned where specific data is given on the amount of laser pointer movement per revolution and its distance from the centre spindle.
This TT is a beautifully engineered machine with, from memory a 22 kg platter driven by a fractional horse power motor via a thread. Hereafter I will call this TT. "TD" .The specifics were 2 mm movement on a radius of 400 mm, per revolution.
With this information it is possible to calculate the retardation torque and hence the drag. From this it is possible to calculate how much the pod moves.

This turntable was indeed my Final Audio VTT1 - which has a 26kg platter system - Platter 15kg, Subplatter 5kg, Copper mat 4.5kg, Clamp 1.8kg all designed to be used together to provide optimum clamping and energy dissipation to ground from the record. An AC motor, driven from a sine & cosine wave generator and Onix OA60 power amplifier drives the platter via a silk thread.

To be clear what was being measured in my post on the Timeline thread – the 2mm lag was generated by setting the TT speed with no stylus playing and then measuring the lag when playing. A 2mm lag at a radius of 400mm is a speed error of 0.08%

If I set the speed with a record playing, which is my normal procedure, then there is no speed error at all as measured on the Timeline, and therefore the variation in stylus drag due to music playing is an immeasurable % age of the total drag.

If we assumed that the variation in stylus drag is plus or minus 20% of the total drag (remember it is not registering at all on the timeline), then the string drive Final Audio VTT1 has at worst about half the wow and flutter of an SP10mk2, and about the same as an SP10mk3, without the induced negative effects of the servos.

Another small matter to correct re your post on the movement of Halcro’s POD –

05-10-15: Richardkrebs
The pods appear to be slightly crescent shaped. If this is the case the CofG will be biased towards the two feet closest to the platter. This will increase the tilt.

This is wrong. If a chunk of mass is removed that is in front of the feet (closer to the platter than the centre of the pod ) then the centre of gravity of the pod shifts back AWAY from the platter. This INCREASES the resistance to tilt.

In summary, your maths is wrong because your calculations are based on a misinterpretation of my Timeline test results.. When the 26kg platter speed was set with the stylus playing, there was in fact no measurable retardation.

Dover.

The essence of an I beam and ply is the separation of the two outer layers (sheets). Any bending causes compression of one sheet and tension in the other. This increases the stiffness of the structure.
As I said earlier. It is a 25 year old design. Were I to design one today, it would use different materials and architecture.

(Dover "To be clear what was being measured in my post on the Timeline thread – the 2mm lag was generated by setting the TT speed with no stylus playing and then measuring the lag when playing. A 2mm lag at a radius of 400mm is a speed error of 0.08%") and ("In summary, your maths is wrong because your calculations are based on a misinterpretation of my Timeline test results.. When the 26kg platter speed was set with the stylus playing, there was in fact no measurable retardation.")

My post calculates the retardation torque required to cause the slowing, from stylus raised to stylus lowered and then applies this to the pod.
What happens AFTER you adjust the speed with the stylus lowered makes no difference to the calculated pod movement.

Re the CofG of the crescent shaped pod. I didn't make this note clear. It appears that the feet have been moved back towards the rear of the pod, moving the CofG closer to the front feet. The photos are a little ambiguous. If this is not the case then yes the CofG will move away from the front pod feet reducing the tilt.

The platter is heavier than I thought. If its radius of gyration is as I assumed, this extra weight will increase the torque required to slow it and by definition increase the pod tilt.