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Spindle selection/requirements

​The spindle interface selection 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 upon 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.


When not machining, the spindle interface requires quick interchangeability. Yet when machining, it is vital that the joint between the 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 - is required to have a stable cutting process when having long tool overhangs or with heavy cutting loads. 

Torque transmission - is key for large diameter milling tools and for turning operations. The load applied at a distance away from the spindle center line (Torque=Force x Radius) needs to be countered by a greater driving contact area.


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 center height and precision


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

Clamping force – N Clamping force – lb​
Sources: HSK Handbook, copyright 1999.
Big Daishowa (Big plus spindle system.)


Coromant Capto​ HSK-A​ Steep Taper (SK)​


Torque transmission 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 center positioning accuracy is required in which case the polygon drive provides the best capability.

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