A Precision CNC Turn-Mill Machining Center with Gear Hobbing Capability

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland A Precision CNC Turn-Mill Machining Center with Gear Hobbing Capability Xianshuai Chen 1,4,a, Dailin Zhang 2, Songmei Yuan 3, Xiao Zhang 4, Jianyu Chen 5,Ruxu Du 1,4 1 Institute of Precision Engineering, The Chinese University of Hong Kong, Hong Kong 2 School of Mechanical Engineering, Huazhong University of Science and Technolog, Wuhan, China 3 School of Mechanical Engineering, Beihang University, Beijing, China 4 Center of Precision Engineering, Institute of Advanced Technology, Guangzhou& Chinese Academy of Science (GIAT), Guangzhou, China 5 Guanghua School of Stomatology, Hospital of Stomatology, Sun Yet-Sen University, Guangzhou, China a xschan@mae.cuhk.edu.hk Keywords: Machining, Synchronization control, Gear Hobbing, Biomedical engineering Abstract. With ever increased demand for reduced sizes and increased complexity and accuracy, traditional machine tools have become ineffective for machining miniature components. Typical examples include dental implants, the parts used in mechanical watch movement, and the parts used in medical endoscope. With complex geometry and tight tolerance, few machine tools are capable of making them. This paper introduces our PC-based CNC Turn-Mill Machining Center. It has 5 axes, an automatic bar feeder, an automatic part collection tray, and a tool changer. In particularly, it has a special synchronization control algorithm that gives not only higher accuracy but also ease of use. In addition, to improve the accuracy, the software based volumetric error compensation system is implemented. Based on the experiment testing, the machining error is ± 3 µm in turning, ± 7 µm in milling and the maximum profile error is less than ± 7.5 µm in gear hobbing. 1 Introduction Machining is one the oldest and yet most commonly used manufacturing processes. As a result, much research has been done and the basic theory of machining is well established, including metal cutting principles [1], machine tool dynamics [2], tool path generation [3], high speed machining and etc. Machining precision miniature parts is however still a challenge. For complex precision miniature parts, such as the dental implants, gears rack used in camera focusing system, as well as gears and pinions used in mechanical watch movements, few machine tools are capable of making them. It is generally believed that smaller machine tools would be effective for machining miniature parts. In recently years, the so-called micro factory has become a hot research topic [4-6]. The idea of micro machine tool may be traced back to 1970s [7]. But the first realization was not found until 1996 in Japan [8]. In 2000, Okazaki and Kitahara built a micro-lathe measured 32 mm in length [9]. Though, it suffered from poor accuracy and limited shape generation capability. Lu and Yoneyama [10] developed a micro-lathe measuring 200 mm in length and successfully turned a 300 µm brass wire to 10 µm in diameter using diamond tools, but it has no other functions, such as CNC. Entering the 21st century, with the rapid advance of precision control technology, building a micro machine tool becomes much easier. Loffler [11] developed micro lathe that is small enough to be used in the chamber of a Scanning Electron Microscope (SEM) to investigate micro cutting process in vacuum condition. Sun, Liang and Du [12] developed a high speed micro lathe with its own CNC software. It can cut 3D sculptures on a bar of 2 mm in diameter. Vogler and et al. [13] developed a meso- All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/05/16,23:44:15)

2 1242 Mechatronics and Applied Mechanics II scale Machine Tool, called mmt. More recently, Davim and et al developed a precision turning machine and used it to machining flexible polyamide composites [14]. Axinte and et al built a 4- axes machine tool, which reached the accuracy about 1 µm [15]. Mecombera and et al worked on the micro milling [16]. These machine tools however are not meant for practical manufacturing. They have limited functions and can only make simple parts as tool changer, automatic workpiece loading / unloading, as well as side / end milling capability are not available. In practice, miniature parts from the industry are complex. Figure 1 shows a number of sample micro parts, including dental implants, a camera gear rack, and a couple of parts used in mechanical watch movements. These parts are approximately 1-3 mm in diameter and the required tolerances are less 10 µm. Moreover, they require multiple cutting steps and special functions such as gear hobbing. Because of the shapes of these parts, other processes, such as chemical milling and MEMS lithography, are imperative. Fig. 1 Typical micro parts, their diameters are about 1 mm and requirement tolerance is 10 µm From the brief review above, it is seen that there are still many unsolved problems in micro machining. This motivates us to design and build a low cost precision CNC turning milling machining center with gear hobbing capability. This paper is organized as follows. Section 2 introduces the machine. The control system is presented in Section 3. This is the key part of the research, as it dedicates the functionality and accuracy of the machine. The machine accuracy testing is shown in Section 4. The cutting tests are shown Section 5. Finally, Section 6 contains conclusions and future research work. 2 Precision CNC Turn-Mill Machining Center The CAD model of our CNC Turn-Mill Machining Center is shown in Figure 2. It has three spindles: W1, W2, W3, and five axes: Z1, X2, Y2, X3, Z3, as well as an auxiliary axis θ (for setting the gear hob). The overall dimension of the machine is (L W H) mm and the work volume is (L W H) mm. The main spindle (W1) is equipped with a guide bush and hence, can automatically feed the workpiece. The angular position accuracy of the main spindle (W1) and the 2nd spindle (W2) is degrees when the resolution of the encoder is 10,000 counts per revolution. The angular position accuracy of the 3rd spindle (W3) is 0.03 degrees when the resolution of the encoder is 10,000 counts per revolution. The position resolution of the tables is 1µm. The tables sit on a marble base for low thermal expansion. The photo of the machine is shown in Figure 3. From the function point of view, the machine can be divided into two parts: one for turning and gear hobbing as shown in Figure 4(a), and the other for milling and grinding as shown in Figure 4(b). The turning operation is shown in Figure 5. The turning tool rack can accommodate up to five turning tools and the tool hange is done by aligning the 2nd table, X2 and Y2, and the workpiece table, Z1.The gap between the tools is 12 mm and therefore, can accommodate 8 mm or 10 mm tool holders. In particular, we use 710 and 740 series of Applitec turning tools [17].

3 Applied Mechanics and Materials Vols Fig. 2 The CAD model of our CNC Turn-Mill Machining Cente Fig. 3 The photo of our CNC Turn-Mill Machining Center (a) (b) Fig. 4 Illustration of the machine being divided into two parts (a) Turning and hobbing (b) Milling and grinding Fig. 5 The turning operation, hobbing operation and milling/grinding operation Invented some 100 years ago, gear hobbing is very efficient for gear production [18]. The gear hobbing principle is as follows: as the hob rotates, the so-called basic rack is generated. Furthermore, with a synchronized workpiece motion, the gear profile is produced. Thus, from geometric point of view, it is the relative motion between the gear hob and the workpiece generates the gear profile as the image of the gear hob. Note that at any time, a number of teeth of the hob are engaged in the cut making the operation very efficient. Gear hobbing is done through the 2nd spindle, as shown in Figure 5. Hobbing is a rather complicated process in which the gear teeth are formed by a series of synchronized engagement between the workpiece and the hob. The key parameter of hobbing is helix pitch, because there is the angle between hob axes and workpiece axes. So, we use encoder to make a mechanism for controlling hobbing angle. In practice gear hobs can be purchased from a number of companies, such as Diametal. The milling and grinding function is done through the 3rd spindle, as shown in Figure 5. In order to achieve higher accuracy, the motions of the 3rd table and the 1st table can be synchronized with linear and circular interpolation. 3 The Control System Precision machine tool needs not only precision components, but also precision control system. We purchased most of the components, such as spindle, table, and servomotor. Though we developed our own control system to ensure the accuracy, as detailed below.

4 1244 Mechatronics and Applied Mechanics II 3.1 Hardware The hardware includes an industrial control computer (Manufacturer: Advantech Technology Ltd.; Model: IPC-610-L), two multi-axis controller cards (Manufacturer: Googoltech Technology Ltd.; Model: GT-400-SV) and 8servomotor drivers (Manufacturer: Mitsubishi; Model: Mr- J2series). The hardware architecture of the control system is shown in Figure 6. Fig. 6 Hardware Architecture of the control system Fig. 7 The photo of our machine control system The two PC-based motion control cards are specially designed for motion control. Each card is capable of controlling up to four axes during independent motion and up to three axes during coordinated motion. In our system, one card is allocated to control the four linear axes and the other is used to control the three rotational spindle axes and one linear axis. Besides, a signal acquisition board is used for setting up the hobbing angle (ө). The maximum sampling rate is 2 MHz. All linear axes are controlled in a semi closed-loop manner based on the feedbacks from the servomotors. All servomotors are from Mitsubishi. The resolution is 0.04µm per count. The sampling period for the linear axes controller card is set at 200µs. Since the control resolution for the main spindle (W1) is 80,000 counts per revolution, closedloop control is impossible when the machine is operating in the turning mode where speeds ranging from 5,000-10,000 RPM are needed since the maximum allowable speed is limited by the sampling frequency of 2 MHz. Therefore, the main spindle will be operated in open-loop manner when the machine is operating in turning mode. On the other hand, speeds ranging from RPM are needed during the gear hobbing mode and in this case closed-loop control is used. For the tool spindles, (W2) and (W3), speeds ranging from 1,000-3,000 RPM are needed in the gear hobbing mode and speeds ranging from 5,000-10,000RPM are needed in the milling mode. Therefore, they are operated in closed-loop manner under all circumstance. The servo period for the spindle controller card is set at 100µs. Figure 7 shows the photo of our machine control system. 3.2 Software The software part of the control system is developed using C++ language and objective- oriented design method. Also FLTK libraries are used to make the interface user-friendly. As an example, Figure 8 shows the jog motion and G code interfaces, respectively. The user interface shows position and velocity, tool management, automatic loading /unloading, home, jog running, spindle control, G code and so on. Fig. 8 Jog Motion and G code Interface

5 Applied Mechanics and Materials Vols One of the unique features of the machine is its ability to make miniature gears. As opposed to traditional gear hobbing machines, the system does not use mechanically means to synchronize the axes but rather a technique called electronic gearing. Figure 9 shows the working principle of gear hobbing. As the hob cutter axis is required to rotate much faster than the workpiece axis during gear hobbing, the servomotor of the hob cutter is assigned as the master axis and the servomotor of the workpiece is assigned as the slave axis. Fig. 9 The working principle of mechanically decoupled gear hobbing Since the two servomotors are not linked mechanically, synchronization is achieved entirely based on the closed-loop control of the two spindle axes. Once the master axis is commanded to rotate at a given speed, the encoder feedback of the master axis will be continuously sampled and slave command will be generated to control the slave axis. Synchronization is achieved by following position constraint at any time instance: C= θ m K θ s (1) Where θm is the angular position of the master axis, K is the required synchronization ratio, θ s is the angular position of the slave axis and C is the constant synchronization bias. If C equals to zero, that is: 0= θm K θ s (2) Then, the total synchronization is achieved. The advantage of mechanically decoupled gear hobbing is obvious: it is flexible and has unlimited choice of synchronization ratio. However, the gains for both the position loop and the velocity loop of the two servomotors must be carefully tuned in order to achieve accurate synchronization. This will be further discussed in the subsequent section. The software supports all standard G codes, based on which various machining operations can be programmed, such as milling, tuning, gear hobbing and etc. Moreover, using RS274 standard, the software can also integrate to commercial CAM software systems, such as MasterCAM. 4 Machine Accuracy Test 4.1 Synchronization Test As mentioned in the previous chapter, the workpiece and tool spindle of the machine are mechanically coupled and synchronization is achieved by controlling the two spindles electronically. This section will present the synchronization algorithm in details with experimental results. Since the two spindle systems are rather similar, an identical transfer function, G(s), is used for both spindle systems. Thus, the instantaneous position of the tool spindle θ t can be expressed as: θ = G( s) θ t i (3)

6 1246 Mechatronics and Applied Mechanics II Where θ i is the command position of the tool spindle. In practice, the instantaneous position of the tool spindle θ t is used as the command position of the workpiece spindle. On other hand, the instantaneous position of the workpiece spindle θ w can be expressed as: θw = G( s) θt (4) By introducing the synchronization ratio K, the expression relating the instantaneous position of the workpiece spindle θ w and the command position of the tool spindle θ i can be obtained as follows: θ w = G 2 ( s) θi K (5) Equation (5) is the governing equation for the master-slave synchronization model used in our machine system. The synchronization model is implemented using C++ language in DOS environment. The sampling is triggered using the hard real-time interrupt function in the UCOS library. The angular positions of the tool spindle θ t and the angular position of the workpiece spindle θ w is sampled every 1 ms after the spindles are commanded to run. The following parameters are used in the experiment: synchronization ratio K= , master Kp = 3, slave Kp = 2, servo sample time ST = 100µs, acceleration a = 0.5 counts/st2 and velocity v = 160 counts/st. The time responses of the two spindles are shown in Figure 10 in the form of synchronization error. From the figure, it is seen that the synchronization error is approximately 45 counts, or 45 (360 / 10000) = Fig. 10 Synchronization error of the spindles Fig. 11 Minimum error for every sample point on hobbing 4.2 Position Accuracy Calibrating the machine tool is another important task.the position accuracy of our machining center measured by Renishaw Machine Analysis system. The results show that the position accuracy of the X axis is ±(5.3/2) =±2.65 µm, Y axis is±(4.2/2) =±2.1 µm, and the one of Z Axis is ±(3.9/2) =±1.95 µm. This error can be cut down by software compensation. In addition, in order to test the average error along the entire work volume of the machine, a set of 3,500 profile points is designed and tested. The Euclidian distance between each extracted sample point to the corresponding designed points is calculated. Figure 11 shows the errors for all sample point. It is found that the average error (RMS) is 2.1µm. In summary, the machine tool is accurate. 5 Cutting Tests 5.1 Turning test A part known as the balance staff is used to benchmark the turning performance and it is a critical component with tight tolerances in a mechanical watch movement. Figure 12 shows the designations of dimension of the balance staff and the minimum dimension of this part is only 0.086mm (A and G). Figure 13 shows some samples of the balance staff. Ten random samples are chosen and the dimensions are measured using a micrometer.

7 Applied Mechanics and Materials Vols The maximum error occurs in A and F, in which the average error is 3µm. The average error is 2.18µm and the standard deviation is small. For comparison, the same part is cut using a Tornos Deco 10a CNC machining center. It is found that the average error is 2.28µm. 5.2 Milling Test Fig. 12 Designation of dimension Fig. 13 Balance staff samples machined for the balance staff by the system A part known as the Date Corrector Pinion is used to benchmark the milling performance and it is a critical component with tight tolerances in a mechanical watch movement. Figure 14 shows the engineering drawing. This part is difficult to machine because it has some concave corners and the surface roughness requirement is high. Figure 15 shows a sample part with a There is a comparison of the part produced by Turnos Deco 10a (left) and our machine (right). It is seen that our machine produced a better profile of the part. This is largely attributed to the fact that our synchronization control works better. Fig. 14 The drawing of Date Corrector Pinion (a) Machined by Tornos Deco 10a (b) Machined our machine Fig. 15 The comparison between Deco 10a and our machine 5.3 Hobbing Test Gear hobbing is an important application of our machine. In the gear hobbing experiments, a high precision custom-made gear hobbing tool is purchased from a Swiss company Diametal. It is designated to machine a pinion with diameter of Φ1.284 mm and module of mm. The corresponding gear profile is shown in Figure 16. Fig. 16 The tool path for hobbing the gear Fig. 17 The magnified sample gear machined by our machine The work piece material is brass. Figure 17 shows a sample gear. The gear profile is measured using a CMM system (Manufacturer: Mitutoyo, Model: Quick Vision Pro).The machined profile consists of 3,600 samples and the measurement accuracy is ± 2µm. As shown in Figure 18(a), the measured gear profile is compared to the designed profile. In

8 1248 Mechatronics and Applied Mechanics II order to better visualize the error, an offset envelope of ±7.5µm is generated around the design profile. Figure 18(b) shows the machined gear profile is well within the envelope. 5.4 Other Tests (a) (b) Fig. 18 The design gear profile for the custom-made cutter hob Our machine has been used for machining various biomedical engineering parts. For example, One is the bending section used in an ureteroscope, the other is a dental implant, the drawing and a sample are shown in Figure 19. The cutting tests are successfully completed, and the maximum error is 7µm and the average error is only 4µm. Fig. 19 The drawing and samples of Ureteroscope part and dental implant 6 Conclusions and Future Work In this paper, we present a CNC turn-mill machining center with gear hobbing capability. The machine has 3 spindles and 5 axes. The key to the success of the machine is the advanced motion control, including multi-axes synchronized feedback control and careful calibration. Based on experiments, the machine is accurate: the machining error is ± 3 µm in turning, ± 7 µm in milling, and the maximum profile error is less than ± 7.5 µm in gear hobbing. This is almost the same as the precision machine tools in the market, which it costs only a fraction. Currently, we are working on the following issues: modeling and minimizing the machining errors; developing new algorithms to hob asymmetric parts; and machining more complex biomedical engineering parts. Acknowledgements This work was partially supported by a direct grant from the Faculty of Engineering at the Chinese University of Hong Kong under the grant number and by a grant from the Major Project of the Science and Technology Ministry of China under the grant number 2010ZX

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