The Lowdown on Locating Pins

Locating pins are utilized in manufacturing setups to precisely position workpieces in various orientations to make it possible to execute a wide range of manufacturing operations or dimensional inspections.  Within this broad definition, the design of locating pins can be customized and optimized for a diverse mix of alignment needs as dictated by the manufacturing requirements.  For this reason, locating pins come in many different shapes and sizes and are fabricated from a wide range of specialized materials and coatings.  Additionally, in fast paced production environments, the time and effort involved in achieving this alignment also play an important role in the selection of the locating pin design; therefore, alignment schemes must be carefully designed to avoid lengthy setup times.  A brief survey of the types of pins and secondary features available for incorporation into pin designs will serve to open up new avenues for the manufacturing engineer to more effectively build alignment systems.

Locating pins are ordinarily differentiated by the shape of the pin head, with each head style accomplishing a slightly different purpose depending on the application.  A single diamond head pin, one of the most popular pin styles, contacts the mating hole at only two locations, allowing for ease of fit while still constraining all planar motion except rotation.  Cylindrical locating pins with generous lead in chamfers can be used where more precision is needed, but the use of these significantly increase set up time because cylindrical locating pin schemes are far more vulnerable to machining errors.  As the size and shape of the drilled hole depart from the specified dimensions, it becomes more difficult to mate the workpiece on multiple cylindrical locating pins which are designed to contact the mating hole at all points.  For this reason, an entirely cylindrical pin alignment scheme may not be necessary or practical.

In contrast, the diamond pin scheme can more easily be sized and is usually sufficient for most alignment applications.  A single diamond head pin can also be used in conjunction with a cylindrical pin to achieve an alignment scheme that is sufficient to quickly constrain a work piece to a high degree of precision. Additionally, when workpieces must be aligned to specified vertical clearances, height locating pins possessing a flat, hardened head and a tightly controlled length are typically employed.  In light of this brief survey, other head styles and shapes such as rounded, cone, and bullet style heads can also be specified as needed.

The shank is the part of the pin that is installed into the mounting fixture and does not contact the workpiece.  A variety of different shank mounting configurations are available, such as the threaded shank for blind hole applications or shanks with various styles of side locating flats or notches for use when side access to the pin is available.  Shanks for press fits are also feasible when the back of the mounting plate can be accessed to facilitate the removal and replacement of the pin.  When access to the underside or side of the mounting plate/fixture is not feasible, various head mounting styles such as the counterbored pin are also available.

A wide variety of other secondary features and options are also available and can be specified to further hone the desired functionality of locating pins for each application.

Locating pins with machined flange features at the base of the head serve a dual functionality because the flange mates with the work piece and provides additional height location.

Stepped head pins, usually utilized in sheet metal fabrication, possess various diameter transitions at the head and allow for the positioning of numerous workpieces at once.

For blind hole positioning applications, locating pins with machined flat channels running down the pin head serve as an air vent system and are typically employed in tight fit-up applications in order to prevent air compression inside the hole.

Locating pins can also be fabricated with an undercut at the base of the pin which allows the work piece to actually rest on the base, should this be desired.   If this is not necessary, pins can also be fabricated with a generous radius at the head/shank transition in order to add structural strength to pins that may be subjected to substantial side loads.

Because locating pins are typically subjected to repeated, sometimes heavy contact loads, they are usually fabricated from hardened tool steels to achieve superior wear resistance when compared with softer materials.   In applications requiring a high degree of corrosion resistance, locating pins manufactured from 300/400 series stainless steels are available.

If the use of stainless steel pins are not feasible there are numerous engineered coatings which can be specified depending on the application, but a TiCN coating is most commonly specified to provide substantial corrosion resistance and harden the surface layer.

Additionally, other coating types can be specified to provide essential electrical insulation where locating pins are required to accurately fixture the workpiece in the pre-weld setup.  One of the most popular, cost effective options is the KCF alloy coating which provides excellent insulation and prevents weld build up between the workpiece and the pin surfaces.

As detailed in this brief survey of locating pin diversity and functionality, there are many features that can be combined into a locating pin design for a variety of applications.  The chart below represents a good starting point in summarizing the extensive number of possibilities available to the manufacturing engineer.  No matter the alignment application, there is a locating pin for the job.