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Machine tool spindle units 1 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 1 1 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 1 2 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 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 automatictool 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 1 3 State of the art Spindles with high power and high speeds are mainly developed for the machining of large aluminum frames in the aerospace industry Spindles with extremely high speeds and low power are used in electronics industry for drilling printed circuit boards PCB 1 4 Actual development areas in industry Current developments in motor spindle industrial application focus on motor technology improving total cost of ownership TCO and condition monitoring for predictive maintenance Another central issue is the development of drive systems which neutralize the existing constraints of power and output frequency while reducing the heating of the spindle shaft Particular attention was paid to the increase of the reliable reachable rotational speeds in the past However the focus has changed towards higher torque at speeds up to 15 000 rpm Because of Increased requirements in reliability life cycle and predictable maintenance the condition monitoring systems in motor spindles have become more important Periodic and or continuous observation of the spindle status parameters is allowing detection of wear overheating and imminent failures Understanding the life cycle cost LCC of the spindles has steadily gained importance in predicting their service period with maintenance failure and operational costs 2 Fields of application and specific demands Spindles are developed and manufactured for a wide range of machine tool applications with a common goal of maximizing the metal removal rates and part machining accuracy The work materials range from easy to machine materials like aluminum at high speeds with high power spindles to nickel and titanium alloys which require spindles having high torque and stiffness at low speeds Cutting work materials with abrasive carbon or fiber reinforced plastics FRP content need good seals at the spindle front end Spindles for drilling printed circuit boards operate in the angular speed range of 100 000 to 300 000 rpm The increase in productivity and speed in this application field over the last few years was possible with the development of precision air bearings Spindles used in die and mould machining have to fulfill the roughing operations high performance cutting HPC at high feed rates as well as the finishing processes high speed cutting HSC at high cutting speeds Depending on the strategy and the machinery of the mould and die shop either two different machine tools equipped with two different spindles are used or one machine is equipped with a spindle changing unit Another possibility is to use a spindle which can fulfill both HSC and HPC conditions but this still remains a compromise regarding overall productivity Aerospace spindles are defined by high power as well as high rotational speeds Today s spindles allow a material removal rate MRR of more than 10 l of aluminum per minute Grinding is a finishing operation where high accuracy is necessary which requires stiff spindles with bearings having minimum runout The present internal cylindrical grinding spindles have a runout requirement of less than 1 mm Spindle units which are used mainly for boring and drilling operations require high axial stiffness which is achieved by using angular contact bearings with high contact angles On the contrary high speed milling operations use spindles with bearings having small contact angles in order to reduce the dependency of radial stiffness on the centrifugal forces Contemporary machining centers tend to have multi functions where milling drilling grinding and sometimes honing operations can be realized on the same work piece The bottleneck for the enhancement of the multi technology machines is still the spindle which cannot satisfy all the machining operations with the same degree of performance Reconfigurable and modular machine tools require interchangeable spindles with standardized mechanical hydraulic pneumatic and electrical interfaces 3 Spindle analysis The aim of modeling and analysis of spindle units is to simulate the performance of the spindle and optimize its dimensions during the design stage in order to achieve maximum dynamic stiffness and increased material removal rate with minimal dimensions and power consumption The mechanical part of the spindle assembly consists of hollow spindle shaft mounted to a housing with bearings Angular contact ball bearings are most commonly used in high speed spindles due to their low friction properties and ability to withstand external loads in both axial and radial directions The spindle shaft is modeled by beam brick or pipe elements in finite element environment The bearing stiffness is modeled as a function of ball bearing contact angle preload caused by the external load or thermal expansion of the spindle during operation The equation of motion is derived in matrix form by including gyroscopic and centrifugal effects and solved to obtain natural frequencies vibration mode shapes and frequency response function at the tool attached to the spindle If the bearing stiffness is dependent on the speed or if the spindle needs to be simulated under cutting loads the numerical methods are used to predict the vibrations along the spindle axis as well as contact loads on the bearings Spindle simulation models allow for the optimization of spindle design parameters either to achieve maximum dynamic stiffness at all speeds for general operation or to reach maximum axial depth of cut at the specified speed with a designated cutter for a specificmachining application The objective of cutting maximum material at the desired speed without damaging the bearings and spindle is the main goal of spindle design while maintaining all other quality and performance metrics e g accuracy and reliability does not always lead to accurate identification of the spindle s dynamic parameters A 3 2 Theoretical modeling Theoretical models are based on physical laws and used to predict and improve the performance of spindles during the design stage The models provide mathematical relation between the inputs F force speed and the outputs q deflections bearing loads and temperature The mathematical models can be expressed in state space forms or by a set of ordinary differential equations In both cases linear or nonlinear behavior of the spindles can be modeled 3 2 1 Mechanical modeling of shaft and housing Finite element FE methods are most commonly used to model structural mechanics and dynamics of the spindles The method is based on discretization of the structure at finite element locations by partial derivative differential equations The analysis belongs to the class of rotor dynamic studies where the axis symmetric shaft is usually modeled by beam elements which lead to construction of mass Me and stiffness Ke matrices Timoshenko beam element is most commonly used because it considers the bending rotary inertia and shear effects hence leads to improved prediction