Method 1:
1. Open the assembly you want to create the external surface for.
2. Under the Tools tab, click simplify.
3. Next, click the Visible Faces button shown below.
4. Click the process button.
5. Next it asks you to Modify Results. You can exclude certain parts of you don’t want them to be included. Otherwise, just hit the Preview button. Then hit Finish.
6. In the Pathfinder, right click on the Simplified Assembly and click Save As. This can be saved either as a part file or a parasolid.
7. If you saved it as a parasolid, to open it go to the Application button, select Open, and browse to the file. When you click to open it you will have to decide how you want to open it. Open it in a part file and it will be an external surface then.
Method 2:
1. Open up a new part file.
2. Click part copy as shown below.
3. Browse to where the assembly is located and click open.
4. On the Part Copy Parameters screen click Copy as Construction Body.
5. Click apply, and ok. In your Pathfinder it should show that it is a part copy. You can now save this part file.
