and amount of machining required. “Symmetrical castings with
X and Y directional features in relation to the axis may be ma- chined faster and at a lower cost on a CNC turning center than complex shaped castings requir- ing machining of features in three or more directions on multi-axis machining centers,” Finn said. “A rigid part with simple surfaces can obviously be machined faster and at a lower cost than a fl imsy casting with complex surfaces.” Tight dimensional tolerances
and low surface roughness values are costly because additional fi nish machining or grinding will be need- ed, Finn said. “Stepped and cross holes will increase the number of machining steps and raise the cost.”
In Other Cases: Cut Machining Time
Machining cost is closely related to machining time, including how
many times a part must be mounted, dismounted and mounted again before it is a fi nished part. “If a part requires multiple setups, it
As-cast mounting holes in this aluminum diecast headlight part were positioned in the main die cavity, without core pins. Movable slides and core pins wear out cycle after cycle and add variability.
is going to aff ect cost,” said Jiten Shah, president of casting design consulting fi rm Product Development Analysis, Naperville, Ill. If casting-in a hole would eliminate a setup, the machin- ing cost savings likely would be worth the additional com- plexity in the casting mold. “We used to say, you can’t
touch a part for less than 50 cents,” Prucha said. “I don’t know
if that price is entirely accurate, but with any of the machine setups, you have an engineering cost.” Tool designers may make modifi ca-
tions to the part to increase machining effi ciency. Casting a few locators (fl at pads on which the machining equip- ment can clamp) onto the part can cut
Sept/Oct 2012 | METAL CASTING DESIGN & PURCHASING | 27
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