SuperFlatten
https://forums.autodesk.com/autodesk/attachments/autodesk/130/245251/1/SuperFlatten%201.1.lsp
...So the default flatten command isn't amazing and sometimes breaks the drawing. An alternative is to run a lisp command which doesn't affect values other than z.
Download the lisp file from the above. Then in cad type 'cuiload' load the lisp file you've downloaded.Once loaded simply type 'superflatten' and select the objects in the drawing you wish to be flattened....
