|April 2000 Tech Feature||www.virtualindian.org|
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By Lyle Landstrom
During my colorful blue collar career where I was laid off, fired, or quit just about every industrial job known to man I picked up a few skills useful to the vintage motorcyclist. Now that I have a real job and a family to support, I needed a part time trade to fall back on to support my motorcycle habit. In the course of working out of my garage to do this, I tried lots of motorcycle related activities. I tried painting: too much time waiting between coats, can't do it all year as it's too cold in the winter, needs a clean environment, and most of all, my taste isn't exactly like anyone else's. I tried wrenching as I have a degree in marine and small engine mechanics: too committed to time constraints, never enough tools, and everyone's bike is their baby and needs special treatment. So I decided on limited parts production as it was perfect for me. I can do the pattern work in the basement with limited woodworking tools. And I can do the foundry work in the garage. Hot metal doesn't care what the ambient air temperature is; but most of all, I can do it on MY time.
Man has been sand casting metal for several thousand years. The ancient Chinese had some of the best bronze castings in the world for any time in history. In what is now Iraq, there were natural draft iron furnaces with so much production that scientists think it altered the atmosphere of our planet. There is a mine in France where man has been mining foundry sand for hundreds of years. It is still considered the best naturally bonded molding sand in the world.
If primitive man can do it in a primitive environment. Then primitive man can do it in a modern environment. The purpose of this article is to show how it can be done.
The first thing needed is an idea. For me this usually the easiest. Sometimes I have too many ideas and nothing gets done. In this case however, I was soliciting the VI list for Sport Scout Big Base engine case information when Jim Wall responded with a set of factory blueprints.
After careful scrutiny of these prints I decided the best course of action was to draw them up in CAD. This was because the originals were hard to read and a CAD program allows manipulation of the drawing from a pattern makers perspective. I also thought that if there was enough interest in the cases, then whomever receives a set would need a good drawing to machine them from.
After the CAD drawing was completed, I decided to pattern up the cam cover first. Jim Wall suggested making the cover deeper so an extra cam lobe could be accommodated. I decided to do this as this extra depth could be machined off for a normal cover. This surface needs machining after casting so it shouldn't be a problem.
Pattern making is like life. In life you live one day at a time but with a clear goal in mind, solving each hurdle in steps. In pattern making, you break up a 3-D object into simpler geometric shapes which are then assembled into the goal - or pattern.
Because metals shrink as the freeze in a mold. Patterns are bigger than the actual part. With aluminum, this shrinkage factor is usually 1/4" per foot. Or about 2%. Because I had the CAD drawing finished for the right case which matches surfaces with the cam cover, I mirrored the cam area of the drawing to give an inside view of the cover. I then plotted it out at 102% of normal. If I had an actual cover to work from, I could have laid it on a Xerox machine and copied it at 102%.
This drawing was then transferred, using carbon paper, to the wood pattern material which I had laminated up the night before. It is usually better to laminate sheets of wood with the grains in each layer perpendicular to each other to avoid warpage over time, rather to use one piece of wood at the correct thickness.
The next step was cutting out the perimeter of the pattern. I set the saw at about 2% draft: the angle needed for a smooth draw from the sand.
I cut out the top surface in a similar manner and sanded it down to the correct thickness on a belt sander.
While tracing the top of the cover, I located the two cam bushing holes and drilled through the top so I could register the bosses on the inside surface of the cover. Next the oil pump surface was traced out from the original cam cover drawing and cut out. The entire assemblage was done in a similar manner. I had to use the original drawing for the oil pump surface as the drawing was too illegible to draw in CAD.
Once the entire pattern was glued up, I gave it a coat of polyurethane sealer. The real test of any product is to use it for an application for which it never was intended. In this case, rather than buying a 1 gallon - minimum order = $50.00 from a pattern shop product, I used a product that can be had at any builders supply store for about $5.00. This sealer serves 4 purposes: 1, it seals the wood to prevent moisture from warping it. 2, it strengthens the wood pattern. 3, it serves as a guide coat when sanding similar to a guide coat when doing motorcycle painting. 4, it gives a very smooth surface so that the pattern can be easily drawn from the sand.
Many sealers can be used. I've had bad luck with some enamels because the sand has a tendency to stick. The polyurethane is also compatible with polyester body filler as used for auto body and gas tank repair. This is another product similar to products sold by the pattern industry but costing many times more. This is how the fillets were made on the inside surface of the pattern. Automotive body filler is also excellent to use in core boxes. It's easier to cut the box out square and fill the radius in with bondo than to use a gouge to cut out the wood.
After the initial bondoing and sanding, more coats of polyurethane were applied. I usually work better at multiple tasks than one so I coated several several patterns at once. Being a naturally lazy person, I only had to clean the brush once for each coat. After several coats were applied with 220 grit dry sanding between the pattern was finished.
Not all patterns are made in one piece like the cam cover. In the picture, note that I have some riser patterns in 2 pieces. This is called a split pattern. I made the wood pattern at a double shrinkage factor (1 for aluminum, one for brass) which will be cast into a aluminum master pattern mounted on a match plate. This will enable me to make multiple pieces in one mold. It's also a lot quicker to produce a mold using this method. The finished risers will be manganese bronze which is very shiny yellow brass yet high strength.
Off to the Foundry (garage)
Because this is a simple pattern with a parting line on the same plane, a simple flask is chosen. The pattern is laid on the bottom board, parting dust sprinkled over the top and the first layer, or lift, of sand applied then rammed with a wood rammer. I use homemade wood rammers and flasks. The reason is because a pattern can be damaged easier with a metal rammer and metal flasks are unessessary for all but the most precise molding. Metal flasks are also very expensive and I've gotten by with homemade equipment so far. The wood flasks will last for years if you're careful. I use alignment pins to align the top (cope) with the bottom (drag) part of the flask.
Once several layers of sand are scooped into the flask and rammed, the sand is struck off level with the top of the flask. It is then rolled over. I like to scratch out the locations of the gates with a pick so I know where to cut them in on the other half of the mold. I also like to use a "bob" to locate the sprue (where the metal is poured in). This bob not only helps register the sprue, but serves as a reservoir of molten material to be drawn in as the metal shrinks in the mold. This counteracts shrinkage. It also gives a nice surface to ram against which reduces the possibility of sand "washing out" as metal is poured in. My bobs are made from the resin plastic left over from patterns when mixed in yogurt containers. The yogurt container serves as a good bob mold and can be used over and over. Eat yogurt, stay healthy, and have mixing containers. The other half of the mold is set and parting compound is dusted over the inside surface. This compound, which can be pretty much any dust, keeps the sand from sticking from one half of the mold to the other, and from it sticking to the pattern.
The upper portion of the mold is riddled with sand and rammed in lifts until it is full. It is then struck off and opened.
The pattern is "rapped" which means wiggled in the sand just a bit and then withdrawn. The bob is withdrawn and the sprue, riser, and gates are cut in. I like old thin wall plumbing pipe to cut in my sprue and risers. This same thin pipe costs big bucks at a foundry supply house. So it pays to cruse the dumps. Some molders use a wood pin which the sand is rammed around just like a pattern, but this way woks better for me.
The sprue is on the right and the riser is on the left. The riser helps gas escape while pouring and also supplies metal to the casting as it cools. Usually the sprue needs to be wider than the thickest section in the casting. The gates are usually at the thick sections as well. There are some exceptions but this is a good general rule of thumb.
After I get a couple of molds finished, I fire up the crucible furnace. I don't fire it up right away because if for some reason the molds don't turn out, there's no place to pour the metal. My homemade ingot molds won't hold an entire melt. I usually ram the last mold while the metal is melting. Usually it's time to pour when I get the last one rammed.
I don't usually pour more than three molds with one melt because the metal is constantly cooling once the gas is shut off. Aluminum needs to be poured around 1350 degrees F and it loses heat rapidly. I check the temperature with a pyrometer. "Eyeballin" by color won't work with aluminum. My scrap rate went to near nil when I finally got a pyrometer. Pouring too hot results in gas porosity and a dull finish. Too cold results in a mis-pour where the metal didn't completely fill the mold and also in a dull finish. Although, these are not the only causes.
I don't stir the aluminum at any time as that creates air entrainment. I don't use flux because it's unnecessary for aluminum. Many books have been written about aluminum casting defects. This shouldn't confuse anyone because usually the simplest solution is the best. I won't delve into this subject any further.
The aluminum is poured into the molds as steadily as possible. You can tell when to start slowing down by keeping an eye on the riser which is filling up with the melt.
Because this was the first time I had poured this pattern and I was experimenting with different gates, combined with my overwhelming sense of curiosity, I "shook out" (removed the casting) from the mold within a few minutes. This is sometimes done in industry as it's a "poor man's" form of heat treating and there's even a technical word for it.
Don't do this with bronze or brass because the castings are so hot that when you open the mold the oil in the sand spontaneously ignites with the air and flares up. It takes about 3 - 6 weeks to regrow eyebrows. It was a drag avoiding the bar for that long.
The sand pretty much falls off the castings. The remaining sand that clings can be cleaned off with a brush. The sand which is burnt needs to be reclaimed in a muller for this type of sand. Most small scale sand casting is done with "green sand" that is, a sand bound with clay and water. Because I do most my casting during the winter months, I use an oil binder. The oil bonded sand is also more permeable which reduces casting defects. It is also not as sensitive to how hard the mold is rammed. Most of all, I can mold when it's below zero.
I waited until the other castings had cooled down somewhat before shaking them out. From the pictures you can see that the casting that was shook out while hot is duller. The hotter aluminum is, the easier it oxidizes. This isn't a problem but for parts which don't require much finishing it's better to wait until they have cooled down so they look better.
Before I got my molding bench at an auction, I used this sandbox which is where I do all my shake outs now. I have rammed up hundreds of molds in this little wood box.
After the first castings are completed. I check the fit against the part they are going to fit to. I have a set of beat up Sport Scout cases and matched them to it. Looks good so far.
One of the problems was because this pattern was made for both double lobe and single lobe cams. The dimensions of the casting gets smaller as more material is milled off. I had to compromise to make the pattern fit both cases.
I also cut one casting down the middle to check thickness, machinability, the fillets and so forth. The sectional view is a good example of the draft on the pattern. This is very important when designing a pattern.
At this point, I have sent three casting to Jim Wall who will machine them. I might have to change the pattern a little based on his recommendations. By the time you will read this, I will have started on the case patterns themselves. Currently I'm patterning the inner primary, a coil bracket, my riser match plate, and a private job unrelated to motorcycling.
I hope to have shown how foundry and pattern work can be accomplished at home with simple tools. It is an activity that grows on itself. I have been able to buy or make items as I need them from the small profits of my garage foundry. There is minimal need for fancy store bought items.
I have discovered there is a small network of home foundry people. I see the same people time after time at foundry related auctions. One guy has his little niche doing model railroad parts, another makes steam engine parts. I've done award plaques in bronze for local football teams among other stuff. I do a lot of novelty type stuff as well.
I'd like the thank everyone who has helped me along the way. Especially to MJ whom I consider my friend and who is a way better foundry man that I am. I'd also like to thank the cops for not carting me off when I added too much zinc to the melt the first time I made brass. It produced a flourescent green plume that drifted about 4 feet off the ground across the street into the park. It was quite the effect. I don't know what was in it. But it sure cured my sore throat.
I'll be more than happy to answer questions to anyone about any items in this article. Stay tuned for Big Base Cases!
to other foundry sites here!
Mirroring the case drawing (below) to get an inside view (above) of the Big Base Scout cam cover (back to text)
Cutting out pattern
Registering (back to text)
Checking cam cover pattern on drawing
Polyurethane coated patterns. Note riser patterns -and Lyle's taste in paintwork, which I find rather attractive, but I see what he meant in the introduction... :-) Moen
(back to text)
Pattern, bottoms-up, on bottom board in lower part of flask (drag)
Sprinkled with parting dust
Rolled over. Note yogurt container "bob" (back to text)
Pattern dusted again, and top part of flask (cope) put on
Sand riddled into cope
Cope lifted off
Pattern and finished mold
Ready to close for pouring
Man that's hot!
Smokin! (back to text)
Dull (hot) shakeout
Shiny (cool) shakeout
Checking new cover against old cases
Casting cut down the middle to check thickness etc
Marshall Foundry, considered the best small
iron foundry link.
This guy casts Indian Chiefs!!!!!
Online moldmaking magazine:
A foundry forum from the artists perspective:
Suppliers to the hobbyist:
This guy's got way too much time on his
Yahoo! Isn't this guy cheerful!!
Casting in a frying pan (almost)
Moldmaking and pewter casting stuff:
The only book you need to get started:
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