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Machinability
is generally assumed to be a function of tool edge life. The main factors which influence the
behavior, 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 as 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 machinability on the basis of
strength. For example, nodular irons are normally 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
a low hardness value, but which has a very abrasive
action on the cutting tool. For example,
the presence of hard phosphate particles embed­ded in a soft, ferritic matrix
reduces tool life considerably.