1、 Development of a 3-axis Desktop Milling Machine and a CNC System Using Advanced Modern Control Algorithms 1. Introduction As new fields such as IT(Information Technology), BT(Bio Technology) and NT(Nano Technology) emerge as a driving force in the industry, the interests in micro-factory system hav
2、e been growing. The micro-factory is a miniaturized flexiblemanufacturing system which consumes minimal space and energy compared to theconventional one, and it is desired to produce micro/meso size mechanical components necessary for IT, BT and NT applications. Major technical units contributing to
3、 micro mechanical machining systems are,to name a few, high speed spindlesystems, micro high precision feeding systems, control systems to generate coordinated motions, tooling and chucking systems, frame design and module allocationschemes based upon optimization for high stiffness. Researchers hav
4、e been trying to put micro technologies together to build micro-factory systems which make micro/mesosize precision parts to meet the needs from the manufacturing industry.1 In this paper, we present a miniaturized 3-axis milling machine and a dedicated CNC system for the machine. The 3-axis milling
5、 machine is constructed as one of micro-factory module and designed to produce high precision micro parts. It has a desktop size of 200300200 mm3 and is serving as our testbed machine.From finite element analysis and an impact hammer test, we have verified that ithas a good structural stiffness and
6、high natural frequencies. A high speed air turbine spindle on the horizontal z-axis can run at up to 160,000 rpm. This 3-axis milling machine was put under real machining tests and it successfully demonstrated its machining capabilities. A CNC system was developed for operation of the 3-axis desktop
7、 milling machine. The CNC system includes a G-code interpreter which can process a basicset of G-codes and M-codes in real-time. The CNC system consists of two parts.The one is agraphical user interface which runs under Microsoft Windows, and the other is a DSP program which interpolates commands an
8、d executes a real-time servo control. Two parts communicate each other through a dual port RAM(Random Access Memory). Job assignments for the two parts are discussed in detailin this paper. To improve the performance of the CNC system for the 3-axis milling machine beyond the traditional PID-type co
9、ntrol, different control schemes have been tested including H control, input shaping control, disturbance observer and cross-coupled control on the 3-axis milling machine. The rest of this paper is organized as follows. Sec. 2 presents the design of the 3-axis milling machine. The results of the fin
10、ite element analysis and the natural frequencies obtained from the impact hammer test are given in this section. In Sec 3, a PC-based CNC system developed for the 3-axis milling machine is discussed. Several modern control schemes including H control design, input shaping control, disturbance observ
11、er, and cross-coupled control are discussed with their experimental results to show their effectiveness and drawbacks in Sec 4. The concluding remarks will be given in Sec. 5. 2. Design of a 3-axis Milling Machine Micro machine tools are required to have high machining accuracy while providing enoug
12、h stiffness. To estimate basic machining performance and stiffness of a micro machine tool, a miniaturized 3-axis milling machine was built and used as a testbed. Fig. 1 shows the 3-axis milling machine and its specifications. It has a mini-desktop size of 200300200 mm3 and its cut-ting volume is 20
13、2020 mm3. The vertically installed XY stage is driven by voice-coil motors, and for the z-axis, a magnetically preloaded air bearing and a linear motor are used. The air spindle runs at up to 160,000 rpm and it is fast enough for high precision machining. A weight balancer using an air bearing cylin
14、der is installed to counteract the gravity force acting on the XY stage in the y-direction. A small cutting force dynamometer is also installed underneath the work table to monitor the cutting process. Fig. 2 shows a picture of the 3-axis milling machine. 2.1 Static and Dynamic Analysis Finite eleme
15、nt analysis was done to investigate the static and dynamic characteristics of the designed 3-axis milling machine using a finite element model as shown in Fig. 3. The computational results showed that the deflection due to its own weight was negligible. When a 10N force was located at the machining
16、position in z-direction, the numerical results showed that the displacement change at the work table would be about 0.07 ., and the back fame would undergo less than 0.02 . deflection in z-direction. It seems that the 3-axis milling machine has good stiffness due to good frame design and a pair of s
17、mall LM guides supporting each x- and y-direction. The modal analysis revealed many important dynamic modes of the 3-axis milling machine. We used the impact hammer test to verify the computed natural frequencies. The measured natural frequencies do not exactly match the computed ones, but the indic
18、ated frequency range from the finite element modal analysis was similar to that from the impact hammer test. Fig. 4 and Table 1 show the measured natural frequencies and corresponding frequency response function of the 3-axis milling machine. It can be seen that the natural frequencies of z-axis sta
19、ge, which is supposed to be low in stiffness due to its air bearing, locate at a range of 250390 Hz. The natural frequency of the XY stage shows at about 400 and 710 Hz and, for the back frame, it is around 440 and 640 Hz. It seems that the designed miniaturized 3-axis milling machine has higher natural frequencies than conventional machine tools.
