SAWS, cutting-tools with toothed edges. The various types of saw may be classified into reciprocating, revolving, and travel ling, i.e., band-saws passing around wheels. The first class in cludes numerous hand- and machine-operated blades, some cut ting only on the one stroke, others cutting both backwards and forwards. The second class covers the circular saws for wood, bone, ivory and metal; the largest of the third class are made up to about 12 ft. in diameter. Large band-saws are used for cutting up big forgings into various outlines. Some saws have the teeth milled or punched out of the solid plate or web; others have teeth fastened in with wedges, so that they are easily replaced in case of fracture, besides being easier to make and temper. What are termed friction saws have either no teeth or but slight notches on the periphery, and they will cut through iron and steel owing to the heat generated by the friction at the great speed of rotation. Diamond saws have a large number of diamonds fixed in pockets on the rim and are employed for stone sawing. The principal difficulties with saws are clearance in the "kerf," which depends on the "set" or side projection of the teeth beyond the web or plate of the saw; and true cutting, which depends partly on guidance and partly on the truth of the blade.
Circular saws for wood are hammered to make a "tension," so that although the saw does not lie true while at rest, its rim runs perfectly true at the appropriate speed for which it has been tensioned.
Saws date from Neolithic times, when they were formed from flint flakes with finely jagged edges; they were followed by metal saws made in bronze or copper. Now steel is employed ex clusively, some for sawing wood being tempered soft enough to be sharpened with a file, while those for cutting metal can only be sharpened with a grinding wheel. The fineness or coarseness of teeth varies greatly, according to the class of sawing. So do the shapes of teeth, some pointing forward with straight or curved edges, others of equal angles and many of special M shapes for heavy cross-cutting of lumber. The most elaborate teeth have "cleaner" teeth interspaced so as to scrape out the sawdust and clean the kerf neatly. In the larger circular saws the inserted class of teeth is very common. These are held in pockets around the disc with a V fastening and a springy holder, or with a wedge fixing. (See WOOD-WORKING MACHINERY ; MA CHINE-TOOLS.)