Riving knife

A riving knife is a safety device installed on a table saw, circular saw, or radial arm saw used for woodworking. 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 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.

A table saw 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 close up and bind the blade. Poor blade or fence 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.

Saw blade "grabbing" occurs more frequently during ripping than cross-cutting, and may manifest with both hand saws and bandsaws. It is more dangerous with a circular saw as areas of the 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, as normal, and the saw may either jam, stall or at worst break the blade. If a table saw grabs, this is likely to be at the rear of the blade, where the teeth are rising up from the table and so may lift the wood upwards, out of control. The wood is then likely to catch the teeth on top of the blade and be thrown forwards, towards the operator. This accident is termed a "kickback".

A second form of 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.

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Wikipedia