23.10 Selecting Blades Blades, regardless of the machine they are on, are designed to cut efficiently and effectively. Select- ing the proper blade is an important part of sawing. Using the wrong blade may ruin the workpiece, dull the blade prematurely, or result in extra work, such as sanding. For example, crosscutting with a rip blade creates problems. Rip teeth are larger and have a different cutting angle that may cause the wood to splinter. The blade you choose depends on the sawing operation. You may be sawing solid wood or ply- wood across or along the grain. You could also be cutting nongrain composites such as hardboard, particleboard, or even plastic. After making a cut, inspect the cut edges of the workpiece. Look to see how rough or smooth they are. Burn marks result from using a dull blade, improperly adjusting the rip fence, or feeding stock too slowly. Inspect the blade frequently to determine how well it is performing. Saw blade performance is based on tooth design and chip load. Chip load is the thickness of a chip that is removed by one cutting edge of the tool. In the case of a saw blade, chip load depends on the: • Number of teeth. • Size of the gullet. • Speed of the blade. • Rate of feed. Chip load is a factor for all types and styles of blades. If wood chips totally fill the gullet, the blade will cut poorly. This is because there is no more room in the blade to hold sawn chips. You can feed faster with large gullet blades, but the sawn edge will be rougher. Too much pressure when feeding causes the blade to heat up due to increased friction. Excess heat can remove the temper from a blade, causing it to dull prematurely. 23.10.1 Circular Blade Circular blades are used on table saws, radial arm saws, power miter saws, and various stationary power saws. Important blade specifications include blade diameter, tooth design (hook angle, cutting edge shape, and number of teeth), kerf width, and the size of the arbor hole. Diameter Machines are made to use blades with a wide range of diameters. Machines are described in terms of the maximum blade diameter installed in the machine, such as a 10″ table saw or a 7 1/4″ por- table circular saw. Smaller diameter blades reduce the maximum depth of cut. Most 10″ (254 mm) blades mounted on a 10″ table saw will cut through 2″ (50 mm) material at 45° and 3″ (75 mm) material at 90°. Hook Angle Hook angle refers to the angle at which the front edge of the tooth contacts the material. This angle is created between the face of the tooth and a line that extends from the tooth tip to the arbor hole. Rip blades may have a hook angle between 10° and 20°. Blades designed for power miter, radial arm, and other pendulum type saws generally have smaller hook angles and may even have 0° or a negative hook angle. A negative angle gives you greater con- trol over the feed rate. Safety in Action Using the Scroll Saw • Fasten loose clothing, secure long hair, and remove jewelry. • Wear safety glasses, goggles, or a face shield. • Make sure the saw blade teeth point down. • Make sure the proper blade type and size is installed. • Keep the floor around work area clean and free from sawdust and scraps. • Change blades (and speed on machines having step pulleys) with switch off and the power disconnected. • Check all adjustments by rotating the motor by hand before turning power on. • The hold-down should press lightly on the material being cut. • Stop the machine before removing excess stock from the table. • Do not attempt to saw large stock without proper support. • Hold material firmly and feed it into the blade at a moderate speed. • Make relief cuts before cutting tight curves. • Shut off the power and let the machine come to a complete stop before leaving it. 400 Section 4 Machining Processes Copyright Goodheart-Willcox Co., Inc.