Turntable platter from plaster

[john blackburn]

magentawhale.

You will need to seal the mirror/mdf join as it is very fine when mixed, capiliary action will draw it into any gap. Plain liquid latex will be ideal for this as it peels easily to remove. Any flashing you do get will snap off easily if thin and then scrape to finish.

When you mix it, pour the mixed plaster into a clean jug before pouring it. This will leave any poorly mixed plaster which has stuck to the mixing vessel behind and give no lumps. If the mould is precisely levelled up before you pour it should give a fairly parrallel result. It mixes very easily as the plaster is so finely ground.

When you pour it you should pour as slowly as possible allowing it to find it's own way out fron the centre of the mould. The slower you pour the more time the air bubbles have to rise to the surface. you can get a finger in there and push the mix into any corners to ensure no bubbles are trapped in the angle.

If you blow gently on the surface as you pour, it will pop the bubbles quicker than if you leave them to pop by themselves.

Good luck

John

[wakibaki]

I cast a few things. Aluminium is fun. You can smelt it in a stainless cup in a furnace made from a flowerpot full of barbeque charcoal, stuck in a large can with the top cut out and a little hole in the side, with a vacuum cleaner as a blower.

I think you may run into some problems casting an object with this shape in this orientation. Better to cast it on edge and deal with the sprue, than end up with a whole lot of bubbles which are hard to get to move toward the hole. Casting on edge has its own problems, as the mass distribution may be uneven. It may be anyway as has already been pointed out. 'Course you can have more than one hole, but then you have multiple sprues.

Better in some ways to leave the top open and let it level by gravity... you'll probably get the small bubbles at the bottom and the big ones at the top. You could turn it upside down in use.

Whatever, some kind of vibrator can help with the bubbles.

You can probably turn it on a potters wheel for levelling. Just secure it at the edges with 3 lumps of soft clay, and work gently. Or ask the potter.

While plaster is a safe and accessable material for casting, I think you'd be better making a turntable from something that starts out flat and is fundamentally homogeneous like MDF, and think about a different casting project, maybe a tonearm plinth in clear acrylic?

I made some plaster Disney characters for the kids a few years back. One thing for sure, your feel for the process develops, so you need a few practise goes.

w

I'd probly start with a large cake tin, overfill it, and quickly remove the excess with a wet straightedge. You'll get some idea if you're going to get any shrinkage.

[mmerig]

Making a mold and then having an unprotected disk of plaster of paris may be more trouble than it's worth. The plaster is not that durable (chips), and does not have much tensile strength either (so it could crack).

It may be just as cheap to get a metal platter that will fit your spindle and fill it with the plaster. Once it's dry you could flatten it using sandpaper wrapped around a board. This is what I did, and although the bottom may not be "within microns flat" and perhaps very slightly out of balance, the effect on sound would likely be negligible. My turntable has a servo DC motor and a meter that shows speed deviations. I don't see any fluctuations (and 0.05 % would be noticeable on the meter). It sounds good too.

Just my two cents.

[ak_47_boy]

I think plaster will warp and crack during drying.

[bear]

Investigate a supply house for those people who do "pottery" "ceramics" and other things that use clay and gypsum products.

There are a wide range of gypsum based products that are nothing much like the "plaster of paris" that is used on walls in your home.

Some of it is intended to be mixed and poured for casting. Some of it has higher tensile strength than concrete! Some of it is used for repairing bridge roadways!

Most of it is rather resonant when hard and dry...

The quality of your casting depends on the mold.
Learn about how to make molds...

It's all online too...

_-_-bear