The 
Fix Together Constraint holds all the parts in the Move Parts list perfectly rigid relative to one another. One part will not rotate or translate away from another if a move is applied. If one of the parts in the list is moved then all the other parts will move with it.
Procedure: Select the parts that need to be fixed together. More than two parts can be listed for this type of move. Their location and orientation will be fixed with respect to each other as the model is built.
•Select the parts in the Move Parts Tab.
