Marking gauge

Also called: gauge, cutting gauge, mortise gauge, mortice gauge, panel gauge

A marking gauge is a tool for scoring a thin line parallel to a straight, planed edge. A sliding fence rides along the edge while a point fixed further along the tool's wooden beam, called the stem, scratches the line into the wood's surface.

The gauge family

Three related tools do this job. Hasluck (1897) named them together: 'Gauges used for marking parallel with an edge that has been wrought straight are the marking, the cutting, and the mortise gauge.' An edge that has been wrought straight is simply one already planed true. An ordinary marking gauge carries a single scoring point, usually steel, though it can be fitted with a pencil point instead. A cutting gauge is fitted with a knife-type cutter in place of a point, and is meant for scoring across the wood fibres, where a point would tear rather than cut. A mortise gauge is set apart by carrying two scoring points, one of them adjustable, instead of one, so it can scribe both sides of a mortise at once; some gauges are made reversible, combining two of these functions in a single tool depending on which face is used. Bird (2004) considers the marking gauge one of the most useful tools in the shop, and one that has been in use for centuries.

Using it

In use, the tool rests on the flat side of its stem with the point held clear of the wood, and the fence is pressed firmly against the true face or edge, the surface or edge already planed straight and flat, as the gauge is run along the work. One method is to grip the workpiece by one end with its true face or edge outward, and run the gauge until it meets the vice jaw. Novices are advised to hold the work in a vice while they first learn the tool, and to try other ways of holding it only once they are skilled. Marking a mortise with a mortise gauge, either an ordinary marking gauge or a mortise gauge can be set up for the width, but where a mortise gauge is used, the shoulder lines are first squared round the work with a knife and try square, and the gauge is then set: Fairham (1920) described setting it 'so as to strike the two gauge lines.' The fence is positioned to fix where, across the thickness of the board, the mortise falls, and it is kept pressed against the board's edge as the gauge runs along. Care matters on finished work: Hasluck (1897) warned never to run the mortise gauge line past the cross lines that mark the mortise's ends, 'as it does not look neat to see the gauge line in a finished job continued beyond the mortises.' Lacking a mortise gauge, a joiner can produce the same two lines by using an ordinary marking gauge twice.

Sharpening the point

A gauge point straight from the factory is rounded, and will tear cross-grain fibres into a ragged rather than a crisp line. Filing half its thickness flat, with that face square to the beam, corrects this; a left-handed user can file the flat in the mirror-image orientation to make the tool easier to use. Properly sharpened, the tool cuts a clean, crisp line whether the stroke runs with or across the grain.

Other uses

Beyond scribing joints, carpenters and cabinetmakers also use the gauge to lay out a board's finished planing dimension: because the scored line is only a shallow cut, a plane shaving peels away cleanly once the plane reaches it, signalling that the target thickness has been reached. Lacking a power thickness planer, a hand-tool worker can bring a board to thickness this way, using the gauge together with hand planes. In marking a drawer joint, the cutter gauge is set slightly under the actual thickness of the drawer side, to allow for the width of the cutter itself, and the line is scored on the drawer front's inner face, then carried all the way around the back piece. In Japanese practice, where the tool is called a kebiki, it can also be used to cut through the wood entirely, rather than only to mark it.