All About Scissors
September 16, 2009
Scissors come in all shapes and sizes and are used for applications as simple as cutting paper and as challenging as cutting Kevlar and for everything in between. Unlike a knife, scissors have two pivoted (or hinged) blades. Surprisingly, most types of scissors are not particularly sharp; it is primarily the shearing force between the two blades which cuts.
How They Work
Mechanically, scissors are a double-lever with the pivot/hinge acting as the fulcrum. For cutting thick or heavy material, the mechanical advantage of a lever can be exploited by placing the material to be cut as close to the fulcrum as possible. For example, if the applied force (your hand) is twice as far away from the fulcrum as the cutting location (the item being cut), the force at the cutting location is twice that of the applied force at the handles. Specialized scissors, like bolt cutters exploit leverage by having a long handle and placing the material to be cut close to the fulcrum.
Most people think scissors are only made from “cutlery grade” (420) stainless steel. In reality, a variety of materials are used for scissor blades including carbon steel (1095), other stainless steels (301, 400 series, 17-4 and 17-7 PH), high speed steels (M2, M4), tool steels (A2, A7, A11, D2, S7), tungsten carbide and even ceramics such as zirconia (ZrO2). Coatings (TiN, TiC, TiCN, boron carbide, Armoloy®) and dry film lubricants (e.g., Teflon®) have also been used for improved cutting performance.
The Role of Heat Treating
Heat treating of both forging and stampings are common, and after all of the screw holes have been drilled, depending on material the scissor, they are heated to temperatures between 1450°F (790°C) and 2000°F (1100°C) and rapidly quenched. Most scissors are heat treated either in furnaces (atmosphere or vacuum), salt baths, or by induction.
Induction hardening can be used to harden only the edge on scissors, or to anneal and soften handles on scissors so that they can be set or processed and is usually performed on less expensive types of scissors.
One popular scissors manufacturer processes his stainless steel scissors blades in an electrically heated mesh belt conveyor furnace running under dissociated ammonia atmosphere. Extremely rapid quenching through the critical carbide precipitation range of 1900°F (1040°C) - 900°F (480°C) is required to produce the desired hardness, with a controlled amount of retained austenite. A tempering operation follows hardening.
Deep cryogenic treatment (-300°F) has bee reported to improve wear resistance on scissor blades. The added expense can be justified for certain applications such as surgical scissors.
History of Scissors
Although commonly ascribed to Leonardo da Vinci, scissors were likely invented in 1500 BC in ancient Egypt. These were like modern shears with the joint at the far end. Modern cross-bladed scissors were invented by Romans around AD 100. The word “scissor” has its origin in the Latin word “cisoria,” meaning a cutting instrument, but the spelling is due to confusion with the Latin word “scissor,” a form of the verb “scinder,” meaning to cut. IH