Stability-based spindle speed control during flexible workpiece high-speed milling

High-speed machining (HSM) is a technology used to increase productivity and reduce production costs. The prediction of stable cutting regions represents an important issue for the machining process, which may otherwise give rise to spindle, cutter and part damage. In this paper, the dynamic interaction of a spindle-tool set and a thin-walled workpiece is analysed by a finite element approach for the purpose of stability prediction.The gyroscopic moment of the spindle rotor and the speed-dependent bearing stiffness are taken into account in the spindle-tool set finite element model and induce speed-dependent dynamic behaviour. A dedicated thin-walled workpiece is designed whose dynamic behaviour interacts with the spindle-tool set. During the machining of this flexible workpiece, chatter vibration occurs at some stages of machining, depending on the cutting conditions and also on the tool position along the machined thin wall.By coupling the dynamic behaviour of the machine and the workpiece, respectively, dependent on the spindle speed and the relative position of both the systems, an accurate stability lobes diagram is elaborated.Finally, the proposed approach indicates that spindle speed regulation is a necessary constraint to guarantee optimum stability during machining of thin-walled structures.     

Investigation of surface integrity in high speed end milling of a low alloyed steel

Understanding the effects of cutting speed, feed rate and cutting depth on surface integrity is very important for the control of workpiece quality. This paper presents a global experimental study of surface integrity in the case of high speed end milling. In the global term, we include measurements of residual stresses, surface roughness and cutting forces. Our observations and conclusions are mainly concentrated on the effect of depth of cut with a set of constant parameters, such as cutting speed, feed rate, and tool/material couple. This set of constants has been determined using the theory of stability lobes. All experiments have been performed with an electro-spindle equipped with magnetic bearings. The results lead to a good understanding of the influence of cutting conditions on surface integrity in high speed milling of a low alloyed steel. The discussion examines a specific point where the residual stress and residual stress gradient are lowest and also the origin of the residual stress value.     

变齿距铣刀高速微铣削的动态特性及稳定性研究

在高速微铣削加工过程中,提高生产效率和零件质量的需求日益强烈,这使得机床一直在系统的动态稳定性极限附近工作,而机床颤振的存在是限制微铣削加工生产率的主要障碍.基于颤振稳定性的准确预测,能采取一些措施来提高动态稳定性极限,例如通过改变铣刀结构.提出了数值分析与铣削实验相结合的方法,采用变齿距微铣刀来研究铣削加工的动态特性和稳定性.另外提出了采用时域仿真的输出力来表征加工稳定性的新方法,采用变齿距铣刀可以非常有效地提高某些速度范围的颤振稳定性.对于选定刀具的加工,这种方法可以用于加工优化,或在设计阶段预测刀具新型结构的性能.     

数控车床主轴变频调速传动控制的优化设计

文章介绍了数控车床主轴变频调速传动控制的设计方法,通过对传动控制的负载特性与容量适配选型、变频调速控制原理及系统的接口设计、干扰的抑制、变频器参数合理设置的分析研究,从而达到优化设计的目的,掌握其运用方法,是确保数控车床主轴传动系统能够稳定、可靠运行的保证。     

Surface shape prediction in high speed milling

Chatter is one of the main causes affecting the surface quality of machined parts. Since it occurs when a machining process is unstable, it is important to select process parameters that promote stability. However, this condition alone is not sufficient to ensure good surface shape because the dynamics of the machining system as a whole also affects the resulting surface. Thus, even in the absence of chatter, significant movement of the tool or the part may occur causing surface defects. This effect is significant in high speed milling. In order to select optimal machining parameters, a computationally efficient simulator has been developed based on a novel machined surface generation model capable of accurate and reliable prediction of the surface shape. The programme written in MATLAB^® predicts the surface shape from which surface finish, waviness, form and position parameters can be determined. A test bench consisting of a relatively low frequency second order dynamic mode is used to hold a test sample. The results obtained on a machining centre show that the predictions favourably compare with experimental results.     

Suppression of regenerative chatter vibration in simultaneous double-sided milling of flexible plates by speed difference

This paper presents a new method to machine flexible plates with high accuracy and high productivity. Precision steel plates are finished conventionally by face milling with electro-magnetic chucks. It is difficult to correct flatness of the flexible plates, because they deform to fit the chuck surfaces when chucked. To solve this problem, the authors have tried simultaneous double-sided milling, but this causes the regenerative chatter vibration. Thus, the new method is proposed and verified to suppress this chatter vibration, in which the regenerative effects on both sides are cancelled out by rotating the two milling cutters at different speeds.     

A theoretical and experimental study of surface generation under spindle vibration in ultra-precision raster milling

In ultra-precision raster milling (UPRM), the impulse spindle vibration induced by the impulse-like cutting forces is intrinsic and special mechanism majorly influencing surface topography. It is fundamentally distinctive with the step spindle vibration induced by the step-like cutting forces in turning. However, no work has been conducted to study surface generation under the impulse spindle vibration in UPRM in depth. Consequently, this paper theoretically and experimentally elaborates that in UPRM, (i) the impulse spindle vibration includes the axial, radial and coupled-tilting spindle vibration with damping; (ii) the excitation frequency of the impulse-like cutting forces, i.e. spindle speed, determines the spindle vibration characteristics, i.e. synchronous or asynchronous spindle vibration; (iii) the coupled-tilting spindle vibration is a predominant factor influencing surface generation; and (iv) the irregular spindle-vibration waves induced by the impulse spindle vibration produce one of the irregular, lattice-like and stripe patterns or their hybrids at a milled surface.     

Surface finish visualisation in high speed, ball nose milling applications

High speed milling systems are capable of producing complex parts that require little or no hand finishing operations. The machined surfaces are smooth to the touch and they are within the required machining tolerances. However, the visual surface appearance of the parts can be a quality issue because the surface patina is generated by a rotating, multi-flute, ball nose milling cutter as it moves over the surface of the part. Current CAM systems provide good simulating procedures to view the overall surface geometry with swept volume procedures, but they do not consider the micro pattern pertaining to the cutting action of individual cutter flutes. The work presented in this paper addresses this consideration and assesses two new methods to satisfy the need to account for, and predict, the surface machining effects from a ball nose milling cutter.     

高速电主轴过盈联结装置的设计

介绍了一种适用于高速高精度数控机床电主轴部件的轴向定位与扭矩传递装置———阶梯过盈套，它是一种可拆式过盈联结装置。文中阐述了该装置的结构特点和设计计算方法，分析了各因素对其承载能力的影响。通过设计实例，验证其设计计算的合理性     

A comparative study on the spindle system equipped with synchronous and induction servo motors for heavy duty milling with highly stable torque control

In order to create a higher torque spindle system for productive milling operations, rotational speed stability against the torque disturbance has been studied with respect to the spindle mechanical design parameters, actuator types and spindle control algorithms. The study showed a remarkable difference in the spindle rotational speed stability against torque disturbance between a spindle system equipped with an induction servo motor and a permanent magnet synchronous servo motor. The results of this study have been obtained by theoretical analysis, numerical simulation and physical experiments, and the experimental study showed that the hybrid actuation spindle achieves longer tool life.     

The effects of spindle vibration on surface generation in ultra-precision raster milling

Spindle vibration has a significant influence on surface quality of ultra-precision-machined components. However, relatively few studies on the particular spindle vibration under the excitation of intermittent cutting forces in ultra-precision raster milling (UPRM) have been reported. In this study, a specialized model for an aerostatic bearing spindle under the impulsive excitation from intermittent cutting forces of UPRM is developed and its derived mathematical solutions reveal that the spindle vibration is impulsive response. The theoretical and experimental results signify that the impulsive spindle vibration produces inhomogeneous scallops forming ribbon-stripe patterns and irregular patterns like run-out on a surface of UPRM. The potential benefits for UPRM are the theoretical supports for optimization and prediction of surface generation through the optimal selection of spindle speed.     

Feed rate optimization for 3-axis ball-end milling of sculptured surfaces

The aim of this research is to improve the productivity of CNC machine tools by optimizing feed rate. To optimize feed rate two programs were used: “ACIS” (with scheme language) and “Visual Basic”. The scheme program for modeling the work piece, tool, cutting edge, and calculating maximum cutting force and the Visual Basic program to control all the activities linked to the ACIS program for estimating optimized feed values. Laboratory tests were conducted to verify the results from the modeling, using an insert-type one-flute ball-end cutter on a CK45 carbon steel work piece. No coolant was used throughout the experimental works. Comparisons were made between the maximum cutting forces, in the “fix” feed rate tests. The results indicate significant increases in productivity, which can be achieved, by using the optimized feed rate method.     

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.     

CNC machine tool surface interpolator for ball-end milling of free-form surfaces

This paper presents a new type of CNC machine tool interpolator that is capable of generating the cutter path for ball-end milling of a free-form surface. The surface interpolator comprises on-line algorithms for cutter-contact (CC) path scheduling, CC path interpolation, and tool offsetting. The interpolator algorithms for iso-parametric, iso-scallop and iso-planar machining methods are developed, respectively. The proposed surface interpolator method gains the advantages for minimizing the data loaded to the CNC machine tool and maintaining the desired feedrate and position accuracy along the CC path.     

手机壳塑料模具紫铜电极高速铣削加工

对手机壳塑料模具紫铜电极的结构和功能进行了研究,分析了电极高速加工的工作过程、工艺路线、装夹方式、刀具选用和切削用量,介绍了紫铜电极高速加工编程的具体步骤,重点阐述了如何选取高速加工工艺参数。实践结果表明,整个加工过程清晰、刀具和切削参数选用合理。     

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.     

铣床变速冲动控制的模拟实现方法

为了使学生能在操作实践中理解并掌握铣床控制功能,摒弃机械操作机构,模拟实现了各电气控制线路。其中变速冲动控制的模拟实现是关键问题之一。采用蠕动型行程开关实现变速冲动控制的模拟实现方法,简单方便、直观有效、又不增加经济投入,学生实训效果较好。     

Static rigid force model for 3-axis ball-end milling of sculptured surfaces

Static rigid force model is used to estimate cutting forces of sculptured surface in a straightforward way, without considering tool deflection, machine tool dynamic behavior and any vibration effects. Two programs were used for calculations, “ACIS” the 3-D geometric modeler and “VISUAL BASIC”. Two programs were edited and used to perform the calculations, the scheme program to model the work piece, tool and cutting edge and to obtain the geometric data and the VISUAL BASIC program design to use ACIS geometric data to calculate the cutting forces. The engaged part of the cutting edge and work piece is divided into small differential oblique cutting edge segments. Friction, shear angles and shear stresses are identified from orthogonal cutting database available in literature. The cutting force components, for each tool rotational position, are calculated by summing up the differential cutting forces. Laboratory tests were conducted to verify the predictions of the model. The work pieces were prepared from CK45 steel using an insert-type ball-end cutter. No coolant was used in any of the experimental works. The cutting forces predicted have shown good agreement with experimental results.     

主轴转速对单点渐进成形精度的影响

研究了不同主轴转速条件下的成形件的回弹量大小,结果表明:在较低的主轴转速范围内,回弹量随主轴转速的增加而增大;在中等主轴转速范围内,回弹量随主轴转速的升高而减小,但变化幅度不大;在较高的主轴转速范围内,回弹量随主轴转速的增加而增大,且变化幅度较大。同时,分析了不同主轴转速范围下成形件的表面质量,结果表明:在低速和高速主轴转速下,成形件表面几乎没有鱼鳞纹,质量较好;在中间主轴转速范围内,成形件的表面质量较差,且鱼鳞纹较多。并且运用负补偿的方法研究了不同补偿值条件下的回弹量大小,结果表明:采用负补偿的方法能够减小成形件的回弹量。     

Tool wear evaluation by vibration analysis during end milling of AISI D3 cold work tool steel with 35 HRC hardness

In this study, the relationship between vibration and tool wear was investigated during end milling. For this purpose, a series of experiment were conducted in a vertical milling machine. An indexable CBN insert and AISI D3 cold work tool steel hardened to 35 HRC were used as material twin in the experiments. The vibration was measured only in the machining direction, which has more dominant signals than in the other two directions. The measurements were taken by using an acceleration sensor assembled on a machinery analyzer. Tool wear was measured by a toolmaker's microscope. It was observed that there was an increase in vibration amplitude with increasing tool wears. This situation was evident especially by monitoring vibration of displacement type. It was also observed that the first three multiplies of tooth passing frequency (1×, 2×, 3×) gave the best information about the tool wear. Results showed that there was no considerable increase in the vibration amplitude until a flank wear value of 160 μm was reached, above which the vibration amplitude increased significantly.