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| Lost Foam Castings |
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| vtsteam:
This is how the housings would be fitted on the mill ways. The photo actually shows an earlier (pink) version of the patterns without the extended leg. |
| Noitoen:
That's hot :) I mean cool. Must use my yard space to try out some casting. |
| PekkaNF:
"CNC hotwire cutter" :clap: Is there absolutely anything you haven't tried yet? maybe square dance :) How much do you need to scale the foam plug to counter shrinkage? Pekka |
| vtsteam:
Thanks Noitoen. Let us know how it turns out! Pekka, thanks! re. shrinkage I'm not sure -- do you mean the metal part shrinkage after casting or the foam shrinkage when hot wiring? 1.) The metal shrinkage will be the same as it is for any type of casting. 2.) The foam shrinkage is generally called kerf in hot wiring (as in sawing). It is actually a combination of evaporation and shrinkage away from the hot wire, which in CNC is a result of radiation from the wire, not contact. with the wire. In hand cutting, contact often results, however, because it is nearly impossible to do that slowly and uniformly enough at low enough heat to achieve non-contact. Usually you set the kerf allowance in the CNC program (like Jedicut, which I use) -- and it stays pretty consistent for a particular brand of foam and foam type and heat setting. But not absolutely so. Since foam is cheap, I generally cut a test part, first, and if that needs some adjustment, I increase the kerf allowance in the program. btw, I wrote a spreadsheet hot wire cutter design program a while back for determining what size wire to use with a particular power source and bow length. You can juggle values around to work out a rig for cutting foam with what you have on hand for electrics, or wire. That works for a hand held bow or a bow driven by CNC, and works for virtually any power source, from batteries to computer PSUs to wall warts. My own weapon of choice for cutting foam is an old style "dumb" 12V auto battery charger, connected through a router speed controller to mains. My wire is single strand stainless steel salt water fishing leader material .011" in dameter. If anyone wants the hot wire power supply spreadsheet program, I'll post it here. I always think it's important when discussing foam cutting details to warn people to cut ONLY polystyrene foam with a hot wire, if you intend to try it. Polyurethanes and polyisocyanurate foams (usually tan or green here in the US, but possibly other colors elsewhere) will yield poisonous fumes if cut with a hot wire. |
| PekkaNF:
Both :lol: They will compound. I have done some manual hot wire cutting for model aeroplanes (had to learn build them as fast as I trashed them). I never came proficient on building nor controlling the planes...but I just made eneough trash to learn something. I have also cast low temp metals after pattern and learned very early to make a little oversize pattern. I heard that patternmakers had some rulers that were scaled few % oversize to produce just right dimenssions after casting. Your projects are really fasinating and indicate how much proficiency you have accumulated by doing all sorts of things. I wish I had your spradsheet program 15 years ago. I build one SCR primary regulated PSU that offers someting like 2 to 27 V out. Did some experimetal cuts, until "whiskers" looked about right and then I knew the right current. I had several bows made of extruded carbon fibre tubes and bigger ones from glas fibre skining poles. I aqquired Nichrome wire from Mexico, Germany and finaly from Finland...I used a lot one type of yellowish styroxfoam that had really hard skin, after Dow chemicals started using recycled material on their blue foam. The recycled was very good for building, but it had some hard granules that left a dent on othervice perfect surface. The hard skin variety was sometimes very good, when a thin wing section was cut out of it and kerf was substituted with glas fibre cloth/epoxy/delaminating strip and weighted down for gluing. But if I cut too close to it or left it into core it would bow. Pekka |
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