The Shop > Metal Stuff
Lost Foam Casting: a Crankcase in Zinc Alloy
Eugene:
Steve,
May I ask a question?
How do you make such beautiful moulds, their accuracy and sharpness looks excellent. I'm on the beginner slopes with hot wire cutting and my efforts look distinctly agricultural when compared to yours. Could it be a CNC foam cutter?
Eug
vtsteam:
Eugene, the pattern is the most important part of any mold. People often get caught up in the technology of sand, or furnace refractories, burners, metal composition, gating and risering, fuels, etc. in casting. Yet knock a couple pieces of wood together for a pattern.
Casting starts with the pattern and its finish.
I spend a lot of careful time with very fine sandpaper, even on the foam cutouts to get them as good as I can before using them. More time than they took to cut out by machine. I use little strips of sandpaper under a strong light. Oudtoors on the porch in summer is perfect!
I do the same with wooden patterns, and use filler and sanding sealer coats, each sanded down, and several coats of spray colored lacquer to finish. Most of my practice for this kind of thing comes from building model airplanes, which I've been doing as a hobby since I was 14.
Oh one other thing about cnc hot wire foam cutting. You need to go VERY slowly for a machine process. 1mm/sec to get good quality cuts. and you need thin wire and low temps to match. I use .011" stainless steel fishing leader as a hot wire.
If you don't have a good pattern, all the time and money in the world invested into the latest metal casting forum technology will just get a person a lump of scabby metal. Sand break outs happen because the pattern is rough. Inclusions ditto.
Conversely, I've found that the crudest casting equipment will turn out good faithful castings with good finish if the pattern is good and the technque is sensible. My iron foundry is made from ordinary firebrick, and clay and sand. The fuel I use to cast in aluminum is store bought charcoal briquets or home made charcoal. For the last few zinc castings I just used wood for a fuel. Ordinary plaster of Paris with no additives, no bake out, air dried for a few hours and ordinary scraps of foam were used for a pattern.
I like simplicity, and I am impatient, and I hate to waste things. I use what is available and try to do a good job with it. I study what happens and I gradually think through why it didn't go as I wanted. Eventually the problems get worked out by trial and error until I understand my own equipment and materials.
Anyway, it is great that you ask, and the answer is a lot simpler then I've ended with here. Just take a little sandpaper and do a very careful job, with a light touch. A few times through it, and you'll get good at it. It's nothing more than that. :beer:
vtsteam:
Eugene, re-reading your question, I see you probably don't have a cnc foam cutter, and that isn't a problem for getting good patterns at all -- you must be using a hand bow, or vertical fixed wire. I did that for many years. The main thing again here is to have good templates --smooth ones, just like the need for smooth patterns in casting.
The way I made foam cutting templates was to cut them out of sheet aluminum with tin snips (after pasting a paper printed pattern on top) and then filing to the line with a fine file. The fine file is essential.
For small patterns (rather than wings) a vertical wire and table is best. After that comes sanding the foam pattern, as it does with CNC, as well.
Low temperatures, slow smooth movement, thin wire are all helpful, but the blank is only a blank, and hand work refines a pattern to what it should be.
awemawson:
Steve,
I don't think that we've had the pleasure of pictures of your CNC hot wire cutter. It sounds interesting, any chance that you could oblige?
vtsteam:
Hi Andrew -- my wire cutter is just a plans built wooden drawer slide style wing cutter -- quite crude, and looks like the original here:
http://www.drayconstruction.com/foamstuffs/
except that I used 3/4" birch plywood instead of the plans shown MDF.
It's not a particularly good or accurate "machine" as you can imagine but it does cut wing cores, and an occasional pattern. I bet practically anybody here could design and build a better one. One big problem with the drawer slides as designed is they protrude off the ends as they slide to the far end, so you need a far bigger area than the cutting area.
In general, I don't like cnc hot wire cutting, and for all tapered wings I now use a pivot wire (manual wire attached to a fixed point, used with a single airfoil pattern). It's much faster and not prone to CNC programming errors -- which can quickly eat up a LOT of good foam. If I had to cut the same wing in production, maybe CNC would make sense. But for occasional one off, it seems I waste 3 panels getting one good one, and it takes hours of first programming, generating G-code, cutting a blank and positioning it for wire cutting specing the taper and finally cutting at 1mm/sec, meanwhile watching it in case something goes wrong.
I wish I could say that it's magic, and easy but frankly, I'd rather cut a one off part with a hot wire by hand.
I'll show you my crude hot wire jig saw instead, which, with a simple foam pattern to do, can cut far more quickly than the CNC would. Back with pics later.....
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