Casting The Turntable Platter - Sweep Pattern Experiment

The next step for the turntable project was to make the platter.  This was the second of the two cast aluminum components to make, and unlike the Technics which is die cast, this too would be sand casted.  However, this time I chose a different molding method called "sweep casting" which is applicable to radially symmetric parts like this.  This was my first time trying this, and I had relatively favorable results.

The sweep pattern is basically a cross section of the part being casted, which is swept around a central axis to remove pre-rammed sand layer by layer.  Think of it like those center pivot irrigation systems, except instead of watering crops, we're removing sand to create a mold cavity.

This has the advantage of saving time since I didn't have to make an entire wooden replica of the turntable platter.  I began by transposing the final dimensions of the platter, plus shrinkage allowance, into a flat piece of plate.  Pardon the atrocious blueprints

Machining The Turntable Base Casting

Now that the turntable base has been cast, it's time to machine the features to make it functional.  I started by setting it up on the milling machine upside down with spacer blocks underneath, and clamping through the middle.  In this setup I faced the bottom contour.

Here is a closeup of the finish, now the base will sit flat for the next setups.

Casting The Turntable Base

As much work as building the pattern for the turntable base was, this was only half the battle when it came to making it.  Casting the actual base posed a whole different set of challenges.  The first of which was the need to construct a flask big enough to make the sand mold.  Standard turntable size is bigger than anything I'd casted, about 14.5"x18", so the flask had to be at least this big with extra room for sand all around.  I decided to fabricate this monstrosity from scrap pieces of steel plate.  I bought a large piece of rectangular tubing and cut it up to create the rounded corners.  For the parting surfaces, I originally welded a strip of 1/4" steel on the edges, but due to problems of the steel deflecting and not aligning, I resorted to screwing small pieces of angle iron onto the strips to form a good flat edge.  On top of all this trouble, the flask ended up being really really heavy.  I was originally going to weld ribs all around the outside perimeter to prevent the walls from bulging out when ramming in the sand, but I either ran out of material, or figured it was already way too heavy.  Even with no sand inside, it almost needs two people to lift it.

Here is the pattern in the flask ready for molding.  Ideally, there would be a bigger gap between the flask and pattern walls so the sand would have more support, but I wasn't interested in making this thing even heavier.

Building The Turntable Base Pattern

I made the turntable base from cast aluminum.  Unlike the Technics 1200 which has a die cast body, I sand casted mine.  This is because die casting is suited for mass production and requires a permanent mold, which is hard work (if you don't believe me, see the soap dish mold project).  Actual die casting also requires really expensive equipment to hydraulically clamp the mold shut and inject the liquid metal under very high pressure. Clearly I wasn't going to go through all that trouble and expense just to make one or two turntables, thus sand casting was the natural choice.  In order to cast it like this, I had to construct a pattern for making the sand mold.

First let's discuss the design of the turntable base.  I chose this weird half-rectangular, half-circular shape because it seemed easier to mold than if it were a rectangle.  It also required less material.  I was determined to cast the base in one piece, and without the use of any sand cores (because for some reason, my foundry technology level hasn't reached that point yet).  The base also had to have this big hole in the middle for a drive belt to go through.  I chose belt drive since I didn't feel confident enough in my electrical skills to attempt a brushless direct drive motor like the Technics.

Now let's get into the pattern.  I started this project years ago.  You may remember a video of me cutting a 16" diameter piece of wood on my lathe, well this is what it was for.

3 "Impossible to Kill" Vegetables for Beginning Gardeners

If you're looking to start gardening but you don't have a green thumb, sit back and watch. You should have no trouble growing these three vegetables as they easy to grow and are tough to kill with neglect. And the best part is: all these can be grown in containers!