Popular Woodworking 2007-08 № 163, страница 42

Follow the action. The riving knife behind the blade moves up and down with the blade. The knife prevents kickbacks and doesn't have to be removed for typical operations, such as cutting joints.

The European table saw has a more workable blade-safety system. It consists simply of a riving knife and a blade cover. Let's clarify some misunderstandings about the purposes of splitters and riving knives. The first is that there seems to be a need for clarification on the actual j ob that these devices do. It is widely believed that the sole purpose of the splitter and riving knife is to keep the kerf open after the cut, therefore keeping the wood from closing on the blade. This purpose is even described as such in the regulations that govern the manufacturing of table saws. In reality, the main job of these pieces of metal behind the blade is to deny the workpiece access to the back of the blade. Without a roadblock the back teeth of the blade can - and often do - bite into the workpiece, pick it up, and throw it toward the user at speeds of up to 120 miles per hour. Many woodworkers have experienced kickback and the number of accidents from this one phenomenon is astronomical. The use of a riving knife or a splitter makes kickback a non-issue.

What is the difference between a riving knife and a splitter? A splitter is attached to the carriage assembly behind the blade. The carriage assembly on U.S. table saws does not rise and fall with the blade. Two problems arise with this arrangement. A splitter is designed to be very close to the back of the blade when the blade is raised to its full height. The distance between the back of the blade and the splitter increases as it is lowered. The usual working height for cutting 4/4 stock leaves about a 2" gap between the blade and the splitter. This unprotected gap isn't ideal, but it is still better than no splitter. The second issue is that a splitter stands above the top of the saw blade thereby forming a barrier when making non-through cuts. Therefore the splitter, along with the rest of the device, needs to be removed from the saw when performing non-through cuts.

In contrast to the splitter, a riving knife is attached to the arbor assembly so it moves along with the saw blade. This means that once the riving knife is adjusted close to the blade, it always stays in this same relationship. Typically, a European riving knife can be adjusted in its closeness to the back of the blade and also in relationship to the blade height. When the riving knife is adjusted the least amount below the top of the blade, it is not an impediment for non-through cutting.

The first industrial table saws made in the U.S. had riving knives. American table saws are just beginning to come back to using a riving knife. A Standards Technical Panel at Under

writers Laboratory, one I have been a member of for a number ofyears, recently passed a proposal for new safety regulations for the table saw. The new regulations specify that, starting in 2008, all newly designed table saws will incorporate a riving knife in the design of the saw. Additionally, after 2014, the regulations require a riving knife on all table saws of the designs currently being manufactured. In both cases the riving knife is required to be below the top of the blade.

A few U.S. front-runners now offer table saws with their versions of a riving knife. The SawStop, Powermatic PM2000, and a Grizzly 12" are the first. On these saws, one difference from European riving knives is that instead of having one adjustable knife, two riving knives are offered. One of the riving knives, when attached, is taller than the top of the blade and holds the blade cover. The other, shorter, riving knife is offered without a blade cover for non-through cuts. The regulations call for a knife that is lower than the top of the blade, and so Powermatic added a second riving knife to the PM2000 after realizing the need. Powermatic has made the blade cover come off easily from its taller knife, but I do not see that there is a need for this. All three of these new table saws offer a quick release for the riving knife that makes the changes effortless.

Blade Covers

The blade cover is a necessary part of safety at the blade. It is a barrier between our hands and the blade, whether or not the blade is running. On European table saws there are a couple requirements for the blade cover. The maximum outside width of the cover is 40mm (1¹/2") when it is mounted to a riving knife. The narrower the blade cover the less intrusive it is on your work. The amount of space between the fence and the blade cover becomes especially precious when ripping narrow work.

You cannot make non-through cuts on the table saw when a blade cover is attached to the riving knife. European regulations therefore made it mandatory for the blade cover to be able to be removed or reattached in less then 10 seconds. Compared to current U.S.-made saws, the ease of removal is significant. On U.S. saws the blade cover is permanently attached to the three-in-one guarding assembly. There are no size requirements for the blade cover and consequently they vary widely. When a non-through cut is needed, the whole guard assembly is typically removed.

Quick-change artist.

By twisting a knob and pulling upward, you can remove the blade cover in just a few seconds. Most U.S. saws have no such feature.

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