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.
| C3 | C4 | C5 | C6 | C8 | C10 |
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®
HSK-A
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 |
BIG-PLUS®
ISO/CAT/BT
7/24 taper | Taper 40 | 12000 | 2703 | 16000 |
Taper 50 | 24000 | 5405 | 12000 |
HSK-A | HSK-A 63 | 18000 | 4054 | 20500 |
HSK-A 100 | 45000 | 10135 | 12500 |
HSK-A 125 | 70000 | 15766 | 9500 |
Coromant Capto® | C5 | 32000 | 7207 | 28000 |
C6 | 41000 | 9234 | 20000 |
C8 | 50000 | 11261 | 14000 |
C10 | 70000 | 15766 | 10000 |
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.