Gluing Wood With Cramps

The type of cramp you should use depends on the size of the wood you are gluing.

G cramps: The most versatile type of cramp, this gets its name from its shape which is like the letter G. G cramps have a mouth capacity of between 25mm and 250mm or 300mm and are mainly used for cramping pieces of wood to a bench and for holding down veneer or laminate while the adhesive dries.

Variations on the basic G cramp include deep-mouthed cramps – which reach further across the workpiece – and small, sliding bar G cramps. The latter are smaller and easier to use than conventional G cramps and are useful for holding small pieces of timber in place.

Sash cramps: These are used to cramp larger pieces of timber, such as doors and window frames. Consisting of two adjustable stops on a long bar, they come in different lengths up to 3m long with extensions. One stop is adjusted by sliding along the bar and securing with a pin: the other tightens like a vice jaw. Because of their size, sash cramps are expensive to buy. They are, however, obtainable from hire shops.

Web cramp: This consists of a 3.6m loop of nylon webbing, running through a ratchet, which can be tightened and released using a spanner or screwdriver. The web cramp cannot apply as much pressure as a sash cramp, but it is cheaper and is quite adequate for light and medium weight gluing jobs.

You can make an improvised form of web cramp by using strong cord and two short pieces of dowel. Tie a double thickness of the string around the object to be put under pressure then use the dowel to twist the strands together until the tension cramps the wood firmly. Use the second piece of dowel to hold the first in place.

Preparation

If you are using liquid animal glue, make sure that it is fresh – most have a limited life and will cease to work properly if they are old.

Most, but not all, adhesives are effective only on surfaces which are free of moisture, dust and grease. Unless the instructions with your adhesive specify special gap-filling properties, the surfaces should also be reasonably smooth.

Always ‘dry assemble’ your work to begin wi Blow out dust from any inaccessible corners of the work, then fit the pieces together. Having made the necessary adjustments and re-cleaned the joints, mark each part to eliminate the possibility of getting things in the wrong order on final assembly. Make sure that you sand and finish off any areas which would prove too inaccessible after gluing.

Mixing and applying adhesives

Before you start mixing and applying your glue or adhesive, make sure that all the necessary cramping equipment is well to hand – it will be too late to search for it once the glue is mixed.

If there are any small holes or cracks in the wood, fill them at this stage. A good filler can be made by mixing a thin glue or adhesive with some sawdust: if you use sawdust from the same wood, the finished joint will be barely noticeable.

When making up a mixed glue or adhesive, use a small piece of wood and an old china teacup or saucer. Follow the manufacturer’s instructions to the letter, taking particular care when water has to be added.

Use another, preferably flat, piece of wood to apply your glue or adhesive or apply it straight from the container. Make sure that each surface to be coated receives an even covering. Glue invariably shrinks as it dries – causing stresses and strains which will weaken the joint unless the coat is even.

Cramping techniques

Unless you are using impact adhesive, you should cramp the wood as soon as the joint has been made. Whichever type of cramp you use, you must be careful that the surface of the wood does not get scratched and damaged by the action of the cramp while the glue is setting. You can either use some newspaper or alternatively, to prevent the metal cramp jaws from bruising the surface of the wood, always ensure that there is a small block of wood between the two. Make your blocks, or cushions, from small offcuts. Keep them as clean and smooth as possible so that they do not accidentally push the wood that is being glued out of shape.

When using a G cramp, make sure that the jaws and cushions are positioned as far over the joint as possible then tighten the cramp to finger-tight. Where two pieces of angled, or wedge-shaped, wood are being cramped, position a second cramp at right-angles to the first to stop the parts slipping.

Use a sash cramp in the same way as a G cramp, this time of course on a larger scale. Make sure the sash is exactly square to the workpiece or distortions may result. During cramping, the bar of the sash will tend to bow in towards the workpiece, so place small wedges underneath to keep it straight.

When using a web cramp, or its improvised alternative, ensure that the webbing runs around firmly fixed parts of the workpiece. Otherwise, you may break one joint as you are trying to cramp another.

If you are making an improvised web cramp, make sure that it is of a really strong material like terylene cord.

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