Chapter 11 Sawing and Cutoff Machines 161 Copyright Goodheart-Willcox Co., Inc. 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 the majority of cutting jobs, the all-hard blade is best for straight, accurate cutting under a variety of conditions. SAFETY NOTE 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 fl y apart, causing injuries. Blades are also made from tungsten and molybdenum steels, and with tungsten carbide teeth on steel alloy backs. The following rules of thumb can be used to select the cor- rect blade: • Use a 4-tooth blade to cut large sections or readily machined metals. • Use a 6-tooth blade to cut harder alloys and for mis- cellaneous 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 of a hacksaw must be mounted to cut on the power (back) stroke. The blade must also lie perfectly flat against the mounting plates, Figure 11-6. A properly tensioned blade provides long blade life and accurate cuts. Many techniques have been developed for properly mounting and tensioning blades. Use a torque wrench and consult the manufacturer’s literature. If information about the 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 indica- tor of whether the blade is tensioned properly. When proper tension is achieved, the pin holes become slightly elongated, Figure 11-7. The blade becomes more firmly seated after the first few cuts and stretches slightly. The blade requires retensioning (retightening) before further 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 specify the length of cut. Mark the stock and mount the work firmly 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. 11.3 Power Band Saw The horizontal band saw, Figure 11-10, is frequently referred to as a cutoff machine. It offers the following advantages over the power hacksaw: • Greater precision. The blade on a band saw can be guided more accurately than the blade on a power hacksaw. It is common practice to cut directly “on the line” when band sawing because finer blades can be used. 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 backstroke. Make sure it is perfectly fl at 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 Goodheart-Willcox Publisher Figure 11-7. Pin holes on a properly tensioned blade are slightly elongated, rather than round.