Riving knife (circular saw)

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This article covers riving knives on circular saws. For other meanings of the term, see the terminology page Riving knife

Table saw with riving knife (black, to the right of the saw blade)

A riving knife is a protective device on a circular saw . Unlike other wedges , a riving knife is consistently evenly wide. However, it often has a triangular or scythe-shaped side view.

construction

The riving knife consists of an upwardly tapering, arched piece of metal (mostly spring steel ), which is firmly attached to the saw table in the cutting direction behind the saw blade, precisely aligned with its plane. It can be adjusted vertically and horizontally to suit different sheet sizes. The inner arc should be set at a distance of 3 to 8 mm from the cutting edge flight circle of the saw blade. The exact regulations for this are specified, for example, in the European standard EN1870-1. Spring steel can withstand deformations better than normal steels. This means that after the elastic deformation, the riving knife can return to its original position.
In order to be effective and to prevent the saw blade from getting jammed, the riving knife must be narrower than the kerf (tooth width) and wider than the blade of the saw blade. This results in the formula for the ideal riving knife width: (kerf + master blade) / 2 .

Protective effect

The riving knife prevents the saw blade from getting jammed in the kerf. The saw blade jams,

• when a workpiece is pushed by hand at some distance from the saw blade. During the advance, the workpiece is pressed against the saw blade (using leverage).
• if a wooden component has internal tensions which, when sawing, cause the workpiece to press against the saw blade from both sides and reduce the kerf.
• if a non-fixed workpiece moves towards the saw blade when sawing with the hand-held circular saw.

The jamming of the saw blade in front of the axis of rotation causes the saw to brake. A table saw jamming behind the axis of rotation leads to the workpiece knocking up. Powerful circular saws can catapult away smaller workpieces at great speed.

In the case of a circular saw, the jamming of the saw blade causes the saw to flip up. In order to avoid contact with the running saw blade in this case, conventional hand-held circular saws have a cover that folds in front of the saw blade under spring pressure. The saw blade rises on plunge-cut saws. In addition, with modern circular saws, the saw blade is braked electrically when the finger slips off the switch.

In any case, serious injuries can occur if the saw blade jams.

The riving knife also prevents accidental contact with the saw blade from behind.

An additional protection is the upper cover connected to the riving knife, usually made of a hardwood part with several widths of the wedge, which acts as an "extension" of the wedge to the front and holds it back if a workpiece is jammed (due to excessive feed pressure). When sawing to a specific blade diameter is according to the applicable accident-prevention regulations (UVV) a cover required, the blade and the splitting wedge covers.

In the case of long and large workpieces such as in the sawmill sector, drive wedges are also used, which are driven into the kerf from behind to prevent it from closing and lateral pressure on the saw blade.