High-speed machining has become a vital component and direction of modern manufacturing technology. Many industrialized countries have started using high-speed cutting machines with spindle speeds reaching tens of thousands of revolutions per minute. China's automotive, tractor, and aerospace industries have imported numerous advanced production lines, machining centers, and high-performance machine tools from abroad, including many high-speed cutting systems. As the spindle speed increases significantly, the traditional BT (7:24 taper) tool system struggles to meet the requirements of high-speed cutting. In response, developed countries have raced to develop new tool systems suitable for high-speed cutting. Among these, the HSK (Germany Hohl Schaft Kegel), the US KM, and Japan's NC5 are widely used, with the HSK system being the most mature and widely adopted.
The HSK tool system uses a hollow short-cone structure and two-sided clamping method, offering superior performance in terms of rigidity, radial runout accuracy, repeated installation accuracy, and clamping reliability. The manufacturing precision of the tool system directly impacts its working performance. This paper analyzes the main differences between the German DIN standard and the ISO standard in the HSK tool system and discusses the influence of positioning accuracy and connection stiffness, providing reference for the development of a domestic new tool system.
The formulation of the HSK tool system standard began in 1987, with a special working group established at Achen University of Technology by over 30 units, including machine tool manufacturers, tool manufacturers, and user companies. Prof. Weck initiated research on a new tool system. After the first round of research, the working group submitted a proposal for an "automatic tool change hollow handle" to the German Industrial Standards Organization in July 1990. In July 1991, Germany published the draft DIN standard for the HSK tool system and proposed the development of relevant ISO standards. However, in May 1992, the International Organization for Standardization (ISO/TC29) decided not to formulate an ISO standard for automatic tool change hollow shanks. After further study, Germany established the official industrial standard DIN 69893 for the HSK tool system in 1993. In May 1996, at the ISO/TC29/WG33 review meeting, the HSK tool system based on DIN 69893 was developed into the ISO standard draft ISO/DIS12164. After several revisions, the official ISO standard ISO 12164 for the HSK tool system was promulgated in 2001.
Before the official ISO standard for the HSK tool system was issued, products were designed and manufactured according to the German DIN 69893 and DIN 69063 standards. However, the ISO standard made several important improvements to the DIN standard, which greatly affect the performance of the HSK tool system. The quality of the taper between the shank and the spindle directly affects its performance and accuracy. Therefore, it is essential to reasonably define the accuracy requirements of the shank taper and the spindle hole. The DIN and ISO standards adopt different approaches in this regard. For example, the HSK-A shank's main control dimensions under the DIN standard are shown in Figure 1, while Table 1 provides the relevant dimensions of the HSK-A63 shank and the spindle taper.
In the DIN standard, the shank taper is controlled by the diameters d2 (large end) and d3 (small end), along with the position dimensions l2 and l3, and the taper (1:10). The corresponding spindle taper hole is controlled by D2 (large end diameter), L2 (sectional position size), and the taper (1:10). The ISO standard, on the other hand, controls the shank taper by d2 (large end diameter), l2 (sectional position dimension), profile tolerance t, and taper (1:9.98). The small end is not specified separately, and the spindle taper hole is also controlled similarly but with a different taper (1:10).
The interference at the large and small ends when the shank taper is matched with the spindle taper is calculated differently under each standard. Under the DIN standard, the maximum interference at the large end is 12 μm, and the minimum is 4 μm. At the small end, the maximum is 8 μm, and the minimum is -1 μm (a gap). Under the ISO standard, the maximum interference at the large end is 17 μm, and the minimum is 7 μm. At the small end, the maximum is 14 μm, and the minimum is 4 μm.
Positioning accuracy and joint stiffness are key indicators for evaluating the performance of the HSK tool system. Both DIN and ISO standards use the end face for axial positioning, resulting in high axial positioning accuracy (<0.001 mm). Radial positioning accuracy depends on the fit between the shank taper and the spindle taper hole. According to the DIN standard, the average interference at the large end is 8 μm, while under the ISO standard, it is 12 μm, making the ISO standard more favorable for ensuring positioning accuracy.
The coupling stiffness between the HSK tool holder and the spindle is closely related to the fit of the taper surface and the clamping force. The deformation curve under static load shows that there are two states of connection stiffness. The A-segment corresponds to lower loads and higher stiffness, while the B-segment indicates increased deformation and reduced stiffness under heavy loads. The ISO standard specifies a larger interference fit, which helps maintain higher stiffness under heavy loads, though it reduces the range of working loads. The DIN standard, with smaller interference, ensures better clamping force transmission and higher stiffness within a wider load range, but may lead to rapid stiffness reduction under heavy loads.
In conclusion, the HSK tool system manufactured according to ISO standards has a larger interference fit, ensuring better positioning accuracy and system stability. The use of different tapers (1:9.98 and 1:10) in the ISO standard contributes to higher rigidity and better performance under heavy loads. While the DIN standard offers a more reliable clamping force, it is more suitable for high-speed, light-load operations. Overall, the ISO standard provides a more balanced and stable performance for high-speed and heavy-load cutting applications.
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