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Casting and Foundry
Backyard Metalcasting.comOne of the best beginners' resources for metalcasting on the Internet. Also be sure to check out the forums (my nickname is Anon).
The Association of Backyard MetalcastingThis is a smaller community than BYMC, but contains some of the most experienced hobby casters on the Internet. Their forum is smaller, again, but there are fewer beginning casters compared to BYMC. Again, my nickname is Anon.
Tim Williams' websiteAn excellent compendium of all sorts of esoteric casting, ceramics, and electronics info. Be sure to check out the pages on alloying.
Foundry Design Calculator, Propane Burner Calculator, and Refractory EstimatorThree enormously useful calculators written by Adam Ziegler.
eFunda Alloy DataGood for looking up the composition and properties of alloys. You only get a few per session before it wants you to buy the service, but that should be enough for most things.
Ron Reil's WebsiteThe inventor of the Reil burner has a wealth of burner and forge information on his (rather hard to navigate) site.
theworkshop.caFrank G. of theworkshop.ca has lots of information about lost foam, and the rest of his site is worth checking out as well.
Suppliers
Legend, Inc.Good prices on ceramic crucibles.
The Cajun ShoppeHigh-pressure regulators and other propane supplies.
McMaster-CarrAnything and everything. Of note are their aluminized foundry gloves and similar high-end safety gear.
eBayIt may take some searching, but foundry supplies pop up on eBay from time to time. In particular, there are usually some good deals on ceramic fiber blanket. Be careful, though--some suppliers repackage commercial supplies and sell them for rip-off prices.
Machining and Fabrication
Mini-lathe.comLots of information about general machining, and tuning up cheap import tooling to perform like real machine tools.
Bedair Machine WorksMore general machining info, and several interesting projects. If there's one fault, it's that he turns everything from giant pieces of stock instead of making rough castings. His scrapyard must be the most well-stocked in the country . . .
How to Make SpringsMore than you could possibly want to know about making springs, and several ways to make custom springs with limited tooling.
Suppliers
Surplus CenterGood deals on motors, blowers, and other assorted mechanical bits and pieces.
Victor MachineryAn extensive selection of machine tool accessories.
EncoSame thing as Victor Machinery, except with different selection and better prices on some things. Always shop around.
Online MetalsExtensive stock of, well, stock. More expensive than a wholesaler, but good for small orders and specialty materials. Also good for convenience.
Ceramics
Digitalfire Ceramic Articles DatabaseThe go-to ceramics reference material. Mostly geared to art and functional-ware ceramics instead of refractory and technical ceramics, but there's still an incredible amount of useful information. If you're thinking about trying slip casting, read the article here first.
Update 10/08: The reference material is in the process of being moved to a new site, which can be found here. Unfortunately, while the articles still appear to be available for free, some of the chemistry information is being moved behind a paid subscription, and has already been stripped from the old site.
Axner Ceramic SupplyIf you're going to send dirt through the mail, this place has a good selection and good prices. A local supplier will be cheaper--ceramic supplies are heavy!