Section 13. qxd

power-head
production
units and the development of 
transfer
mechanisms. Power-head units,
consisting of a frame, electric driving motor, gearbox, tool spindles,
etc., are available for many types of machining operations. Transfer
mechanisms move the workpieces from station to station by various
methods. Transfer-type machines can be arranged in several configu-
rations, such as a straight line or a U pattern. Various types of
machine tools for mass production can be built from components;
this is known as the 
building-block
principle. Such a system combines
flexibility and adaptability with high productivity. (See 
machining
centers.
)
Numerical control (NC),
which is a method of controlling the
motions of machine components by numbers, was first applied to
machine tools in the 1950s. Numerically controlled machine tools are
classified according to the type of cutting operation. For instance, in
drilling and boring machines, the positioning and the cutting take
place sequentially (point to point), whereas in die-sinking machines,
positioning and cutting take place simultaneously. The latter are often
described as 
continuous-path
machines, and since they require more
exacting specifications, they give rise to more complex problems.
Machines now perform over a very wide range of cutting conditions
without requiring adjustment to eliminate chatter, and to improve
accuracy. Complex contours can be machined which would be almost
impossible by any other method. A large variety of programming sys-
tems has been developed.
The control system in NC machines has been converted to computer
control with various software. In 
computer numerical control
(CNC), a
microcomputer is a part of the control panel of the machine tool. The
advantages of computer numerical control are ease of operation, sim-
pler programming, greater accuracy, versatility, and lower maintenance
costs.
Further developments in machine tools are