taper sizes 30, 40, 45, 50, 60. The specifications describe the position of the driving key and flange and the thread of the draw-in bolt that holds the shank in the spindle. The tooling is referred to as Quick Change; National Machine Tool Builders' Association, 1927; NMTB; American Standard Machine Taper, ANSI B5.18; DIN 2080 / IS 2340; ISO R 290-2583. There are slight variations in threads and flanges (JIS B 6339: MAS 403); and the European standards (e.g., ISO taper) use metric draw threads. The NMTB tool shanks had the 7 in 24 taper, but they also had a constant diameter tail (pilot) at the end of the shank that was described in the 1927 patent. Subsequent design variations dropped the tail (making the shank shorter) and put a V-groove in the flange that aided automated tool changing. Modern designs started using power drawbars that gripped pull studs (also known as retention knobs) that were screwed into the tool shank rather than screw-in drawbars. The power drawbar would grip the pull stud rather than screwing into the tool shank. The more modern toolholder designs became known as the Caterpillar "V-Flange", CAT, V-Flange, ANSI B5.50, SK, ISO, International (INT), BT, ISO 7388-1, DIN 69871, NFE 62540. Once again, there are slight variations in the tooling. Although the basic taper dimensions are the same, there are differences in the flanges, draw-in thread sizes, and pull studs; the international versions use metric sizes. HSK taper HSK toolholders were developed in the early 1990s. HSK stands for :de:Hohlschaftkegel; German for "hollow shank tapers". Steep tapers tend to loosen at high speed, as their solid shanks are stiffer than the spindles they fit into, so under high centrifugal force, the spindle expands more than the toolholder which changes the overall length: That is, as the spindle 'expands' the toolholder tends to move deeper into the spindle in the z-axis which can cause the production of parts that are out-of-tolerance. HSK's hollow shank is deliberately thin and flexible, so it expands more than the spindle and tightens when rotating at high speed. Furthermore, the HSK holder is dual contact: It engages with the spindle on both the taper and the top of the flange which prevents axial movement when thermal growth and/or centrifugal force of the spindle occurs. The flexibility is also used to provide accurate axial location. An HSK toolholder has both a tapered