A dowel is a solid cylindrical rod, usually made of wood, plastic, or metal. In its original manufactured form, a dowel is called a dowel rod. Dowel rods are often cut into short lengths called dowel pins. Dowels are employed in numerous, diverse applications including as axles in toys, detents (e.g., in gymnastics grips), structural reinforcements in cabinet making, and supports for tiered wedding cakes. Other uses include:
- As furniture shelf supports
- As moveable game pieces (i.e., pegs)
- As supports for hanging items such as clothing, key rings, tools, toilet roll dispensers and picture frames
- To precisely align two objects in a dowel joint: a hole is bored in both objects and the dowel pin is inserted into the aligned holes
- As a core to wrap cable or textiles around
For modest manufacturing volumes, wood dowels are typically manufactured on industrial dowel machines. Such machines employ interchangeable cutting heads of varying diameters, thus enabling the machines to be quickly retooled to manufacture different diameters of dowel (e.g., 1⁄4 in or 6.35 mm, 1⁄2 in or 12.7 mm, 2 in or 50.8 mm, etc.). Each head consists of a hollow chuck with cutting knives protruding inward, through which the wood passes as the knives cut it into cylindrical shape. The mechanism is open-ended, with material guides at the machine's entry and exit to enable fabrication of continuous dowel rod of unlimited length. Some dowel machines have powered feed devices that automatically feed material into the machine, while others require hand feeding.
High-volume dowel manufacturing is done on a wood shaper, which simultaneously forms multiple dowels from a single piece of rectangular stock (i.e., wood). These machines employ two wide, rotating cutting heads, one above the stock and one below it. The heads have nearly identical cutting profiles so that each will form an array of adjoined, side-by-side "half dowels". The heads are aligned to each other and one head is shaped to make deeper cuts along the dowel edges so as to part the stock into individual dowel rods, resulting in a group of dowel rods emerging in parallel at the machine's output.
The wooden dowel rod used in woodworking applications is commonly cut into dowel pins, which are used to reinforce joints and support shelves and other components in cabinet making. Some woodworkers make their own dowel pins, while others purchase dowel pins precut to the required length and diameter.
Dowel-based joinery typically employs fluted dowel pins. A fluted dowel pin has a series of parallel grooves cut along its length. The fluting provides channels through which excess glue—which is used to secure the dowel pin in its hole—can escape as the dowel is inserted, thereby relieving the hydraulic pressure that might otherwise split the timber when the mating pieces are clamped together.
Various systems, such as Dowelmax, have been devised to aid in dowel joinery. In some cases, alternative joinery methods may be used in place of conventional dowel pins, such as Miller dowels, biscuit joiners, and proprietary tools such as the Domino jointer.
Wooden dowels have been used in manufacturing and woodworking for many centuries. One of the earliest documented uses of wooden dowels was in Japanese shrines in AD 690, which were constructed using only wood, wooden dowels and pegs, and interlocking joints. Around AD 1000, Leif Ericson sailed across the North Atlantic in a ship that was largely constructed of overlapping planks held together by wooden dowels and iron nails. The wooden dowels did not rust and thus were more reliable than iron for long expeditions.
Dowel pins are often used as precise locating devices in machinery. Steel dowel pins are machined to tight tolerances, as are the corresponding holes, which are typically reamed. A dowel pin may have a smaller diameter than its hole so that it freely slips in, or a larger diameter so that it must be pressed into its hole (an interference fit).
When designing mechanical components, mechanical engineers typically use dowel holes as reference points to control positioning variations and attain repeatable assembly quality. If no dowels are used for alignment (e.g., components are mated by bolts only), there can be significant variation, or "play", in component alignment.
Typical drilling and milling operations, as well as manufacturing practices for bolt threads, introduce mechanical play proportional to the size of the fasteners. For example, bolts up to 10 mm (0.394 in) in diameter typically have play on the order of 0.2 mm (0.008 inches). When dowels are used in addition to bolts, however, the tighter dimensional tolerances of dowels and their mating holes—typically 0.01 mm (0.0004 inches)—result in significantly less play, on the order of 0.02 mm (0.0008 inches). Manufacturing costs are inversely proportional to mechanical tolerances and, as a result, engineers must balance the need for mechanical precision against cost as well as other factors such as manufacturability and serviceability.
In automobiles, dowels are used when precise mating alignment is required, such as in differential gear casings, engines, and transmissions.
Bolts in a bolted joint often have an important function as a dowel, resisting shear forces. For this reason, many bolts have a plain unthreaded section to their shank. This gives a closer fit to the hole and also avoids some problems with fretting wear when a screw thread bears against an unthreaded component.
A cross dowel is a cylindrically shaped metal nut (i.e., a metal dowel) that is used to join two pieces of wood. Like other metal nuts, it has an inside threaded hole, although the hole is unusual in that it passes through the sides of the dowel. One or both ends of the dowel are slotted, with the slots oriented parallel to the threaded hole through which the bolt will pass.
In a cross dowel application, the two pieces of wood are aligned and a bolt hole is drilled through one piece of wood and into the other. A dowel hole is drilled laterally across the bolt hole and the cross dowel is inserted into it. A screwdriver is inserted into the slot at the end of the cross dowel and the dowel is rotated so that its threaded hole aligns with the bolt hole. The bolt is then inserted into the bolt hole and screwed into the cross dowel until the wood pieces are held tightly together.