Section 13. qxd

surface integrity
of a part.
Typical detrimental effects may be lowering of the fatigue strength of the
part, distortion, changes in stress-corrosion properties, burns, cracks, and
residual stresses. Improvements in surface integrity may be obtained by
post-processing techniques such as polishing, sanding, peening, finish
machining, and fine grinding.
Vibration
in machine tools, a very complex behavior, is often the
cause of premature tool failure or short tool life, poor surface finish,
damage to the workpiece, and even damage to the machine itself.
Vibration may be 
forced 
or
self-excited.
The term 
chatter
is commonly
used to designate self-excited vibrations in machine tools. The excited
amplitudes are usually very high and may cause damage to the
machine. Although there is no complete solution to all types of vibra-
tion problems, certain measures may be taken. If the vibration is being
forced, it may be possible to remove or isolate the forcing element
from the machine. In cases where the forcing frequency is near a nat-
ural frequency, either the forcing frequency or the natural frequency
may be raised or lowered. Damping will also greatly reduce the
amplitude. Self-excited vibrations are generally controlled by increas-
ing the stiffness and damping of the machine tool. (See also Secs. 3
and 5.)
Good machining practice requires a rigid setup. The machine tool
must be capable of providing the 
stiffness
required for the machining
conditions used. If a rigid setup is not available, the depth of cut must
be reduced. Excessive tool overhang should be avoided, and in milling,
cutters should be mounted as close to the spindle as possible. The length
of end mills and drills should be kept to a minimum. Tools with large
nose radius or with a long, straight cutting edge increase the possibility
of chatter.