Magnesium Machining Practices and Precautions
The possibility of fire when machining Magnesium is always a real concern. However, if proper precautions are taken, this risk can be reduced greatly.
Magnesium fire result from combustion of the metal due to heat generation beyond the "Heat of Incipient Fusion". A magnesium fire is very violent and burns with a bright white glow. It has the ability to extract oxygen from liquids and damp powders, so extinguishing by suffocation is necessary. Use of water containing fluids for coolants is not recommended. In fact, coolants are not recommended or necessary in most cases. Where coolants are desired, only mineral oit cutting fluids should be used.
The following list of safety precautions and machining tips for machining magnesium are recommended in conjunction with normal safety practices.
A Class D (Ansul metal fire) Fire Extinguisher should be in the immediate vicinity of the machining activity
Cutting tools should be kept sharp to prevent excessive heat generation
Cutting tools should NOT be allowed to dwell on the magnesium part being machined. This will result in formation of very fine particles of magnesium which can be easily heated to combustion by friction from the cutting tool.
Generally, high speeds, heavy feeds and heavy depth of cuts help reduce the fire hazard by reducing heat generation and the creation of heavy chips and turnings. Due to magnesium's high rate of thermal transfer, heat is dissipated rapidly and it is virtually necessary for the entire chip to be heated to the point of combustion for a fire to result. So, the larger the chip, the greater the amount of heat necessary for combustion.
If a fire does result, confine it to a small area and smother it with the extinguisher media. Do not blast it and spread it over the shop. For this reason, we use Dry Ansul Sand in moisture proof containers for pouring directly on any small fire that may occur.
Following are the particulars on tool sharpening, feeds, speeds and depth of cuts recommended for magnesium. These cover the basic machining operations of Drilling, Reaming, Boring and Milling.
Drilling: Shallow-hole drilling (depth is less than 5 times drill diameter) presents few problems and consequently, only a few modifications are necessary for high quality drilled holes. Standard point angels of 118 degrees and chisel edge angles of 120 to 135 degrees which give a relief angle of approximately 12 degrees will give the best cutting action. It is extremely important, regardless of the type of drill used, that the cutting edges be kept SHARP. Deep-Hole drilling can be performed with great speed and precision due to the excellent machining characteristics of magnesium. To extract chips from the hole, it is recommended to use high-helix drills of 40 to 45 degrees. If standard drills of low helix angles are used, it will be necessary to withdraw the drill frequently to clear the chips. The standard drill point angle of 118 degrees is the most satisfactory. Drilling speeds in the range of 75 to 400 surface feet per minute are satisfactory and higher speeds can be used. The feeds used in drilling magnesium should be heavier than those for other metals to secure proper chip formation. Small drills work best with light feeds, as they give slightly coiled or ribbon-like chips which feed out through the drill flutes without jamming. Heavier feeds should be used on large drills to prevent jamming of the chips.