Identification of angular and positional deviations inherent to 5-axis machining centers with a tilting-rotary table by simultaneous four-axis control movements

This paper describes a method for identifying the eight deviations inherent to five axis control machining centers by means of simultaneous four-axis control movements. Some methods to identify the deviations have been proposed. However, a simultaneous four-axis control technique using a ball bar instrument has not been applied to the measurement of relative displacements between the main spindle and the worktable. Furthermore, the method for assessing the deviations from the trajectories has not been proposed. Thus, in this paper, a calibration method based on the simultaneous four-axis control technique is proposed for five axis control machining centers with a tilting rotary table. To confirm the validity of the proposed method, simulations were conducted. The trajectories were obtained by means of a mathematical model into which the eight deviations were substituted. In the first step, four of the eight deviations were estimated by an observation equation for which two measurement trajectories and six reference ones were used. In the second step, the remaining four deviations were geometrically calculated using the values estimated by the observation equation. As a result, it was found that the proposed method was sufficient to identify the deviations accurately.     

Prediction and identification of rotary axes error of non-orthogonal five-axis machine tool

This paper proposes an efficient and automated scheme to predict and identify the position and motion errors of rotary axes on a non-orthogonal five-axis machining centre using the double ball bar (DBB) system. Based on the Denavit-Hartenberg theory, a motion deviations model for the tilting rotary axis B and rotary C of a non-orthogonal five-axis NC machine tool is established, which considers tilting rotary axis B and rotary C static deviations and dynamic deviations that total 24. After analysing the mathematical expression of the motion deviations model, the QC20 double ball bar (DBB) from the Renishaw Company is used to measure and identify the motion errors of rotary axes B and C, and a measurement scheme is designed. With the measured results, the 24 geometric deviations of rotary axes B and C can be identified intuitively and efficiently. This method provides a reference for the error identification of the non-orthogonal five-axis NC machine tool.     

Analysis of circular trajectory equivalent to cone-frustum milling in five-axis machining centers using motion simulator

The present paper describes the effect of the half apex angle of the cone-frustum on the motion trajectory under simultaneous five-axis motion and the effect of the sensitive direction of the ball bar when the motion trajectory is measured along the three-dimensional circular conical path. In the present paper, simulation of the measurement by means of a ball bar instrument is mainly conducted using a motion simulator developed previously. In particular, a precise mathematical model was developed to express the pitch errors of the axes of rotation of the five-axis machining center having a tilting rotary table driven by worm gears. In the experiment and simulation, primarily the center position and half apex angle of the cone-frustum were varied. In addition, two sensitive directions of the ball bar were investigated. The motion simulator incorporating the pitch error model can express the detailed trajectories obtained by the ball bar, even if the half apex angle and center position of the cone-frustum and the sensitive direction of the ball bar were changed. Then, the influence of the frictional force of the linear axes of motion, and the backlash and pitch error of the axes of rotation on the circular trajectories were analyzed. In particular, for the case of a half apex angle of 45°, the trajectory due to the errors of the axis of rotation is strongly affected by the sensitive direction of the ball bar.     

Enhancement of geometric accuracy of five-axis machining centers based on identification and compensation of geometric deviations

The present paper describes the enhancement of kinematic accuracy of five-axis machining centers with a tilting rotary table. Geometric deviations inherent to the five-axis machine are calibrated through the actual trajectories measured by two different settings of a ball bar in simultaneous three axis motion. Measurement using a cylindrical coordinate system is superior to measurement using a Cartesian coordinate system from the viewpoint of the number of measurements. In order to verify the effectiveness of the calibration method, the inherent geometric deviations measured on the cylindrical coordinate system were corrected through the post processing of NC data for cutting the cone-frustum.The relative displacement between the tool center point and the workpiece was detected by the ball bar. Based on the experimental results, it is confirmed that the radius, center position, and roundness of the three-dimensional circular trajectory are improved when the inherent geometric deviations are corrected.     

A machining test to calibrate rotary axis error motions of five-axis machine tools and its application to thermal deformation test

This paper proposes a machining test to parameterize error motions, or position-dependent geometric errors, of rotary axes in a five-axis machine tool. At the given set of angular positions of rotary axes, a square-shaped step is machined by a straight end mill. By measuring geometric errors of the finished test piece, the position and the orientation of rotary axis average lines (location errors), as well as position-dependent geometric errors of rotary axes, can be numerically identified based on the machine׳s kinematic model. Furthermore, by consequently performing the proposed machining test, one can quantitatively observe how error motions of rotary axes change due to thermal deformation induced mainly by spindle rotation. Experimental demonstration is presented.     

A method of testing position independent geometric errors in rotary axes of a five-axis machine tool using a double ball bar

Ensuring that a five-axis machine tool is operating within tolerance is critical. However, there are few simple and fast methods to identify whether the machine is in a “usable” condition. This paper investigates the use of the double ball bar (DBB) to identify and characterise the position independent geometric errors (PIGEs) in rotary axes of a five-axis machine tool by establishing new testing paths. The proposed method consists of four tests for two rotary axes; the A-axis tests with and without an extension bar and the C-axis tests with and without an extension bar. For the tests without an extension bar, position errors embedded in the A- and C-axes are measured first. Then these position errors can be used in the tests with an extension bar, to obtain the orientation errors in the A- and C-axes based on the given geometric model. All tests are performed with only one axis moving, thus simplifying the error analysis. The proposed method is implemented on a Hermle C600U five-axis machine tool to validate the approach. The results of the DBB tests show that the new method is a good approach to obtaining the geometric errors in rotary axes, thus can be applied to practical use in assembling processes, maintenance and regular checking of multi-axis CNC machine tools.     

Sculptured surface machining of spiral bevel gears with CNC milling

Gears are crucial components for modern precision machinery as a means for the power transmission mechanism. Due to their complexity and unique characteristics, gears have been designed and manufactured by a special type of machine tools, such as gear hobbing and shaping machines. In this paper, we attempt to manufacture the spiral bevel gear (SBG: the most complex type among the gear products) by a three-axis CNC milling machine interfaced with a rotary table. This consists of (a) geometric modeling of the spiral bevel gears, (b) process planning for NC machining, (c) a tool path planning and execution algorithm for both 4-axis and 3/4-axis (three out of four axes) controls. Experimental cuts were made to ascertain the validity and effectiveness of the presented method with a CNC milling machine controlled by the 3/4-axis control mode.     

Ballbar dynamic tests for rotary axes of five-axis CNC machine tools

This paper proposes a new ball bar test method for the inspection of dynamic errors of rotary axes in five-axis CNC machine tools. The test circle is defined in a workpiece coordinate system and the ball bar test is performed by simultaneously driving of linear–rotary axis couple. The effects of the center position and the radius on the setting values, rotational range and measurement sensitivity of the rotary axis were investigated. The proposed ball bar test is performed in two steps: the circular positioning and the circular tracking with a continuous feed. Axial dynamic errors are obtained by subtracting the measured tracking errors from the positioning errors. A ball bar test system (BBTS) was developed to plan the tool path and the tool orientation, to communicate with the five-axis CNC controller and to process the measured data. Error patterns were simulated regarding the gain mismatch, backlash and tracking direction to help a fast diagnosis of the error sources. Simulations and experimental results prove the effectiveness of the new test method.     

Construction of an error map of rotary axes on a five-axis machining center by static R-test

This paper proposes an efficient and automated scheme to calibrate error motions of rotary axes on a five-axis machining center by using the R-test. During a five-axis measurement cycle, the R-test probing system measures the three-dimensional displacement of a sphere attached to the spindle in relative to the machine table. Location errors, defined in ISO 230-7, of rotary axes are the most fundamental error factors in the five-axis kinematics. A larger class of error motions can be modeled as geometric errors that vary depending on the angular position of a rotary axis. The objective of this paper is to present an algorithm to identify not only location errors, but also such position-dependent geometric errors, or “error map,” of rotary axes. Its experimental demonstration is presented.     

Error map construction for rotary axes on five-axis machine tools by on-the-machine measurement using a touch-trigger probe

Position-dependent geometric errors, or “error map,” of a rotary axis represent how position and orientation of the axis of rotation change with its rotation. This paper proposes a scheme to calibrate the error map of rotary axes by on-the-machine measurement of test pieces by using a contact-type touch-trigger probe installed on the machine's spindle. The present scheme enables more efficient and automated error calibration, which is crucial to implement periodic check of rotary axes error map or periodic update of its numerical compensation for five-axis machine tools. The uncertainty analysis of the error calibration is also presented with a particular interest in the influence of error motions of linear axes. The experimental demonstration is presented.     

Assessment of machine tool trunnion axis motion error, using magnetic double ball bar

A method is proposed in this paper to assess the axis motion errors of a trunnion-type A-axis using the magnetic double ball bar (DBB) as the measuring instrument. The proposed method consists of five DBB tests with a single setup for all of the tests and the exclusive motion of the trunnion axis during data acquisition. The single setup helps to reduce non-productive time by limiting the intervention of the operator within the machine workspace whereas the exclusive trunnion axis motion prevents the data from being contaminated by other axes motion errors within each test. Simulations show that setup errors cause eccentricities and radius changes of the ball bar data when viewed on a polar plot. Finally, the proposed method is applied to a VL30 Mitsui-Seiki vertical machine tool to identify its trunnion axis motion errors. The results show the effectiveness of the proposed method as well as its ease of use and the short time required.     

Identification of two different geometric error definitions for the rotary axis of the 5-axis machine tools

Geometric errors are clearly among the critical error sources in 5-axis machine tools and directly contribute to the machining inaccuracies. According to the definition of geometric errors of the rotary axis, different understandings have been exist in published studies. It is extremely dangerous as it makes the comprehension of the geometric errors ambiguous and may make the geometric error identification and compensation less effective. This phenomenon has not been noticed so far. In this paper, two different commonly used geometric error definition and modeling methods are firstly identified and analyzed, named as “Rotary axis component shift” and “Rotary axis line shift”. The features and relationships of these two error modeling methods are analyzed. After a detailed comparison, “Rotary axis component shift” is more suitable to definite the geometric errors of rotary axis. An experiment has been conducted on a 5-axis machine tool to show the correctness of our work. The results show that the identified geometric errors of rotary axis based on the two error models are greatly different and need to be concerned.     

Prediction of the dynamic behaviour of machine tools during the design process using mechatronic simulation models based on finite element analysis

Increased productivity, higher velocities and acceleration for feed and cutting motions are requirements for innovative machine tools. At the same time the production process must achieve reduced form and position deviations of the work-piece. Therefore knowledge of the dynamic behaviour of machine tools during the design process is essential to develop high-performance machines. Using finite element analysis and mechatronic simulation, taking the mechanical, electrical and control systems into account, is the first step for optimisation. Developing the control parameters using these simulation techniques is one of the major steps in detecting the mechatronic characteristics. This paper presents a method for developing the control parameters concerning tool to work-piece deviations of mechatronic simulation models including disturbance variables. As an example a 2-axis CNC test stand for feed drive axes will be visualised with its simulation and measurement results in the time and the frequency domain.     

5-Axis tool path smoothing based on drive constraints

In high speed machining, the real feedrate is often lower than the programmed one. This reduction of the feedrate is mainly due to the physical limits of the drives, and affects machining time as well as the quality of the machined surface. Indeed, if the tool path presents sharp geometrical variations the feedrate has to be decreased to respect the drive constraints in terms of velocity, acceleration and jerk. Thus, the aim of this paper is to smooth 5-axis tool paths in order to maximize the real feedrate and to reduce the machining time.Velocity, acceleration and jerk limits of each drive allow to compute an evaluation of the maximum reachable feedrate which is then used to localize the areas where the tool path has to be smoothed. So starting from a given tool path, the proposed algorithm iteratively smoothes the joint motions in order to raise the real feedrate. This algorithm has been tested in 5-axis end milling of an airfoil and in flank milling of an impeller for which a N-buffer algorithm is used to control the geometrical deviations. An important reduction of the measured machining time is demonstrated in both examples.     

Arc-intersect method for -axis tool paths on a 5-axis machine

An inherent problem with simultaneous 5-axis machining is that it often suffers from dramatic reductions in feed rate when the tool axis is in the vicinity of the singularity point of the machine; during large orientation changes over small distances; during rotary axes reversals and from interpolation of the tool axis vector. $3\frac{1}{2}\frac{1}{2}$-axis machining offers an alternative strategy that can be used to overcome these problems and still maintain some of the salient features of 5-axis machining to improve machining times over 3-axis ballnose machining. In $3\frac{1}{2}\frac{1}{2}$-axis machining, during cutting the machine moves only its three linear axes while the two rotary axes are locked, resulting in a fixed tool orientation. Locking the rotary axes generates fewer fluctuations in the feed rate than simultaneous 5-axis machining and results in a more consistent surface finish with lower variations in cutting force and torque. A new tool positioning strategy called the Arc-Intersect Method (AIM), which can also be applied to simultaneous 5-axis machining, is presented here for $3\frac{1}{2}\frac{1}{2}$-axis machining on simultaneous 5-axis machines using toroidal or flat endmills. A cutting test was performed and the part was measured with a CMM to check for accuracy and to measure the cusp heights. Machining times were compared to 5- and 3-axis tool paths and cutting torque measurements were compared between $3\frac{1}{2}\frac{1}{2}$- and 5-axis machining.     

A strategy for identifying static deviations in universal spindle head type multi-axis machining centers

This research was conducted in order to investigate the effectiveness of the checking method specified in ISO 10791-6 and to propose an additional method for identifying the geometric deviations inherent to five-axis machining centers with a universal spindle head. The specified method is not sufficient for assessing the influence of individual geometric deviations. Therefore, additional measurements are needed in order to use this checking method effectively. The spherical motion in the additional measurements is a modified version of the ISO standard. Some of the deviations are presumed based on the result of the additional measurements, and so the method of identifying the remainder of the geometric deviations was designed by analyzing the link geometry of the machine. The identification procedure consists of three steps. In the first and second steps, after performing two measurements, six out of ten deviations are identified by means of an observation equation. In the third step, four of the ten deviations are identified by means of the link geometry. The exactness of the proposed procedure for identifying the deviations inherent to the five-axis machine is confirmed through simulations.     

5轴卧式加工中心回转工作台设计与应用

大型5轴卧式加工中心机床回转工作台是机床在加工过程中驱动工件回转的部件,回转精度及可靠性直接影响机床的整体性能。针对5轴机床在联动切削过程中B轴移动的特点,选用大型可调间隙蜗轮蜗杆进行传动,传动机构配置有减速机构以增加驱动力矩,回转机构通过大型轴向径向组合轴承支撑,通过高精度光栅尺进行位置反馈,得到快速进给5 r/min、定位精度为3″、重复定位精度为1.5″的高精度回转工作台。     

螺旋锥齿轮数控加工参数转换计算方法

通过对螺旋锥齿轮机械式铣齿机与CNC铣齿机之间加工调整参数转换原理的研究，推导了这两类机床运动轴数学模型；提出了求解各运动轴表达式系数的速度求导法。与其它方法比，该方法简便、易行，而且计算量小。最后给出的用MATLAB所编程序计算的结果及加工仿真实例说明了该文方法的正确性和精确性。     

Comparison between multi-point and other 5-axis tool positioning strategies

A method of generating sculptured surfaces at multiple points of contact between the tool and the workpiece was developed and proven viable by the current authors in previous work. They denoted this finish machining method, “Multi Point Machining”, or simply MPM. This paper compares MPM with two other 5-axis tool positioning strategies; namely: the inclined tool, and the principal axis method. It is also compared with 3-axis ball nose machining. Comparisons are conducted using computer simulations and experimental cutting tests. Results obtained show that MPM produced scallop heights that are much smaller than those produced by the other tool positioning strategies.