Warping
Also called: warp, cupping, bowing, twisting, winding, in winding
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Warping is what happens when a piece of wood loses its flat, straight shape as moisture inside it changes: a board can twist along its length, cup across its width, or bow along its face. Almost everything that causes it traces back to grain and moisture.
What causes warping
Distorted or irregular grain, and abnormal wood such as juvenile wood (wood grown near the tree's centre in its early years) or reaction wood (wood formed under stress, often on the underside of leaning trunks or branches), make distortion worse. A green, unseasoned timber with spiral grain, where the fibres run at an angle around the trunk rather than straight along it, will twist as it dries, and a tree that grew this way needs its timber sawn extra thick to keep the resulting boards from twisting. Uneven moisture within a board sets up internal stress that can roughen its surface, raise the grain, and bring on checks, cracks, cupping, and distortion. Some individual timbers are known for how they behave: one species has moisture-drawing gum inside its vessels that leaves it liable to twist along its length and distort across its width as temperature changes, which is why it suits cheap bedroom furniture better than pieces meant for heated rooms. Highly figured timber, prone to twisting and distortion because its grain is contrasting and erratic, is generally unsuited to solid-wood use for the same reason, although a hardwood valued for its handsome grain and texture can often still go into furniture even when it shows some twisting or distortion. Not every timber behaves this way: one deep-red, moderately hard, straight- and fine-grained species works easily, resists marine borers (it is teredo-proof, resistant to the marine worm Teredo), and does not twist or distort.
Checking a board for warp
A board's flatness is checked with a pair of winding sticks, two matched, straight sticks set across the board near each end and sighted along like gunsights: any twist in the surface shows as the sticks appear to lean out of parallel with each other. Making the sticks longer than the board they sit on makes any twist much easier to spot. Boning strips, a matched pair of sticks roughly a foot long, are used the same way as winding sticks. Marks made from a pair of winding sticks set on a board can also be checked by holding a straightedge against them, to show whether the surface is already flat before any planing begins.
Designing joints against warp
Furniture joinery builds in resistance to twisting wherever a joint might work loose. A haunch, a short stub added at the top of a tenon (the tongue that projects from one member to fit into a slot, called a mortise, cut in another), bridges the top of a mortised leg and helps keep the mortise from cracking open; the extra width it adds also lowers the chance of the rail twisting inside the leg, and haunching or dividing a wide tenon resists twisting without needing a large mortise cut into the frame. A wide tenon base is kept broad partly to resist the joint twisting, and cutting shoulders along a tenon's edges adds further resistance to twisting on top of the glue bond to the mortise cheeks. Making a mortise chisel thicker also helps stop the chisel itself from twisting as it is used. Scarf joints, which splice two lengths of timber end to end, need the same care in timber framing: the scarf joint nicknamed the whistle cut handles shearing loads well but needs fasteners to resist pulling and twisting forces, and any scarf joint without enough resistance to twisting can open up once the jointed timbers have seasoned.
Working with wood that wants to move
Steaming timber beforehand is a common way to cut down its tendency to distort, though doing so also weakens it. Once a board is planed to its final thickness, wrapping it in plastic keeps it from distorting before use, and the same trick works for small prepared parts sealed in plastic wrap while they wait. An irregularly shaped part that needs an accurate final form is best rough-cut first and left time to shrink and distort before it is sized exactly. If a board twists anyway, the remedy is to plane down its high corners until it sits true. How a wide board is cut from the log also matters: wide flat-sawn boards, cut with the growth rings roughly parallel to the face, will distort somewhat, unlike quartersawn stock, cut with the rings closer to perpendicular to the face, though a board cut straight through the centre of the log also tends to stay fairly flat on both faces. Before boards are glued edge to edge, one face of each is first planed flat and free of twist. The same dry run applies to whole assemblies: a build sequence commonly includes a trial, glue-free assembly to confirm joints, dimensions, squareness, and twist before final gluing, and a framed, matchboarded door's main frame is assembled the same way before it is glued. A sash window's head and pulley stiles, the vertical frame members its counterweighted sashes slide against, are planed straight and untwisted, then gauged to width along with their tongues.