shank, and a flange with a mating surface. The shank is short (about half as long as other machine tapers), with a shallow taper (a ratio of 1:10), and slightly too large to allow the flange to seat fully in the socket. The thin walls, short shank and shallow taper provide a large opening in the back of the tool. An expanding collet fits in there, and mates with 30° chamfer inside the shank. As the drawbar retracts, it expands the collet and pulls the shank back into the socket, compressing the shank until the flange seats against the front of the spindle. This provides a stiff, repeatable connection because it utilizes the centrifugal force inside the spindle. As centrifugal forces increase the expanding collet within the HSK forces the walls of the toolholder shank to stay in contact with the spindle wall. The HSK design was developed as a nonproprietary standard. The working group that produced the HSK standard consisted of representatives from academia, the Association of German Tool Manufacturing and a group of international companies and end users. The results were the German DIN standards 69063 for the spindle and 69893 for the shank. The HSK working group did not adopt a specific product design, but rather a set of standards that defined HSK toolholders for different applications. The group defined a total of six HSK shank forms, in 9 sizes. Sizes are identified by the diameter of the shank's flange in millimeters. These diameters are taken from the R10′ series of preferred numbers, from 25 to 160 mm. Today, the shank forms are designated by the letters A through F and T. The main differences between the forms are the positions of the drive slots, gripper-locating slots, coolant holes and the area of the flange. A is the basic form. The B-form shank is a variant for high-torque applications, and has a flange one size larger relative to its shaft diameter. (Thus, an A-40 shank will fit into a B-50 socket.) Forms C and D are simplified variants of A and B for manual use, eliminating features to accommodate automatic tool changers like a V-groove and associated orientation slots, and a recess for an RFID chip. Forms E and F flanges and tapers are similar to forms A and B, but designed for very high speed machining (20,000 rpm and up) of light materials by eliminating all