A riving knife or splitter is a safety device on table saws, circular saws and radial arm saws used for woodworking. They are both comparable in their safety function, but they are distinct and different devices, each with its pros and cons.
Splitters are fixed in relation to the saw table, riving knives are fixed relative to the blade and move with it as blade depth is adjusted.
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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, along the grain. As wood is ripped, but before the two pieces are completely separated, pre-existing stresses in the wood are released. The partially-cut board may move, twist, or very commonly the gap or kerf closes up and the two separate halves of the cut press together and jam around the saw blade.
This "grabbing" is a feature common to all ripping operations and may occur with hand saws, or bandsaws too. 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 circular 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".[1]
A second form of kickback may occur if the saw's fence is mis-adjusted, so as be slightly closer to the rear of the blade than the front, rather than being accurately parallel. In this case the fence pushes the wood into the saw blade, leading to a similar result.[2] This is especially likely when cross-cutting sheet materials that are wider than the cut length. These may pivot on the table and jam against the blade. If a proper cross-cutting jig is not being used, the fence should at least be adjusted (either slid forwards or a false fence added) so that the end of the fence stops alongside the blade.[3] Beyond the cut there is thus a free space for the cut-off to pivot into, without binding.
A third form occurs when a loose piece of wood, freshly cut free, slips against the back of the blade. Apart from the measures above, this "falling board"[4] may require an assistant to control it.
Splitters are stationary blades mounted behind the blade, and of similar thickness. If the kerf grabs inwards, it will first grab onto the splitter rather than the moving blade.[5]
The thickness of a splitter should ideally be greater than that of the saw blade, but thinner than that of the saw kerf. As modern saw blades use inserted teeth of tungsten carbide that form a wider kerf than the metal of the saw's disk, this is now a possibility.[5] Old, or low-cost, saw blades may use a disk that is approximately the same thickness as the kerf they leave, making them more susceptible to grabbing and kickback. These blades may be avoided for just this reason. Even an inserted tooth blade is still full-thickness across its rear teeth, and so their thinner disks are no guarantee against accident.
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.[6] 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:[7]
The way it achieves all of this is that it 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.
As well as table saws, riving knives are also fitted to hand-held circular saws, and to cross-cut saws ("chop saws").[8]
In 2009, Underwriters Laboratories (UL) will require that all new table saw designs include a riving knife.[9]
Featherboards are sprung fingers that apply downward pressure to hold the workpiece against the table. They can reduce the risk of a kickback developing, but will not restrain the board if one does occur.
Some US table saws are fitted with sharpened ratchet teeth on free-swinging pawls attached to the guard. These are intended to restrain a board during a kickback, but require awkward adjustment and are ineffective compared to the simple splitter.[10]
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