Section 13. qxd

parts are first roughed out on the lathe, then finished accurately to size
by the cylindrical grinder. The work is carried on centers, rotated slowly,
and traversed past the face of a grinding wheel.
Universal grinders
are cylindrical machines arranged with a swiveling
table so that both straight and taper internal and external work can be
ground. 
Drill grinders
are provided with rests so mounted that by a simple
swinging motion, correct cutting angles are produced automatically on the
lips of drills; a cupped wheel is usually employed. 
Internal grinders
are
used for finishing the holes in bushings, rolls, sleeves, cutters, and the like;
spindle speeds from 15,000 to 30,000 r/min are common.
Horizontal surface grinders
range from small capacity, used mainly in
tool making or small production work, to large sizes used for produc-
tion work.
Vertical surface grinders
are used for producing flat surfaces on produc-
tion work. 
Vertical 
and
horizontal disk grinders
are used for surfacing.
Grinding machines are used for 
cutting off
steel, especially tubes, structural
shapes, and hard metals. A thin resinoid or rubber-bonded wheel is used,
with aluminum oxide abrasive for all types of steel, aluminum, brass,
bronze, nickel, Monel, and Stellite; silicon carbide for cast iron, copper,
carbon, glass, stone, plastics, and other nonmetallic materials; and dia-
mond for cemented carbides and ceramics.
Belt grinders
use a coated abrasive belt running between pulleys. Belt
grinding is generally considered to be a roughing process, but finer fin-
ishes may be obtained by using finer grain size. Belt speeds generally
range from 2,000 to 10,000 ft/min (600 to 3,000 m/min) with grain sizes
ranging between 24 and 320, depending on the workpiece material and
the surface finish desired. The process has the advantage of high-speed
material removal and is applied to flat as well as irregular surfaces.
Although grinding is generally regarded as a finishing operation, it is
possible to increase the rate of stock removal whereby the process
becomes, in certain instances, competitive with milling. This type of
grinding operation is usually called