I learned 3D modeling a few years ago, and then 3D printers came into common use, and so much of the pattern work I do now is 3D printed (I can still make manual patterns from wood and steel, but am transitioning to mainly 3D printed patterns).
If you know 3D modeling, then you just add a bit of machining allowances (in the 3D model) to the surfaces that will need to be machined, and add 3-5% draft angle on the surfaces parallel to the direction that the pattern will be pulled from the sand.
A 3D printed pattern needs to be scaled up by a factor of about 1.015 to allow for metal shrinkage.
For patterns that will have a core, a coreprint is added to the pattern, which is just a protrusion that looks like a boss on either end of where the core goes, to support both ends of the core.
For the first flywheel pattern that I attempted, I used myfordboy's pattern video, which is a manual process using wood.
Myfordboy goes to a lot of trouble to add mass to the rim of aluminum flywheels that he casts, or he uses zamak.
I have found that using gray iron for flywheels (and everything else too) is really the way to go (for me anyway).
I use gray iron electric motor end bells for cast iron scrap.
Avoid using sash weights unless they are a known high grade gray iron (American sash weights are trash metal).
I know of one guy who has used old heating radiator iron to cast a motor in iron, but typically the higher-phosphorus irons should be avoided for engine work.
Generally, if a piece of iron will break cleanly with a sledge hammer, then I suspect it will be ok to use for engine parts.
Ductile iron will melt in a backyard furnace, but you have to watch for hardness and adjust additives (such as ferrosilicon).
Iron scrap does not have to be clean in order to make quality castings, although some will say otherwise.
Any paint, rust, etc. will come off of the melt as slag, and melting rusty iron is not a problem.
I don't use any flux with iron.
Below is an cut ingot of gray iron that I poured, and a few broken small pieces that were poured.
You want a clean uniform gray section without white spots (chills).
Typically you do not reduce iron scrap to ingots, as many do with aluminum.
The iron should be used directly from its scrap form, and only excess melt material should be poured into ingot molds.
Here are some broken up electric motor end bells.
No need to clean the paint or rust off of the scrap.
The key to melting iron is to use a good 3,000 F refractory such as Mizzou, and using a Morgan clay-graphite "Salamander Super" crucible.
I use an oil burner to melt iron, as do most others who do iron work, and a varialbe speed Toro leaf blower for combustion air.
The heat from an iron melt is intense, and so IR eye protection is required (welding shaded glasses or facemask), and heat shields on everything that is close to the crucible or furnace (a piece of square sheet metal will protect the gloved hand from the heat).
Good leathers or heat resistant synthetic clothing (I am a leather guy; old school I guess) and leather boots are required.
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