Choosing The Right Joint For The Job
Before embarking on any project you will have to decide on the type of woodworking joint you will be using.
There are seven primary woodworking joints…..Butt, Dado & Rabbet, Tongue & Groove, Dovetail, Finger, Mortise & Tenon and Lap.
As we look at each joint you will see which joint is best suited to a particular project.
Butt joints are formed very easily, although they are not as strong, or as aesthetically pleasing. as other joints.
As the name implies, a butt joint is formed when the surfaces of two pieces of wood are butted together.
Fortunately modern woodworking glues are incredibly strong, so the joint would only fail through being twisted or sheared, rather than being pulled apart.
You can reinforce the joint with screws, nails, dowels and splines, so that it is capable of carrying a heavy load.
Boards are often butted together lengthwise to make tabletops, cutting boards and workbenches. These butt joints are very strong as they run in the same direction as the grain of the timber.
Dowels are often used to reinforce and align butt joints in the construction of cabinets.
Dado & Rabbet
These are interlocking joints which are used for a wide variety of tasks. A rabbet is a square groove which is cut along the edge of the timber, whereas a dado is a square groove running across the grain.
Rabbet Joints are an improvement on butt joints as they partially interlock. They are exceptionally strong due to the greater surface area available to apply glue and the inherent strength of an interlocking joint. They are often used to conceal the edges and ends of panels especially in the construction of bookcases.
Dado Joints are used almost exclusively to support the ends of shelves in bookcases or dressers.
Dado and rabbet joints can be combined to form a joint which resists shear very effectively.
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Tongue & Groove
Tongue and groove boards are often used to form the backs of cabinets and dressers, as the joints are strong as well as hiding the mechanics of the joint.
A groove is cut along one edge of the timber with a router and two grooves are cut in the opposite face to form the tongue.
Preparing a dovetail joint is probably the greatest challenge that a newbie woodworker is faced with. However, as with any skill, practice makes perfect.
Dovetail joints have two parts, the pin and the tail. Preparation is key….. make a drawing of the joint, ensuring that there is half a pin top and bottom.
The slope of the pin should be 1 to 6. The tails can be as much as three times their width apart.
Finger joints, also known as box joints, are mostly used to join pieces of equal thickness together.
Although not as strong as dovetails, they are easier to fit together, especially if you make the straight pins using a table saw with a dado blade set.
It’s wise to cut some test slots in scrap timber to ensure the fingers fit together snugly.
Mortise & Tenon
The great strength of a mortise and tenon joint is derived from the tenon being an integral part of the completed workpiece.
It’s also a very versatile joint, as you can make the tenon any size you want and even reinforce it if necessary.
However, there are two rules to follow when deciding on the width of the mortise. Firstly, it should be no more than half the thickness of the workpiece and secondly, leave a minimum of 3/8″ between the edge of the mortise and the face of the workpiece.
The best modus operandi is to prepare the mortise first using a drill and then complete the job by neatening the hole with a chisel. The joint can be reinforced using wedges or dowels..
Lap joints are favoured for frame construction as they have a large gluing area compared to butt joints and the two pieces lock mechanically giving additional strength.
There are three main types of lap joint…..corner half lap, ‘T’ half lap and cross lap.
Corner half laps are made by cutting two wide rabbets in the ends of two workpieces.
A ‘T’ half lap joins the end of one board having a wide rabbet to the middle of another board which has been cut with a matching wide dado.
Cross laps are half lap joints where both workpieces are cut with dados and the joint is within the length of the workpiece.