Scissors
From Wikipedia, the free encyclopedia
Scissors are a tool used for cutting thin material which requires little force. They are used for cutting, for example, paper, cardboard, metal foil, thin plastic, food, cloth, rope and wire. They are also used for cutting hair and nails.
Unlike a knife, scissors have two pivoted (or hinged ) blades. Most types of scissors are not particularly sharp; it is primarily the shearing between the two blades which cuts. Children's scissors are even less sharp, and are often protected with plastic.
Although often used interchangeably with "scissors," the term shears is reserved by those in the industry for scissors longer than 15 cm[1].
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[edit] Mechanics
Mechanically, scissors are a first-class, 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 (i.e., the hand) is twice as far away from the fulcrum as the cutting location (e.g., piece of paper), 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 but placing the material to be cut close to the fulcrum. Scissors cut material by applying a local shear stress at the cutting location which exceeds the material's shear strength.
[edit] Kitchen scissors
Kitchen scissors, also known as kitchen shears, are similar to common scissors. The main difference is the location of the fulcrum. Kitchen scissors have the fulcrum located farther from the handles to provide more leverage and thus more cutting power. High quality kitchen scissors can easily cut through the breastbone of a chicken.
[edit] Left handed scissors
Most scissors are best suited to use with the right hand, but left handed scissors are designed for use by the left. Left handed scissors obviously have handles which are comfortable to hold in the left hand, but they have other differences, more subtle yet equally important.
Because scissors have overlapping blades, they are not symmetric. This asymmetry is true regardless of the orientation and the shape of the handles: the blade that is on top always forms the same diagonal regardless of orientation. Human hands are also asymmetric and worse, when closing the thumb and fingers do not close vertically, but have a lateral component to the motion. Specifically, the thumb pushes out and fingers pull inwards. For right-handed scissors held in the right hand, the thumb blade is closer to the body so that the natural tendency of the right hand is to force the cutting blades together. Conversely, if right-handed scissors are held in the left hand, the natural tendency of the left hand would be to force the cutting blades laterally apart.
Furthermore, with right-handed scissors held by the right-hand, the shearing edge is visible. When used with the left hand, the cutting edge of the scissors would be behind the top blade, and one cannot see what is being cut.
Some scissors are marketed as ambidextrous. They have symmetric handles so there is no distinction between the thumb and finger handles, and they have very strong pivots so that the blades simply rotate and do not have any lateral give. However, most "ambidextrous" scissors are in fact still right-handed. Even if they successfully cut, the blade orientation will block the view of the cutting line for a left-handed person. True ambidextrous scissors are possible if the blades are double-edged and one handle is swung all the way around (to almost 360 degrees) so that the back of the blades become the new cutting edges[2].
Using scissors designed for the wrong hand is very difficult for most people — even for left-handers who have become accustomed to using the more readily available right-handed scissors. They have to unnaturally force the blades together to cut and stretch their necks over the top blade to see what is being cut. This unnatural motion can also cause marks on the hand, sores, and eventually calluses.
[edit] History
Scissors were likely invented in 1500 BC in ancient Egypt[3]. These were likely shears with the joint at the far end[4]. Modern cross-bladed scissors were invented by Romans around AD 100.
An immense stride in perfecting scissors was taken in 1761 when one Robert Hinchliffe produced the first pair made of cast steel, hardened and polished. The forming of the bows was his greatest difficulty. Somehow he solved problems with the hole. A ready sale was found for his wares in London and other markets. He lived in Cheney Square, and was reputed to be the first person who put out a signboard proclaiming himself "fine scissor manufacturer".[5].
In a part of Sweden (today Finland) an ironworks was started 1649 in the hamlet "Fiskars" between Helsingfors and Åbo (fi: Turku). In 1830 a new owner started the first cutlery works in Finland, now a part of Russia, making among others scissors with the trade mark FISKARS. In the sovereign Finland FISKARS Corporation introduced in 1967 new methods in the manufacturing of scissors.[6].
[edit] Manufacturing
The blades of scissors are manufactured to be slightly curved; this provides the best cutting performance. The grinding of the curved blades has always been the most demanding task in the manufacturing of scissors. Before the 1960s, mechanizing the grinding process proved difficult. It is a tricky task to move the blade along a curved path against the grinding wheel, and "automated" scissors grinders developed up until that time imitated the manual work. Therefore the machine could hold only one blade at the time, and even then skilled hands were needed to provide the finishing touches.
Leonardo da Vinci has in his notebooks described a wooden grinding wheel with leather circumference coated with a mixture of oil and emery abrasives [1]. This type of grinding wheel has been in use for a long time in cutlery works, see picture "1962".
In 1967, the Fiskars Corporation,[7] Finland, introduced a new method using surface grinders for scissors. The method is based on the discovery that the slightly curved surface of the blade can be generated by parallel lines, see the picture "1963".
Also the contour and the edge can be ground with a formed grinding wheel as shown in the picture "1967". Instead of moving one blade along a curved path, the surface grinder can keep many blades with a magnetic holding device in a rectilinear motion. The edge is in contact with the grinding wheel during one thousandth part of a second and flooded with cutting fluid for several seconds during the motion of the table.
By the former "automated" grinders the edge was in contact with the grinding wheel during several seconds. This method could only operate with very soft non-ceramic grinding wheels. The method of formed grinding wheel was not possible with soft wheels.
At the World Exhibition in Seville, Spain, in 1992, the vice-president of Fiskars Corporation explained that a man at Fiskars scissors factory in 8 hours grinds the same number of scissors as during one year in 1962 [2].
Scissors with ergonomic handles date from designs developed in around 1880. Modern versions from 1967 include using plastic handles instead of brass, the flat form of the blades and joining the blades with a rivet instead of a screw. Owing to the precision of the surface grinders one grinding operation is sufficient for each surface and contour of the scissors.
[edit] Specialized scissors
- Pinking shears are scissors with a serrated cutting edge.
- Tin snips are scissors for cutting through sheet metal, for example tin plate.
- Pruning shears and secateurs are scissors for cutting through branches of tree and shrubs.
- Trauma shears, or 'Tuff Cuts', are robust scissors used in emergency medical response and rescue.
In the following, the blades meet and compress the material instead of shearing it so they are technically not scissors
- Bolt cutters cut through heavy wire and bolts.
- Nippers cut small pieces out of tile.
A hemostat resembles a pair of scissors but has a locking clamp instead of a blade; it is used in surgery and does not cut at all.
