Magnesium has been used in manufacturing notebook computer frames, video cameras, digital cameras, PDAs and other consumer electronics products because of its high strength to weight ratio. When magnesium is alloyed with aluminum, the resultant material is very light and strong, and easily machinable.
The main concern in machining magnesium alloy is the danger of fire ignition when dry cutting. Fire may occur when the melting point of the alloy (400-600 degrees Celsius) is exceeded during machining. The small chips and fine dust generated during cutting are also highly flammable and pose a serious fire risk if not properly handled.
There are several points to note when machining magnesium:
Firstly, use a lower cutting speed when compared to cutting aluminum. The workpiece temperature goes up with an increase in cutting speed and also smaller undeformed chip thickness. In other words, the slower the machining speed and the larger the chips, the lower the workpiece temperature will be. Due to this reason, some companies have modified woodworking tools for machining magnesium so as to achieve larger chips and lower fire hazard.The cutting tools used should have relief and clearance angles that are sufficiently large to prevent unnecessary cutting tool-workpiece friction, thus lowering the heat generated during the cutting process.
Second, keep the machining center clean. Cleaning the machining centers regularly and storing the magnesium chips correctly are important aspects of machining magnesium. Keep a container of cast iron chips near by when machining magnesium, If fire occurs, smother the fire with the cast iron chips.
Thirdly, if coolants are necessary for high speed machining, do not use water-based lubricants. Instead use a light mineral oil, or a water-soluble cutting fluid such as Castrol Hysol MG specially formulated for machining magnesium. Some companies in Japan use semi-dry machining via a misting system.
The fourth point is to monitor the workpiece temperature during machining. Experiments were carried out using thermocouples mounted into the workpiece to monitor the workpiece temperature during machine. Dry cutting of magnesium alloy thin walls was achieved using cutting speed of 440m/min for roughing and 628m/min for fine finishing.
Despite the fire hazards, as competition from overseas low-cost production bases intensifies, and magnesium becomes increasingly used in electronics products, most machining job shops could very well find machining of magnesium a niche worth pursuing.
Author Ken Yap is a director of Suwa Precision Engineering Pte Ltd in Singapore and represents precision component manufacturers from Suwa, also called "The Oriental Switzerland" in Japan due to its Swiss resemblance for rich watch-making industry, its mountainous terrain and its precision component making industry. For more information on precision manufacturing, visit his homepage at http://www.suwaprecision.com.
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