CNC: Milling holder tapers (the story behind the names BT30, CAT40, etc.)
The taper used generally used is the NMTB, also called NMT or NT. It was defined by the NMTBA, the National Machine Tool Builders Association (now called the Association for Manufacturing Technology) of the USA. It is a 7/24 taper, 16.7112 degrees.
BT, HSK, ISO (also called DIN 2080) and CAT (also called DIN 69871) tooling all have this taper, but differ in the other details, like the areas for holding by the tool changer.
Tooling comes in different sizes: NMTB-25, 30, 35, 40, 45, 50 and 60. NMTB-25 is the smallest, used on small machines, while NMTB-60 is the largest. For a single taper type, the different sizes are scaled versions of the same basic shape, like the BT holders in the picture below.
Pic. source: Cadem NCyclopedia multimedia CNC training software
Cadem NCyclopedia has extensive explanations on CNC holder types and tapers. It is great for CNC programming training and CNC machinist training.
