From Wikipedia:
"
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).[
citation needed] 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).[
citation needed] 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."
Since dowel pins typically only come in fractional sizes, drill blanks and precision gage pins can be used for other sizes.