Machining Fundamentals 160 Copyright Goodheart-Willcox Co., Inc. 11.2 Power Hacksaw A power hacksaw, Figure 11-3, uses a reciprocating motion to move the blade across the work. The blade cuts on the backstroke. Types of feeds available include: • Positive feed—produces an exact depth of cut on each stroke. The pressure on the blade varies with the number of teeth in contact with the work. • Definite pressure feed—exerts a uniform pressure on the blade regardless of the number of teeth in con- tact with the work. The depth of the cut varies with the number of teeth contacting the work. Gravity feed is an example of definite pressure feed. Feed can be adjusted to meet varying conditions. For best performance, the blade and feed must be selected to permit high-speed cutting and heavy feed pressure with minimum blade bending and breakage. Standard power hacksaws are available in sizes from 6″ × 6″ (150 mm × 150 mm) to 24″ × 24″ (900 mm × 900 mm). The saws can be fitted with many accessories. A swivel vise per- mits angular cuts to be made quickly, Figure 11-4. Quick- acting vises allow faster manual clamping of the workpiece. Power stock feed, power clamping of the workpiece, and automatic cycling can automate the cutting operation. Auto- matic cycling moves the work out the required distance, clamps it, and makes the cut automatically. The cycle is repeated on completion of the cut. High-speed cutting requires the use of a coolant. The coolant reduces friction, increases blade life, and prevents chip-clogged teeth. Cast iron and some soft brass alloys, unlike most materials, do not require coolant. 11.2.1 Selecting a Power Hacksaw Blade Proper blade selection is important. Use the three-tooth rule: at least three teeth must be in contact with the work at all times while cutting. Large sections and soft materials require a coarse-tooth blade. Small or thin work and hard materials require a fine-tooth blade. For best cutting action, apply heavy feed pressure on soft materials and large work. Use light feed pressure on hard materials and work with small cross sections, Figure 11-5. Blades are made in two principal types: flexible back and all-hard. The choice depends on use. A flexible back blade should be used when safety requirements demand Goodheart-Willcox Publisher Figure 11-3. An industrial reciprocating power hacksaw. Worakit Sirijinda/Shutterstock.com Figure 11-4. A swivel vise permits angular cuts. Heavy feed pressure Large work or soft metals Light feed pressure Small work or hard metals Goodheart-Willcox Publisher Figure 11-5. Apply heavy feed pressure on soft metals and large work. Use light pressure on hard metals and work with small cross sections.