The history of machining involves constantly evolving technology, with machines, cutting tools and components advancing at different paces and often leapfrogging one another. As spindle speeds began to increase in the early 1990s, a group of international manufacturers, academics and the Association of German Tool Manufacturing came together to make sure toolholder performance didn’t fall behind. Their work produced the German DIN standards for HSK toolholders and spindles. 

Here’s the fun part: there’s not just one style of HSK toolholder. The group defined a total of six (!) HSK styles, represented with the letters A through F. Each style features different characteristics for targeted applications, but in a nutshell: styles A, B, C and D were designed for standard applications, and styles E and F were designed for higher speed applications. 

In this article, you’ll find a quick primer on HSK, as well as deeper dives into the three most popular HSK forms: A, E and F. 

The Basics of HSK

The defining characteristic of HSK toolholders is their hollow 1/10 taper. During high-speed rotation, spindles can tend to bellmouth slightly, sucking a traditional steep taper toolholder (CT, BT, NMTB) into the spindle and negatively impacting machine precision. The hollow taper of HSK toolholders is designed to expand with the spindle, with the elastic deformation of the taper, ensuring consistent and solid contact with the spindle wall.

Additionally, when an HSK drawbar clamps the toolholder, the toolholder is drawn into the spindle, making a second contact surface between the toolholder flange and spindle face. This helps ensure positioning accuracy, and provides additional support against radial forces during heavier cutting.

HSK toolholders come in a range of sizes directly related to the flange diameter in millimeters: 25, 32, 40, 50, 63, 80, 100, 125 and 160. 

HSK A: The Most Common Style

Of the six styles of HSK toolholders, the A style is the most common. Designed for moderate to high spindle speeds and moderate torque, HSK A toolholders feature:

• Drive slots in the shank: for higher torque transmission. Additionally, these slots are unequal so that the toolholder can only be oriented in one direction in the spindle.

• Slots and keyways in the flange: for high-speed automatic tool changes

• Bore on the flange: for a toolholder ID chip

• Internal coolant tubes: for high-pressure coolant

HSK A toolholders are in stock at HAIMER in the following sizes:

• HSK 32A

• HSK 40A

• HSK 50A

• HSK 63A

• HSK 80A

• HSK 100A

• HSK 125A

HSK E: For High-Speed Performance

Built for higher spindle speeds, the HSK E style removes all sources of unbalance found in HSK A style toolholders. This includes:

• No drive slots: torque transmission is only through the spindle–toolholder connection

• No keyways in the flange: only a symmetrical slot, resulting in slightly slower tool changes

• No bore in the flange: for toolholder ID chips

The perfectly symmetrical HSK E form makes it ideal for extremely high-speed, lower-torque machining applications. HSK E style holders are in stock at HAIMER in the following sizes:

• HSK 25E

• HSK 32E

• HSK 40E

• HSK 50E

HSK F: For Demanding High-Speed Applications

Also for high-speed, lower-torque applications, HSK F style toolholders are identical to the HSK E toolholders except for one major factor: the tapers are actually one size smaller than an E style shank of the same size. So an HSK 63F toolholder will have the same size taper as an HSK 50E toolholder. 

Why does this matter? The reduced taper size leaves space for a larger flange face, allowing for greater flange-to-spindle face contact area. HAIMER stocks HSK F style holders in the following configurations:

• HSK 63F

• HSK 80F (with Makino style drive pins)

And remember, A, E, and F might be the most popular styles of HSK toolholders, but there are other options as well. If you would like to learn more about the advantages HSK toolholders can bring to your shop, contact Haimer today to discuss the particulars of what you need. Whether you’re working with standard applications or high-speed applications with more complicated variables at play, HAIMER can provide the right toolholder for the job. 

Previously Featured on Haimer's blog.

You can find Haimer's line of toolholders, accessories, adapters and extensions on MSCDirect.com.