The machining of castings requires a different approach to the machining of wrought metal components. Castings are never totally flat, square, round or straight. A secondary datum system may be required to define the machining requirements.
When a flat face is required, add material to the casting to give a machining allowance.
When fixtures are necessary to hold the part for machining, the target datum points (see Datum Systems) will be used to set the casting to establish start points for machining.
On parts with no obvious location areas for machining, machined lugs should be added for holding the casting. These can be removed completely with a secondary machining operation.
Renishaw probes - Micro Metalsmiths' machining centres are fitted with Renishaw probes. Probing establishes the position of detail prior to machining. Cast datums have to be established from "touch points" and probing those points produces accurate machining, repeatable from component to component.
Lead-in chamfers on cast holes aid tapping.
Reaming - blind holes cannot be reamed to full depth, due to the lead angle on reamers. Holes with full depth tolerance will need boring, which usually adds cost.
Where castings require machining on faces, bosses or any type of raised edge, chamfers cast round the periphery help to minimise de-burring operations and so reduce cost. This also applies to feed gates which are to be removed by machining. Setting them on a pad 0.5 mm (0.020") high with chamfers minimises the need for de-burring.