taper: B10 = small end of MT1 (D = 10.095 mm) B12 = large end of MT1 (D = 12.065 mm) B16 = small end of MT2 (D = 15.733 mm) B18 = large end of MT2 (D = 17.780 mm) B22 = small end of MT3 (D = 21.793 mm) B24 = large end of MT3 (D = 23.825 mm) The number after the B is the diameter (D) of the large end of the taper to the nearest mm, and 'about' 1mm larger than the large end of the socket (~2mm in the case of B22 and B24) NMTB taper family The National Machine Tool Builders Association (now called the Association for Manufacturing Technology) defined a steep taper that is commonly used on milling machines. The taper is variously referred to as NMTB, NMT or NT. The taper is 3.500 inches per foot and is also referred to as "7 in 24" or 7/24; the computed angle is 16.5943 degrees. All NMTB tooling has this taper but the tooling comes in different sizes: NMTB-10, 15, 20, 25, 30, 35, 40, 45, 50 and 60. These tapers were apparently also specified in ASA (now ANSI) B5.10-1943. NMTB is a "self releasing" or "fast" taper. Unlike the more acute self holding tapers above, such tapers are not designed to transmit high torque; high torques are carried by driving keys engaging slots on the flange. The purpose is to allow a quick and easy change between different tools (either automatically or by hand) while ensuring the tool or toolholder will be tightly and rigidly connected to the spindle, and accurately coaxial with it. The larger end adjacent to the tool makes for more rigidity than is possible with Morse or R8 tapers fitted to comparable machines. Patent 1794361 (filed 25 March 1927) describes milling machine spindle and tool shapes using a steep taper. The patent was assigned to Kearney & Trecker Corporation, Brown & Sharpe, and Cincinnati Milling Machine Company. The patent wanted a taper that would freely release the tool and found that a taper of 3.5 in 12 had that property. The patent also used the keys and slots and a tail on the tool shank to prevent the tool shank from falling out of a horizontal mill's spindle while the operator connected the drawbar. ANSI B5.18-1972 specifies some essential dimensions for milling machine spindles and tool shanks using