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Machine tools and tooling considerations

How to choose machine spindle

How to choose machine spindle

How to choose machine spindle interface is a key decision as this often defines the limitations on metal cutting efficiency. There is no quick answer to which interface is best – it is really dependent on the components to be machined and the operations being carried out. It should not be assumed that the standard spindle options with a machine are necessarily the best interface choice.


Machine spindle requirements

When not machining, the machine spindle interface requires quick interchangeability. Yet when machining, it is vital that the joint between the machine spindle and coupling is solid, even if cutting forces do their best to disrupt this interface. It is important to have an interface which gives good bending stiffness and torque capabilities.

  • Bending stiffness: Required for a stable cutting process when having long tool overhangs or with heavy cutting loads
  • Torque transmission: Large diameter operations are most sensitive. The load applied at a distance away from the spindle centre line (Torque=Force × Radius) needs to be countered by a greater driving contact area
  • Accurate tool centre position: To provide repeatability and secure production which is especially important for turning operations

The coupling characteristics to withstand high bending or radial cutting forces are:

  • Flange contact diameter: Flange face contact increases the platform base reducing the leverage of the cutting force
  • Clamping force: The greater the clamping force to restrain a coupling, the greater the cutting force required to “topple” the coupling
  • Cross-sectional area: Reduction of the tool diameter relative to the flange contact diameter will reduce the stiffness of the tool
  • Torque transmission: Most clearly evident with large diameter tools and turning, inability to withstand torque will immediately mean loss of centre height and precision

Machine spindle interface history

The machine spindle interface has evolved with the evolution of machines. Some of the key mile stones we can see which influenced the changes are:

  • NC control leading to automatic tool changing and tool storage. This lead to the steep taper adopting pull studs and gripper grooves
  • Higher spindle revolutions
  • Multifunctional machining, turning, milling and drilling with the same interface

The first and well known interface was the Morse Taper, developed for drilling as long back as 1868. Thereafter the steep taper 7/24, also called ISO taper, was introduced in 1927. Gripper grooves and pull studs got added for tool change in the 60’s with three regional variations; MAS-BT in Asia, ISO/DIN in Europe and CAT-V in America.

The downside with the steep taper is bending stiffness and rpm capability due to low clamping force and lack of face contact with the spindle nose. This lead to new developments in the 90’s with BIG-PLUS® (developed in Japan by BIG Daishowa ), HSK (developed in Germany by the DIN committee) and Coromant Capto® (launched in 1990 and are the only system developed for all type of applications – turning, milling and drilling – from day one).


Machine spindle interface types

The table below shows the four main couplings and a step by step evolution from the traditional steep taper to Coromant Capto®. All interfaces, except BIG-PLUS®, are standardized as DIN, ISO or ANSI.

Taper angle​Flange contact​ Clamping method​​ Torque transmission​​
Steep taper16.26°​​No​​Pull stud​​Drive keys on flange contact​​
BIG-PLUS®16.26°​​Yes​​​Pull stud​Drive keys on flange contact​​
​HSK-A​5.7°​​Yes​​Internal segment clamping​​​Drive keys on taper​
Coromant Capto​®​​2.88°​​Yes​Internal segment clamping​​​Polygon

Steep taper



Coromant Capto®


Steep taper 7/24

For steep tapers, the taper angle is always the same. Gripper groove and thread for the pull stud can vary. Available as CAT, ISO, DIN and MAS BT.


BIG-PLUS® is developed for machining centre applications. The taper and gripper groove is the same as the traditional steep taper, however, through tight tolerances face contact is achieved increasing bending stiffness. A standard steep taper holder can fit in a BIG-PLUS® spindle however it is not recommended to mix. Available as CAT, ISO, DIN and MAS BT.



HSK (DIN 69893) is developed for machining centres. It features flange contact and hollow taper segment clamping removing the need for pull studs. The drive keys have various configurations depending on the variant and in some cases none for high speed applications.

  • Type A: General machining, high bending loads and moderate torque, automatic tool change
  • Type B: Stationary application, moderate bending loads, high torque, special applications, automatic tool change
  • Type C: General machining, high bending loads and moderate torque, manual tool change (Ref. Type A)
  • Type D: Stationary application, moderate bending loads, high torque, special applications, manual tool change (Ref. Type B)
  • Type E: High speed application, light and fast spindles, low bending moments and torque, automatic tool change, easy balance
  • Type F: Moderate speed applications, machining of soft materials, medium bending moments and torque, automatic tool change, easy balance
  • Type T: Rotating and static applications with tighter tolerances in the driving slot (for tool positioning). No "neck" is required and therefore improved bending moment abilities

Note! Most machines stating HSK-T spindle interface still require the neck for automatic tool changing and magazines – this means that HSK A/C/T tools are required











Coromant Capto®

Coromant Capto® (ISO 26623) covers the benefits of both HSK and BIG-PLUS® but has removed the need for drive keys by instead driving through a tapered polygon with face contact. The strong coupling cross section provides room for segment clamping with the highest clamping force capabilities providing unrivalled bending stiffness, torque transmission and centre position accuracy.

The improved radial precision and torque transmission was required to cover the demands for the three target application areas:

  • Machine spindle interface – machining centres and vertical lathes
  • Modular coupling – machining centres
  • Manual quick change system – lathes

Coromant Capto® is the most common interface for multi-task machines as it can cope with both static (turning) and rotating (milling/drilling) applications.


Machine spindle recommendations for different machine types

Machining centres (only rotating)

It is recommended to use a spindle interface with face contact. BIG-PLUS® and HSK-A provide good enough stability for the majority of rotating applications in machining centres. For heavy duty applications, Coromant Capto® C10 should be considered before a larger coupling like HSK-A 125 or SK60. For high rpm applications, HSK-E or F should be considered.


Multifunctional machines (static and rotating)

Coromant Capto® is the only coupling capable of handling the torque and bending stiffness requirements for static and rotating applications.


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There are different spindle clamping mechanism solutions as well as spindle interface production. Sandvik Coromant works actively with machine tool builders to support integration of Coromant Capto® into machines. The primary focus is on the machine styles and interfaces below where the benefits of Coromant Capto® are the greatest.

​Turning centre ​**
​Heavy duty lathe
​Vertical turning lathe
​Multi-task machine
​Machining centre with turning
Machining centre, heavy duty​
* Long boring bars on large turning centres

Clamping force comparison

The internal segment clamping used for hollow tapers (HSK and Coromant Capto®) can provide higher clamping force than the pull stud can provide on steep taper. The table shows the clamping force applied to the respective interfaces. The increased cross sectional area and coupling length of Coromant Capto® allows much higher clamping forces compared to HSK-A.


Sources: HSK Handbook, copyright 1999.
Big Daishowa (Big plus spindle system.)


Coromant Capto®


Steep taper (SK)​


Torque transmission and bending stiffness comparison

Steep taper – the drive keys on a large radius provide good drive for rotating applications

HSK-A – a small contact area with the slots on the taper rather the flange diameter (small radius) so is not recommended for high torque applications.

Coromant Capto® – for turning applications centre positioning accuracy is required in which case the polygon drive provides the best capability.

The size of the coupling will provide limitations on bending stiffness and maximum rpm. A large coupling provides good stiffness but lower rpm while a small coupling allows higher rpm but lower bending stiffness.

The diagram below shows the limitations for the respective couplings from theoretical (FEA) calculations for when the face contact is lost (bending stiffness) and fatigue limit is reached (maximum torque).

Blue: Maximum bending moment (Nm)
Red: Maximum torque (Nm) within fatigue limit

BIG-PLUS® and HSK provide good enough stability for the majority of rotating applications but for multi-task machines, Coromant Capto® is the only coupling capable of handling the torque and bending stiffness requirements.

​Coupling ​ Size and
clamping type​ ​
Clamping force
(pull stud or draw bar)​
Max rpm
(depending on
spindle and bearings)
​N ​lbs
7/24 taper ​
Taper 40​12000​2703​16000​
​Taper 50​240005405​​12000
HSK-A​HSK-A 6318000​​405420500​
HSK-A 100​​4500010135​​12500
HSK-A 125​​7000015766​9500​
Coromant Capto®​C532000​7207​28000​

Static test comparing bending stiffness and torque resistance

The well reputed RWTH Aachen University in Germany carried out a series of static tests in the machine tool laboratory (WZL) comparing bending stiffness and torque resistance of the different spindle interfaces.

Coromant Capto® was measured with two clamping forces: the same as for HSK-A, (22 kN for C6 and 50 kN for C10) and then the higher standard clamping force (45 kN for C6 and 80 kN for C10).

Bending Stability​
Deflection [mm/m]​
Bending moment [Nm]​​
C6- 45kN​
C6- 22kN​
HSK-A 63- 22kN​
7/24 taper, size 40-15kN​
Bending Stability​
Deflection [mm/m]​
Bending moment [Nm]​​
C10- 80kN​
C10- 50kN
HSK-A 100- 50kN​
7/24 taper, size 50-25kN​

The results show that even with the same clamping force as HSK-A the stronger coupling exhibited improved bending stiffness, but with the higher clamping force the toppling force was 2.88 times better for C6 compared to HSK-A 63 and 2.15 times better for C10 compared to HSK-A 100.

Torque Stability​
Deflection [mm/m]​
Torsional moment [Nm]​ ​ ​​
C6- 46kN​​
HSK-A 63- 22kN​
7/24 taper, size 40-15kN​
Torque Stability​
Deflection [mm/m]​
Torsional moment [Nm]​
C10- 50kN
HSK-A 100- 50kN​
7/24 taper, size 50-25kN​

The graphs show that Coromant Capto® C6 has 2.29 times better torque resistance than HSK-A 63. The twisting angle was 7.1 times better. Corresponding figures for C10 were 1.85 times better torque resistance and 4.0 times better twisting angle than HSK-A 100.

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