Crash Course in Machining – Tool Tapers

Crash Course in Machining – Tool Tapers

A tool holder for milling has three main parts: the taper, the flange and the collet pocket. The taper is the conical part that is introduced into the spindle. The flange is the part that serves to hold the tool holder when it is in the tool changer storage magazine of the machine tool. The collet pocket is the part of the tool holder that actually holds the cutting tool.

Several technological solutions have been developed from the need to standardise how the cutting tool is held in machine tools. These standards are BT, CAT (also known as V-Flange), ISO (also known as SK) and HSK. One should also mention the R8 collet tool holder technologies and the Morse taper collet which are used in manual machines.

The conical forms such as BT, CAT and ISO, are usually used in conventional milling machines. In contrast, the HSK tool holders are more suitable for high speed milling. The conical tool holders as BT, CAT and ISO are coupled to the spindle via “two cones in contact”. The centrifugal force that increases with the rotational speed of the spindle, together with the thermal effects, tends to push the cone into the spindle, thus increasing the retention force. The displacement that the cone experiences within the spindle may provoke imprecision in the machining.

ISO tool holders are more widespread in Europe for traditional machining. They are clamped to the machine by means of hydraulic or pneumatic actuators. The dimensions of the ISO cones are standardised and classed as ISO 30, ISO 40 and ISO 50, according to the external diameter of the cone. In the US, however, CAT tool holders are more widely used in traditional machining.

The CAT tool holder was created by the Caterpillar Corporation in order to standardise the tool holding systems in their machine tools. These holders are designed according to the size of the cones and are classified as CAT-30, CAT-40 and CAT-50. The BT tool holder is similar to the CAT, with slight differences, such as the fact that it is symmetrical with respect to the rotary axis of the spindle, which improves performance.

HSK, now in the ISO 12164-1:2001, stands for hollow taper shank in German. This tool holder has certain advantages over the others in terms of HSM where rotational speed is greater than in conventional milling. As rotational speed increases, the centrifugal force increases the retention force on the cutting tool by means of milled drive keys that are located within the shank of the mechanism. The keys tend to expand in the interior of the tool holder because of the centrifugal forces in such a way that the metal-on-metal contact is reinforced and pressured within the spindle axis. Thus, the contact stiffness is increased and more aggressive cutting operations can be undertaken. The retention force of an HSK tool holder can be up to twice that of a conventional tool holder, such as, for example, a BT. In the BT interface, the clamp pincers are on the outside of the shank, so they tend to lose contact and retention force as centrifugal force increases. The HSK tool holders outperform the ISO tool holders by up to 5 times in terms of radial stiffness. In addition, HSK tool holders offer greater repeatability.

There are various designations for HSK tool holders. There is a number to indicate the external diameter of the flange (25, 32, 40, 50, 63, 80 up to 100 mm), which determines the stiffness, followed by a letter ranging from A to F. The letters most suitable for high speed machining are A, B, E and F:

  • Letter A is used for applications requiring moderate torque and high rotational speed with automatic tool change;
  • Letter B is for high torque, high speeds and automatic tool change;
  • Letter C is for moderate torque and high rotational speed, but manual tool change;
  • Letter D is for high torque, high speed and manual tool change;
  • Letters E and F are also for HSM applications, but with low torque and very high speeds. E and F designate HSK tool holders that are designed for very high speed operations that either have no framework or for cutting operations that may negatively affect the equilibrium of the machining process, resulting in the appearance of chatter.
An HSK63A tool holder

Finally, there is also a wide range of other tool holders that have been specially designed by companies, like the Sandvik Coromant Capto, Kennametal KM or Komet ABS. These tool holders feature a polygonal-shaped coupling, and are used in lathes and in milling centres, for a quick tool change.

When manipulating these tool holders it is important to pay particular attention to cleaning and maintaining the cones (especially high-precision ones, such as those for HSK), since swarf and lubricant can negatively affect their performance. If the contact surfaces are insufficiently clean, the quality of the contact decreases, which will impede the cone settling within the spindle. It is advisable to blow the collet clean before proceeding to clamp it.


At CNC Proto we offer state-of-the-art Multi-Axis Turning and Milling machining services that produce outstanding quality parts to the tolerances you specify. Try us now by following the link and get a quote for your parts in 24h!

Fernando Duarte Ramos

With an MSc. in Mechanical Engineering, Fernando has worked 8 years at CERN designing the next generation of particle detectors alongside top European engineers and physicists. Being passionate about CNC machining, he started CNC Proto, an online CNC machining service in Europe dedicated to providing fast turnaround, high quality parts at a competitive cost.
Close Menu