Getting joints to fit snugly is one of the major objectives in carpentry, and nothing introduces the techniques so well as the halving joint. As for the perfect finish, that’s the role of the mitre.
There are many situations in woodwork when you need a joint that’s fast and simple, but also neat and strong. And this is where halving joints come into their own. Despite their simplicity, they’re very effective joints because the two pieces of wood are cut so they interlock together, either face to face or edge to edge, making the joint as strong as ¡ª if not stronger than ¡ª the timber itself. They are used almost exclusively for building frameworks, joining the rails (side pieces) either at a corner or in a cross absolutely flush. You end up with a frame that’s neat enough to be on show and sturdy enough to need no reinforcement
Mitre joints, though not strictly speaking considered halving joints as there’s no interlocking, are halved to make up a perfect 90¡ã angle. In this section, only the simple mitre is dealt with.
Strength of joints
There are three things that affect the strength of a halving joint ¡ª the size of the timber, the quality of the timber, and any reinforcement you add.
The size of timber is important because it governs the amount of adhesive in the joint; the greater the areas glued together, the stronger the joint will be Usually problems only arise when you are trying to 10in thin pieces of timber together ¡ª it’s almost impossible to get the joint to stay rigid. Regarding timber quality, hardwoods rarely present a problem, but with softwoods, splitting can occur which will seriously weaken the joint. You should, therefore, reject timber containing knots, cracks and other potential weak spots.
In many cases, the correct adhesive is all the reinforcement you need ¡ª use a good quality PVA woodworking adhesive, or, if the joint will be subjected to heat or moisture, a urea formaldehyde woodworking adhesive. If still greater strength is required ¡ª this is more likely on corner halving joints than on cross halvings – you should drive screws through the overlaps, or, for a more natural look, drill a hole right through and glue in a length of dowel. Both the dowels and screws are set like the spots on a dice to stop the joint twisting.
Simple butt joints must be reinforced in some way to have strength, but with mitred butt joints this would defeat the decorative aim Because of this, they are normally reserved for situations where strength is not required ¡ª picture frames and decorative edgings, such as door architraves for example.
You can use either a chisel or a piece of glasspaper to take off any unevenness in the timber, although it’ll be quicker to use a chisel to clear out the edges so that the corners are absolutely square. When the pieces finally fit neatly, spread adhesive on both faces of the joint and hold them in place with a G-cramp (protecting the wood’s surface with scrap timber) until the glue has set. Remove the cramp, and add any reinforcing screws or dowels that may be needed, drilling pilot holes first.