Section 13. qxd

machinability.
This is best defined in terms of
(1) tool life, (2) power requirement, and (3) surface integrity. Thus, a
good machinability rating would indicate a combination of long tool
life, low power requirement, and a good surface. However, it is diffi-
cult to develop quantitative relationships between these variables.
Tool life is considered as the important factor and, in production, is
usually expressed as the number of pieces machined between tool
changes. Various tables are available in the literature that show the
machinability rating for different materials; however, these ratings are
relative. To determine the proper machining conditions for a given
material, refer to the machining recommendations given later in this
section.
The major factors influencing 
surface finish
in machining are (1) the
profile of the cutting tool in contact with the workpiece, (2) fragments
of built-up edge left on the workpiece during cutting, and (3) vibration
and chatter. Improvement in surface finish may be obtained to various
degrees by increasing the cutting speed and decreasing the feed and
depth of cut. Changes in cutting fluid, tool geometry, and tool material
are also important; the microstructure and chemical composition of the
material have great influence on surface finish.
As a result of mechanical working and thermal effects, 
residual stresses
are generally developed on the surfaces of metals that have been
machined or ground. These stresses may cause warping of the workpiece
as well as affect the resistance to fatigue and stress corrosion. To mini-
mize residual stresses, sharp tools, medium feeds, and medium depths of
cut are recommended.
Because of plastic deformation, thermal effects, and chemical reac-
tions during machining processes, alterations of machined surfaces may
take place which can seriously affect the