Advanced workpiece materials

Group K

In the ISO classification set up in 1958 the group K workpiece materials were classed as workpieces that when machined generated crack type chips.

For typical group K workpiece materials, such as cast iron castings, the vibration created when crack type chips are produced can easily cause the cutting edge to chip. Generally, crack type chips leaves a poor surface finish, and as a result this can cause abrasive tool wear. Note however, that due to the way in which the chips are generated, the cutting resistance is not as high as when compared to machining steels or stainless steels.


Machinability of workpiece materials

Recently, high-grade cast irons, materials included in K group as castings, have increased adhesiveness and strength. Consequently, some cast irons are now seen as difficult-to-cut materials.

For high-grade cast irons such as ductile and malleable cast iron, crack type chips are not always produced which makes chip control an important consideration.

High-grade cast irons also produce high cutting resistance, which can easily result in chipping, heat generation and crater wear. A particular feature when machining ductile and malleable cast irons is that, in comparison with general steels, the crater wear is produced at a point closer to the cutting edge. Plastic deformation of the cutting edge that causes flank wear should also be a consideration when cutting these materials.


Other cast irons available include hardened cast irons, such as solid-solution chilled cast iron, and heat-treatable pearlite malleable cast irons. Both of these materials are as hard as steel. Due to the fact that they include graphite within their structure, high frequency vibrations occur when machining. Therefore they are often considered to be difficult-to-cut materials.


Another feature of cast irons is the uneven surfaces. This is caused because the surfaces undergo thermal expansion and contraction and become very hard due to heat oxidation during the casting process. The cast surface is also referred to as surface scale and is one of the main causes of problems experienced when machining cast irons.

There are many products manufactured by casting iron in moulds made of sand. When the molten metal is poured into a sand cast there are occasions when the sand mixes with the surface of the metal. Surface scale contaminated with sand has a poor machinability level, sometimes resulting in short tool life.