Dowel pins are useful for locating a part into a repeatable, fixed position. They are often used to align parts that need to be more precise than simply lining up screws, as screw holes generally are used for fastening, and not precise alignment. However, pinned joints are not as precise as kinematic couplings, which are used for more precision and repeatability in the optics field and other high tolerance applications. Pinned joints are tolerant to within 1 μm, with a very low cost or complexity to design, which makes them a popular alignment method.
Pinned joints usually consist on a press fit side and a slip fit side. The holes for the press fit side are circular, and the holes for the slip fit side are usually a circular hole paired with a slotted hole. The reason for the slotted hole, as opposed to a similarly sized circular hole, is to account for the concentricity tolerance. While the slip fit tolerance allows for the size of the dowel pin, the slot allows for misalignment. This can also be done with a diamond shaped pin if two circular holes are preferred. Again, the diamond shape simply allows for misalignment of the second pin.
Hole and Slot
Round pin and diamond pin
A chart below is provided for metric dowel pin sizes. These tolerances are normally down through the “Limit” option in SolidWorks, indicating the maximum and minimum diameters of the dowel pin holes, with the tolerance being the difference. Standard dowel pin sizes coming soon!
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