(Optional) Select one or more surface parts or a components.Selecting
objects before starting the tool will limit the tool's results to those
objects.If you select a component, all surface parts in the component
that have touching faces will be merged.
Click Stitch in the Solidify group on the Repair tab.The edges of faces that can be merged are highlighted.
(Optional) Change the Maximum distance slider in the Options panel to
adjust the distance between parts that are automatically detected.
Select the Check for coincidence check box to find any edges or planes
that are coincidental to each other.
Click the highlighted faces that you want to merge.The faces are
merged when you select them.
(Optional) Click the Select Geometry tool guide to select faces that
were not automatically detected.
Stitch in the Solidify group on the Repair tab.