The Differences Between CAT, BT and HSK Tooling Tool Holders

CAT & BT tooling- often referred to as “V-flange” by tooling manufacturers and machinists alike - is the most accepted and widely used standard of conventional milling tooling in the U.S. today. The “CAT” designation was developed by industrial conglomerate Caterpillar Inc. in order to standardize their machinery and has become the U.S. national standard. The “BT” series, on the other hand, is the Japanese equivalent and is more prevalent internationally, particularly in Europe since it was developed there. 

What do CAT and BT have in common?

CAT and BT series holders are similar in that they both have the same 7:24 taper ratio and require the use of a retention knob, or pull stud, to secure the holder within the machine spindle (CAT holders generally have a standard thread while BT’s are mostly metric). 

How are CAT and BT different?

The difference between the two styles lies in the flange design where the BT bottom flange is thicker and engaged differently by the tool changer. Typical variations are CAT40, CAT50, BT30, and BT40 where the numerical portion signifies the taper dimension (the larger the number, the larger the holder). Because they are so similar, many CNC machine manufacturers and industrial tool suppliers offer the choice between CAT and BT spindles and there is some debate as to the efficacy of doing so. So why choose one over the other? Ease of tooling procurement. CAT tooling is widely distributed in the U.S. while BT holders can be more easily procured internationally. But functionally speaking, there is little difference between the two.

Limitation of the CAT and BT systems

There are some important limitations to note, however, when using CAT and BT systems. Both engage the CNC machine with a single-contact surface with a small gap existing between the flange of the tool holder and the spindle face. As a result, there can be inaccuracies that result from higher speeds, that exceed 800 RPM’s, as centrifugal forces pull the holder up toward the spindle. While balanced holders help alleviate this problem, newer CNC clamping systems without this imitation- such as HSK- are rapidly gaining traction.

HSK ("Hollow Shank Taper") is distinctly different

HSK is a German standard- translating to “Hollow Shank Taper” in English- that bears no resemblance to the CAT and BT styles mentioned above. The taper is much shorter and engages the spindle in a different fashion (i.e. no retention knob) with a cleverly designed dual contact method. The HSK system contains an internal clamping mechanism providing both radial and axial connection (thus the “dual contact”) providing a tremendous rigidity lacking in most CAT/BT set-ups. This allows for improved repeatability leading to increased tool life in high speed machining applications- the Holy Grail for production manufacturers. Within the HSK family, there are 6 main styles (designated by letters “A-F” for differing applications) with HSK-63A being the most widely used today. While this style is becoming more popular, it does have some limitations. Due to the newer technology, HSK machine and tooling costs remain expensive and thus is not as popular among smaller manufacturers. However, for those engaged in machining high-value and precision parts, the HSK technology can offer tremendous value.