Mechatronics Approach for Desk-Top CNC Milling Machine Design Riza GURBUZ

Transcription

1 Solid State Phenomena Vol. 144 (2009) pp Online available since 2008/Sep/26 at (2009) Trans Tech Publications, Switzerland doi: / Mechatronics Approach for Desk-Top CNC Milling Machine Design Riza GURBUZ Cankiri Karatekin University Cankiri College, Cankiri Meslek Yuksekokulu, Cankiri, 18200, Turkey Keywords: CNC, Machine Tool, Mechatronics Design Abstract. The marriage of computer, electronic technology and traditional machining disciplines has given birth to revolutionary new disciplines, Mechatronics. Therefore Mechatronics requires mechanical, electronic and computer knowledge together. The main aim of this article is to present the mechatronics approach for desk-top CNC milling machine design. Construction of computer controlled (CNC) Machine differs greatly from that of conventional machine tools. This difference arises from the requirement of higher performance levels. Machine Structure, guide ways, feed drives, spindle and spindle bearings, measuring systems, machine control unit, software and operator interface, gauging and tool monitoring should be considered as mechatronics approach. Servo motors, motor drivers and motor control unit were used in this CNC milling machine to make it flexible and easy programming. Ball screws were used to X-Y Z axes to eliminate backlash of the slides. Non-contact home and Limit switches were used to prevent possible damages against over travels. Necessary torque and power have been calculated to select the motors by taking care of the load, ball screws, motor inertia and required cutting tools and etc. and developed CNC milling machine has been tested several times for reliable machining of machine parts. Introduction Mechatronics in its widest sense could relate to anything from robot to CNC machining centers. In the field of mechatronics, it is primarily concerned with systems failing within a limited power range and requiring precision in one or more aspects A complete mechatronic system would be composed of mechanical and electrical parts, overlaid with sensors, which record information, microprocessors, which interpret, process and analyze the information, and assemblies which then react upon this information [1, 2]. Fig. 1. Schematic diagram of a CNC Machine Tool Mechatronics design and construction of computer numerically controlled (CNC) machine differs greatly from that conventional machine tools. The difference arises from the requirement of 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 TTP, (ID: , Pennsylvania State University, University Park, United States of America-13/06/14,13:22:24)

2 176 Mechatronic Systems and Materials II higher performance levels. The CNC machines often employ the various mechatronics elements that have been developed over the years. However, the quality and reliability of these machines depends on the various machine and electronics elements and subsystems of the CNC machine. Figure 1 shows a schematic diagram of the working principle of an NC axis of a CNC machine and the interface of a CNC control [3]: A CNC system basically consists of the following parts: 1-Central processing unit (CPU), 2-Servo-control unit, 3-Operator control panel, 4-Machine control panel, 5-Other peripheral devices, 6-Programmable logic controller (PLC), CNC Machine Elements The machine structure is the load carrying and supporting member of the machine tool. All the motors drive mechanisms and other functional assemblies of machine tools are aligned to each other and rigidly fixed to the machine structure. The machine structure is subjected to static and dynamic forces and it is, therefore, essential that the structure does not deform or vibrate beyond the permissible limits under the action of these forces. All components of the machine must remain in correct relative positions to maintain the geometric accuracy, regardless of the magnitude and direction of these forces. The machine structure configuration is also influenced by the considerations of manufacture, assembly and operation. The following are some of the important constituent parts, and aspects of CNC machines to be considered in their designing [3]. a) Machine structure, b) Guide ways, c) Feed drives, d) Spindle and spindle bearings, e) Measuring systems, f) Controls, software and operator interface, g) Gauging and tool monitoring. Calculation of Drive Requirements on Feed Motor Shaft Fig. 2. Photo of main parts of the CNC Milling Machine

3 Solid State Phenomena Vol For the selection of an optimum servomotor and drive, it is necessary to find the drive requirements on the motor shaft. These are as below: a) Load Torque, b) Load Inertia, c) Maximum Speed. Generally, the following procedure is followed to find the load torque, load inertia and maximum speed requirements on the motor shaft and then it is selected proper motors in according to total inertia and maximum speed of table at the cutting process [3, 4, 5]. Main parts of CNC milling machine has been shown figure 2 and Fig. 3. It was used solid works to design the CNC machine as a detailed and It has been observed all movement the axes on the simulation process of the program before the producing the CNC machine. Fig. 3. Solid Works Design of CNC Milling Machine Fig. 4. Components of a Feed Drive of a CNC Machine Slide Fig. 5. Solid works Design of Feed Drive System

4 178 Mechatronic Systems and Materials II Table 1. Some Values to Calculate the Required Motor Power t a = Acceleration time of motor 0.1 s W = Weight of sliding mass, 200 N W c = Weight of coupling, 0.2 N p B = Pitch of ball screw 5 mm/rev R = Reduction rate on the X-Y-X axes 1 ρ = Density of ball screw (table) kg/m 3 L B = Length of the ball screw on X-Y 0.5 m D B = Diameter of ball screw 0.02 m f = Resistance force of ball screw (Calc.) 80 N µ = Coefficient of friction (table) 0.15 η = Efficiency of ball screw (table) (0.90) N M = Maximum rotational speed of motor 3000 rpm a = Maximum depth of the cut (Aluminum) 5 mm V = Maximum cutting speed (Aluminum) 50 m/min F t = Maximum cutting Force (calculated) 1100 N P = Selected A.C. Servo Motor Power (3) 400 W Ps = Spindle Motor Power 1750 W Rs = Reduction rate of spindle speed 3:1 Some values can be taken from product catalogues, some of them can be taken from machine design or machine cutting books and some can be calculated by using concerning equations and constant values of machine tools and machining materials Torque for cutting at maximum speed of table T C = ( F t.p B )/(2π Rη ) =( )/( 2π )= 0.97 Nm Torque for sliding mass T L =(W + f ) P B /(2π Rη )=( ) 0.005/(2π 1 0.9)=0.25 Nm Total torque for sliding mass and cutting process T T = T L + T C = 1.22 Nm Inertia of coupling J C = (W c D c 2 )/8 = ( )/8 = kgm 2 Inertia of ball screw Jb= (π ρ L B D 4 B )/3=(π )/32= kgm 2 Inertia of sliding mass, J s = W(P B /2π R) 2 = 200(0.005/2π 1) 2 = kgm 2 Total load inertia of moving parts

5 Solid State Phenomena Vol J L = J L. 1 + J B + J C = kgm 2 Acceleration power of motors P a = (2π N M /60) 2.(J L /t a ) = 124 W Required Motor Power for cutting P 0 = ( 2π N M T T )/60 = 383 W Required motor power for cutting is 383 Watt. Selected motor power is 400 Watt which has been shown on table 1. Therefore there is not any problem in cutting process of machine parts. X-Y and Z axes motors have been directly mounted to ball screws. It has been used trigger belt on the mounting spindle motor to spindle. Spindle speed has been decreased 3 times by using 3:1 rated trigger belt. Torque of motor should be satisfactory to cut the machine parts on the X-Y-Z axes. G-M Coded Programming Windows PC-based interface can be communicated with any CAD/CAM G-Code outputting system. Most major control units have CNC Interface, including: a built-in G-Code editor and tool path preview. In this system, Standard G-M coded program was adapted to motor driver of the CNC Milling machine and all the commands have been translated from English to Turkish. Main commands of CNC program is shown on the Fig. 5. Fig. 5. Computer Screen View of CNC Control Commands Desk-top CNC milling machine is P.C. controlled engraving machine and it has ability for machining such materials as aluminum, steel, wood, plastics,it is capable of working 2 axes simultaneously allowing such operations as engraving of two-dimensional fonts, signboards, company logo etc. Some features of the desk-top CNC milling machine are engraving of fonts, windows platform, graphical Interface, commutability with standard CAD system, supported file formats such a DFX, G-M Code programming and PC based and manual control. Manual control are used to make the tool offset before automatically machining of the machine parts

6 180 Mechatronic Systems and Materials II Conclusion and Recommendation Desk-top CNC Milling machine has been designed by using solid works program. A.C Servo motors and motors drivers have been selected by calculating inertia of motors, ball screw, load etc. Computer Control program has been adapted for A.C. motor drivers. AC Servo motors have been mounted to ball screws and X-Y-Z axes. Special A.C. Motors, drivers and motion control program (G-M Codes) have been used for making flexible programming. Recalculating ball screws have been used to eliminate the backlash in the drive system. Limit switches have been located on slides to prevent over travels. It is definitely recommended to use ball screw to eliminate backlash and to use A.C. Servo Motor and drivers for closed loop control for CNC Machine Another area is for a flexible CNC programming of machine parts to use the G-M Coded software. Desk-top CNC milling machine has been tested to observe standard of optimal performance, speed and accuracy in different type applications and has been effectively using on the CNC machine workshop of Mechatronics Research and Training Center since References [1] W. Bolton, Mechatronics, Electronic Control Systems in Mechanical and Electrical Engineering, ISBN , Addison Wesley Longman, (1999). [2] S. Devdas, A.K. Richard Mechatronics System Design PWS Publishing Company, (1997). [3] S.Karunakaran, N.S.Dasharathy, Mechatronics and Machine Tools, Mc Graw Hill Book Company,(1999). [4] Gürbüz, R., Retrofitting of the Conventional lathe to CNC Lathe Conference Proceedings, 7. International Machine and Manufacturing Congress, (1996), p [5] Gurbuz, R., Mechatronics and Mechatronics Application, Selection of Motor and Motor Drivers for Converting a General Purpose Lathe to CNC Lathe, Gazi University Technical Education Faculty, Journal of Polytechnics, Number: 471,(2001), p. 39.

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