Let's use iron Pickaxe Heads as an example. This will work for most metals and all tool parts though (Thaumium items can still be made with a normal stencil).
1. Make a normal Pickaxe Head stencil, then make a cheap Pickaxe Head. Stone or wood will work.
2. Get a Smeltery set up with a casting table under a faucet.
3. Place your cheap Pickaxe Head on the casting table.
4. Melt some gold or aluminum brass in the smeltery.
5. Right click the faucet to pour the gold or brass over the cheap Pickaxe Head. This will give you a cast for the item you want.
6. Remove the pickaxe head and the cast from the casting table, then put just the cast back on it.
7. Smelt some iron in the Smeltery, then use the faucet to pour that into the cast you've just made.
8. Remove the iron Pickaxe Head and the cast from the casting table. The cast can be reused later.
