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Posted by on Sep 6, 2017 in Tell Me Why |

Why Are There Many Kinds of Saws?

Why Are There Many Kinds of Saws?

Saws are of many shapes and sizes, according to the special purposes for which they are to be used and the materials they are meant to cut. Most are designed to cut through wood but others have the particular qualities needed for cutting metals and stones. There are two main types of hand-saw. Both have steel blades with wooden handles, but the teeth are shaped differently to fit the job they have to do.

One is a cross-cut saw for cutting across the grain of the wood. The leading edges of the teeth slope backward and are also bevelled, or angled, transversely. In other words, they are cut away at the edges to give an oblique angle like the edge of a chisel, thus producing a sharp-pointed profile or shape designed to avoid splintering the wood. The second type is the rip or tenon saw, which is used for cutting along the grain of the wood. This has a smaller blade, strengthened by a steel strip along the top edge, and smaller teeth. It is used, as the name suggests, in cutting tenon or slotted joints.

Smaller hand saws, like the fretsaw, are used for cutting intricate shapes. These have a narrow blade stretched across an open frame. The hacksaw, which is used for cutting metal by hand, also has an open frame, but its blade is deeper and more closely-toothed.

hacksaw

Machine saws are of three main types: first, a larger, stronger hacksaw operated by an electric motor through a crank and connecting rod; second, a circular saw, which has a rotating, disc-shaped blade with teeth on the circumference; and third, a band saw which has a blade formed like an endless flexible band and tightly stretched over pulleys.

The band saw, which has fine teeth along one edge, operates at a higher speed than the circular saw. It can cut round curves of quite small radius because the blade is so narrow.

For stone-cutting in quarrying there are swinging gang-saws with teeth like chisels and flat-bladed circular saws, which are either fed with a mixture of hard sand and water or fitted with rims of a hard abrasive substance called carborundum.

The hand-held power saw driven by an electric or petrol engine is used for felling trees and cutting logs. This is also known as a chain-saw because it is much like a bicycle chain equipped with saw teeth.

hand-held power saw

Saws were at first serrated materials such as flint, obsidian, sea shells and shark teeth. In ancient Egypt, open (unframed) saws made of copper are documented as early as the Early Dynastic Period, circa 3,100–2,686 BC. Models of saws have been found in many contexts throughout Egyptian history. Egyptian saws were at first serrated, hardened copper which cut on both pull and push strokes. As the saw developed, teeth were raked to cut only on the pull stroke and set with the teeth projecting only on one side, rather than in the modern fashion with an alternating set. Saws were also made of bronze and later iron.

In the Iron Age, frame saws were developed holding the thin blades in tension. The earliest known sawmill is the Roman Hierapolis sawmill from the third century AD and was for sawing stone. According to Chinese legend, the saw was invented by Lu Ban. In Greek mythology, as recounted by Ovid, Talos, the nephew of Daedalus, invented the saw. In archeological reality, saws date back to prehistory and most probably evolved from Neolithic stone or bone tools. “The identities of the axe, adz, chisel, and saw were clearly established more than 4,000 years ago.”

Once mankind had learned how to use iron, this became the preferred material for saw blades of all kinds; some cultures learned how to harden the surface (“case hardening” or “steeling”), prolonging the blade’s life and sharpness. Steel, made by quenching hot iron in water, was used as early as 1200 BC. By the end of the 17th century European manufacture centred on Germany (the Bergisches Land) and in London and the Midlands of England. Most blades were made of steel (iron carbonised and re-forged by different methods).

In the mid 18th century a superior form of completely melted steel (“crucible cast”) began to be made in Sheffield, England, and this rapidly became the preferred material, due to its hardness, ductility, springiness and ability to take a fine polish. A small saw industry survived in London and Birmingham, but by the 1820s the industry was growing rapidly and increasingly concentrated in Sheffield, which remained the largest centre of production, with over 50% of the nation’s saw makers. The US industry began to overtake it in the last decades of the century, due to superior mechanisation, better marketing, a large domestic market, and the imposition of high tariffs on imports. Highly productive industries continued in Germany and France.

Early European saws were made from a heated sheet of iron or steel, produced by flattening by several men simultaneously hammering on an anvil, after cooling; the teeth were punched out one at a time with a die, the size varying with the size of the saw. The teeth were sharpened with a triangular file of appropriate size, and set with a hammer or a wrest. By the mid 18th century rolling the metal was usual, the power for the rolls being supplied first by water, and increasingly by the early 19th century by steam engines.

The industry gradually mechanized all the processes, including the important grinding the saw plate “thin to the back” by a fraction of an inch, which helped the saw to pass through the kerf without binding. The use of steel added the need to harden and temper the saw plate, to grind it flat, to smith it by hand hammering and ensure the springiness and resistance to bending deformity, and finally to polish it.

Most hand saws are today entirely made without human intervention, with the steel plate supplied ready rolled to thickness and tensioned before being cut to shape by laser. The teeth are shaped and sharpened by grinding and are flame hardened to obviate (and actually prevent) sharpening once they have become blunt. A large measure of hand finishing remains to this day for quality saws by the very few specialist makers reproducing the 19th century designs.

Content for this question contributed by Beth Puget, resident of Pittsburgh, Allegheny County, Pennsylvania, USA