CNC – Why Gray Cast Iron is used in machine tool parts
Vibration in machine tools causes poor part accuracy and surface finish, and leads to accelerated failure of parts like bearings. It is therefore important that a large part of the machine is made of materials with high vibration damping capacity, to reduce the amplitude of the vibrations. Damping is the conversion of mechanical energy into heat (remember the energy conservation mantra “Energy cannot be created or destroyed. It can only be converted from one form to another” ? ).
Gray Cast Iron has extremely high damping capacity, and is therefore the preferred material for machine tool beds, tables, columns, etc. (as also engine cylinder blocks, for the same reason). You can see its damping capacity compared to other materials in the graph below.
The damping behaviour is because of the large graphite flakes. Gray CI is easily machinable as it tends to crack easily across the graphite flakes that have poor strength. The carbon in Gray CI is in the uncombined graphite form, while in White CI it is in the combined Cementite (Iron Carbide) form.
Gray cast iron, flake graphite iron and flake graphite cast iron all refer to the same material. Graphite is the same stuff that’s in pencils, by the way.
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