Factors affecting machinability

Machinability is generally assumed to be a function of tool edge life. The main factors which influence the behaviour, and thus the life of the edge of a cutting tool, are:

- the mechanical, characteristics of the material being machined, such as its strength, hardness and metallurgical structure;

- the state of the casting, involving the skin finish, critical dimensions, machining allowances, slag inclusions, the presence of scabs, rust, dirt, etc.;

- the nature of the machining techniques being used;

- the characteristics of the machine-tool being used, such аs machine efficiency, available power, and the rigidity of the setup.

Other factors aside, it is primarily the structure of the metal which determines its resistance to the cutting action of the tool, i. e. the potential rate of metal removal, and the resulting abrasion on the tool, i. e. the life of the cutting edge.

Structure, strength and machinability are interrelated to some extent – in general, increased strength implies reduced machinability. This basic relationship must be understood, otherwise difficulties may be experienced in the machine shop if the designer has specified a material with a higher strength than is necessary. Nevertheless, care should be taken in rating machihabity on the basis of strength. For example, nodular irons are normallу considerably stronger than flake -graphite types, but are likely to be easier to machine. It is therefore recommended that structure, rather than strength, be adopted as the basis for machining practice.

Hardness provides a more reliable guide to machinability than does strength, for hardness depends mainly on the matrix structure of the casting. Again, however, the relation is of a general nature only, for it is possible to have a metal which exhibits low hardness value, but which has a very abrasive action on the cutting tool. For example, the presence of hard phosphide particles embedded in a soft, ferric matrix reduces tool life considerably.