What is a riving knife?

A riving knife is a piece of safety hardware on a table saw. It usually consists of a single thin piece of metal, slightly thinner than the blade kerf, mounted directly behind the rotating table saw blade. As material passes through the spinning saw blade, the two halves pass on either side of the riving knife. The riving knife keeps the pieces separated and prevents them from pinching back together. This helps avoid dangerous kickback, which could cause the workpiece to be thrown back in the face of the operator at high speed.

Why are riving knives important?

Table saws are one of the most dangerous tools in the typical workshop. There is the obvious danger of injury from the spinning saw blade. But another common cause of accidents is harder to see.

Related: Table Saw Safety: Tips to Live By

Kickback is caused when the rear teeth of the saw blade catch the workpiece as they spin up and towards you. While the front teeth are always engaging the workpiece, they are pushing it down against the top of the table saw. The rear teeth are only pushing it against air, so if they catch the workpiece, they can fling it up and back towards the operator at up to 100 miles per hour. Ideally, the workpiece slides smoothly past the spinning blade once it has been cut by the front teeth, without engaging the rear teeth at all. But a couple factors are working to cause problems.

The act of pushing the workpiece through the saw blade causes a small rotational force, no matter how hard you may try to only push straight forward. This force wants to rotate the rear of the workpiece into the rip fence and the front of it into the rear of the saw blade. Kickback can result, no matter what type of material is being cut.

When cutting wood, another factor comes into play. Since wood was once a living organism, its internal structure is not completely regular. As a tree grows, different internal stresses occur in its cellular structures. And later, as the wood dries, it does so somewhat unevenly, creating more stresses. When a piece of wood is cut in two, some of those internal stresses are unleashed, and the wood may bend or warp in new directions. If it happens to bend so that it pinches together on the saw blade, kickback can occur.

A riving knife helps prevent both of these situations by forcing the two cut pieces of material to stay apart. Since they are kept away from the rear teeth of the blade, the risk of kickback is almost completely eliminated.

Are riving knives required in the U.S.?

Yes and no, at least for home or hobbyist use. There is no law that says a table saw must have a riving knife. In practical terms, though, any table saw marketed for sale in the U.S. needs to have one. (Note: Nothing on this site should be construed as legal advice. If in doubt, always consult your attorney.)

That’s because of UL 987, Stationary and Fixed Tools, an industry standard covering table saws. While compliance with the standard is voluntary, most businesses and other organizations will only purchase UL-certified products when possible. And many insurance policies will not cover accidents resulting from the use of non-UL-compliant products. So, in effect, its standards are requirements for any manufacturer hoping to sell its products in the U.S.

Related: Table Saw Buying Guide: What to Look For

UL 987’s sixth edition, published in January 2005, added a requirement for riving knives on all new table saws submitted for UL certification as of January 31, 2008. And it also required riving knives be added to the design of all previously-certified table saws by January 31, 2014.¹ Previously, UL 987 had only required a splitter (which it calls a “spreader”). Now, a combination riving-knife/blade-guard unit is considered to be in “riving knife position” if the blade guard is removed to allow dado or other non-through cuts. It’s in “spreader” position if the blade guard is attached.²

Employees using table saws at work in the U.S. are covered by Occupational Safety and Health Administration (OSHA) regulations. A regulation at 29 CFR § 1910.213(c)(1), Woodworking Machinery Requirements codifies the requirement that table saws used in employment settings have a spreader. This requirement has not been updated to require riving knives in the way that UL 987 has.² But any commercially-available table saw purchased these days will have a riving knife as its spreader rather than a simple splitter.

What's the difference between a riving knife and a splitter?

A splitter performs a very similar task to a riving knife. It keeps the two halves of a sawn piece of wood from pinching back together.

There are a couple important differences between the two, namely their effectiveness at lower blades heights and their ability to handle through-cuts.

Effectiveness at different blade heights

A splitter is generally mounted to a fixed part of the saw like the trunnion. It remains the same height no matter how high or low the saw blade is positioned. Since table saw blades are circular, the lower they go, the longer the effective distance between the back of the blade and the splitter. This means more opportunity for the workpiece to pinch back together before it reaches the splitter, increasing the risk of kickback.

Riving knives, however, are mounted to the same arbor assembly as the blade, so they move up and down in unison with the blade. So the distance between the blade and the riving knife, usually just a fraction of an inch, always stays the same. This never gives the two halves a chance to pinch together, greatly reducing the kickback risk.

Handling through-cuts

Non-through cuts like dados are made by lowering the saw blade low enough that it only cuts part way through the workpiece. Since a splitter is always the same height, when the workpiece is not cut all the way through, it will inevitably bump into the splitter as it exits the blade. This makes splitters useless for non-through cuts. For such cuts, you’d need to remove the splitter. And since a splitter can be a hassle to remove, the temptation to just leave it off is strong, increasing the risk of dangerous kickback.

Many riving knives, by contrast, are either already exactly as high as the saw blade, or can be easily adjusted to that height. And since they move up and down with the blade, they can be used for non-through cuts. The uncut material simply passes over the top of the riving knife. Since you don’t need to remove them, you continue to receive the safety benefits no matter what kind of cuts you make.

So while a splitter provides some measure of kickback protection, a riving knife is useful in more circumstances and provides better safety overall.