Tramming Post Style Mill/Drill

[Dr Stan]

Have you trammed the mill?  If not what ever feature you machine on a part will be out of square with the bottom.  Even something simple like drilling holes requires the head to be trammed (sq with the table).

https://www.youtube.com/watch?v=JfANyiS8Hs4

He shows how to do so on a mill with a ram.  Round arm mills like your BP and my Fray are more difficult, but the overall procedure is essentially the same.