Vintage DD turntables. Are we living dangerously?

Why Halcro's favored topology, which is not at all original to Halcro, has anything to do with Copernicus, I have never understood, but that's ok. It's catchy.

Now that's a good question to put to the Copernican Thread....😎

05-10-15: Richardkrebs
The linear temperature coefficient of expansion of Aluminium is 0.000023m/m degrees C and is indeed approx. double that of steel.
Using this figure on a LO7D and assuming a 5 degree C delta, we get a change in distance to spindle of around 0.04mm.

This is wrong. When examining dimensional stability one needs to take into account materials engineering. If aluminium is produced by a rolling or extrusion process, then the dimensional stability is directional - much lower along the roll and higher across the roll. Furthermore if a metal is produced by a casting process then it is usually substantially more dimensionally stable than that manufactured through a rolling/extrusion process.
The L07D is a cast foot. I think you will find the engineers went to great cost to produce a cast for this very reason.

Richardkrebs - one thing puzzles me. You made your turntable from acrylic sheet, which has a Youngs modulus 60 times lower than steel ( which means it is 60 times less rigid ) and has a temperature coefficient of expansion 6 times higher than steel. Given that your TT is a triangulated structure and the SP10 motor is mounted in the centre of the acrylic sheet, it would appear that you have mounted your SP10 motor on a trampoline. This seems at odds with your stated design goals of absolute dimensional stability.

Furthermore, on the plinth you made, the arm is mounted closer to one of the three structural legs. So not only is your TT plinth expanding and contracting at a higher rate than say a cast chassis like the Melcos & Microseikis of this world, or even the L07D, your VTA is constantly changing due to the large differential in vertical structural rigidity between the centres of gravity of the SP10 motor and the arm. In layman's terms in your plinth the SP10/platter will move up and down at a greater rate than the tonearm when excited.

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.