Tuesday, April 13, 2021

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.

The next problem I encountered involved the big hole in the middle of the pattern.  I was told in Foundry Academy that I could mold patterns with window pockets like this by ramming the sand from one side, then scraping down the excess on the other before ramming the second half.  The idea was that if you put enough parting compound, then the two halves of sand wouldn't stick together- WRONG.  I think this idea would work under the proper conditions, but I had several factors working against me here.  Firstly, I was using inadequate sand.  I thought what was left of the 100 lbs I had would be enough, but it didn't even fill the flask.  In a pinch, I bought the closest, finest sand available from the hardware store, which was play sand.  This of course was too coarse and shouldn't be used for sand casting, but at the time I was getting desperate and wanted to finish the project.  The second factor was the awkward shape of the casting.  While a draft analysis would reveal no undercuts where sand could get stuck, there are parts where the sand had to be very tall and narrow, or otherwise not have the proper support.  The coarse sand compounded this problem, since it wasn't as cohesive.  Thirdly, if you think about how I had to ram the sand, the pressure through the middle of the pocket would just break off the big chunk from the other side that needed to be supported.  See the diagram below.

Here is the pattern right before ramming the second half of sand.  Notice how I meticulously scraped the protruding sand to a nice conical shape. 

I attempted molding it like this two or three times before giving up.  Still wanting to cast something that day, I decided to pour metal into the remaining half of the mold that was partially intact.  This resulted in a big half-turntable-shaped ingot for remelting later.

To fix the molding problem, I decided my best chance was to seal up the window pocket in the pattern.  I fabricated a sheet metal "shield" to act as the parting line.  The metal was strong enough to support the sand ramming, but thin enough that metal would not flow through and fill the gap.  This shield had to be a weird shape, so it took some TIG welding and fabrication skills.

I managed to simplify the shield design into two pieces, here is the first one.

Then after positioning and tack welding it in the pattern, I made the second piece to fill the remaining gap.

Here tack welding it all in place.  I did the remaining welding off the actual pattern as to not accidentally light it on fire.

I then ground it smooth as possible and filled in the corners with hot glue, and some chunks of wood.

Firing up the foundry.  This photo shows two more modifications I had to make to cast this.  The crucible seen in the background had to be extended to hold more metal, so I welded some pieces of steel to make it about 2" taller.  Also the pouring shank was modified so it could be lifted by two people, due to the increased weight.  I discuss these and other concurrent foundry upgrades in this video.

I didn't get any photos of the final molding, but here is the flask after the aluminum was poured.  It turned out I really overestimated how much metal I needed, and probably didn't even need to extend the crucible at all.  There was still enough left over to fill most of my ingot trays, even after spilling some on the ground.

Shaking out the casting.  Yeeeeeah boi!!!!!

At last!  It finally came out correctly.  Here it is after cutting off the sprues.  I still had a lot of cutting and grinding to do to clean it up.  Notice how only part of the sheet metal shield portion filled with aluminum.  Also note the layer of sandy looking stuff in the corners on the platter support platform.  This is actually vitrified parting compound.  When molding, I was really not interested in the mold failing a third time (since it took at least an hour to make the mold), so I literally poured parting compound into the corners to functionally increase the corner radius and aid the pattern release.  Well, it worked, but it left a pretty thick layer of this stuff.  Grinding it off was interesting because it would slightly spark, unlike the aluminum beneath.

Next Step - Machining The Base

Previous Step - Making The Base Pattern

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