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Read many CNC and Machining books from Google books and others.
Though some are not complete, they still provide valuable information for free.

Select a Book CNC Programming Handbook 2nd Edition by Peter Smid CNC Machining Handbook By James Madison Theory and Design of CNC Systems CNC Control Setup for Milling and Turning Fundamentals of Machining and Machine Tools Machinists' and Metalworkers' Pocket Reference American Machinists' Handbook and Dictionary Lathe Work for Beginners Modern Machine Shop Practice Vol 1 PDF Modern Machine Shop Practice Vol 2 PDF Marks Free Technical Ebooks links - must see! ASM Handbook - Aluminum and Aluminum Alloys