14-10-2010, 10:42 AM
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A B S T R A C T
This paper presents the state-of-the-art in machine tool main spindle units with focus on motorized spindle units for high speed and high performance cutting. Detailed information is given about the main components of spindle units regarding historical development, recent challenges and future trends. An overviewof recent research projects in spindle development is given. Advanced methods ofmodeling the thermal and dynamical behavior of spindle units are shown in overview with specific results. Furthermore concepts for sensor and actuator integration are presented which all focus on increasing productivity and reliability.
Introduction
Machine tool spindles basically fulfill two tasks:
_ rotate the tools (drilling, milling and grinding) or work piece
(turning) precisely in space
_ transmit the required energy to the cutting zone for metal
removal
Obviously spindles have a strong influence on metal removal rates and quality of the machined parts. This paper reviews the current state and presents research challenges of spindle technology.
Historical review
Classically, main spindles were driven by belts or gears and the rotational speeds could only be varied by changing either the transmission ratio or the number of driven poles by electrical switches. Later simple electrical or hydraulic controllers were developed and the rotational speed of the spindle could be changed by means of infinitely adjustable rotating transformers (Ward Leonard system of motor control). The need for increased productivity led to higher speed machining requirements which led to the development of new bearings, power electronics and inverter systems. The progress in the field of the power electronics (static frequency converter) led to the development of compact drives with low-cost maintenance using high frequency three-phase asynchronous motors. Through the early 1980’s high spindle speeds were achievable only by using active magnetic bearings. Continuous developments in bearings, lubrication, the rolling element materials and drive systems (motors and converters) have allowed the construction of direct drive motor spindles which currently fulfill a wide range of requirements.
Principal setup
Today, the overwhelming majority of machine tools are equipped with motorized spindles. Unlike externally driven spindles, the motorized spindles do not require mechanical transmission elements like gears and couplings. A motor spindle mainly consists of the elements shown in Fig. 2. The spindles have at least two sets of mainly ball bearing systems. The bearing system is the component with the greatest influence on the lifetime of a spindle. Most commonly the motor is arranged between the two bearing systems. Due to high ratio of ‘power to volume’ active cooling is often required, which is generally implemented through water based cooling. The coolant flows through a cooling sleeve around the stator of the motor and often the outer bearing rings. Seals at the tool end of the spindle prevent the intrusion of chips and cutting fluid. Often this is done with purge air and a labyrinth seal. A standardized tool interface such as HSK and SK is placed at the spindles front end. A clamping system is used for fast automatic tool changes. Ideally, an unclamping unit (drawbar) which can also monitor the clamping force is needed for reliable machining. If cutting fluid has to be transmitted through the tool to the cutter, adequate channels and a rotary union become required features of the clamping system. Today, nearly every spindle is equipped with sensors for monitoring the motor temperature (thermistors or thermocouples) and the position of the clamping system. Additional sensors for monitoring the bearings, the drive and the process stability can be attached, but are not common in many industrial applications.
Machine tool spindle units
A B S T R A C T
This paper presents the state-of-the-art in machine tool main spindle units with focus on motorized spindle units for high speed and high performance cutting. Detailed information is given about the main components of spindle units regarding historical development, recent challenges and future trends. An overviewof recent research projects in spindle development is given. Advanced methods ofmodeling the thermal and dynamical behavior of spindle units are shown in overview with specific results. Furthermore concepts for sensor and actuator integration are presented which all focus on increasing productivity and reliability.
Introduction
Machine tool spindles basically fulfill two tasks:
_ rotate the tools (drilling, milling and grinding) or work piece
(turning) precisely in space
_ transmit the required energy to the cutting zone for metal
removal
Obviously spindles have a strong influence on metal removal rates and quality of the machined parts. This paper reviews the current state and presents research challenges of spindle technology.
Historical review
Classically, main spindles were driven by belts or gears and the rotational speeds could only be varied by changing either the transmission ratio or the number of driven poles by electrical switches. Later simple electrical or hydraulic controllers were developed and the rotational speed of the spindle could be changed by means of infinitely adjustable rotating transformers (Ward Leonard system of motor control). The need for increased productivity led to higher speed machining requirements which led to the development of new bearings, power electronics and inverter systems. The progress in the field of the power electronics (static frequency converter) led to the development of compact drives with low-cost maintenance using high frequency three-phase asynchronous motors. Through the early 1980’s high spindle speeds were achievable only by using active magnetic bearings. Continuous developments in bearings, lubrication, the rolling element materials and drive systems (motors and converters) have allowed the construction of direct drive motor spindles which currently fulfill a wide range of requirements.
Principal setup
Today, the overwhelming majority of machine tools are equipped with motorized spindles. Unlike externally driven spindles, the motorized spindles do not require mechanical transmission elements like gears and couplings. A motor spindle mainly consists of the elements shown in Fig. 2. The spindles have at least two sets of mainly ball bearing systems. The bearing system is the component with the greatest influence on the lifetime of a spindle. Most commonly the motor is arranged between the two bearing systems. Due to high ratio of ‘power to volume’ active cooling is often required, which is generally implemented through water based cooling. The coolant flows through a cooling sleeve around the stator of the motor and often the outer bearing rings. Seals at the tool end of the spindle prevent the intrusion of chips and cutting fluid. Often this is done with purge air and a labyrinth seal. A standardized tool interface such as HSK and SK is placed at the spindles front end. A clamping system is used for fast automatic tool changes. Ideally, an unclamping unit (drawbar) which can also monitor the clamping force is needed for reliable machining. If cutting fluid has to be transmitted through the tool to the cutter, adequate channels and a rotary union become required features of the clamping system. Today, nearly every spindle is equipped with sensors for monitoring the motor temperature (thermistors or thermocouples) and the position of the clamping system. Additional sensors for monitoring the bearings, the drive and the process stability can be attached, but are not common in many industrial applications.
