Choosing a 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 really depends 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.
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 that offers good bending stiffness and torque capabilities.
Bending stiffness: Required for a stable cutting process with 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 center line (torque = force x radius) needs to be countered by a greater driving contact area.
Accurate tool center position: To provide repeatability and secure production, which is especially important for turning operations
The coupling characteristics for withstanding 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, an inability to withstand torque will immediately mean loss of center height and precision
Machine spindle interface history
The machine spindle interface has evolved with the evolution of machines. Some of the key milestones that influenced the changes are:
- NC control leading to automatic tool changing and tool storage. This led to the steep taper adopting pull studs and gripper grooves
- Higher spindle revolutions
- Multifunctional machining, turning, milling and drilling with the same interface
The well-known, first interface was the Morse Taper, developed for drilling as long ago as 1868. Thereafter, the steep taper 7/24, also called the ISO taper, was introduced in 1927. Gripper grooves and pull studs were added for tool change in the 60s, 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 a low clamping force and lack of face contact with the spindle nose. This led to new developments in the 90s with BIG-PLUS® (developed in Japan by BIG Daishowa), HSK (developed in Germany by the DIN committee) and Coromant Capto® (launched in 1990 and the only system developed for all types 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 taper||16.26°||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 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 center 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, mixing is not recommended. Available as CAT, ISO, DIN and MAS BT.
HSK (DIN 69893) is developed for machining centers. It features flange contact and hollow taper segment clamping, eliminating 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; therefore, bending moment abilities are improved
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® (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 unrivaled bending stiffness, torque transmission and center 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 centers and vertical lathes
- Modular coupling – machining centers
- 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.