Riving knife

A riving knife is a safety device on table saws used for woodworking.

Table saws are typically used for two functions - cross-cutting and ripping. Cross-cutting is slicing a board across its width and across the grain of the wood. Ripping is cutting a board lengthwise, with the grain. A riving knife prevents a phenomenon known as kickback. This occurs when the wood is caught by the rear edge of the table saw blade, lifted off the table and propelled backwards toward the operator.

Two circumstances usually cause kickback: The closure of the kerf behind the blade due to the relief of stresses in the wood as it is cut, or a binding of the wood between the blade and a vertical fence used to guide the wood into the blade during the ripping operation.

Table saws are sometimes equipped with some kind of "splitter", a stationary blade of metal or plastic that holds the kerf open behind the blade. The safety function of a riving knife is the same as a splitter - it prevents the slot cut into kerf from closing behind the blade on a rip, or allowing the stock that may bind between the blade and fence from getting caught by the teeth on the back of the blade

A riving knife differs from a simple splitter in some important ways:

* It doesn't need to be removed from the saw when cross-cutting or doing a blind (non-through) cut as it doesn't extend above the top of the saw blade. If it isn't removed, the operator can't 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's independent of (and won't interfere with) other blade guards and dust collectors

The way it achieves all of this is that is mounted on the same mechanism that mounts the blade, allowing it to move with the saw blade as it's raised, lowered and tilted. To work properly, the knife should be just slightly less than the width of the blade, and is just slightly shorter than the blade.

In 2009, Underwriters Laboratories (UL) will require that all new table saw designs include a riving knife. [http://ulstandardsinfonet.ul.com/tocs/tocs.asp?doc=s&fn=0987.toc Stationary and Fixed Electric Tools UL 987] ]

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