3- and 4-Contact Point Spindle Bearings - a new Approach for High Speed Spindle Systems

One important demand on spindle systems in modem machine tools is to realise higher rotational speeds in order to increase the machining efficiency. In conventional spindle bearings the contact angles on inner and outer ring deviate extremely from one another with rising rotational speeds due to centrifugal forces. Axial shift of trie inner ring (elastic mounted bearing) and increasing normal forces in the contact zones on the outer ring are typical consequences leading to high internal bearing loads and a reduced life span. Based on these problems bearings with a new inner geometry are studied. Instead of two contact zones these bearings have three or four contact zones to ensure constant contact angles and reduced normal forces on the outer ring. In this paper analytic operation studies and first experimental tests are presented.     

An investigation of the use of heat pipes for machine tool spindle bearing cooling

The results of an experimental investigation into the practicality of using a heat pipe installed in the spindle of a milling machine to remove the heat produced in the spindle bearings which is capable of causing thermal distortion and cutting error are presented in the paper. Measurements of the variation of bearing temperature with time are reported at four different spindle speeds when there was no heat pipe installed, when the heat pipe was cooled by air and when the heat pipe was cooled by an ice/water mixture. Analysis of the results by a simple heat transfer model indicates that the particular heat pipe used was capable of removing up to 160 W with a corresponding 50% reduction in the rise of the bearing temperature above the temperature of the surrounding air at steady operating conditions.     

Thermal characteristics of the spindle bearing system with a gear located on the bearing span

High cutting speeds and feeds are essential requirements of a machine tool structure to accomplish its basic function which is to produce a workpiece of the required geometric form with an acceptable surface finish at as high a rate of production as is economically possible. Since bearings in high speed spindle units are the main heat source of total cutting system, in this work, the thermal characteristics of the spindle bearing system with a tilting axis were investigated using finite element method to improve the performance of the spindle bearing system. Based on the numerical results, a specially designed prototype spindle bearing system was manufactured. Using the manufactured spindle bearing system, the thermal characteristics were measured and compared to the numerical results. From the comparison of the numerical results with the experimental results, it was found that the finite element method predicted well the thermal characteristics of the spindle bearing system.     

An integrated thermo-mechanical-dynamic model to characterize motorized machine tool spindles during very high speed rotation

High speed machining (HSM) is a promising technology for drastically increasing productivity and reducing production costs. Development of high-speed spindle technology is strategically critical to the implementation of HSM. Compared to conventional spindles, motorized spindles are equipped with built-in motors for better power transmission and balancing to achieve high-speed operation. However, the built-in motor introduces a great amount of heat into the spindle system as well as additional mass to the spindle shaft, thus complicating its thermo-mechanical-dynamic behaviors. This paper presents an integrated model with experimental validation and sensitivity analysis for studying various thermo-mechanical-dynamic spindle behaviors at high speeds. Specifically, the following effects are investigated: the bearing preload effects on bearing stiffness, and subsequently on overall spindle dynamics; high-speed rotational effects, including centrifugal forces and gyroscopic moments on the spindle shaft and, subsequently, on overall spindle dynamics; and the spindle dynamics on the cutting point receptance. The proposed integrated model is a useful tool for differentiating quantitatively different effects on the spindle behaviors. The results show that a motorized spindle softens at high speeds mainly due to the centrifugal effect on the spindle shaft.     

Milling machine spindle analysis using FEM and non-contact spindle excitation and response measurement

In this paper a method for analysing lateral vibrations in a milling machine spindle is presented including finite-element modelling (FEM), magnetic excitation and inductive displacement measurements of the spindle response. The measurements can be conducted repeatedly without compromising safety procedures regarding human interaction with rotating high speed spindles. The measurements were analysed and compared with the FEM simulations which incorporated a spindle speed sensitive bearing stiffness, a separate mass and stiffness radius and a stiffness radius sensitive shear deformation factor. The effect of the gyroscopic moment and the speed dependent bearing stiffness on the system dynamics were studied for different spindle speeds. Simulated mode shapes were experimentally verified by a scanning laser doppler vibrometer. With increased spindle speed, a substantial change of the eigenfrequencies of the bearing-related eigenmodes was detected both in the simulations and in the measurements. The centrifugal force that acted on the bearing balls resulted in a softening of the bearing stiffness. This softening was shown to be more influential on the system dynamics than the gyroscopic moment of the rotor. The study performed indicates that predictions of high speed milling stability based on 0 rpm tap-test can be inadequate.     

An elastic deformation model of high speed spindles built into ball bearings

Kinematic characteristics and elastic deformation properties of ball bearings such as stiffness, axial, and radial deflections of the rings, speeds of balls, contact angles, and loads, all vary strongly with rotational speed, temperature, and axial preload. This paper presents an elastic deformation model of spindle units built into ball bearings, which are running on high rotational speed and axial preload. For this, software for simulation of the high speed spindle units was developed and introduced into a practical use. A computer aided analysis of the model was carried out and experiments were made, which showed a significant effect of high rotational speed, cutting load, bearing axial preload, and a new effect for which the criteria for a choice of bearing preload are given.     

Evaluation of an internally spring preloaded four contact-points bearing for spindle units

In this paper, the concept of an internally spring preloaded four contact-points bearing for the use in high precision and high speed applications is investigated. It is designed as a compact self-contained fixed bearing unit that is free of play, easy to mount and resistant against temperature differences between the inner and outer ring. The concept of the bearing, its functionality and properties are introduced and the results of an experimental analysis of a first prototype are presented. The prototype bearing, which is based on a hybrid 7014 spindle bearing with an additional spring preload unit, was manufactured by hard turning. Its properties are compared to those of similar two and three contact-points spindle bearings. Typical requirements in modern high precision applications, e.g. machine tool spindle units, and limiting characteristics of bearings, such as displacement, stiffness, friction torque and operating temperature, are considered. Finally, the potential of the new bearing concept is discussed.     

A new method to quantify radial error of a motorized end-milling cutter/spindle system at very high speed rotations

The radial error motion of a machine tool cutter/spindle system is critical to the dimensional accuracy of the parts to be machined. The spindle's radial error motions can be measured by mounting a sphere target onto the spindle as a reference. A set of sensors is used to measure displacements of the reference sphere in various directions to determine spindle error motions. This measurement technique can be reliably carried out when the spindle is at rest or at low rotational speeds. However, at very high speeds, the reference sphere must be carefully centered and balanced to avoid introducing additional error motions. In addition, the sensors must be held with very rigid mounts in order to avoid measurement errors caused by vibrations. For high-speed end milling spindles, the spindle is operated with a cutter. The cutter must be removed when mounting a reference sphere. Because the cutter itself can introduce errors due to centering and unbalancing effects, the error motions measured by the reference sphere method do not include the error caused by the cutter. This paper introduces a new and practical method to provide an indicator of the radial error of a motorized end-milling cutter/spindle system at very high speed rotations without the need of a reference sphere. This indicator of the radial error is based on the size of the cutting marks produced by the end mill, which is attached to the spindle. The cutting marks are circular, and their diameters are related to the radial error of the cutter/spindle system. Quantitative precision analysis was carried out to confirm the accuracy and repeatability of this new measurement technique. This technique has been implemented in order to determine the effects of the spindle speed, the level of unbalanced mass, and the spindle stiffness on the cutter/spindle's radial error. The results reveal that the centrifugal force generated by the unbalanced mass is the main factor causing the increase in radial error. One way to compensate for the effect of unbalanced mass is to increase the spindle stiffness. Experimental results confirm that a higher front bearing preload can render the spindle stiffer, thus reducing the radial error of the cutter/spindle system. Finally, it should be pointed out that the proposed cutting mark measurement cannot replace the sphere method because it cannot provide time-resolved or angle-resolved information as those obtained from polar charts. However, the proposed cutting mark measurement can provide the characterization of the spindle with the cutter attached. As a result, both methods can complement each other to provide a more complete picture of the behavior of the cutter/spindle system at high speeds.     

Determination of the dynamic behaviour of micro-milling tools at higher spindle speeds using ball-shooting tests for the application in process simulations

The dynamic behaviour of milling processes can be analysed using process simulations based on measured frequency response functions. However, the determination of these functions for micro-milling processes is challenging due to small tool diameters of 1 mm or less, the influence of higher spindle speeds on the dynamic behaviour, and runout errors. Therefore, an approach for analysing micro-milling tools based on an excitation using bearing balls with a diameter of 1 mm, shot by compressed air, is presented. The measured dynamic response is applied to a geometric physically-based process simulation in order to analyse tool vibrations in a micro-milling process.     

立磨主轴装配和轴承游隙的测量与调整研究

针对某新型立磨关键核心部件主轴装置结构特点和装配精度要求,论述了主轴装置的装配方法及顺序,研究了立磨主轴轴承游隙的测量和调整方法。实践表明:该方案科学合理,有效保证了产品的装配质量和工期要求。     

A New Method for Evaluating Error Motion of Ultra Precision Spindle

Conventional measuring methods for evaluating error motion of spindle rotation are inadequate to meet the current needs of ultra precision spindle bearing systems.In this paper, therefore, a new measuring method for spindle rotational accuracy based on a three points method was proposed. This method made it possible to reduce considerably the time and effort required in measuring the spindle rotational accuracy. Measurement of error motion to the nano-meter order was attained. Furthermore, this method was proved to be an effective method for measuring the out-of-roundness of a testpiece.     

Modeling of spindle-bearing and machine tool systems for virtual simulation of milling operations

This paper presents a general, integrated model of the spindle bearing and machine tool system, consisting of a rotating shaft, tool holder, angular contact ball bearings, housing, and the machine tool mounting. The model allows virtual cutting of a work material with the numerical model of the spindle during the design stage. The proposed model predicts bearing stiffness, mode shapes, frequency response function (FRF), static and dynamic deflections along the cutter and spindle shaft, as well as contact forces on the bearings with simulated cutting forces before physically building and testing the spindles. The proposed models are verified experimentally by conducting comprehensive tests on an instrumented-industrial spindle. The study shows that the accuracy of predicting the performance of the spindles require integrated modeling of all spindle elements and mounting on the machine tool. The operating conditions of the spindle, such as bearing preload, spindle speeds, cutting conditions and work material properties affect the frequency and amplitude of vibrations during machining.     

Programming spindle speed variation for machine tool chatter suppression

This paper presents a novel method for programming spindle speed variation for machine tool chatter suppression. This method is based on varying the spindle speed for minimum energy input by the cutting process. The work done by the cutting force during sinusoidal spindle speed variation S³V is solved numerically over a wide range of spindle speeds to study the effect of S³V on stable and unstable systems and to generate charts by which the optimum S³V amplitude ratio can be selected. For on-line application, a simple criterion for computing the optimal S³V amplitude ratio is presented. Also, a heuristic criterion for selecting the frequency of the forcing speed signal is developed so that the resulting signal ensures fast stabilization of the machining process. The proposed criteria are suitable for on-line chatter suppression, since they only require knowledge of the chatter frequency and spindle speed. The effectiveness of the developed S³V programming method is verified experimentally.     

Numerical investigation of chatter suppression via parametric anti-resonance in a motorized spindle unit during milling

A new concept for increasing process stability during milling processes is presented utilizing a parametric anti-resonance. The beneficial effect is examined by numerical calculations in the time domain for an example spindle which is supported conventionally by ball bearings and is equipped with an additional active magnetic bearing that enables the realization of a deliberate parametric excitation. In the present study the stiffness of this active support bearing is changed periodically with time. The parametric excitation is tuned to a parametric anti-resonance which triggers the transfer of vibration energy between the vibration modes of the underlying system without this additional time-periodicity. It is highlighted how vibration energy is transferred from the first bending mode of the rotor, which is the most critical one by means of regenerative chatter, to a mode with higher damping in order to increase the effective damping of the entire spindle drive unit.     

Vibration based preload estimation in machine tool spindles

Prevention of catastrophic bearing failures caused by excessive thermally induced preload is one of the key issues in the design and operation of high speed spindles. Temperature monitoring and shutting the spindle off in hazardous conditions is at present the most common method of avoiding bearing seizure. However, due to a rapid heat built-up and measurement delay, this solution in not reliable. An improved protection can be achieved either by custom systems with springs or hydraulic actuators that maintain a steady preload, or by monitoring the instantaneous preload and appropriate, predictive adjustment of machining conditions. While the former approach increases the cost of spindles and degrades their potential reliability, monitoring and proactive control of the actual preload eliminates these disadvantages. Its feasibility is predicated upon fast, reliable and accurate in-process preload estimation. A suitable estimation algorithm proposed in this paper employs vibrations of the spindle housing measured by means of accelerometer(s) and analyzed in the context of mechanical multi-degree-of-freedom model of the spindle assembly treated as a component of the entire machine tool. Representative experimental results are presented.     

电机后置式电主轴热态特性的分析与研究

文章提出了一种新型电主轴结构,并对该电主轴在油脂润滑和油-气润滑条件下轴承的温升进行了计算与分析,最后对轴承球材料和润滑方式的选择提出了一些建设性意见.     

Bearing load analysis and control of a motorized high speed spindle

Angular contact ball bearings are the most popular bearing type used in the high speed spindle for machining centers. Because the bearing load is increased rapidly with the raised spindle speed due to the centrifugal force and temperature raise, proper initial preload and especially operating-induced load control of the angular ball bearing is important to the rigidity, accuracy and life of the spindle. The bearing layout, preload mechanism an on-line load bearing control are discussed in this paper. The management of the centrifugal force and thermally-induced bearing loads is especially emphasized. An active bearing load monitoring and control mechanism that consists of an integrated strain-gage load cells and piezoelectric actuators has been developed and tested. This active control and monitoring mechanism on-line adjusts the bearing load according the cutting conditions. Experiments were conducted to identify the proper initial bearing preload range. Optimal preload for the lowest bearing temperature raise existed for a specified spindle speed. The optimum preload, however, should be raised when the operational speed is increased.     

Effect of the thermal contact resistance on thermal behaviour of the spindle radial bearings

Evaluation of effects the thermal resistance occurring at the bearing/spindle journal interface has on working clearance, temperature and power losses of the NN3018K radial bearing and on temperature of the B7018C angular bearing. Formulas determining the resistance value for the bearing mounted directly on the spindle and for a sheet of insulating material placed between the two components. A computational model of the bearing assembly. Analytically determined influence of insulating material on thermal behaviour of the bearings over a wide range of speeds up to the limit speed.     

Dynamics of ultra-high-speed (UHS) spindles used for micromachining

Micromachining dynamics commonly dictate the attainable accuracy and throughput that can be obtained from micromachining operations. The dynamic behavior of miniature ultra-high-speed (UHS) spindles used in micromachining critically affects micromachining dynamics. As such, there is a strong need for effective techniques to characterize the dynamic behavior of miniature UHS spindles. This paper presents a systematic experimental approach to obtain the speed-dependent two-dimensional dynamics of miniature UHS spindles through experimental modal analysis. A miniature cylindrical artifact with 5 mm overhang is attached to (and rotating with) the spindle to enable providing the dynamic excitations to and measuring the resulting motions of the spindle. A custom-made impact excitation system is used to reproducibly excite the spindle dynamics up to 20 kHz while controlling the impact force. The resulting radial motions of the spindle are measured in two mutually perpendicular directions using two independent fiber-optic laser Doppler vibrometers (LDVs). To ensure the mutual orthogonality of the measurements, the two lasers are aligned precisely using an optical procedure. A frequency-domain filtering approach is used to remove the unwanted spindle motion data from the measurements, thereby isolating the dynamic response. The spindle dynamics is then represented in the form of frequency response functions (FRFs). A global curve-fitting technique is applied to identify natural frequencies and damping ratios. The developed approach is demonstrated on a miniature UHS spindle with aerodynamic bearings, and dynamic characteristics are analyzed at different spindle speeds and collet pressures. The spindle speed is shown to have a significant effect on dynamic response, especially at higher spindle speeds, while the collet pressure is observed not to have any significant effect on the spindle dynamics. It is concluded that the presented approach can be used to characterize the dynamics of miniature UHS spindles effectively.     

小模数滚齿机用电主轴的设计与研究

小模数数控滚齿机去掉了传动用的齿轮副、蜗轮蜗杆副等传动元件,刀具轴和工件轴直接与电机轴相联。几何误差和热误差是小模数滚齿机的主要误差来源,为此,设计了一种新型的电主轴。该电主轴采用了主轴电机置于主轴后轴承之后的形式,缩小了主轴轴承位置的径向尺寸,使轴承摩擦产生的热量的散发,减少了电机发热对轴承变形产生的影响;主轴轴承采用了静压轴承的形式,提高了主轴轴承的耐磨性和回转精度。该电主轴的设计能够提高小模数滚齿机的生产效率和加工精度。