CONSTRUCTION AND EVALUATION OF LOW-COST TABLE CNC MILLING MACHINE

Transcription

1 CONSTRUCTION AND EVALUATION OF LOW-COST TABLE CNC MILLING MACHINE Ivo Pahole 1, Luka Rataj 2, Mirko Ficko 3, Simon Klančnik 4, Simon Brezovnik 5, Miran Brezočnik 6, Jože Balič 7 1,2,3,4,5,6,7 University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, Maribor, Slovenia Abstract: The paper presents the low-cost design of the table CNC milling machine. It describes the structure of the machine suitable for domestic modeling. In the continuation the tests and checking of static rigidity, positioning accuracy and repeatability are discussed. Further, the control of the machine is presented. In the end the description and presentation of the manufacture of the test product and findings follow. Key words: machine tools, CNC, milling 1. INTRODUCTION Nowadays the custom order as well as small-series production demands adaptable machines. Here, the numerically control machine tools (CNC machine tools) have assumed the principal role. Until recently, the design and construction of CNC machine tools had been a field of activity of high technology companies. Today the technology of CNC control is characterized by accessible prices and is technologically so ripe that even individuals can design and construct CNC controlled machines [3]. Of course, such home made machines are not intended for series production, precise machining, machining of technologically demanding materials or pieces of great dimensions. However, they can effectively replace conventional machines intended for modeling, hobbies or certain types of production [1]. Higher capacity and accuracy of control of CNC machine tools, if compared with conventional machines, has had a considerable influence on the development of function components, frameworks, guides, bearings, change-speed gears. The entire metallic structure must be rigid and the assemblies must function kinematically and geometrically flawlessly. The paper presents the design and construction of the CNC machine tool for modeling purposes. It is composed in the following order. The introductory section is followed by the purpose and description of the CNC milling machine. The description continues with checking of static rigidity of the machine. On the basis of measurements the positioning accuracy of the machine was determined as described in the fifth chapter. The machine 143

2 control is discussed in the fifth chapter. Next, the programme tool Mach intended for CNC machine operation is presented. The eighth chapter presents the results of the test example. The paper ends with a short conclusion. 2. PURPOSE AND DESCRIPTION The machine is applicable in making printed circuits and engravings [4]. Due to the size of the working area it is also suitable for making smaller 3D products. Because of the clear view and simple use the machine is also suitable for pedagogical purposes, particularly, for learning CNC programming [3]. Structurally, the machine is designed from three component parts: Lying coordinate table from grey cast iron allowing the motion in X- and Y- axis. Vertical column from aluminium allowing the motion in Z-axis. It accommodates the main spindle and the control electronic equipment with power supply. Base plate from structural steel connecting both parts and affording stability to the machine. For the main spindle a low-price milling cutter is used. Due to smaller power of the main spindle and cutting forces it is possible to use the tools of smaller dimensions and to machine the materials such as wood, plastic materials and aluminium which are easier to work. Further, only small milling depths are possible. Taking into account those limitations, the bipolar step motor with adequate control available at acceptable price suffices for driving the individual machining axis. In fact, the controller is a personal computer with installed suitable software functioning according to the principle step-direction. The ball bearing threaded spindle with ball nut allows the transformation of rotation of the step motor into translational movement. From the motor the torque is transmitted through a toothed belt (Fig. 1). Fig. 1. Design of feeding motion Table 1. Technical data on the machine Controls Unipolar microstepping hobby CNC Working range X-axis 180 mm Working range Y-axis (transverse) 140 mm Working range Z-axis (vertical) 250 mm Accuracy repeatability 0.05 mm Main spindle speed range rpm 144

3 Motion-steplessly all axes Driving spindle Machine length Machine width Machine height Machine weight mm/min 1000 W 500 mm 400 mm 550 mm 70 kg Dovetail slide guides featuring the lateral slackness adjustment are used for translational movements of X and Y axes. Two round guide columns, on which the main spindle support slides, are used for translational movement of the vertical Z axis. The machine is fitted with limit microswitches limiting the motion of machining axes outside the working range and preventing damages to the machine itself. The used graphic user interface is the programme Mach3 running on the control personal computer. Table 1 shows the technical data on the developed CNC milling machine. 3. CHECKING OF STATIC RIGIDITY AND ACCURACY-REPEATABILITY When checking the static rigidity, the deflection of the cantilever beam accommodating the machine main spindle was found out. Checking was performed as shown in Fig. 2. When loading was applied, the measuring dial test indicator measured the beam deflection of 0.13mm. Similar measurements were performed in different and extreme points in the direction of the machining axis Z. Results showed the same deviations, so it can be claimed that the column and the guides are rigid enough and are not excessively distorted. The weakest link is the base plate connecting the column of the machining axis Z and the X-Y coordinate table (Fig. 2). Fig. 2 Representation of static rigidity measuring in the direction of the machining axis Z and base plate indicated in red 145

4 The positioning accuracy-repeatability was measured by means of the analog dial test indicator (Fig. 3). The measurements were performed several times on three different spots and on different axes. The results showed that the deviations differ on different spots. The values vary from 2 µm on the central part of guides to 4µm at guide ends. Fig. 3 Measuring of repeatability on X and Y axes The tables 2, 3 and 4 show the results of accuracy measurements on the axes. individual machine point 1 st measurement 2 nd measurement 3 rd measurement Table 2 Measurements of positioning accuracy on x-axis y40 y-4 z80 x-90 y0 z mm 0.00 mm 0.01 mm 0.00 mm 0.00 mm 0.00 mm mm 0.00 mm 0.02 mm point 1 st measurement 2 nd measurement 3 rd measurement Table 3 Measurements of positioning accuracy on y-axis y40 y0 y-40 z mm 0.00 mm 0.00 mm 0.02 mm 0.01 mm 0.00 mm 0.01 mm 0.00 mm 0.00 mm point 1 st measurement 2 nd measurement Table 4 Measurements of positioning accuracy on z-axis y0 y-40 z0 x40 z mm 0.00 mm 0.00 mm 0.01 mm 0.00 mm 0.01 mm 146

5 4. MACHINE CONTROL In the frame of the numerical control, for which the machine incorporates the measuring elements i.e. the so-called digital position detector, a special control loop is usually used for each axis. In our case the step motors are used, therefore, additional measuring elements and control loop are not needed, since the step motor can operate with the control directly without the feedback control loop [2]. A drawback of this equipment is that the errors of the transmission elements are neglected and that the actual position of the feeding axis deviates from the desired position (consequently, the machine accuracy is worse than if the feedback control loop is used) [4]. The controls Hobby CNC Chopper Driver Board Kit was selected for the use on the machine so that the 3 and 4 step motors can be controlled through the computer LPT interface. The characteristics of the selected controls are: max.input voltage 42VDC, min. input voltage 12VDC, recommended input voltage 24VDC, max.current 3 A in each phase, min 500 ma step and direction control, compatible with software: DeskNC, TurboCNC, Mach and similar programmes. The CNC machine is used with programme package Mach3. It is distinguished by simple programming, loading and decoding of programme. It can be operated by directional keys on the keyboard. For transmitting the signal from the personal computer to the controller the series LPT interface like for older printers is used. The machine can be controlled with the programme manually or through the G-code of written commands. At the beginning, first the programme has to be set up. The parameters from the machine have to be set, namely: gear ratio, spiral pitch and type of step motor providing the planned dimensions on the monitor coincide with the actual dimensions on the machine. The programme has the option of controlling four axes: X-linearly, Y-linearly, Z- linearly and the fourth i.e. circular motion. 5. EXAMPLE AND FINDINGS Herebelow, the test product made by means of the developed CNC machine, is described (Fig. 4). Fig. 4 CAD model and final product 147

6 When making the test product, the machine construction has proved to be successful as proved by certain measurements. For better machine accuracy it would be necessary to: install position indicators and to use the feedback control loop, reinforce the base plate, reinforce the cantilevered beam. For greater motions the step motors would have to be replaced by servomotors. For machining harder materials the main spindle of higher outputs and torque would have to be used. It has been found out that the price of the described construction of the machine is favourable, since much less funds have been spent for construction of the machine than would be spent for the purchase of a similar machine on the market. 6. LITERATURA [1] Vojko Andrejašič, Janez Kopač, Peter Krajnik. Gradnja 4 osnega CNC frezalnega stroja. Orodjarstvo (2008) vol. 11, pp [2] Ivan Zagradišnik, Bojan Slemnik: Električni rotacijski stroji, učbenik, Fakulteta za Računalništvo in Informatiko, Maribor, 2001 [3] Ivo Pahole, Igor Drstvenšek, Mirko Ficko: Programiranje numerično krmiljenih strojev rezkanje, Navodila za vaje, Fakulteta za strojništvo, Maribor, 2006 [4] Luka Rataj: Tehnološka zasnova obdelovalnega CNC stroja za pedagoške namene: diplomsko delo visokošolskega strokovnega študijskega programa, Fakulteta za strojništvo Maribor,