One of the main reasons for specifying a diecasting, particularly a zinc alloy diecasting is to avoid the finish machining needed with most other forming processes. However finish machining of zinc alloy diecastings is still required on occasions, particularly to produce features that are not readily cast without sliding cores in the die. For parts produced in high quantities the cost of extra moving die elements is easily justified but for lower volume parts it is sometimes more cost effective to machine the casting. Zinc diecasting alloys are economical to machine and high quality finishes are readily obtained. Reference should be made to the “Machining” section of this document for ideal parameters for the range of machining processes. Probably the most important thing to remember when machining zinc is the prevention of heat build up at the cutting edge, by maintaining tool sharpness and applying adequate coolant.
Compared to other diecasting alloys, zinc alloys machine faster and with less tool wear than aluminium alloys and without the fire risk involved with magnesium diecastings.
Compared to materials formed by other processes, zinc alloy diecastings have the advantage that accurate datum surfaces can be provided for clamping and hence it is generally easier to achieve accuracy in the machining operation.
One machining operation that is commonly performed on zinc alloy diecastings is tapping of holes to produce female threads. Most frequently the hole is produced by a core in the die rather than by drilling after casting. Unless minimising torque relaxation of the fastener screwed into the threaded hole is critical, it is usually better to use fluteless thread forming taps. These produce no swarf and hence avoid the necessity of a process stage to ensure its elimination from the diecasting. In fact the avoidance of swarf by avoiding machining is another plus for zinc alloy diecastings. Swarf remaining on components of electrical, electronic, pneumatic and hydraulic components is a common cause of product failure.