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A HOME-BUILT CHARCOAL-FIRED FOUNDRY

by Gene Elliott (c) 2000  (edited by Steve Kaehler)

gene@harbornet.com

(Cautionary Notes & Warnings)

Each spring SRS folks around Seattle talk about and sometimes pull off planning a summer camping expedition into the great outdoors called Great Escape And Retreat (G.E.A.R.) to sit around a campfire, enjoy the beauty of nature, and talk about robots. We happen to live in an area surrounded by campgrounds and parks that beg to be used.  Not wanting to ignore the "call of the wild", G.E.A.R. happened this year at South Whidbey Island State Park from July 28 to 30. During this weekend, I made some simple aluminum castings using a home-built, charcoal-fired foundry. This article describes how the foundry was built and what was done at the camp out.  The commemorative SRS/POBO/OSER 2000 G.E.A.R. medallion above (left) was cast in a sand mold made using the wooden pattern (right).

Since I’ve always wanted a small, portable foundry set-up that I could take along for field demos and other special projects, building this furnace gave me the perfect excuse to provide my very own portable small parts casting foundry. The picture shows the main apparatus involved.  From left to right, the molding flasks, foundry, the blower hose, and blower.

I happened to have an old, empty 30-pound Freon bottle lying around, so I cut off the top with a carbide rotary cutter (from Harbor Freight) and drilled and cut a hole along the side for the Tuyere (blower hole) pipe. I welded in the Tuyere pipe, mixed up the refractory (heat-resistant) compound and filled it to the bottom of the blow hole, inserted the wall mold, and packed in more refractory.

The refractory lining is a cement-like substance that can withstand the high temperatures involved with molten metal. I recommend using the commercial stuff rather than brewing your own. I bought mine from the Pyramid Product Co. for my first furnace, but attempted to brew my own for this one. I used "Fire Clay" (tan colored powdery mixture used in fireplace building) from the local brick supplier. My original mistake was using ordinary concrete mix for the cement since the rocks cannot withstand high temperatures, exploding and spalling the sidewalls and lid (popping out large chunks). To overcome this problem I lined furnace with "K-wool" wet blanket which is commonly used in oil furnaces as the liner. I filled the cracks and spalls with a pre-mixed refractory patching compound that I bought at Johnstone supply.
 

The lid was made from a strip of soft sheet iron bolted at the seam with holes drilled every few inches. For the lifting device, I put some tabs equally spaced around the outside and wove wire through the holes for support of the refractory (much like the wire mesh used in sidewalks and floors to hold things together). For the lid hole, I cut out a round piece of pine about 3" diameter (large enough to drop in an occasional aluminum can).When everything was dry and molds removed, I built a small fire inside with newspaper and small sticks to start the refractory compound curing process (this is when I discovered my mistake with the concrete mix).  Once I removed the concrete and patched the refractory, things worked fine, so now the fun begins.

I built my molding flasks (cope & drag) from cheap 1x4 pine about 8" x 10" and installed guides on the sides to keep things aligned. The Ingot molds (far left) were attached to a piece of fiber board and have a nice draft so the pattern can be easily pulled out of the sand. The sand I used was "Petro Bond" also bought at Pyramid, it's ready any time you’re ready to make a casting, requiring no mixing or fuss, and it can be reused over and over.  The parting compound (white looking powder on the sand in the molding flask picture) is used to help release the pattern from the sand. My melting pot  was a small cast iron pot with the handle cut off and bolted on the back to help when lifting and pouring the molten metal. My original thoughts on the ‘melting pot’ came from this old photograph though I downsized it a little from the picture.  I drilled and tapped holes and installed carriage bolts for the lifting areas, while coat hanger wire was used for the actual lifting, stirring, and venting as the foundry worked.

So I set up the foundry, threw in about eight lumps of BBQ charcoal, splashed on some lighter fluid, tossed in a match, put in the pot, and covered it with the lid. When the flames died down some I started the blower (an old hair dryer) and gently dropped in some aluminum to start melting. It took about 15-20 minutes to get a pot full of molten aluminum. The sand molds for the object being cast and the ingot molds were ready. From previous experimentation, I've learned to pour steady and quick to fill the molding flasks with the hot metal and then pour the excess into the ingot molds for reuse next time.  The original pattern and untrimmed casting results are shown below.
 
 

The charcoal-fired foundry is an idea developed by Dave Gingery who has written several books on the subject of foundry work and the results. These books can be purchased from Lindsay Publications, Inc. (P.O.Box 538, Bradley, IL 60915-0538. Ask for No. 163, Charcoal Foundry by Dave Gingery.)

This is not intended to give you all the knowledge of sandcasting and the dangers (almost none if one is careful), but to excite everyone with the possibilities of making things out of scrap aluminum that might actually be useful in robotics.  We had fun with my demo foundry at GEAR and even though I was camped (with a little wood) in a Winabego next to the restrooms where "Ranger Rick" was cleaning and restocking, nobody even realized we were doing foundry work in the State Park (probably a good thing!).  I could provide all kinds of advice on the DO'S and DONT’S (besides the info below) but the most important thing is to have FUN and be SAFE!
 
 

 Referenced Sources:

**** Disclaimer & Cautionary Warnings ****

Disclamer: Neither the author, nor the SRS, egroups, Dave Gingery, Lindsay Books, or anyone else assumes any responsibility for any general health or life threatening events one gets into related to casting, foundry making, burns, forest fires, or from reading and applying information contained within this article.

Molten metal (lead ~800F, aluminum ~1200F) can cause severe burns if spilled or splattered upon someone!  Use extreme caution when handling.  Wear thick shoes, long pants, heavy gloves, and eye protection.  Do not attempt to handle castings without allowing adequate time for cooling.  Keep children away from heat sources, parts, tools, and other dangerous items.  Above all else, use sensibility and common sense.  If you lack either of these, don't even think of trying this.

Foundry Safety Equipment
by Gordon Lawson

A home shop foundry has different requirements from a commercial foundry. For instance, a home shop usually will not have to pass an OSHA inspection.

This is no excuse, however, to not follow some simple guidelines. In this regard, I have outlined a list of safety equipment. Each item has proven its worth.

Leather work boots, preferably with steel toes. A friend, who was helping me, almost came to disaster because he accidently stepped on a hot piece of aluminum
slag. (In bright sunlight, hot items don't look as hot.) He now has a souvenier tennis shoe which he shows to his friends. On the bottom is a hole the size of a quarter.

Trousers, not shorts and long sleeve shirt or welder's vest. While reducing some salvage lead pipe into convenient sized ingots for future use, I added some wet lead
to the molten mixture. Kaboom! is a mild word for what happened next. Fortunately, it was 785 degree lead and not 1150 degree aluminum. I "only" suffered minor
burns on my legs, arms and neck.

Welder's gloves or other leather gloves with long gauntlets up the arm. I couldn't begin to recount the number of times I have parboiled my hand because "I just need
to do this little thing and my gloves are over on the table and inconvenient..." The foundry is hot! Aluminum is mild compared to brass or iron, but it is still about three
times hotter than the flash point of paper.

Long-handled pliers (or tongs) and three foot 1/4" rod. The pliers are for adding things to the crucible and adjusting coals, etc. The rod is used to mix the crucible's
contents and move dross to the side just before pouring.

The above equipment should be adequate for a charcoal foundry. An electric or gas-fired foundry may have different requirements.