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Machine tool spindle units1 IntroductionMachine tool spindles basically fulfill two tasks:rotate the tools (drilling, milling and grinding) or work piece (turning) precisely inspace transmit the required energy to the cutting zone for metal removalObviously spindles have a strong influence on metal removal rates and quality of the machinedparts. This paper reviews the current state.and presents research challenges of spindle technology.1.1.Historical reviewClassically, main spindles were driven by belts or gears and the rotational speeds could only bevaried by changing either the transmission ratio or the number of driven poles by electricalswitches.Later simple electrical or hydraulic controllers were developed and the rotational speed of thespindle could be changed by means of infinitely adjustable rotating transformers (Ward Leonardsystem of motor control).The need for increased productivity led to higher speed machiningrequirements which led to the development of new bearings, power electronics and invertersystems. The progress in the field of the power electronics (static frequency converter) led to thedevelopment of compact drives with low-cost maintenance using high frequency three-phaseasynchronous motors.Through the early 1980s high spindle speeds were achievable only by usingactive magnetic bearings. Continuous developments in bearings, lubrication, the rolling elementmaterials and drive systems (motors and converters) have allowed the construction of direct drivemotor spindles which currently fulfill a wide range of requirements.1.2. Principal setupToday, the overwhelming majority of machine tools are equipped with motorized spindles.Unlike externally driven spindles, the motorized spindles do not require mechanical transmissionelements like gears and couplings.The spindles have at least two sets of mainly ball bearing systems. The bearing system is thecomponent with the greatest influence on the lifetime of a spindle. Most commonly the motor isarranged between the two bearing systems.Due to high ratio of power to volume active cooling is often required, which is generallyimplemented through water based cooling. The coolant flows through a cooling sleeve around thestator 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 isdone with purge air and a labyrinth seal.A standardized tool interface such as HSK and SK is placed at the spindles front end. Aclamping 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 tobe transmitted through the tool to the cutter, adequate channels and a rotary union become requiredfeatures 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 formonitoring the bearings, the drive and the process stability can be attached, but are not common inmany industrial applications.1.3. State of the artSpindles with high power and high speeds are mainly developed for the machining of largealuminum frames in the aerospace industry. Spindles with extremely high speeds and low powerare used in electronics industry for drilling printed circuit boards (PCB).1.4. Actual development areas in industryCurrent developments in motor spindle industrial application focus on motor technology,improving total cost of ownership(TCO) and condition monitoring for predictive maintenanceAnother central issue is the development of drive systems which neutralize the existing constraintsof 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 ofIncreased requirements in reliability, life-cycle and predictable maintenance the conditionmonitoring systems in motor spindles have become more important. Periodic and/or continuousobservation of the spindle status parameters is allowing detection of wear, overheating andimminent failures.Understanding the life cycle cost (LCC) of the spindles has steadily gained importance inpredicting their service period with maintenance, failure and operational costs.2. Fields of application and specific demandsSpindles are developed and manufactured for a wide range of machine tool applications with acommon 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 withhigh power spindles, to nickel and titanium alloys which require spindles having high torque andstiffness 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 to300,000 rpm. The increase in productivity and speed in this application field over the last fewyears was possible with the development of precision air bearings.Spindles used in die and mould machining have to fulfill the roughing operations (highperformance 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 dieshop either two different machine tools equipped with two different spindles are used or onemachine is equipped with a spindle changing unit. Another possibility is to use a spindle which canfulfill both, HSC and HPC conditions, but this still remains a compromise regarding overallproductivity.Aerospace spindles are defined by high power as well as high rotational speeds. Todaysspindles 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 spindleswith bearings having minimum runout. The present internal cylindrical grinding spindles have arunout requirement of less than 1 mm.Spindle units which are used mainly for boring and drilling operations require high axialstiffness, which is achieved by using angular contact bearings with high contact angles. On thecontrary, high-speed milling operations use spindles with bearings having small contact angles inorder to reduce the dependency of radial stiffness on the centrifugal forces.Contemporary machining centers tend to have multi functions where milling, drilling, grindingand sometimes honing operations can be realized on the same work piece. The bottleneck for theenhancement of the multi-technology machines is still the spindle, which cannot satisfy all themachining operations with the same degree of performance. Reconfigurable and modular machinetools require interchangeable spindles with standardized mechanical, hydraulic, pneumatic andelectrical interfaces.3. Spindle analysisThe aim of modeling and analysis of spindle units is to simulate the performance of the spindleand optimize its dimensions during the design stage in order to achieve maximum dynamicstiffness 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 housingwith bearings. Angular contact ball bearings are most commonly used in high-speed spindles dueto their low-friction properties and ability to withstand external loads in both axial and radialdirections. The spindle shaft is modeled by beam, brick or pipe elements in finite elementenvironment. The bearing stiffness is modeled as a function of ball bearing contact angle, preloadcaused by the external load or thermal expansion of the spindle during operation. The equation ofmotion is derived in matrix form by including gyroscopic and centrifugal effects, and solved toobtain natural frequencies, vibration mode shapes and frequency response function at the toolattached to the spindle. If the bearing stiffness is dependent on the speed, or if the spindle needs tobe simulated under cutting loads, the numerical methods are used to predict the vibrations alongthe spindle axis as well as contact loads on the bearings.Spindle simulation models allow for the optimization of spindle design parameters either toachieve maximum dynamic stiffness at all speeds for general operation, or to reach maximum axialdepth 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 bearingsand spindle is the main goal of spindle design while maintaining all other quality and performancemetrics, e.g. accuracy and reliability.does not always lead to accurate identification of the spindles dynamicparameters； A.3.2. Theoretical modelingTheoretical models are based on physical laws, and used to predict and improve theperformance of spindles during the design stage. The models provide mathematical relationbetween 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 differentialequations. In both cases linear or nonlinear behavior of the spindles can be modeled.3.2.1. Mechanical modeling of shaft and housingFinite element (FE) methods are most commonly used to model structural mechanics anddynamics of the spindles. The method is based on discretization of the structure at finite elementlocations 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 leadto construction of mass (Me) and stiffness (Ke) matrices.Timoshenko beam element is most commonly used because it considers the bending, rotaryinertia and shear effects, hence leads to improved prediction o