TABLE OF CONTENTS The discussion above on constraint is necessary in order to understand the role of assembly features in assembly. Simply put, assembly features are the carriers of constraint between parts. When parts are joined, degrees of freedom are constrained. The shape of the feature determines which degrees of freedom are constrained and which remain free.
Several scenarios for joining parts via features can be envisioned. The simplest one provides a feature that by itself constrains all six degrees of freedom. A square peg in a blind square hole can do this if we consider only the mathematics. From a practical point of view, such a feature may not be the best choice. First, it contains several two-sided constraints, although we can soften their effect if we can tolerate a small amount of locked in stress or a small gap in the joint. Second, if there is a small gap, then repeated alternating loads will lead to a kind of battering wear that will gradually destroy the feature. [14]
A more robust way to join two parts is to use multiple features, each of which provides constraint in some of the six degrees of freedom. The car seat discussed in Section 4.C.5.b provides an example of this. Three degrees of freedom are constrained by the 25-mm pin hole joint and the surrounding floor ( X and Y normal to the hole axis and Z along it), two more by the 25-mm 40-mm pin slot...
