many teeth to be in contact with the material. The more
teeth in contact, the greater the feed pressure required
to force them into the material. Excessive feed
pressure will cause off-line cutting.
The POWER HACKSAW is found in all except
the smallest shops. It is used for cutting bar stock, pipe,
tubing, or other metal stock. The power hacksaw (fig.
12- 13) consists of a base, a mechanism for causing the
saw frame to reciprocate, and a clamping vise for
holding the stock while it is being sawed. Two types
of power hacksaws are in use today: the direct
mechanical drive and the hydraulic drive.
The capacity designation of the power hacksaw
shown is 4 inches by 4 inches. This means that it can
handle material up to 4 inches in width and 4 inches
Three types of feed mechanisms are in use today.
They are as follows:
1. Mechanical feed, which ranges from 0.001 to
0.025 inch per stroke, depending upon the class and type
of material being cut.
2. Hydraulic feed, which normally exerts a
constant pressure but is so designed that when hard
spots are encountered, the feed is automatically stopped
or shortened to decrease the pressure on the saw until
the hard spot has been cut through.
3. Gravity feed, which provides for weights on the
saw frame. These weights can be shifted to increase or
decrease the pressure of the saw blade on the material
All three types of feed mechanisms lift the blade
clear of the work during the return stroke.
The blade shown in figure 12-14 is especially
designed for use with the power hacksaw. It is made
with a tough alloy steel back and high-speed steel
teeth-a combination which gives both a strong blade
and a cutting edge suitable for high-speed sawing.
These blades vary as to the pitch of the teeth
(number of teeth per inch). The correct pitch of teeth
for a particular job is determined by the size of the
section and the material to be cut. Use coarse pitch
teeth for wide, heavy sections to provide ample chip
clearance. For thinner sections, use a blade with a pitch
that will keep two or more teeth in contact with the
work so that the teeth will not straddle the work. Such
straddling will strip the teeth. In general, you select
blades according to the following information:
1. Coarse (4 teeth per inchfor soft steel, cast
iron, and bronze.
2. Regular (6 to 8 teeth per inch)for annealed
high carbon steel and high-speed steel.
3. Medium (10 teeth per inch)for solid brass
stock, iron pipe, and heavy tubing.
4. Fine (14 teeth per inch)for thin tubing and
Speeds and Coolants
Speeds on hacksaws are stated in strokes per
minutecounting, of cm-use, only those strokes that
cause the blade to come in contact with the stcck.
Speed changing is usually accomplished by means of
a gearshift lever. There maybe a card attached to your
equipment or near it, stating recommended speeds for
Figure 12-13.Power hacksaw.
Figure 12-14.Power hacksaw blade.