CalibrationofCNC_省略_nebydirectmethod_Abd.docx

Calibration of CNC milling machine by direct method Abdul Wahid Khan*, Wuyi Chen * School of Mechanical Engineering and Automation, Beihang University Beijing 100083, China. ABSTRACT Calibration refers to the system of quantity value determination of instruments, equipments and test devices according to industrial requirement， based on metrological characteristics. In present research critical parameter which affects the accuracy and product quality of a CNC milling machine, was investigated and quantified by using direct method. These parameters consist of position dependent or position independent parameters, like linear displacement errors, angular errors of linear axes, straightness error of linear axes and squareness error between the axes. Repeatability, lead screw and resolution error of the CNC milling machine were also quantified to provide additional information to the user, because in absence of this additional information a misconception persists causing a major contributor to the inaccuracy and quality of the product. Parameters were measured and quantified by using a laser interferometer and artifacts as working standards under controlled environmental conditions on a manufacturing CNC milling machine. Polynomial regression analyses were carried out for finding the coefficients to predict the errors at each and every desired position which is quite usefiil for compensation and enhancing the accuracy of a machine system. Machine accuracy detailed chart was also made to assess and assure the accuracy, capability or for accuracy monitoring of the CNC milling machine Key words: Error characterization, Calibration, Laser interferometer, Measurement method, Quality control, Metrology 1. INTRODUCTION Calibration is defined as set of operations which establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by standards I. It determines the relation between the output of the machine tool, instrument or test device and the value of the input quantity, attribute or measurement standard. Calibration is the only comprehensive indicator which depicts a detailed picture regarding accuracy of machine tools which is one of the most important indices to assess the quality and capability of machine tools. Accuracy parameter significantly affects all criterion of machine performance including quick acting, energy efficiency, metal consumption, reliability and durability. On basis of calibration results a qualification or capabilities are ascertained to permit the machine tool, for further processing of compatible accuracy requirement. The results of calibration make possible either the assignment of values of measurand to the indications or the determination of corrections with respect to indications 2. Calibration of machine tools is important for both acceptance testing and error characterization and for compensation etc. 3-7. Basically there are only three common methods reported 8 which are well known and in practice for calibration of machine tools. The first method is direct or parametric method and quite popular for quantifying various error terms independently. Second method is known as volumetric calibration method and able to quantify the error between actual and commanded motion at specific desired point in workspace of a machine tool. It uses some sort of kinematic reference standards such as doable ball bar, disk etc. and mostly used for acceptance testing and for the periodic checks. Third method is based on measuring an artifact or standard part, such as ball plate, cubic box, tetrahedron etc. and known as artifact calibration method or hybrid calibration method. This method is also applicable for acceptance testing and periodic checks. The parametric method is the only well known method which is reliable and provide realistic information about elemental accuracy of machine tools and most popular and appreciated by the machine tool builders and the users for error characterization and error compensation. So in current research authors implemented the parametric method on a CNC milling machine for error characterization in which 21 parametric position dependent and position independent errors of CNC milling machine were quantified and besides these the repeatability, resolution and lead screw errors were also assessed as an additional information. Authors tried to pay attention to calibrate manufacturing machines which is rare attended before in manufacturing environment history * hanvigaiY/'email .com ; * * wv c henTlb uaa.cciu.cn 2008 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Applications, edited by Shenghua Ye, Guangjun Zhang, Jun Ni, Proc. of SPIE Vol. 7160 716010 © 2009 SPIE CCC code: 0277-786Xy 9/$18 doi: 10.1117/12.807066 Proc. of SPIE Vol. 7160 716010-1 although calibration is very common in metrological machines and metrological machines calibrated periodically . 2. OVERVIEW OF MACHINE TOOLS ERRORS Errors in machine tools originate from various sources and causes degradation of machine tool accuracy whereas accuracy is the only main metrological characteristics of machine tools which can be defined as the closeness of the agreement between the results of a measurement and a true value of the measurand 1. As the closeness of agreement spread out the error enlarged and show the degradation of machine tool However degradation of accuracy means degradation of product quality. For characterizing or improving the machine tool accuracy it is essential that one must familiar with the sources which seriously effect and originates errors. 2.1 Errors and their sources in machine tools. According to the nature of the machine tools errors can be divided into two main categories that are quasi-static and dynamic errors. Quasi-static error which is the major sources of error in machine tools consists of geometric error, kinematic error, stiffness error and thermal error. The quasi-static errors are considered as a major source of error and contribute as much as 70% of the total machine errors 9. Geometric error is introduced due to imperfection or imprecision of structural elements and components used in assemblies or subassembly level whereas kinematic error is introduced by the motion of the rigid bodies to reach the exact desired position. Geometric and kinematic errors are considered as interrelated errors. Stiffness error is introduced due to lack of stiffness of machine bodies or having some extent of elastic behavior under loading and unloading conditions. Thermal errors are mainly the major cause of dimensional errors having a non linear behavior and due to its non linear behavior it is quite difficult to estimate exactly. Dynamic errors originated from spindle motion, vibration affecting the machine structure and itself vibration in internal components of machine tools and controller errors fall in this category. Besides these main errors there is some other errors including cutting force induced errors, tool wear errors and fixturing errors. 2.2 Errors categorized on Position basis. As quasi-static error contributes as a major source of error in a machine tool which can be minimized at its design stage and improvement of the system carried out through some preventive actions or compensation etc, so main concentration of the researcher is on this field in which errors can be quantified and avoided by taking preventive actions and improving the flaws at design stages and processing stage. Abbaszadeh-Mir et al. 10 classified the rigid body geometric errors in to two groups. The first group was the position independent geometric errors which were also called link errors such as misalignments, angular offsets and position distance between rotary axes. The second group was the position dependent geometric error parameters that varied with the position of the machine slides. Position independent errors can be calculated by below given equation (1). E = 3(n - 2) (1) “E” represents the error and un” represents the number of joints Whereas the total position dependent errors in a machine tool can be calculated by the formula given below E-6n (2) “E” represents the error and “n” represents the number of joints 3. TOPOLOGY OF CNC MILLING MACHINE AND ERRORS IDENTIFICATION A CNC milling machine as mentioned in figure-1 consists of three prismatic joints named as X, Y and Z slides oriented and move along the X, Y and Z global coordinate system respectively. Y slide joins with the base of the CNC milling machine and X-slide which accommodates the workpiece rides on the Y-slide. Z-slide directly joins with the base of Proc. of SP旧 Vo丨 .7160 716010-2 CNC milling machine and move along the global Z-coordinate system while carrying the spindle motor which mounted on the Z-slide. When the machine coordinate is set at reference zero position all slides reference coincide to zero and display X, Y and Z at zero position. According to the equation (2) the machine has total 18 dependent errors whereas the total independent position errors which are calculated as per equation (1) are found 3. Prismatic joints X, Y and Z slides fall in the category of rigid body system which has six degree of freedom in the space. When a prismatic joint moves, it may have six degree of errors from aforesaid OT r causes. 6-DOF errors in a prismatic joint are composed of the three along the axis X, Y, Z denoted by Sx9Sy9 8Z and the three around the X, Y, Z axis called roll, pitch and yaw errors £x, y, Sz respectively. It can be presented and observed by the coordinate frames by appointing the reference frame on the guide ways and appointing the moving frame on the slides. The translational and rotational errors can be observed in the moving frame through the relative movement of reference frame elaborated in figure-2 and mentioned in Table-1. Proc. of SPIE Vol. 7160 716010-3 Table-1 Error description in CNC milling machine S.No Error type Notation 1 Linear displacement errors 6x(x), Wy), and 6z(z) 2 Vertical straightness errors 5v(x), 5x(y), and (z) 3 Horizontal straightness errors 5z(x), Wy), and Wz) 4 Roll angular errors ejc(x), Ey(y), and ez) 5 Pitch angular errors ev(x), (y), and (z) 6 Yaw angular errors Ez(x), Ez(y), and Ev(z) 7 Squareness errors Sxy, Syz, Szx where, W5W is the linear error, subscript is the error direction and the position coordinate is inside the parenthesis, ucw is the angular error, subscript is the axis of rotation and the position coordinate is inside the parenthesis. 4. CALIBRATION THROUGH DIRECT METHOD Direct calibration method or error quantification is carried out at elemental basis and total positions dependent and position independent parameters can be measured and quantified individually. This approach addresses the problem of computing deformation of machine members individually because the errors meterage of these parameters is usually impossible to analyze precisely by using some other techniques or methodology and quantification of elemental errors is the only possible solution which helps out to find the genuine major source, causes and their contribution in accuracy of machine tool. In direct calibration technique, the structure of the machine is considered as a kinematical model and is then analyzed using the rigid body kinematics so each error can be measured through conventional or by using available modem equipment such as laser interferometer, its accessories and electronic level etc. as described by Week and Bibring 11 and by Sarotori and Zhang 12. Table-2 lists suitable methodology choice. Similar measurement guide line and help are available in some standards documents like ASME B.89.1 and ISOI0360 standards 13, 14. Direct method is considered quite useful for error elimination, minimization through adjustment or compensation. The main advantage of this method is that it presents the direct verification and evidences of mechanical accuracy of a machine tool or its prismatic joints and considered as an authentic method to provide error diagnosis or compensation Table-2 Measuring methods for determining parametric errors of CMMs Errors Measurement Equipment or Methodology Positional Errors Laser interferometer, Step gauge, and Gauge blocks, end bars, ball arrays. Straightness error Straightness interferometer, Mechanical and optical straight edge, Alignment telescope with target, Alignment laser. Displacement indicator or sensors, taut wire etc. Pitch and yaw errors Differential interferometer, Mechanical and Electronic level. Autocollimator, Measurement of positional error along lines with different Abbes offset. Angular laser interferometer. Roll Errors Electronic levels, Reference Flat, Measurement of straightness errors of two parallel lines. Squareness errors Optical and Mechanical Squareness standard, Length standard inclined under defined angles. Diagonal measurements. Proc. of SRIE Vol. 7160 716010-4 5. CALIBRATION OF CNC MILLING MACHINE BY USING DIRECT METHOD Calibration of a CNC milling machine (model XK7132) was carried out by using direct calibration method in which 21 parametric errors were measured directly. The CNC milling machine was equipped with a Fanuc controller. The stroke ranges were X=500mm, Y= 300mm and Z=350mm with resolution of lm, position accuracy lOim and repeatability lnm. For direct calibration a laser interferometer of Renishaw ML-10 along with optics and accessories was adopted as a work standard, which was featured with HeNe laser beam with nominal wave length 0.633 im (in vacuum), and the long term wavelength stability was better than O.lppm. EC-10 environmental compensation unit for air temperature and air humidity compensation, and interfacing card along with data logging and evaluating software were also used. Experiment was carried out under controlled environmental conditions to minimize the effects of random errors and to get the reliable results since the measurement is sensitive by operating temperature and its gradient. Material temperature sensors and air temperature sensor were mounted to avoid temperature effect on measurement process. Three prismatic joints X, Y and Z slides of the machine were calibrated as shown in figure-3 and in figure-4. Fig. 3. Straightness measurement by using laser interferometer Fig. 4 Displacement error measurement through laser interferometer Proc. ofSPIEVol. 7160 716010-5 Laser interferometer was used as per standard operating procedure and available guide lines mentioned in the operating instruction manual. Instrument was preheated according the instructions and tried to shorten the distance between laser head and laser interferometer so that measuring errors could be minimized. Optics was aligned in go