Sunday, August 16, 2009

Dowel Pins and Locating Pins

Dowel pins are the fasteners used to secure two parts together. They are available in both Metric and English sizes, and carry specifications such as diameter, length, and materials. Most dowel pins are made of stainless steel, plastic, , hardened steel, or ground steel. Plastic dowel pins are made of thermoplastic or thermosetting polymers with high molecular weight. Stainless dowel pins are chemical and corrosion resistant, and have relatively high pressure ratings.

Dowel pins are often used as precise locating devices in machinery. Stainless dowel pins are machined to tight tolerances, as are the corresponding holes, which are typically reamed. A dowel pin may have a larger diameter so that it must be pressed into its hole or a smaller diameter than its hole so that it freely slips in.

When mechanical design engineers design the mechanical components, typically they use dowel holes as reference points to control positioning variations and attain repeatable assembly quality. If no dowel pins 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 at least 0.2 mm play for bolts up to 10 mm. If dowel pins are used in addition to bolts, the play is reduced to approximately 0.01 mm.

In automatic machinery, dowel pins are used when precise mating alignment is required, such as in differential gear casings, engines, transmissions and indexing mill. Not only high precision will be achieved, but also it can reduce time to exchange the machine parts. Imagine if you have 50 units of product holders that are required to mount into the indexing mill, without dowel pins, you have to take most of the time to adjust the position of the product holders online. With the dowel pins, you can easily set the units off-line and just mount them into the indexing mill. They are widely used for SMED concept (Single Minute Exchange of Die).

Single Minute Exchange of Die (SMED) is one of the many lean production methods for reducing waste in a manufacturing process. It provides a rapid and efficient way of converting a manufacturing process from running the current product to running the next product. This rapid changeover is key to reducing production lot sizes and thereby improving flow

To locate the parts precisely, normally diamond locating pins are used in conjunction with a round locating pin. The round locating pin holds the part in position, and the diamond pin hold the part to keep it from rotating around the round locating pin.

It is poor design practice to use two round pin four-way locators, as the tolerance stack-up from the center of one pin to the other pin will make mounting the part impossible. Two round locating pins should only be used when one is place in a hole and one is place in a slot. If two holes are to be used as locating features, use one round and one diamond pin.

The long axis of the diamond locating pin should be positioned perpendicular to a line drawn between the center of the round locating pin and the center of the diamond locating pin. Any other orientation will allow the part to swing from side to side and produce inaccurate results.

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