Page 699 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

680 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing

M2 HSS 64-66 HRC saw. The thinner the stock, the finer the saw teeth should be, and
Electron-beam weld the greater the number of teeth per unit length of the saw. Cutting
fluids generally are used to improve the quality of the cut and the
Flexible alloy-steel life of the saw.
backing
Types of Saws. Hacksaws have straight blades and reciprocating
motions. Developed in the 1650s, they generally are used to cut off
bars, rods, and structural shapes. They may be manual or power op-
(H)
erated. Because cutting takes place during only one of the two recip-
Carbide rocating strokes, hacksaws are not as efficient as band saws
insert (described later). Power /vac/esaw blades are usually 1.2 to 2.5 mm
thick and up to 610 mm long. The rate of strokes ranges from 30 per
minute for high-strength alloys to 180 per minute for carbon steels.
(D) The hacksaw frame in power hacksaws is weighted by various mech-
anisms, applying as much as 1.3 kN of force to the workpiece to im-
FIGURE 24.21 (a) High-speed-steel teeth weld- prove the cutting rate. Hand hacksaw blades are thinner and shorter
ed onto a steel blade. (b) Carbide inserts brazed than power hacksaw blades, which have as many as 1.2 teeth per mrn
to blade teeth. for sawing sheet metal and thin tubing.
Circular saws (also called cold saws in cutting metal) generally
are used for high-production-rate sawing, a process called cutting off Cutting-off op-
erations also can be carried out with thin, abrasive disks, as described in Section 26.4.
Cold sawing is common in industry, particularly for cutting large cross sections. Cold
saws are available with a variety of tooth profiles and sizes and can be fed at any angle
into the workpiece. In modern machines, cutting off with circular saws produces rel-
atively smooth surfaces with good thickness control and dimensional accuracy due to
the stiffness of the machines and of the saws. The inner-diameter cutting saw shown
in Fig. 24.25f is used widely to cut single-crystal silicon wafers in microelectronic
devices (see also Section 28.4).
Band saws have continuous, long, flexible blades and thus have a continuous
cutting action. Vertical band saws are used for straight as well as contour cutting of
flat sheets and other parts supported on a horizontal table (Fig. 24.25 d). Also avail-
able are computer-controlled band saws with the capability of guiding the contour
path automatically. Power band saws are available as well; they have higher produc-
tivity than power hacksaws because of their continuous cutting action. With high-
speed steel blades, cutting speeds for sawing high-strength alloys are up to about
60 m/min and 120 m/min for carbon steels.
Blades and high-strength wire can be coated with diamond powder (diamond-
edged blades and diamond-wire saws) so that the diamond particles act as cutting
teeth (abrasive cutting); carbide particles also are used for this purpose. These blades
and wires are suitable for sawing hard metallic, nonmetallic, and composite materi-
als. Wire diameters range from 13 mm for use in rock cutting to 0.08 mm for preci-
sion cutting. Hard materials also can be sawed with thin, abrasive disks and with
advanced machining processes (Chapter 27).

Friction Sawing. Friction sawing is a process in which a mild-steel blade or disk
rubs against the workpiece at speeds of up to 7,600 m/min. The frictional energy
is converted into heat, which rapidly softens a narrow zone in the workpiece. The
action of the blade, which can have teeth or notches for higher cutting efficiency,
pulls and ejects the softened metal from the cutting zone. The heat generated in
the workpiece produces a heat-affected zone (Section 30.9) on the cut surfaces.
Thus, the workpiece properties along the cut edges can be affected adversely by this