There are a few different ways that you can combine part files, depending on the software that you are using. If you are using Autodesk Inventor, you can use the Combine command to combine parts into a single file. If you are using Solidworks, you can use the Combine Parts command to combine multiple parts into a single file. If you are using PTC Creo, you can use the Combine tool to combine multiple parts into a single file.