Smells like foundry

haveblue

I had been eager to pick up a class again this semester at MATC, and when browsing through the classes this past summer, I figured I’d give the foundry class a try.  We did a little bit of foundry work way back in high school shop class (I’m sure those days are long gone thanks […]

I had been eager to pick up a class again this semester at MATC, and when browsing through the classes this past summer, I figured I’d give the foundry class a try.  We did a little bit of foundry work way back in high school shop class (I’m sure those days are long gone thanks to a lawsuit-happy society), and I’ve seen a number of Rick Chownyk’s backyard metal casting demos during past CNC Workshops.

As it turned out, the class isn’t so much a class as it is an open workshop – most of the students have been taking the class for years (two of them started taking it 28 years ago) as a way to easily make parts for their own projects (one of the fellows is a live steam locomotive enthusiast, and always has something interesting that he’s molding).  Just the sort of environment I was eventually hoping to find!  I quickly found that as with so many opportunities, I really didn’t have a clue as to what I wanted to make – I had signed up with the intent to learn, not to do. Sure, there was a bit of learning, but weeks later, I’ve only been able to figure out a single thing that I’d actually like to cast (a fixture block for machining an upcoming project). As such, every class has started with me pawing through the cabinet of patterns, wondering what to try this week. Not that this hasn’t been helpful – I managed to screw up 2 weeks in a row by focusing so much on forming a good parting line around a complex pattern that I forgot to actually remove the pattern from the mold before the aluminum was poured in. Fortunately, the temperature difference is enough between the aluminum pattern and the molten incoming aluminum that the two didn’t fuse (my tendency to create very narrow runners also helped in this regard).  Here’s a brief photo collection of some of my successes and failures.

For those of us new to the class, we started out with flat patterns - items flat on one side that were easy to make molds of. I grabbed a sunflower and horsehead from the cabinet of patterns. Rubbing the patterns down with graphite helps them to release nicely from the mold.
After ramming the sand in the cope and pulling it off the base, the cope is then flipped upside-down. The other half of the flask is placed on top and parting compound is dusted over the surface so that the mold halves will separate cleanly.
The drag is then rammed on top of the cope. After this, I used a brass tube to cut gates (holes for the incoming metal to flow down), a spoon to cut runners (channels that connect the gates to the mold cavity itself), and a piece of coathanger wire to poke vents (to allow air to escape ahead of the incoming metal) in the drag (which is the top half of the mold assembly).
This is the actual crucible of molten aluminum in the induction furnace. Due to the IR sensitivity of the camera, it really doesn't look this pink to the human eye. When the crucible is full of aluminum and the furnace is on high power, there is a very pronounced meniscus of molten metal writhing around due to the induction - it's actually quite reminiscent of the T-1000 from Terminator 2.
The mold, freshly poured. The casting sand is bonded together with oil, so flames are quite often seen lazily burning away as the mold cools. Steel plates or other heavy objects are sometimes placed on top of a mold so that the top half of the mold doesn't actually float up on the molten metal, ruining the cast.
Once the mold halves are pulled apart I get to examine my handiwork.
The horse head came out great, but the sunflower didn't fare so well. Since I tried putting the gate (the hole in the sand through which the molten metal enters) right behind the part itself, I had pushed the sand out around the gate just enough to keep the cavity from filling properly.
A few classes later, I thought I'd try doing a matchplate mold. A matchplate is simply an insert plate between the cope and drag halves of the mold that consists of the both the part cavity as well as the runners to the parts. You can ram both sides of the mold without having to remove the pattern, as the matchplate makes its own parting line. When making a matchplate mold, you only have to worry about cutting the gates and risers, so moldmaking can be done much more quickly.
I had been wanting to try doing a lost foam casting, and I realized that the fixture block I needed for an upcoming project would be a great item to make with this technique (I wasn't finding any scrap pieces of aluminum large enough at work, anyway). I hacked a piece of pink styrofoam insulation into the rough shape, superglued a thin piece of foam onto the end to act as a sprue, and then went to work covering it in a mixture of thinned drywall compound and playground sand.
After sticking the foam core into a bucket filled with sand (with the sprue sticking out the top), I placed an empty can over the end of the sprue for the metal pour.
After letting the metal cool for about 10 minutes, I started pouring off excess sand.
Once fully cooled with the drywall compund chipped away, it didn't look too bad. Not great (lots of sand inclusions), but it should be just fine for fixturing.
Hitting all the sides on a belt sander removed some of the harder and more abrasive deposits prior to near-shape milling.
I used a crummy cheap endmill to clean up all 6 faces of the block, not wanting to subject a good endmill to the abrasiveness of sand inclusions. I'll fully true it up once I get my fly cutter in. The black line is where I'll make a bandsaw cut later on - the fixture block will wind up just being a wedge, but I wanted to leave a 'tail' on it so that I could hold it in the vise more easily. The part still has a gritty feel at this state, so maybe I'll need to remove a bit more of the surface in order to hit better quality metal. When milling the faces, I noticed that the metal wasn't forming into normal chips, but was crumbling (much like machining cast iron). A low quality casting to be sure, but for a first attempt at lost foam, I'll run with it.

2 thoughts on “Smells like foundry”

    1. The thought has certainly occurred to me! If I could come up with a matchplate or two of parts, casting blanks of various Mendel pieces could be done very quickly. They’d still need holes cross-drilled, and draft angles might require slight redesign of some parts, but it is possible.

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