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uncle_bob
I’m not planning on using the aluminum plate for structure. It will just sit on top of the plywood and act as ballast.
34 lbs seems light for ~5 foot tall structure, is there a future use for it besides ballast?
Heating a large mass will aid in maintaining a consistent enclosure temperature.
(Cool aluminum disk(tu))
I'm using cement, and to achieve a flat surface, it will be cast on top of a pane of glass.
It's possible that the open mold side could be made parallel if you level the pane of glass beforehand,
and add a layer of self leveling cement.
I'm estimating that the total weight will be between ~50 to 100 lbs.
When the assembly drawing is completed I'll know where the center of the mass is located.
I'll make adjustments at that time.
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uncle_bob
I have not settled on an exact lay up yet.
If you sandwich fiberglass cloth and epoxy between the plywood that will help keep it stable.
Orientate the plywood to a 90 degree bias of the skin grain.
I imagine that glue alone will work very well.
Wood is among the most thermally stable materials in the world, exceeding steel by a factor of 2.
But moisture absorption disrupts the dimensional stability of wood,
mostly tangentially, and radially, expansion due to moisture parallel-to-grain is very stable.
So sealing the wood is a good practice, after drying it.
I'm using cedar for the arms.
Its coefficient of thermal expansion parallel-to-grain is 0.0000028 in./in./degree F.
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