Using the UNION Command to Join to 3D Solids in AutoCAD
Help File in the AKN:
https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2017/ENU/AutoCAD-Core/files/GUID-C38426A3-B4CA-4788-A6B9-F132DD705CA0-htm.html
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Source: Autodesk
