Timber can acquire a condition known as 'case hardened'. When the material is sawn, internal stresses are released. A further cause is wet wood, or wood with twisted or interlocked grain.

Sometimes the kerf can narrow as it leaves the cutting teeth. When this happens it can nip with great force against the upwards-running part of the saw causing the workpiece to be violently thrown upwards and forwards towards the sawyer.

If the kerf spreads, the now-wider workpiece presses against the riving knife and the face of a 'long' fence. This can increase the feed force required. If there is no riving knife the sideways pressure can can push the workpiece against the uprunning teeth and cause a kickback. For the remedy seeFences.

The principal purpose of the riving knife (splitter) is to prevent damage and injury from both the above situations, but it also serves as a form of rear guard to prevent offcuts and knots from being caught by the upwards-running teeth and thrown towards the sawyer or bystanders.

Once the leading edge of the workpiece reaches the knife, it also serves to guide the cut and prevents the uprunning teeth from unnecessarily scoring the kerf.

Although helpers 'taking off' should not be able to reach this far, it can serve to prevent their hands from touching the up-running teeth.

Adjustment

It should be adjusted so that its distance from the saw teeth does not exceed 8mm at the table level. This distance should be small enough to prevent offcuts or knots from being picked up by the teeth.

Construction

Riving knives should have a chamfered leading edge, and should be thicker than the body of the saw blade, but slightly thinner than the width of cut.

They should be rigid and set accurately in line with the saw. It should be shaped so the inner edge follow as closely as practicable the contours of the largest saw blade designed to be used on the machine.