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 that 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
Both forging and stampings are common. After all of the screw holes have been drilled, depending on the material of 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. It is usually performed on less-expensive types of scissors.
One popular manufacturer processes stainless steel scissor blades in an electrically heated mesh-belt conveyor furnace running under dissociated ammonia atmosphere. Extremely rapid quenching through the critical carbide precipitation range of 1900-900°F (1040-480°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 been 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 the 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