A dovetail jig is a specialized device made for woodworkers to use in making dovetail joints faster and more accurately than they can by hand. Very similar to a template in function, a dovetail jig provides a guide against which a woodworker can move a cutting tool, such as a saw or a router. Some dovetail jigs allow a woodworker to cut the pins and tails of a dovetail joint simultaneously
As used in woodworking, jigs are devices woodworkers use to control the movement of other tools. Jigs are created and used when the woodworker must make the same cuts repeatedly, as when cutting deck balusters or chair legs. A woodworker will commonly make a jig for such tasks to ensure that the pieces cut are as identical as possible, in much the same manner that a locksmith will use a key as a jig when cutting its duplicate.
A dovetail joint is commonly used to join two pieces of wood together at right angles, such as the joints connecting the sides of a drawer to the front and back. Used since mankind first learned to work wood, according to the historical evidence, a dovetail joint consists of one or more fan-shaped tenons, or "pins," on one workpiece, inserted into correspondingly-shaped mortises, or "tails" cut into another. Dovetail joints are very popular because of their resistance to having their components separated.
While a dovetail joint is a secure and reliable joint in woodworking, cutting one by hand is a demanding task and a measure of a woodworker's skill. A dovetail joint works best when the pieces fit snugly and there's no play at all between them; cutting the multiple pins and tails to such exacting specifications freehand is difficult and time-consuming. A dovetail jig reduces the work to an assembly-line process, requiring only careful measurement, a secure clamp and a steady hand.
Carpenters and woodworkers have routinely made their own dovetail jigs to help in the production of the most common dovetail joints they make. A shop-made dovetail jig can be as simple as a series of slots cut into a piece of wood, which is clamped to the workpiece and used as a guide for a hand-held router with a special tapered cutting bit called a dovetail bit. A shop-made dovetail jig designed for use with a handsaw could be as simple as a template cut into a thin piece of wood or acrylic. When clamped to the workpiece, a handsaw — generally a tenon saw, which is specially made to cut joints — is used to make multiple cuts into the workpiece, which is then cleaned out with a chisel. Shop-made jigs are generally built from wood, and may require periodic replacement.
When a woodworker must make a large quantity of dovetail joints, as in cabinetry and other types of furniture, a commercial dovetail jig is indispensable because it drastically reduces the amount of time devoted to the task. Moderately-priced to high-priced, these jigs are designed to let the woodworker cut from a single pin-and-tail combination to a long series without having to change the jig's setup, which is often one of the most time-consuming aspects of woodworking. Commercial dovetail jigs are also adjustable in other areas; most will allow changes to the depth and width of the tails and pins. Many commercial dovetail jigs also operate by clamping together the two workpieces to be joined, making the cuts simultaneously to ensure a good fit. Commercial dovetail jigs are almost universally made of steel, aluminum, or brass, with similarly sturdy fasteners and adjusting hardware, and can be expected to last a very long time with moderate to heavy use.