BT, CAT & HSK: What's the important differences for my #CNC?

HSK is short for the German words "Hohl Shaft Kegel" or, in English, Hollow Shank Taper.

There are several major differences between "steep taper" toolholders, like NMTB, CAT & BT and HSK. 

• Taper: BT and CAT-V holders typically use a 7:24 taper while HSK uses a shallow 1:10 taper
• Dual Contact: CAT and BT only have taper contact with the spindle while HSK both Taper and Flange contact the spindle
• Drawbar: CAT and BT holders are held into the spindle by draw bar fingers that wrap around the outsideof the retention knob (pull stud) while with HSK the drawbar fingers are inside the hollow shank.

Dual Contact
One of the big differences between HSK, short taper toolholders is the way the tool fits into the machine tool spindle. HSK uses a simultaneous fit between the short taper and the face of the spindle. The connection is very rigid. HSK provides dual contact between the spindle face and taper while a conventional V-taper only makes taper contact. 

A standard V-taper tool system is designed to make contact along a fixed taper in the machining center spindle. The tool is held firm against this taper by the drawbar inside the spindle of your CNC. When a conventional holder is seated in the CNC spindle, there is approximately a 3 mm gap between the tool holder flange and the spindle face. 

As machining spindle speeds have increased, BT and CAT tooling systems tend to lose accuracy due to centrifugal forces from running at high RPM. The mouth of the machining center spindle can grow, "bell mouthing". As it grows the BT and CAT tool is under constant drawbar pull, moves up the expanded taper. This causes the Z-axis offset to change. This can also cause the toolholder to stick in the spindle.
The HSK contacts the spindle taper and flange on the spindle face to make a solid union in both the axial and radial planes. In operation, HSK tool holders are resistant to axial movement because the face contact prevents the toolholder from being pulled up into the spindle at high speed.
Cutting tools generally takes higher radial forces because the flange contact and taper contact combine to resist deflection.

Drawbar
The HSK drawbar "fingers" reach inside the Hollow Shank. One of the big advantages of HSK is the "Merry go Round" effect on the drawbar fingers and how centripetal forces affect it. As the RPM is increased on the HSK toolholder the drawbar fingers actually use become a tighter connection on the inside of the flange and increase the pressure in the spindle connection.