Scott F. answered • 08/01/25
Mechanical Design Engineer - Solidworks
Machined parts should always be located following the 3-2-1 principal. 3 points to establish a plane (datum A), 2 points to establish a line within this plane (datum B), and 1 point to to establish a point along the line (datum C).
There are many ways to accomplish this when locating machined parts.
A flat machined flat face against another flat face is commonly used to establish the plane (datum A). The geometrical flatness tolerance of the surfaces will determine how precise the location of this plane actually is.
The line and point are commonly established using two dowel pins. In the base part, the pins are typically pressed-fit into holes, and in the mating part, one pin resides in a precision slot, and the other a precision hole. The slot should be oriented so the long axis points towards the hole. All tolerances affect the final location of the machined part and need to be considered in the design and drafting details. This includes the location tolerances of the holes and slots, the fit tolerances of each pin (pin diameter and hole diameter), the perpendicularity of the pin holes, and the straightness of the pins.
Other methods to establish location include machining locating "features" into the parts. For example a pocket can be machined into one part, and the second part can have faces which are pushed up against the interior edges of the machined pocket, in two directions, when the features are in contact with each other a location has been established. If this method is used, its a good idea to incorporate spring plungers (or other similar spring mechanisms) to ensure that the surfaces are actually in contact during assembly.
