IntroductionSome of the design objectives in the development of the HSK tool and spindle interface were improved machining accuracy, better repeatability, and reduced machining tolerances to allow production of more accurate parts.

With the HSK design, these objectives are achieved by:
• The unique design of the tool and spindle interface.
• A well-developed design with tool contact on the taper and on the face provides a stiffer and more rigid connection between tool and spindle.
• Gripping the tool on the inside of the taper increases clamping force at high speeds.
• Establishing proper tolerances. Tolerances for the HSK are defined in DIN 69063 for the spindle taper and in DIN 69893 for the tool holder taper.

The HSK taper as a systemIn order to benefit from the advantages of the HSK system in the manufacturing environment, an understanding between the machine tool manufacturer, the tool holder manufacturer, and the end-user is required to ensure that certain criteria are met. In addition, it is of interest to investigate new and improved tool holder designs to achieve the maximum benefits from the system.

The machine builder must ensure:
• Spindle rigidity and spindle accuracy
• Dimensional accuracy of spindle taper, spindle face, drive keys per DIN 69063
• Proper tool clamping force to meet the desired machining parameters, and reliability of the power drawbar mechanism
• Established procedures to ensure long service life [maintenance schedule, lubrication schedule, preventive maintenance schedules]
• Cleanliness of the system
• Customer training to convey the unique and important requirements of the HSK system in order to obtain the desired benefits

The tool manufacturer must ensure:
• Compliance of taper tolerances to DIN 69893
• Process control during manufacturing
• Use of improved cutting tool clamping systems with reduced run-out such as hydraulic and heat shrink tool holders or adaptations of these (see our "cold shrink" tool holders)
• Offer balanced tool holders or balanceable tool holders together with easy-to-use balancing systems

The end-user must understand:
• Using machines and tooling with HSK is not "business as usual".
• Manufacturing the HSK tooling is difficult and new for some tool manufacturers. Selection of a tool holder manufacturer who has a proven record and is known for quality can eliminate the problem of non-conforming tools. Non-conforming tools might not be clamped properly by the drawbar, or the drawbar position indication might give false signals, or lead to early spindle wear.
• Maintenance procedures given by the machine builder must be followed.
• Training the operator is very important.
• Maintaining cleanliness of tooling and spindle taper is a major requirement. Machine accuracy will suffer, the tool life will be shortened in a contaminated environment.
• Ensure proper coolant filtration as specified by the manufacturer of the machine.
• Nicks and dings on the tool taper cannot be tolerated. Proper tool holder handling and storage is essential.
• The coolant connection between drawbar and tool holder [coolant tube] should be flexible to avoid damage or early wear to the gripper seal.
• Regrinding of a worn spindle taper might be difficult or impossible.

Measuring gauges ensure consistent complianceTo ensure proper operation and close tolerance machining, inspection procedures must be in place. We also recommend checking tooling and the spindle nose periodically for wear and conformity. For preventive maintenance, we recommend periodic verification of the power drawbar clamping force. Gauges for measuring the tool holder taper and machine clamping must be available to the maintenance department. Ideally, all gauges should be from the same manufacturer for consistency.