What is a riving knife, and why must I never remove it?

Kickback is not a mystery and it is not bad luck. On a circular saw or a table saw the back of the blade is travelling upwards and backwards relative to the work. If the kerf closes and grips the blade there, the blade's own rotation drives the work backwards at the operator, or drives the saw backwards out of the cut. Everything about a riving knife follows from that one fact.

Safety

Step by step

1. Understand what happens at the back of the blade.At the front, the teeth are moving down into the workpiece and the cutting force presses the work onto the table or the saw onto the work. At the back, they are moving upwards and out. Any contact there lifts and throws.
2. Understand what closes the kerf.Internal stress released as the cut opens the board; a board sagging because it is unsupported; a workpiece pivoting off a fence; a wet board swelling. All of them squeeze the slot shut behind the blade.
3. See what the riving knife does about it.It is a plate the same thickness as the blade plate — thinner than the kerf, thicker than nothing — fixed immediately behind the blade. As the kerf tries to close, it closes onto the knife instead of onto the blade. The timber can no longer reach the teeth at the back.
4. Note the difference between a riving knife and a splitter.A riving knife is mounted to the blade assembly, so it stays at a fixed small distance behind the teeth whatever the blade height and bevel. A splitter is fixed to the table and stays put while the blade moves, so it only works at full blade height. A riving knife is much better and is why it has become standard.
5. Note that the guard is a separate thing.The crown guard over the top of the blade stops your hand reaching the teeth and deflects debris. The riving knife stops the workpiece being thrown. They do different jobs and both matter.
6. Check yours is set correctly.It should sit in line with the blade — not offset to either side — and within a few millimetres of the teeth. On a circular saw, its top should not stand above the blade's teeth. Check alignment by sighting along the blade with the saw isolated.
7. Check it matches your blade's kerf.The knife must be thinner than the kerf the blade cuts and no thinner than the blade plate. Swapping between full-kerf and thin-kerf blades can leave a knife that jams in the cut or one that no longer holds the kerf open.
8. Handle non-through cuts properly.Grooves, rebates and housings do not cut all the way through, so a full-height riving knife cannot follow the blade. Many table saws provide a low-profile riving knife that sits below the top of the blade for exactly this. Use it, and use featherboards and a push stick. Never simply take the knife off and carry on.
9. Support the work so the knife never has to do its job.The knife is the backstop, not the plan. Bearers either side of the cut, a free offcut, a straight fence and a sharp blade mean the kerf never closes in the first place.

Tips

• If your riving knife has been removed by a previous owner, the part is usually available from the manufacturer and is worth ordering before the saw is used again.
• A bent riving knife is as bad as none, because it deflects the work sideways into the blade. Sight along it after any incident.
• Never stand directly behind the line of the cut on a table saw. Kickback travels along the blade's plane, at speed.
• A track saw's riving knife retracts to allow the plunge and springs into the kerf as the cut proceeds — that is one reason a track saw is safer for plunge cuts than a hand-held circular saw.

Common mistakes

• Removing the knife because a jig or a sled fouls it — The jig is the thing that should be redesigned. A saw run without its knife has no defence against a closing kerf.
• Fitting a thin-kerf blade and leaving the standard knife in place — If the knife is thicker than the kerf, it jams in the cut and forces the workpiece off line — which is itself a kickback risk.
• Treating the crown guard as the safety feature and the knife as optional — They protect against different things. The guard protects your hand; the knife stops the workpiece being fired at you.
• Free-hand cutting on a table saw without the fence or a mitre gauge — Nothing keeps the work aligned with the blade, so it twists into the back of the blade. The knife reduces the consequence but does not remove the cause.

Questions people ask