A gravity cast is just a pour, you get what you get, there are cavities in them, sometimes very large ones. Also they are cast in sand and the sand sometimes gets in the casting itself and weakens the affected area. When you machine a casting you make it nice and smooth but you just don't know for sure what is inside the part, wheel in this case. They are probably not tested for cavitation either. So as long as the machined surface is smooth they send them on to the next guy.Originally Posted by Avetiso
If the cast is done via pressure or centrifuge then the molten alloy is pressed into the mold via pressure. This usually makes them much more solid.
I've machined gravity cast aluminum and had 1 inch holes show up inside them out of nowhere, just an air pocket. I've also hit rocks, metal of a different material, all kinds of crazy shit sometimes. You gotta figure if the guy that is making them is pissed there is no tellin what he or she will put in there. The cheaper the casting the cheaper the employee was paid to make it and the more likely you'll find imperfections of some sort in them.
ALSO...most castings are two pieces put together. So if they connect them carelessly then you'll be riding on a wheel that is weak right down the middle.
I don't machine wheels.....not yet at least. We have a client that we are talking to right now though. I can't disclose any further information but we'll see how those casting look like. I'll probably be the one making them so I'll have a much better insight into the whole process in a few months.
