Meaning
Precision locating components designed with a hemispherical tip provide point contact against a mating flat surface or V-groove to ensure accurate, repeatable part positioning. Utilizing spherical contact pins in assembly or metrology fixtures prevents the over-constraint of molded polymer parts. Because plastic parts are prone to thermal expansion and minor dimensional variations, these elements allow for slip-free alignment without binding.
This design is widely used in kinematic registration systems to establish a repeatable datum.
Contact Mechanics
The spherical geometry ensures that contact is theoretically limited to a single point, which reduces friction and eliminates translational play. When using spherical contact pins, the force is distributed over a very small area, which requires that the mating surface be of sufficient hardness to prevent indentation. If the plastic part itself is being positioned, the clamp force must be regulated to avoid marking or deforming the polymer surface.
Fixture Application
Setting up a three-point kinematic mount involves placing the part on three of these locating elements to define a plane. Incorporating spherical contact pins in this manner allows the part to be removed and replaced with sub-micron repeatability. This level of precision is necessary when conducting secondary operations such as laser engraving or multi-shot assembly.
Wear Consequence
Contamination by mold release agents or abrasive plastic dust on the tips can cause slight deviations in part positioning. Regular cleaning and verification of the spherical contact pins are necessary to ensure the continued accuracy of the registration fixture.
