Bevel gauge
Also called: sliding bevel, bevel
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A bevel gauge, also called a sliding bevel or T-bevel, is a hand tool for marking and copying angles that are not square, and for carrying one such angle from place to place until it is needed again.
What it is
A bevel gauge looks like a try square, the L-shaped tool joiners use to mark and check right angles, but where a try square's blade is fixed at ninety degrees, a bevel gauge's blade, usually called its tongue, can move, tilt and slide to any angle. Like a try square, it has a blade and a head joined by a pivot, and that pivot locks at any angle the user sets it to. Once locked, the gauge adjusts so it can carry that angle from one place to another.
Setting the angle
A sliding bevel can be set in either of two ways: by copying the angle from an existing board, or by matching a line already drawn in pencil. Its setting can also be found by drawing the angle out at full scale first, viewed in profile, and setting the tool to match the drawn line.
Marking and layout
Once set, a bevel gauge can guide the planing of an edge in the same way a try square does. Hasluck (1907) puts it plainly: "Plane off these edges, using the bevel in the same way as the try square." It can carry an angle over to the end of a piece that needs to be joined, and it can both mark out angles on a workpiece and set a saw blade for an angled cut. When marking out dovetail pins, the sloped ends cut on a board to interlock with another piece and resist a pulling load, a bevel gauge should be kept against a 45-degree reference line while angles are transferred from a drawing. The tool serves layout beyond joinery too: it can find the angle needed for shop-made dust baffles, and where a soffit's face, the underside surface of an arch or doorhead, shares its angle with a door frame's plan, one bevel setting will mark both.
Limits
In handrail layout work, a bevel gauge cannot always be set before the joint it belongs to is cut: if that joint is cut straightaway, a bevel cannot be applied squarely to it, so a temporary joint is made first and the bevel taken from that. A bevel gauge can also be somewhat awkward to use when marking dovetails on narrow boards.