Investigation of the working accuracy of a CNC router

Transcription

1 Investigation of the working accuracy of a CNC router a, Endre Magoss a a Institute of Machinery and Mechatronics, University of West Hungary ABSTRACT The machine and workpiece vibrations, the quality of raw material and the accuracy of machine movement all affect the accuracy of the CNC working center in the wood industry. Three additional factors influence the precision of the router positioning: the rigidity of the machine; the accuracy of the machine control and the accuracy of driveline. We defined a 3- dimensional error map to determine the machine positioning accuracy in the working space and we measured the vibration levels and spectrums of the machine. The measurements were performed by a 5D UniTeam Professional 30 CNC working center at the Institute of Machinery and Mechatronics, University of West Hungary. To determine the positioning accuracy and the vibrations of the machine by different working setups, we used an Agilent Laser Interferometer and an SKF Microlog CMVA40 measuring unit respectively. We have determined that the operational gap appears by the helical gear rack engagement when the machining unit approaches zero point on the X-axis. The same behavior can be observed on the Y-axis by the ball screw as well. We concluded that there is a correlation between the rigidity of the machine and accuracy of the displacement. We could observe greater inaccuracy - both X and Y directions- on the console when the machining unit is near the endpoint. The vibration level of the router depends on the positioning of the router; consequently we got higher vibration velocity values when approaching the end point. Keywords: accuracy of CNC router, vibration, laser interferometry INTRODUCTION The computer moves the machining tool to the specified position after the operator specified the tool route and the machining parameters (speed of rotation, feed rate). Our investigations spelled out how we were able to affect the individual actuators, encoders, linear or rotary encoders and machine structure the machine tool movements to the predetermined position. Therefore, we determined the degree of deviation from the predetermined parameters. Normally, the operator places the workpiece into a position on the machine table (most of the time the nearest location to the operator), enabling the CNC router to work at the same working space every time. This implies that the mechanical parts of the machine will abrasion at the same segment due to the various stresses. Higher repetitions of production increase the gap between the machine elements. The accuracy and the reproducibility are the most important factors for the operator and the factory because the dimension of the workpiece has to be within the tolerance of the quality assurance prescribed at series production. The purpose of our investigation is to develop a monitoring system, which provides accurate information for the operator. The main goal of our study is to provide information about the technical condition of the CNC router and to gain information about the part of the working area where the machining should be performed (where the smallest error during machining will be).

2 We used the ANSI/ASME standard [1], which provides procedures for the performance evaluation of CNC machining centers for our investigation. We have studied the laser interferometry measurement literatures that describe how to perform a CNC working center accuracy investigation with the automatic method [2; 3] MATERIALS AND METHODS Materials The basic structure of the UniTeam Professional 30 CNC router (Figure 1.) is electric arcwelded consisting of a large cross-section steel component. The X-axis lines (helical gear rack and the line of the working plane) are connected to the machine frame. The movement of the X-axis performed by a specially positioned servo motor with helical gear and a reduction gearing with backlash compensation equipment running on the helical gear rack. The Y-axis line is connected to the console, which ensures the movement with a ball screw and a servo motor. The Z-axis is mounted on the moving Y-axis line where the working unit and the exhaust unit is located. The motion on the Z-axis is implemented by a servo motor, a ball screw and an anti-backlash worm gear. [2] A plywood raster table is mounted to the machine frame, and the gripping of the workpiece is ensured by vacuum. Methods Figure 1. UniTeam Professional 30 CNC working center Vibration analysis and accuracy examinations by laser interferometer were carried out during our investigation. We established how the machine structure behaves for vibration during the vibration test. The vibrations were generated by high speed rotation of the working tool [3]. The measurements were carried out systematically at various points on the frame, while the machine tool and the console were also at various points at all time. We defined two different rotation speeds for the machine tool at 9450 and RPM, thus we eliminated the vibrations from the eigen frequency.

3 We used a Microlog CMVA40 data collector and an SKF CMSS293 piezoelectric accelerometer for the vibration analysis. Later which has been developed by SKF for bearing tests. We performed the accuracy assessments with an Agilent heterodyne He-Ne laser interferometer, with a laser wavelength of nm long. The principle of interferometry measurements in the following (Figure 2): The beam passes through a semi-permeable lens at the first step. This lens is mounted in place of the machining tool. The semi-permeable lens transmits the part of the laser beam to a retroreflektor, which is fixed at the end of the raster table. The other part of the laser beam turns 90 degrees by the semi-permeable lens towards the reflecting mirror. This mirror is mounted next to the lens. Both reflectors direct the beams back to the interferometer. The incoming beams come from two different routs; these two beams interfere with each other from which we obtained the extent of displacement.. Figure 2. Measurement with laser interferometer The interferometry measurements have manual and automatic measurement methods as well. We followed the manual mode, so that along a line, the measurements were carried out 3 times in a row. There are different factors influencing the machining accuracy. The accuracy is affected by the machined raw material, the machining tool, the technology and the structure of CNC working center. The influencing factors of the working tool and the machine structure can be explored with the two measurement methods previously presented. The following factors that have an affect on the machined tools are: the cutting edge, the geometry of edge, the diameter of tool and the running precision of edge. If the listed factors change during the machining, it will show on the vibration analysis results. Meanwhile the factors that influence the structure of CNC working center are: the design of machine frame (cast, welded), the structure (portal, console, cross table), the type of movement elements (hydrostatic slide way, ball screw, helical gear rack engagement), the type of encoder systems (direct, indirect) and the type of the tool holder can be measured by leaser interferometer and vibration analyzer. Smaller factories use a so-called machining test for measuring the accuracy. These machining tests have a programmed route; the general shape of the route is either wave, step or square.

4 The degree of error is decided by comparison methods or less accurate measuring devices at these types of tests. Since the measurement used is inaccurate, it causes the accuracy of the machine to be reduced. The measurements are always carried out at low feeding speeds, since they would be more inaccurate at larger feeding speeds due to swings and vibrations. EXPERIMENTAL RESULTS The imbalances from the rotation of the machine tool result in oscillation during the spectrum analysis. Through the outcome of the spectrum analysis one can determine that the CNC router at Institute of Machinery and Mechatronic is in good working order. The oscillation; however, can result in the inaccuracy of machining, at least on the surface roughness.. Figure 3 demonrtrated the the outcome of the vibration analysis. During the measurement the machine was at the 0 position at X-axis, the rotation speed of the tool was 9450 RPM and the machine tool was at various positions as seen on the Figure 4. Figure 3. Measuring results of the spectrum analysis Figure 4. Measuring position of the spectrum analysis

5 We got higher vibration velocity values when approaching the end point at the example measurement and overall as well. This is due to the structure of console. The vibration level of the router depends on the positioning of the router. The results of the interferometry is a maximum of 0.08 mm difference both in X and Y direction. This means that elements we did not observe abrasion and slack which in fact would affect the machining accuracy. The machine control elements, mainly the rotary encoder and motor break, operate properly. The CNC uses different moving elements in X and Y directions. We examined which one is the most precise machine part. The actuator is a helical gear rack engagement on the X-axis and ball screw on the Y-axis. Significant differences were not observed between the parts. The observed maximum deviation of the two parts was about mm on Y direction, which is due to the ball screw mechanism. We have determined that an operational gap appears by the helical gear rack engagement when the machining unit approaches zero point on the X-axis. The same behavior can be observed on the Y axis at the ball screw too. This is shown on Figure 5. On this figure the M1, M2 and M3 (from 0 point) are the measurement numbers 1-3 when the machining unit moves towards zero point. Error [mm] Machine position [mm] Figure 5. Linear error on X-axis CONCLUSION The obtained error values from the measurements on the UniTeam Professional 30 CNC router generally do not affect the expected quality in the wood industry. We have determined that an operational gap appears by the helical gear rack engagement when the machining unit approaches zero point on the X-axis. The same behavior can be observed on the Y axis at the ball screw too. We concluded that a correlation exists between

6 the rigidity of machine and accuracy of the displacement. We could observe greater inaccuracy - both X and Y directions- on the console when the machining unit is near the endpoint. The vibration level of the router depends on the positioning of the router. Consequently we got higher vibration velocity values when approaching the end point. Our investigations provide information about technical condition of CNC router and also provide information about the part of the working area where the machining should be performed with smallest error. We are planning to examine the surface roughness of the wood in our next investigation. The measurements will be carried out at various machining parameters. Our goal is to find engagement and correlations between vibration analysis and surface roughness. Acknowledgement -financed by the European - Development of the complex condition -10/ LITERATURE [1] ASME B5.54, Methods for Performance Evaluation of Computer Numerically Controlled Machining Centers, ASME, [2] C. Guiquan, Y. Jingxia, N. Jun (2001) A displacement measurement approach for machine geometric error assessment. International Journal of Machine Tools & Manufacture Vol. 41, p [3] H.F.F. Castro, M. Burdekin (2003) Dynamic calibration of the positioning accuracy of machine tools and coordinate measuring machines using a laser interferometer. International Journal of Machine Tools & Manufacture Vol. 43, p [4] P. Kovacs Laser interferometer contor of CNC working center MSc thesis University of West Hungary, 2012 [5] G. Utassy - Determine the frequency of errors at wood working machines - MSc thesis University of West Hungary, 2011.