David Merriman's 57" Seaview part 2b

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	David Merriman's 57" Seaview part 2


	Here the first half of the RTV has cured out, the masters were removed, the clay backing scrapped away, and the masters re-installed within their cavities. Mold release spray was applied, a masking tape damn wrapped around it, and the second half of the RTV tool poured.


	The two halves of the two-piece disc tool were then pulled apart and the masters removed and placed in storage. With a sharp knife I cut out the channels (runners) between the central sprue hole and base of each cavity. These runners permit the passage of the metal from the tools center outward into each cavity. (I accidentally broke the antenna master during its extraction from the tool. The horn of the master hung up in a tool half and when I pried just a bit too much, the antenna screen snapped off the mast. But, not to worry: the antenna master had done its job of giving form to each tool half. If needed to make another tool, this master will be easy to repair).


	The initial metal casting tool (two more would later be made containing cavities for the Observation Compartment fittings and girder network) took the form of a two-piece disc that arrayed the masters around the periphery of the tool. Centrifugal force would drive molten metal from the central pouring sprue out and into the cavities. Here you see the two halves of the completed tool next to the masters used to give form to the tool cavities.


	The Spin Casting Machine My 'spin casting machine' is nothing more than a modified blood separation centrifuge bought second-hand at a local surplus store. Pay particular attention to the bowl shaped shield that surrounds the spinning tool in the photo: A very, very important safety device! Ellie and I have both been burned by molten metal slung from an 'exploding' tool that unseated during a pour. As I review the above text, I glance at a five inch festered scab on my right leg, from the knee up - my most resent reminder that molten white metal and flesh do not mix well! Fact is, sometimes these tools leak, and when a disc type spinning tool leaks, it throws molten metal, at high velocity, EVERYWHERE! Hence: the spray shield. But... as they say, that's life in the big city. Note the two little holes either side of the central sprue hole on the tool? Securing studs set into the mounting plate of the machine pass through these holes. A tool is set down on the mounting plate, the studs passing right through the tool. Over the top of the tool is fit a circular metal plate. Thumbnuts are then run down the studs, sandwiching the tool between the two metal plates, compressing the tool halves together, holding them securely onto the motor shaft mounting foundation. The metal I cast is white metal - the same stuff you get today at hardware stores labeled as 'leadless solder'. White metal is typically 95% Tin, 5% Antimony, or similar ratios of the two. Very easy to get - you don't have to go mail order to any specialty house to get this alloy!


	You'll notice the container of Baby Powder and a brush near the machine. Talc absorbs water. Water, even microscope droplets condensing onto the tool cavities while their open, is the biggest reason one gets dimples and distortions on the surface of cast metal parts. Every time a metal tool is prepared for casting the open faces of each tool half is sprinkled liberally with talc, the powder pushed into all cavities with a brush. The talc is then knocked loose by shaking the tool halves vigorously (Not blown off, that would contaminate the tools with water again) and then assembled. The spin casting tool with a white metal shot of parts between the two halves. Note that all this sits atop the modified blood separation centrifuge I use to spin the tool as I pour in the molten metal.


	Examine the photo and you see a cast 'tree' of parts just pulled from the disc tool, the tools two halves sitting askew on the spin casting machines mounting plat. I'm always amazed at how well the molten metal does at reaching into even the tightest crevice within a tools cavity. The radar part is a good example of the utility of spin casting: the centrifugal force generated worked to drive the molten metal into this very tight and convoluted cavity, achieving a complete fill and a perfect cast metal piece. A wonderful process! Just don't get the molten metal on you. I have been questioned as to how rough the metal casting process is on my rubber tools. The answer is a simple one: metal casting, using white metal anyway, seems to degrade the rubber tools little. Now, let me qualify that statement: I use an RTV silicon rubber designed for high temperature work (BJB's TC-5050 - the same rubber I use for my epoxy and polyurethane resin tools). Not just any old silicon RTV rubber will do. The BJB TC-5050 will withstand continuous temperatures of up to six hundred degrees. Proof of point: Some of my old metal casting tools have performed hundreds of shots and the parts that come out today are as crisp and flash free as the parts produced from these tools ten years ago, when they were new. (Only wish that polyurethane resins were so gentle on the rubber!) Now, I should report that the older metal casting tools are a bit 'harder' than when new. But, that has not diminished their ability to register when assembled or to produce parts with all the detail as when the tool was new. The metal parts produced for the Teskey/DeBoer SEAVIEW enhancement package are trimmed away from their attached runners with the aid of side-cutter, flash filed back where needed, and the parts pickled, primed, and painted. Part 2 continues

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