A riving knife is a safety device installed on a

table saw A table saw (also known as a sawbench or bench saw in England) is a woodworking tool, consisting of a circular saw blade, mounted on an arbor, that is driven by an electric motor (either directly, by belt, or by gears). The blade protrudes t ...

circular saw A circular saw is a power-saw using a toothed or abrasive disc or blade to cut different materials using a rotary motion spinning around an arbor. A hole saw and ring saw also use a rotary motion but are different from a circular saw. ''Cir ...

, or

radial arm saw A radial arm saw is a cutting machine consisting of a circular saw mounted on a sliding horizontal arm. Invented by Raymond DeWalt in 1922, the radial arm saw was the primary tool used for cutting long pieces of stock to length until the introdu ...

used for

woodworking Woodworking is the skill of making items from wood, and includes cabinet making (cabinetry and furniture), wood carving, woodworking joints, joinery, carpentry, and woodturning. History Along with Rock (geology), stone, clay and animal parts, ...

. Attached to the saw's arbor, it is fixed relative to the blade and moves with it as blade depth is adjusted. A ''splitter'' is a similar device attached to a

trunion A trunnion (from Old French "''trognon''", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development. Alternatively, a trunnion is a shaft that positions a ...

on the far side of the saw and fixed in relation to the saw table, which must be removed to make any non-through cuts or dados within the depth of the wood.

Function

is typically used for cross-cutting and ripping; cross-cutting slices a board across its grain width-wise, ripping cuts lengthwise along the grain. Various conditions experienced while cutting either way can cause a partially cut board to move, twist, or have the saw blade's

kerf A saw is a tool consisting of a tough blade, wire, or chain with a hard toothed edge. It is used to cut through material, very often wood, though sometimes metal or stone. The cut is made by placing the toothed edge against the material and mov ...

close up and bind the blade. Poor blade or

fence A fence is a structure that encloses an area, typically outdoors, and is usually constructed from posts that are connected by boards, wire, rails or netting. A fence differs from a wall in not having a solid foundation along its whole length. ...

alignment, operator error, or pre-existing stresses in the wood released by cutting may cause these different and dangerous conditions. A riving knife rides within the kerf, pivoting on the saw's arbor in relation to blade height, to maintain an even gap between the two cut sides of the board, preventing jamming which could cause the stock to be forcefully ejected rearward toward the saw's operator. Kickback can pull the operator's hand into contact with the saw blade, as demonstrated by

Popular Mechanics ''Popular Mechanics'' (sometimes PM or PopMech) is a magazine of popular science and technology, featuring automotive, home, outdoor, electronics, science, do-it-yourself, and technology topics. Military topics, aviation and transportation o ...

Forms of kickback

Saw blade "grabbing" occurs more frequently during ripping than cross-cutting (cuts made to wood or stone across its main grain or axis). It can occur with both

hand saw In woodworking and carpentry, hand saws, also known as "panel saws", are used to cut pieces of wood into different shapes. This is usually done in order to join the pieces together and carve a wooden object. They usually operate by having a ser ...

s and

bandsaw A bandsaw (also written band saw) is a power saw with a long, sharp blade consisting of a continuous band of toothed metal stretched between two or more wheels to cut material. They are used principally in woodworking, metalworking, and lumb ...

s but is more dangerous with a

as areas of the circular blade close to the cutting area are moving in different directions. If a bandsaw grabs, the wood is pressed safely down into the machine table (though the saw may jam, stall or break the blade). If a table saw grabs at the rear of the blade where the teeth are rising up from the table, it may rapidly lift the wood upwards. The wood is then likely to catch the teeth on top of the blade and be thrown forwards at high speed towards the operator. This accident is termed a "kickback". Table saw kickback may occur if the saw's fence is not parallel with the blade, but is slightly closer to the rear of it than the front, causing the fence to push the wood into the rear of the blade. This is especially likely when cross-cutting sheet materials that are wider than the cut length, which may pivot on the table and jam against the blade. If a proper cross-cutting jig is not being used, the fence should be adjusted (either slid forward, or a false fence added) so the end of the fence stops alongside the blade, leaving a free space for the cut-off to pivot into without binding. Kickback may also occur when a loose piece of wood, freshly cut free, slips against the back of the blade. Apart from the measures above, this "''falling board''" may require an assistant to control it.

Riving knife versus splitter

A splitter is a stationary blade of similar thickness to the rotating saw blade mounted behind it to prevent a board from pinching inward into the saw kerf and binding on the saw blade, potentially causing a dangerous kickback. Like a riving knife, its thickness should be greater than the body of the saw blade but thinner than its kerf. Blades with a narrow kerf relative to their body are more susceptible to grabbing and kickback. A riving knife has these advantages over a splitter: * It does not need to be removed from the saw when cross-cutting or doing a blind (non-through) cut as it does not extend above the top of the saw blade. If it is not removed, the operator cannot forget to put it back on. * It sits closer to the back edge of the blade, making it much more effective – less space for the stock to shift into the path of the blade * It provides some additional protection for the operator – blocking contact to the back edge of the blade – in those situations where the stock is being pulled from the outfeed side of the saw. * It is independent of (and will not interfere with) other blade guards and dust collectors It achieves all of this by being attached to the saw's arbor, allowing it to move with the saw blade as the blade is raised, lowered and tilted. Riving knives are also fitted to some hand-held electrical circular and powered miter or cross-cut saws (known generically as "chop saws"). As of 2008, Underwriters Laboratories (UL) requires that all new table saw designs include a riving knife.

Other anti-kickback devices

Featherboard

featherboard A featherboard is a safety device used when working with stationary routers or power saws such as table saws or bandsaws. The purpose of a featherboard is to apply pressure against a workpiece, keeping it flat against a machine table or fence ...

is a safety device that applies sideways pressure holding the workpiece against the saw fence. It can reduce the risk of a kickback developing, but will not restrain the board if one does occur.

Kickback pawl

Some US table saws are fitted with sharpened ratchet teeth on a free-swinging pawl attached to the guard which restrain a board during a kickback. This combination may require awkward adjustment and is ineffective compared to the splitter.