On the Assemble command bar, click the Mate button on the Relationship Types list.
Do one of the following:
If you are replacing a relationship you deleted from an existing part, select the part to be repositioned.
If you are placing a new part into an assembly, select the face to be mated.
Select a part in the assembly.
On the part in the assembly, select the face to which the first part is to be mated.
Press Enter.
Note:
You can apply the mate relationship with a floating offset or a fixed offset.
For a fixed offset, type an offset value on the command bar in the Offset Value box. You can change the offset value later.
For a floating offset, you can apply an additional relationship to control the offset value. For example, you can apply an align relationship.
You can use the Flip option on the command bar to change the mate relationship to a planar align relationship.