Tool holder is a device that serves as a connector between the machine spindle and tools or other accessory tools. The main standard spindle models include BT, SK, CAPTO, BBT, HSK, among others. BT and BBT are Japanese standards and are now widely used. SK (DIN6987) is a German standard.

Traditional tool holders include ER type, high-force type, side-lock type, face mill type, drill chuck, and Morse taper shank.
Modern types include hydraulic tool holders, thermal shrink-fit tool holders, and PG (cold press) type.

HSK Tool Holder
The HSK tool system is a new type of high-speed short-taper tool holder. Its interface uses simultaneous taper and face contact for positioning. The tool holder is hollow, with a short taper length and a taper ratio of 1:10, which facilitates lightweight and high-speed tool changing. As shown in Figure 1, due to the hollow taper and face contact positioning, it compensates for the radial deformation differences between the spindle bore and the tool holder during high-speed machining and completely eliminates axial positioning errors, making high-speed and high-precision machining possible. This type of tool holder is increasingly commonly used in high-speed machining centers.

KM Tool Holder
The structure of the KM tool holder is similar to the HSK tool holder, also featuring a hollow short-taper design with a taper ratio of 1:10, and it uses simultaneous taper and face contact for positioning and clamping. As shown in Figure 2, the main difference lies in the clamping mechanism used. The KM clamping mechanism has been patented in the United States, providing greater clamping force and higher system rigidity. However, since the KM tool holder has two symmetrical arc grooves on the taper surface (used during clamping), it is relatively thinner and some components have lower strength. Moreover, it requires very high clamping force to function properly. Additionally, patent protection for the KM tool holder structure has limited its rapid adoption and widespread use.

NC5 Tool Holder
It also adopts a hollow short-taper structure with a taper ratio of 1:10 and uses simultaneous taper and face contact for positioning and clamping. Since torque is transmitted through the keyway on the cylindrical front end of the NC5 tool holder, and there is no torque-transmitting keyway at the rear, its axial dimension is shorter than that of the HSK tool holder. The main difference from the previous two tool holders is that the NC5 does not use a thin-walled structure; instead, an intermediate taper sleeve is added at the taper surface. While KM and HSK tool holders rely on thin-wall deformation to compensate for manufacturing errors and ensure reliable contact of both the taper and face, the NC5 tool holder achieves this through axial movement of the intermediate taper sleeve. The axial movement is driven by a disc spring on the tool holder's end face. Due to the strong error compensation capability of the intermediate taper sleeve, the NC5 tool holder has slightly lower requirements for the manufacturing precision of the spindle and the tool holder itself. Furthermore, the NC5 tool holder has only one screw hole for installing the pull stud, with a thick wall and high strength, allowing the use of增压 clamping mechanisms to meet heavy-duty cutting requirements. The main drawback of this tool holder is the additional contact surface between the tool holder and the spindle taper bore, which reduces positioning accuracy and rigidity.

CAPTO Tool Holder
The CAPTO tool holder, produced by Sandvik Company, does not have a conical shape but rather a tri-lobed taper with rounded lobes and a taper ratio of 1:20. It also features a hollow short-taper structure and uses simultaneous taper and face contact for positioning. The tri-lobed taper structure enables slip-free torque transmission in both directions, eliminating the need for drive keys and thus avoiding dynamic balance issues caused by keys and keyways. The large surface area of the tri-lobed taper results in low surface pressure on the tool holder, reducing deformation and wear, thereby maintaining good accuracy. However, machining the tri-lobed taper bore is difficult and costly, and it is incompatible with existing tool holders. The fit may also self-lock.