Understanding the Differences Between Tool Holders: BT, SK, ISO, PSC, and HSK

When selecting a suitable tool holder, there are several factors to keep in mind: What are the main differences between each taper type? Generally, which machines are these tool holders compatible with? How are they mounted on the machines, and why? What types of outputs are available for each type?

Below, I’ve summarized some of the most frequently asked questions I encounter.

What are the main differences between each taper type? ▼

Tool holders have different taper designs because they are meant to be compatible with various types of machines and machining applications. Different machines may feature different spindle tapers, which determine the type of tool holder that can be used. Additionally, different cutting tools, such as end mills, drills, and taps, may require specific types of tool holders to be securely mounted on the machine spindle.

Some common tool holder taper types include BT, SK (DIN69871), ISO (DIN2080), HSK, and PSC (Polygon Shank). The choice of taper depends on the specific application and machine being used. Selecting the appropriate taper is crucial for ensuring compatibility and reliable performance.

Which machines are these tool holders typically used on? ▼

• BT (also known as BT MAS): These tool holders are commonly used on machining centers from Japan, Korea, Taiwan, and the U.S.
• SK (also known as DIN69871): SK tapers are widely used in German and European machining centers. They can also be used on U.S. machining centers designed for CAT40/CAT50 tools.
• ISO (also known as DIN2080): ISO tool holders are typically found on conventional (manual) milling machines equipped with manual or automatic drawbars.
• HSK (also known as DIN69873): These tool holders are primarily used on high-speed CNC machining centers, particularly those with spindle speeds of 10,000 RPM or higher. Developed in Germany, HSK tapers are popular in Europe and other regions due to their exceptional performance in high-speed machining. The hollow-shank design of HSK provides a more direct and efficient power transfer to the tool, resulting in improved precision, stability, and cutting performance.
• PSC (Capto-compatible): PSC tool holders are ideal for CNC machining applications. Their polygon shank design ensures a strong and stable clamping mechanism, enhancing tool-holding force and stability during machining.

How are these tool holders mounted on the machines, and why? ▼

• BT, SK, and ISO tool holders: These tool holders feature a steep taper and are typically clamped into the machine using a pull stud on the rear of the taper, secured by a drawbar inside the spindle. This traditional method is reliable but may lose precision at extremely high speeds. These tapers often have a ratio of 1:10.
• HSK tool holders (short for "Hollow Shank Taper" in German): HSK tapers are classified as dual-contact designs, meaning they maintain contact with both the spindle taper and the flange on the spindle face. This ensures a robust connection in both axial and radial directions, preventing movement during machining and making them ideal for higher speeds. HSK tapers have a ratio of 1:10 and are available in various types, such as HSK-A, HSK-C, HSK-E, and HSK-F. The primary factors influencing the choice of HSK taper are the speed and torque requirements.

What types of outputs are available for each taper type? ▼

• BT, SK, and HSK tapers: These are the most commonly used tool holders in the manufacturing industry. They typically offer a comprehensive range of options, including standard performance holders such as side-lock holders, collet chucks, and face mill holders. High-performance options, like hydraulic chucks and shrink-fit holders, are also widely available.
• ISO (DIN2080) tool holders: These are somewhat outdated, so you’ll mainly find more traditional options such as side-lock holders and collet chucks. This is largely due to the spindle speed limitations of DIN2080 machines, which make the use of high-speed tooling systems impractical.
• PSC (Capto-compatible) tool holders: These are used for both CNC milling and turning machines. For milling, standard options include ER collet chucks, side-lock holders, and face mill holders. For turning, available options include boring bars, turning tools, and threading tool holders.

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