Machinery's Handbook, 31st Edition
Dowel Pins 1879 Dowel-Pins.— Dowel-pins are used either to retain parts in a fixed position or to pre - serve alignment. Under normal conditions a properly fitted dowel-pin is subjected solely to shearing strain, and this strain occurs only at the junction of the surfaces of the two parts which are being held by the dowel-pin. It is seldom necessary to use more than two dowel-pins for holding two pieces together and frequently one is sufficient. For parts that have to be taken apart frequently, and where driving out of the dowel-pins would tend to wear the holes, and also for very accurately constructed tools and gages that have to be taken apart, or that require to be kept in absolute alignment, the taper dowel-pin is preferable. The taper dowel-pin is most commonly used for average ma - chine work, but the straight type is given the preference on tool and gage work, except where extreme accuracy is required, or where the tool or gage is to be subjected to rough handling. The size of the dowel-pin is governed by its application. For locating nests, gage plates, etc., pins from 1 ∕ 8 to 3 ∕ 16 inch (3.2–4.8 mm) in diameter are satisfactory. For locating dies, the diameter of the dowel-pin should never be less than 1 ∕ 4 inch (6.35 mm); the general rule is to use dowel-pins of the same size as the screws used in fastening the work. The length of the dowel-pin should be about 1 1 ∕ 2 to 2 times its diameter in each plate or part to be doweled. When hardened cylindrical dowel-pins are inserted in soft parts, ream the hole about 0.001 inch (0.025 mm) smaller than the dowel-pin. If the doweled parts are hardened, grind (or lap) the hole 0.0002 to 0.0003 inch (0.005–0.0076 mm) under size. The hole should be ground or lapped straight, that is, without taper or “bell-mouth.” Assembly with Dowel Pins starting on page 1885. British Standard for Metric Series Dowel Pins.— Steel parallel dowel pins specified in British Standard 1804:Part 2:1968 are divided into three grades which provide different degrees of pin accuracy. Grade 1 is a precision ground pin made from En 32A or En 32B low carbon steel (BS 970) or from high carbon steel to BS 1407 or BS 1423. Pins below 4 mm diameter are unhardened. Those of 4 mm diameter and above are hardened to a minimum of 750 HV 30 in accordance with BS 427, but if they are made from steels to BS 1407 or BS 1423 then the hardness shall be within the range 600 to 700 HV 30, in accordance with BS 427. The values of other hardness scales may be used in accordance with BS 860. Grade 2 is a ground pin made from any of the steels used for Grade 1. The pins are nor mally supplied unhardened, unless a different condition is agreed on between the pur chaser and supplier. Grade 3 pins are made from En 1A free cutting steel (BS 970) and are supplied with a machined, bright rolled or drawn finish. They are normally supplied unhardened unless a different condition is agreed on between the purchaser and supplier. Pins of any grade may be made from different steels in accordance with BS 970, by mutual agreement between the purchaser and manufacturer. If steels other than those in the standard range are used, the hardness of the pins shall also be decided on by mutual agreement between purchaser and supplier. As shown in the illustration at the head of the accompanying table, one end of each pin is chamfered to provide a lead. The other end may be similarly chamfered, or domed. If a dowel pin is driven into a blind hole where no provision is made for releasing air, the worker assembling the pin may be endangered, and damage may be caused to the associ ated component, or stresses may be set up. The appendix of the Standard describes one method of overcoming this problem by providing a small flat surface along the length of a pin to permit the release of air. For purposes of marking, the Standard states that each package or lot of dowel pins shall bear the manufacturer’s name or trademark, the BS number, and the grade of pin.
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