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.[1]
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.[1]
Function
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 kerf 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.[1]
Forms of kickback
Saw blade "grabbing" occurs more frequently during ripping than cross-cutting, and may manifest with both hand saws, or 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".[2]
A second form of kickback may occur if the saw's fence is mis-adjusted, so as to 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.[3] 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.[4] 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"[5] 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.[6] Like a riving knife, its thickness should be greater than the body of the saw blade but thinner than its kerf.[6] Blades with a narrow kerf relative to their body are more susceptible to grabbing and kickback.
A riving knife has numerous advantages over a splitter:[7]
- 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 saw blade as the blade is raised, lowered and tilted.[8]
Riving knives are also fitted to some hand-held electrical circular and powered miter or cross-cut saws (known generically as "chop saws").[9]
As of 2009, Underwriters Laboratories (UL) requires that all new table saw designs include a riving knife.[10]
Other anti-kickback devices
Featherboard
A featherboard is a safety device that applies downward pressure holding the workpiece against the saw table. 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.[11]
References
- 1 2 3 What is the difference between a Table Saw Splitter/Spreader and a Table Saw Riving Knife?
- ↑ Kirby, Ian (1998). The Accurate Table Saw. Cambium Press. pp. 10–11. ISBN 978-0964399952.
- ↑ Kirby, Accurate Table Saw, p. 40
- ↑ Kirby, Accurate Table Saw, p. 96
- ↑ Kirby, Accurate Table Saw, p. 23
- 1 2 "Circular saw benches - Safe working practices" (PDF). Health & Safety Executive, UK.
- ↑ "Fight Kickback with a Riving Knife". Fine Woodworking.
- ↑ "Riving Knife Selection" (PDF). DEWALT Industrial Tool Co.
- ↑ "Safe use of manually operated cross-cut saws" (PDF). Health & Safety Executive, UK.
- ↑ Stationary and Fixed Electric Tools UL 987
- ↑ Kirby, Accurate Table Saw
