Gluing up

Also called: gluing, glueing, cramping up, assembly, clamping

Gluing up is the stage of a job where separate pieces of wood are coated with adhesive, brought together and held under pressure until the glue has set. Done well it disappears into the joint; done badly it leaves a joint weaker than the wood around it, so the trade treats it as a step with its own rules of timing, pressure and preparation.

Why grain direction matters

Glue bonds strongly where two long-grain surfaces meet, the faces running along the length of the fibres, and only weakly wherever end grain is involved, the surface left where the fibres are cut across. Bonding one long-grain face to another gives a joint as strong as the wood itself, while a joint that mixes long grain and end grain, or glues end grain to any other direction, bonds far more weakly. This is why joint design matters as much as the glue: a butt joint, where two flat faces are simply brought together, should be shaped so the joinery itself brings long-grain faces into contact rather than leaving the glue to do that work alone, and a tenon is sometimes left with extra bulk so it cannot lever out the end grain of the joint it sits in when the piece is assembled.

Timing

Two spans of time govern a glue-up. Open assembly time runs from when glue is first spread until the two surfaces are brought together; closed assembly time runs from closing the joint until full clamping pressure is reached. A glued joint should be clamped up as soon as possible after the glue is applied, and many ready-mixed adhesives need no minimum waiting time at all when the glue is spread on a single surface, so closing the joint soon after spreading is the usual advice. One technique for a stronger bond is to spread glue, wipe off the excess, pause briefly, then apply a fresh coat for the final assembly; sizing, an earlier coat of glue applied before final assembly, is recommended for mitre joints in particular. Applying too much clamping pressure can squeeze more glue out of the joint than is wanted, which works against the aim of the pause-and-recoat method.

Clamping and checking

Checking that the pieces are square is a step to take while a joint is being glued up, not afterwards. When clamping a panel made of several boards, clamps go on from the middle outward toward each end. Some joints call for their own holding arrangements: a closed-string staircase, where the strings run unbroken past the treads, can use a purpose-made cramping jig to hold the strings square while the glue sets, and a dovetail joint benefits from chiselling away the inner corners of the pins and tails once the glue is dry, to stop the wood bursting out there, then planing the joint flush by working from the outer corners inward. Not every maker uses alignment aids: one glues boards edge to edge with no splines, dowels or biscuits at all.

Adhesives

Choosing an adhesive means weighing the wood species, the type of joint, the working properties wanted, performance, strength and cost together, rather than reaching for one glue by habit. An older urea-formaldehyde resin glue was sold as strong, highly water-resistant, non-staining and usable cold, with a generous working time before it set. For glued-laminated structural timber, a resorcinol-phenol adhesive is the one most often chosen for gluing the wide faces of the laminations together, though other adhesives may be used if they have been tested and shown to meet the required performance and durability standards; the wide faces are best planed immediately before gluing, not left from an earlier stage of preparation.

Setting up the workshop for it

A bench kept aside for assembly work can be plain softwood and needs no vice, but it must stay strong, stable and exactly level. Some workshops keep a separate bench for glue-up alone, particularly for coating veneers by hand. Clamps are worth keeping within easy reach, ready for immediate use, and long sash cramps store better hung on hooks or held in a rack than left loose on the floor.