BK1 is an extension of files related to CAD programs published by Autodesk. BK1 are files used to store a backup copy of a project made in the program. Such a file is generated when the program crashes unexpectedly or ends its operation unexpectedly and prematurely.

Under normal conditions, the program creates backups in BAK files. If there is already a backup copy of BK1, the next ones will get the extension BK2, BK3,…, BKA,…, BKZ, BAA, etc. These files are created, among others. by AutoCAD Mechanical, Mechanical Desktop or AutoCAD Map.