Mysteries of the riving knife

Above: Marking the periphery of the blade on a Felder tablesaw.

Words and photos: Len Crane

The riving knife is the most important, most mysterious and most misunderstood part of a tablesaw. The value of this thin scimitar-shaped piece of high-tensile steel cannot be understated. Understanding the technical issues and correctly setting the riving knife will ensure that best practice is achieved and maintained. It will prevent a serious accident, and could even save your life. The theory and practical issues have one thing in common, a very strong emphasis on safety.

Statistics are proof that failure to properly fit guards to woodworking machines is often the basic cause of many serious injuries and deaths within the timber industry. The riving knife, or splitter as it is sometimes called, is the major safety device on the tablesaw, along with the top (hood) guard. Setting the riving knife correctly is critical if the device is to do its job properly. A well-documented case in Victoria saw the riving knife come loose, contact the spinning sawblade and eject through the steel top guard, embedding itself into the ceiling of the workshop. Make no mistake; failure to do the job right could lead to serious consequences.

Technical issues set the benchmarks for the design, making and setting up of the tablesaw. Safe plant regulations, State Occupational Health and Safety (machinery) requirements, and Australian Standards all play a part in providing guidance and advice for machinery users. Recently, prescriptive regulations made way for on-site risk assessments leading to the safeguarding and use of equipment. This places the onus on you, the operator or user, to provide a safe working environment. In the home workshop the only person to suffer is you. In the workplace the scenario is entirely different. Here, when the chips are down and you are facing a magistrate, you will be asked whether or not you met or exceeded Australian Standards. If you did not, you will be asked to produce professional advice as to the measures you took to provide a safe working environment on the tablesaw. To avoid litigation, most professional users adopt Australian Standards AS1473-1991.

Unfortunately the tablesaw is seldom looked on as a precision tool. The end product of the saw bench almost always requires detailed sizing and finishing after the saw has done its job, ergo, the tablesaw need not be an accurate machine. This sort of thinking is seriously lacking a logical engineering approach. The saw bench and all its component parts must function as a precision tool simply to meet acceptable safety standards. To meet these standards, the tablesaw, its supports and adjustable components, must be rigid and substantially constructed. When these adjustable components are loosened, adjusted to the new required position and locked down, they must be as one with all other components.

When wood is cut longitudinally, stresses are often relieved and the two sawn boards may well bend and twist in one, two or even three planes, leaving very little resemblance to the original board. There is always the danger of the kerf closing and gripping the rising teeth and forward-rotating blade, and being driven towards the operator at a speed often nearly that of the tip speed of the saw.

The set-up

To meet Australian Standards the riving knife is mounted on the carriage for the bearings and shaft, and rises and falls through the table with the saw. It always remains in the same position relative to the sawblade. It must be the same shape as the periphery of the sawblade and set no more than 12mm away from the edge of the blade. It should also be set so that the top of the riving knife is level with the top of the sawblade, or at most, no more than 12mm below the top of the blade. This is to allow for rebates and grooves to be cut. The 12mm is merely a conversion from the original regulation of half an inch.

The riving knife should be the same thickness as the width of the saw kerf, so it is a comfortable fit into the kerf. A thicker knife may cause the operator to force the wood into the sawblade, and any good woodworker knows you should never force a machine tool. The leading edges should be bevelled to facilitate the knife entry into the kerf. The knife should be mounted on the saw carriage so it is exactly in line with the sawblade. The mountings for the knife should have provision for positional adjustment in lateral/longitudinal and vertical planes. This is usually done with elongated slots machined into the knife.

1. To set the riving knife, use a pen to mark the periphery of the blade.

2. Move the periphery mark back to set the height of the riving knife.

How it works

Let’s now look at how the saw and the riving knife should interact. The workpiece is fed into the saw with a positive downward pressure onto the table. As the wood is cut it passes the rising teeth at the rear of the saw and the riving knife enters the kerf. Because the riving knife is the same thickness as the kerf width it will keep the inner faces of the kerf away from the rising saw teeth. This prevents the rising teeth from marking the inner faces—this is one of the three primary functions of the riving knife.

Another primary function of the riving knife is to resist, or more ideally to defeat, the tendency for the rising teeth to lift the workpiece up and allow it to engage with the high speed and powerfully driven upper teeth of the saw, almost always converting the workpiece into a lethal weapon. Having the top guard set just above the thickness of the workpiece will also help keep the timber on the machine table.

These days the use of thinner sawblades to save waste requires the use of thinner riving knives that are a little flexible, laterally. Any uneven force applied to the side of the knife will flex the knife to one side allowing the rising teeth of the saw to abrade one face of the kerf and, if severe enough, may tend to lift the workpiece with all the inherent dangers as described earlier. It is therefore important to avoid using the machine without the aid of the fence.

Unfortunately, the fitting and aligning of the correct knife can be tedious and time consuming and that costs money. There was a time in my youth, when there were no tungsten-tipped saws and any reasonable sized workshop would employ either a full time saw doctor or a machinist with tool sharpening and setting skills. One of their many tasks was to sharpen the carbon steel blades and set them to suit the riving knife on the machine to which it was needed, so some of the standards discussed above were maintained. Nowadays, where tipped blades are the rule it should be realised that each blade must have its very own matched riving knife, each engraved with an identifying number. When the blade is sharpened it’s set, that is, the kerf it cuts should be checked against the thickness of its companion-riving knife. If a variation has occurred, the blade, if its design permits, should be reset to suit the knife.

The third main function of a properly set up riving knife, albeit static, is to prevent the operator’s assistant, the tailer out, when returning the workpiece for the next cut, from inadvertently engaging the rising teeth and creating a projectile at the peril of the operator, or worse still, touching the back of the sawblade.  Safety is really what the riving knife is all about. Avoid the casual attitude of removing it and then forgetting to replace it.

First published in Australian Wood Review, issue 36, September 2002

The late Len Crane was a designer and maker of fine furniture who like to teach and write technical articles.