Machining Fundamentals 162
A
flexible-back blade
should be used when
safety requirements demand a shatterproof blade.
These blades should also be used for cutting odd-
shaped work if there is a possibility of the work
coming loose in the vise.
For a majority of cutting jobs, the
all-hard blade
is best for straight, accurate cutting under a variety
of conditions.
SAFETY NOTE T
When starting a cut with an all-
hard blade, be sure the blade does
not drop on the work when cutting
starts. If it falls, the blade could
shatter and fly apart, causing injuries.
Blades are also made from tungsten and molyb-
denum steels, and with tungsten carbide teeth on
steel alloy backs. The following “rule-of-thumb” can
be used for selecting the correct blade:
• Use a 4-tooth blade for cutting large sections
or readily machined metals.
• Use a 6-tooth blade for cutting harder alloys
and miscellaneous cutting.
• Use 10- and 14-tooth blades primarily on
light-duty machines where work is limited
to small sections requiring moderate or light
feed pressure.
11.2.2 Mounting a Power Hacksaw Blade
The blade must be mounted to cut on the power
(back) stroke. The blade must also lie perfectly fl at
against the mounting plates, Figure 11-6. If long life
and accurate cuts are to be achieved, the blade must
be properly tensioned.
Many techniques have been developed for prop-
erly mounting and tensioning blades. Use a torque
wrench and consult the manufacturer’s literature.
If the information (proper torque for a given blade
on a given machine) is not available, the following
methods can be used:
• Tighten the blade until a low musical ring is
heard when the blade is tapped lightly. A high-
pitched tone indicates that the blade is too
tight. A dull thud means the blade is too loose.
• The shape of the blade pin hole can serve as
an indicator of whether the blade is tensioned
properly. When proper tension is achieved,
the pin holes will become slightly elongated,
Figure 11-7.
The blade will become more fi rmly seated after
the fi rst few cuts and will stretch slightly. The blade
will require retensioning (retightening) before fur-
ther cutting can be done.
11.2.3 Cutting with a Power Hacksaw
Measure off the distance to be cut. Allow ample
material for facing if the work order does not spec-
ify the length of cut. Mark the stock and mount the
work fi rmly on the machine, Figure 11-8.
If several sections are to be cut, use a stop gage,
Figure 11-9. Apply an ample supply of coolant if the
machine has a built-in coolant system.
Blade must be
seated solidly
against
mounting plate
Direction
of cut
Tap blade lightly
and listen to ring
Goodheart-Willcox Publisher
Figure 11-6. The blade must be adjusted to cut on the back
stroke. Make sure it is perfectly flat against the mounting plates
before tensioning. Tighten the blade until a low musical ring is
heard when the blade is tapped with a small hammer. Since
blades have a tendency to stretch slightly after making a few
cuts, tension should be checked and, if necessary, adjusted.
Mounting
pin
Elongated
mounting pin hole
Saw
frame
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Figure 11-7. Pin holes on a properly tensioned blade will be
slightly elongated, rather than round.