I know the general issues involved in cast finishes, but not specifics.
What I’m wondering most is “If you’re having to make a CNC machine pass over the casting to get your surface finish.... what are you really gaining from the casting process in the first place?” OK, perhaps less waste than machining a billet, but in terms of units produced per hour, how much are we saving?
Especially if one isn’t removing that much material in the first place... (eg, on the iPhone case, we’re not talking of hogging down a 2” billet of aluminum... to a high-speed CNC, I doubt that the cycle time would be much increased if the iPhone case were twice as thick as it is now....)
Well, polish cutting of cast parts is pretty common; machine time isn't the big expense at most places, since the cost of time/labor is so low. It's about capacity, and that's where a mixed production model (cast then machine) comes into play, because I can quickly cast 10,000 pieces and spend just 1/10th the normal machining time doing the final polish, as opposed to fully cutting on a CNC.
Nearly 100% of the cold-forged parts I design are subsequently turned on a lathe or CNC machined. The cold-forging does 98% of the work, the lathe or CNC cleans up the finish and makes it pretty. And the total time for production is lower than machining from the start.