Dowel Pins: a steel pin, usually cylindrical, that can keep machine parts in proper alignment or fasten them together. Inserted into receiving holes, dowel pins hold together parts of an assembly with friction. They may also be used as location pins. These pins are common in the automotive and aerospace industries as well as in furniture making.
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.
There are a variety of specifications, military, ISO, DIN, ASME that pins may be made to. And size can even vary by dowel pin material. Metric dowel pins are often found in two size. In DIN 6325 standard the dowel pins are slightly larger than the nominal value. For example a 3 mm dowel pin will range from 3.002 to 3.008. In the ISO 2338 standard the dowel pins are slightly smaller – 3mm nominal range is 2.986 to 3.000. The terminology (e.g. “oversized”, “standard”) is not entirely consistent across suppliers. In inch pins “oversized” refers to pins that are more significantly oversized for worn out dowel pin holes. The most common inch sized pins are slightly oversized, and “Undersized” versions are also available.
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.
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