Defining the correct boundary conditions for a dynamic analysis is one of the most critical steps that will determine the accuracy of the solution. Where do you cut the model off? Many are tempted only to model their own part and apply some boundary conditions at the attachment interface. This may not be a wise choice. You may need to model some of the other guy’s part to get accurate boundary conditions on your part.
Imagine that you were designing some cantilevered bracket mounted to the side of a wall that was to support a 10lb object. If the wall were 12 inch thick concrete it may be OK to assume rigid boundary conditions at the interface to the wall, however, if the wall were 18-gage sheet metal, then a rigid boundary condition would not be very accurate because the wall may be very flexible at the attachment point. Now imagine that the wall structure is rigid but only has a mass of 10-20 lbs. You may also need to model the wall structure in this case because the two structures will interact.
The Rule-of-Thumb is that you should pick the boundary of your model where you find and order of magnitude change in stiffness and mass.
