One of the most common ways of workholding is by location with reference to an external surface. This is also known as locating to a datum surface. Flat surfaces are commonly used for locating. However, there can be various types of locators depending on the application.
Locating Pin Method
The most common method used for locating is with the help of locating pins. Locating pins are available in a variety of standard sizes and shapes depending on the application they are to be used in. For example a “diamond” head pin is used as a feature to help with the locating operation in situations with machining inaccuracies while a flat face with chamfered head style pin is used in applications with short strokes. The different kinds of pins available can be seen in the figure below.
With the use of locating pins a principle known as the “3-2-1 rule” was put into practice. It is named as such because of a procedure with three steps which uses three points of known locations, then two, then one fixed point to locate the part. An example of this can be seen in Figure 5 where three pins are used to form the bottom datum in the z axis, two are used in the x axis and one pin forms the remaining y direction.
For applications with work pieces that experience heavy external forces and are prone to shifting another method exists to help secure the part. In this case a different pin shape would needed. Along with this, the number of pins used would need to be increased to prevent excess movement. This layout can be seen in Figure 6.
Locating pins are one of the most crucial components in a fixture design. Not only do they restrict movement, but also ensure precise positioning for consistency. In the next article we will expose a common issue that may affect the quality of your end-product.
In Part 3 of our Workholding Technology series we will examine ways of locating with the intention of avoiding burrs and reducing the negative effects burrs can cause.