Cramp

Also called: clamp, sash cramp, g-cramp, g cramp, f-cramp, holdfast

A cramp is a tool that grips two or more pieces of wood and holds them together under pressure: usually while glue sets, a joint is pulled tight, or work is held still for cutting. British joiners use the word cramp where American ones say clamp, and the trade keeps dozens of shapes for different jobs.

Kinds of cramp

The commonest general-purpose cramp is the G cramp, a C-shaped frame with a screw at one end that presses a shoe against the work; it suits jobs in a tight space or where the cramp must reach in from an edge. For longer, flatter work such as door frames a sash cramp is used: a long bar with a fixed jaw at one end and a screw-tightened jaw that slides along it, which can be set between a rail and a leg's foot to pull an overly high joint into place. A heavier cramp than the sash cramp stands more strain and its jaws reach deeper, letting it grip thicker stock; as Ellis (1908) put it, 'It will stand a greater strain, and the jaws have a deeper reach than those of the sash cramp, enabling it to be used on thicker stuff.' Ellis also names the Scotch cramp as a type kept for heavier work. Other shapes serve particular joints: a mitred cramp, into which a tabletop is tenoned, keeps the top flat and hides the end grain, the surface left where wood is cut across its fibres, from view; a counter cramp, fixed permanently beneath a counter (a fitted worktop) in the traditional way, draws its joint tight. A pipe clamp, made from a length of plumber's pipe fitted with clamping heads, is used in building post-and-rung chairs by hand to seat a tenon, the projecting end of one piece that fits into a socket in another, fully home, and, set outside the posts, to correct one driven in too far. A vise-grip clamp, a self-locking pliers-type clamp, can simply be bolted to a tabletop as a plainer stand-in for a purpose-built jig. Hand-screws, wooden clamps with a long screw at each end so the jaws can be angled to the work, complete the usual kit.

Home-made cramps

Not every cramp is bought. Jones (1920) describes building a cramping frame at the bench: the centre piece is cut longer than needed so the surplus can be trimmed off later, and fixing a centre piece at the long end of the clamp makes it easier to adjust the width of the bottom bars. In place of the usual loop, one such clamp had, in Jones's words, 'a yard or so of strong cord knotted through it instead of the loop.'

Protecting the work

A cramp bears down hard, and bare metal jaws will dent or bruise timber, so cushioning is standard practice. A scrap piece of wood set between the jaws and a dovetailed workpiece stops the cramp marking it, and plastic pads that fit over the jaws serve the same purpose. Where a hot glue iron, a heated iron used to soften glue, passes near a cramp, a strip of paper laid between the work and the cramp bar stops it staining the wood. Slight irregularities in a surface can be flattened out by setting a block underneath it, made slightly thicker than the cramp bar's own height, so the pressure falls where it is needed.

Beyond gluing

Cramps do more than close glue joints. In carcase assembly, building a box-like frame such as a cabinet body, the parts are cramped and wedged while the whole is checked for square in both directions. A support block can be wedged in place and the joining plates of a biscuit joint clamped together over it. Jigs are commonly held with a cramp too, clamped by one end to either a fence or a tabletop depending on whether the jig needs to be held down or held in; one shop-made jig's clamp takes a 5/16 inch brass threaded insert in its mounting hole to match whatever insert the rest of the jig uses. In veneering, one type of cramp presses down blisters, raised unstuck patches in a glued veneer, and reaches in to glue work away from an edge, further than a standard G cramp can go. Cramps turn up away from glue altogether, too: before a saw is sharpened it is seated in a fitting clamp or saw vise, one strong enough to hold the blade rigid and stop it chattering under the file, while still letting the worker tighten it quickly, with just enough tooth showing above the jaws for the file to clear the top.

Equipping a shop

Cramps are commonly bought and used in matched pairs. Hasluck (1897) judged that a shop holding a couple of pairs of differing capacity, together with an assortment of hand-screws, 'might be considered furnished with cramps.'