Modifying the interface properties
Whenever the last feature of one part and the first feature on the following part are a pin and hole, the application creates a pin/hole interface. By default, the pin is assumed to be centered in the hole. However, the clearance between the pin and the hole is often an important consideration in a tolerance analysis, so you have a number of options for how to represent that interface: centered, right (tangent), left (tangent), or floating.
First, let’s consider the screw in the pan. The screw is inserted into the hole and tightened down in the standoff, so the screw is randomly located within the clearance of the hole. This situation is best represented with the float option.
1. Select Float from the menu in the attachment column for the Pan/Screw interface.
With the Float command, the application introduces a variable that represents the random variation of the location of the pin in the hole. Notice that a double-arrow in the Dimension Loop Diagram indicates that this interface is floating.
Now, consider the screw in the PCB. In this case, the screws are inserted into the holes and loosely threaded into the standoffs. Before the screws are tightened, the PCB can slide left or right until a hole comes in contact with a screw. Since we are trying to predict the probability of fit for this case, we should set the properties of this pin/hole interface to maximize the clearance between the plug and the pan.
2. Select Left from the menu in the attachment column for the Screw/PCB interface
Notice that the vertical line joining the pin and hole in the dimension loop diagram is at the left tangent location. The PCB is pushed away from the interface that we are measuring (indicated by the dashed vertical lines) so that the PCB hole is touching the pin on the left side. Notice also that the nominal value of the measurement changes when you change the interface from centered to left tangent.