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Milling tool holders are important components in the CNC milling process, providing the necessary support and precision for cutting tools. Understanding the different standards and types of milling tool holders is important for optimizing performance, ensuring compatibility, and improving productivity in manufacturing processes. From the classic CAT and BT holders to the innovative HSK designs, each type has its specific applications, advantages, and limitations.

This article goes into the different milling tool holder standards, exploring their characteristics and the contexts in which they excel. Whether you are a seasoned machinist or new to the field, gaining insight into these tools will empower you to make informed decisions that elevate your machining capabilities.

Spindle Taper Standards for Milling Tool Holders

The main standards include:

• CAT / CT (ANSI B5.50) The most popular taper standard of CNC milling tool holders in Canada, this North American standard features inch dimensions, including an inch-threaded drawbar. Structurally, it is almost identical to the DIN 69871 standard.
• DIN 69871 or SK (also sometimes known as ISO) is a German standard of CNC milling machine tool holders taper. This standard has become the foundation for many modern clamping systems. A special groove on the flange of the toolholder allows for automatic tool changes on CNC milling machines. The SK chuck is most commonly found on machines from European manufacturers. Coolant supply can be through the center of the toolholder (type AD) or through the center or flange (type AD/B).
• BT (MAS 403) The BT standard was developed by the Japan Machine Tool Builders’ Association to create a unified standard for toolholders that would ensure high precision and reliability during machining. The primary goal of creating this standard was to improve the compatibility and standardization of tools and machines from various manufacturers.

CAT / CT (ANSI B5.50), DIN 69871-SK and MAS 403 BT standards have the same 7:24 taper, but they differ in the design of the flange, which helps visually identify the toolholder standard. The SK and CT toolholder has symmetrical rings on the flange, and the positioning groove is cut all the way through. In contrast, the BT toolholder has a larger upper ring compared to the lower one, and the groove is partially cut and rounded at the end. All three standards offer common taper sizes: 30, 40, and 50 (e.g., CAT30, CAT40, SK40, BT50, etc.). The larger the size, the bigger the taper. The taper size is always determined by the spindle of your CNC machining center.

• HSK DIN 69893 This standard for milling toolholders was developed for high-speed and high-precision machining on CNC machines. HSK features a short, hollow taper shank, which is its key characteristic. Due to its lightweight design and short shank, these toolholders provide high rigidity, making them ideal for high-speed milling. The short shank and rigidity of the connection reduce vibrations and minimize tool deflection during machining. The HSK standard includes several types of toolholders: A, B, C, D, E, and F, each designed for different applications and operating conditions.

Milling Tool Holders Types

The most popular types of holders across all standards include:

• ER Collet Chucks. This type is among the most popular types of toolholders due to their universality. They can hold any tool with a cylindrical shank, such as end mills, drills, Weldon shank tools, and machine taps. The tool is clamped using a special split ER collet, which is secured within the chuck itself. ER collets and chucks are available in the following standard sizes: ER8, ER11, ER16, ER20, ER25, ER32, ER40, and ER50. The larger the collet size, the wider the clamping range of the tool. For example, an ER32 chuck can hold cutting tools with diameters ranging from 1/8” to 3/4” (3–26 mm), while an ER25 collet can accommodate tools with diameters from 1/8” to 5/8” (1–16 mm).
• Endmill (Weldon) Chucks. They designed to hold tools with a flat on their cylindrical shank. These chucks are easy to operate - simply insert the tool into the chuck hole and tighten it with a screw. The screw securely clamps the tool, preventing it from being pulled out or rotating during machining. A unique feature of this chuck is that it is designed to fit only a specific tool diameter.  
• Facemill Holders. The holder features a calibrated mounting surface in the form of a cylinder and a flat face, which is used to position the milling cutter. It has two keyways that transmit torque from the machine and a bolt, typically with a cross-shaped head, to secure the cutter to the holder body.
• Shrink Fit Holders. This type is perfect for high-speed and high-precision machining, where minimal runout and reliable clamping are important. They are suitable for use with solid carbide tools with cylindrical shanks. The working principle of a shrink fit holder involves the thermal expansion method to clamp the tool. To be able to operate you need a special heating device. The tool is then inserted, and as the holder cools down, it contracts, tightly gripping the tool shank. The release process is done in a similar way. Shrink fit holders provide exceptional clamping accuracy due to the uniform and symmetrical grip around the entire circumference of the tool shank. The even clamping force ensures a high-rigidity connection between the tool and the machine, reducing vibrations and runout. This is particularly important for high-precision operations and when working with materials that require tight tolerances. Additionally, the compact design of shrink fit holders allows the tool to easily access narrow or complex machining areas.
• Hydraulic Chucks. Today they are considered a cutting-edge solution among milling chucks. They are known for their high clamping force, excellent versatility, and ease of use. As the clamping screw is turned, pressure is created in the chamber, which reduces the inner diameter of the chuck. This ensures a secure tool hold with minimal radial runout. In terms of ease of use, hydraulic chucks are comparable to Weldon milling chucks and operate on the same principle: simply insert the tool and tighten the screw. To improve the versatility of hydraulic chucks, it is recommended to use special collets, which provide a clamping range of 1/8” to 1-1/4” (3–32 mm), depending on the size of the hydraulic chuck itself.

Features and Applications

These holders enable precise tool alignment, minimizing vibration and ensuring optimal performance. Their features vary widely, including different shank sizes, taper types, and locking mechanisms, allowing for compatibility with a range of milling machines. Applications of milling tool holders span across various industries, from automotive and aerospace to general metallworking. They play an inportant role in enhancing productivity and accuracy, whether in high-speed machining or detailed finishing work. Understanding the features and applications of CNC milling tool holders is important for professionals looking to optimize their machining processes and achieve superior results.