Assembly is one of the most important techniques in carpentry. In fact, you need to know how you’re going to put each job together even before you make the parts.
I f you want a timber joint that’s permanent, rigid and invisible, glue it together. True, furniture can often incorporate screws and assembly fittings in all their different varieties. But these may obstruct clean lines or free access, and lack strength. In such circumstances you need adhesives.
Even decorative and semi-decorative items which don’t have to bear much load – such as beading and architraves – may still need to be glued as well as nailed or pinned.
But it’s free-standing wooden objects – chairs, tables, sideboards which especially need glued joints to help them resist the stresses they suffer from all directions. A dining-chair, for example, will be dragged across the floor forwards, backwards and sideways, sat on, stood on, leant on, tipped back and lounged in, so that the legs, back and seat joints are forced apart.
However, although modern adhesives are often stronger than the timber they join, you can’t expect them to do all the work. You’ll always get more strength from cutting a proper joint, as well as glueing it, than from just butting two pieces of wood together with adhesive in between. The point to remember when designing glued joints is that the larger the area the glue is spread over, the better it will hold.
Likewise, you can’t fall back on adhesives to make good a joint which you haven’t cut to a tight fit – although some adhesives do have the ability to fill smallish gaps.
Modern industrial adhesives come in a great many formulations and have many different properties, but two types – PVA and urea-formaldehyde (UF) – take care of most do-it:yourself needs. Broadly speaking PVA is used for timber indoors and UF for timber outdoors.
Cramping (another word for clamping) goes hand-in-hand with glueing. Unless the joint is nailed, pinned or screwed as well, you’ll need something to hold the structure together until the adhesive hardens. Nails or pins driven in halfway and later removed are a simple form of cramp. So are heavy weights; but you often need greater pressure than these can provide, and to be able to exert it in two or three different directions at once (for example, along the length and across the width of a box). Here, G-cramps will be adequate for most small jobs and sash cramps for most large jobs but in unusual situations, you may have to improvise and use whatever arrangement seems to grip most firmly.
There are at least two approaches to the problem of holding together a square or rectangular framework if sash cramps aren’t available. One is the mitre cramp. This consists of four blocks, each with a right-angled notch cut in it, which fit over the corners and have a cord passing through them to pull the frame together. The other method is to use wedges. For example, place the frame between two parallel battens, each firmly fixed, and drive in pairs of wedges to tighten it up against them.
Alternatively, fit a second frame over the one you’re assembling, and use wedges between the two.
Working in stages
In order to ensure the squareness of a frame, such as a window-frame, or a box, such as a freestanding cupboard made from panels, you’ll need to glue and cramp the whole thing in one operation. Sometimes, however, you’ll be able to glue components or sub-assemblies (individual frames or boxes) separately and join them together later.
A chair, for example, often consists of two identical side-frames, joined to each other with cross-rails at front and back. Each frame would comprise two uprights (the front and back legs), and two connecting rails -one at seat height and the other at calf height.
The best way to work would be to glue them and leave them to set completely. Then take both frames, insert the cross-rails and glue and cramp this bigger assembly in the same way. To break the job down methodically into steps like this is far more sensible than trying to assemble the whole chair in one go. If you try to do so, you’ll give yourself a lot of trouble, because you’ll be working in at least two directions at once. However, as furniture is three-dimensional, awkward assembly procedures are sometimes unavoidable.
Often you’ll need to sand and perhaps stain certain components before assembly, since it may be impossible afterwards.
When glueing, the secret is to have everything ready beforehand. First you should knock the structure together ‘dry’ (unglued) and make quite sure it all fits. (Tight joints shouldn’t be forced all the way home at this stage.) Lay all the cramps you need in position, adjusted to size, with enough spare room to insert whatever you’re glueing. Have a wooden mallet handy to tap pieces gently into position if necessary. Be clear about the order of assembly, so that you won’t be left with a part that you can’t insert into what you’ve already glued. Once the adhesive is applied and starting to set, you’ll have no time for last-minute searches for protective blocks or a wet rag to mop up excess adhesive.
You need only coat one surface, unless you’re using a contact adhesive, but do it thoroughly. When you’ve finished, leave everything to set in a place where no one will move it, knock it or kick it. In cold weather, wood adhesives take longer to set, so make sure the environment is warm too. ^
There are several different types of adhesive you can use for glued and cramped assemblies, and all have different characteristics.
PVA is a general-purpose adhesive universally used for indoor jobs because it does not need to be specially mixed. It comes as a thick white liquid. It takes at least an hour or two to set properly but you can’t slide the joint around after more than 15 or 20 minutes.
UF (urea-formaldehyde) is much more waterproof than PVA (better for outdoor work), rather stronger, and better at filling gaps. It comes as a powder which must be mixed with water. Once mixed, it ‘goes off – even before use – in an hour or less, unlike PVA; but again the joint should be left cramped overnight to ‘cure’ (set) completely.
Epoxy resins usually consist of a separate syrup and paste which harden quickly when mixed. They are extremely strong and will bond widely different materials (eg, steel/glass/wood/ceramics). Some need very brief cramping. Because of their cost and rather fiddly method of application, they are best kept for small jobs in which wood is joined to another material.
Scotch glue is animal glue made from hide etc, and was the only one in use until the invention of synthetics. It comes as granules or sheets which have to be soaked, then boiled up in a pot. It cracks and gives way with age, and is not water-resistant. It dries very quickly as it cools, and melts under heat. Today, its main uses are in hand-veneering and antique restoration, not in general woodworking.
Contact or impact adhesives are mostly used in laying plastic laminates and similar materials. Because they are spread on both surfaces and allowed to dry before the surfaces are pressed together to form an immediate bond, no cramps are needed – unless perhaps air is trapped and needs to be pushed out.
TIP: KEEP SURFACES CLEAN
Whenever you glue up an assembly, take great care to confine the adhesive only to mating surfaces. You can wipe away excess PVA, UF or scotch glue with a damp cloth, but removing epoxy or contact adhesives usually requires a special solvent; try meths on epoxies, petrol on contact adhesives.
TYPES OF CRAMP
Cramps used for glued joints are usually metal, with jaws which are tightened by a screw action.
G-cramps are the most common and will take assemblies of up to 305mm (12in). –
Their variants include edge cramps, which exert pressure in two directions at right angles to each other and are useful for fixing edging materials, and quick-release cramps, which can be re-adjusted more speedily than ordinary G-cramps.
Sash cramps have two heads – one fixed (but adjustable like a vice) and another which can be pegged into any of a number of positions on the sash bar according to the size of the job. Their capacity is up to 1370mm (54in), although larger versions are available. They can also be extended by bolting two bars together with their movable heads removed. Assembly of any box or furniture carcass usually requires several sash cramps. Cramp heads are also available for fixing to suitable lengths of wood 25mm (1 in) thick-a cheaper alternative.