World Academy of Science, Engineering and Technology 42 2008
Design of Hydraulic Circuit for CNC Lathe Machine Converted from Conventional Lathe Machine Zin Ei Ei Win, Than Naing Win, Jr., and Seine Lei Winn
numerical control is the process of manufacturing machined parts using a computerized controller to command motors which drive each machine axis. In no field of engineering development has progress been so rapid in that of hydraulic operation. Therefore, hydraulic devices and control systems have become more and more important due to automation and mechanization. Similarly, in changing the tool in CNC lathe machine, hydraulic is used to control the manufacturing processing of this machine. In controlling processing, there are two types of control system on NC/CNC machines viz. open loop and closed loop. The overall accuracy of the machine is determined by the type of control loop used. The opened loop control system does not provide positioning feedback to the control unit. The movement pulses are sent out by the control unit and are received by a special type of servomotor command. Since this control system only counts pulses and cannot identify discrepancies in positioning, the control has no way of knowing whether the tool has reached the proper location or not. The machine will continue this inaccuracy until somebody finds the error. The opened loop control can be used in application in which there is no change in load conditions, such as the NC drilling machine. The advantage of the open loop control system is that it is less expensive, since it does require the additional hardware and electronics needed for positioning feedback. The disadvantage is the difficulty of detecting a positioning error. In the closed loop control system, the electronic movement pulses are sent from the control to the servomotor, enabling the motor with each pulse. The pulses are detected and counted by a feedback device called a transducer. With each step of movement, a transducer sends a signal back to the control, which compares the current position of the driven axis with the programmed position. When the number of pulses sent and received match, the control starts sending out pulses for the next movement. Closed loop systems are very accurate. Most have an automatic compensation for error, since the feedback device indicates the error and the control makes the necessary adjustments to bring the slide back to its position. They use AC, DC or hydraulic servomotors. These various motors are mounted by hydraulic circuits or system. The term ‘hydraulic circuit’ is a group of components such as pumps, actuators, control valves, accumulators, restrictors, and pipelines.
Abstract—Nowadays, products can be produced by modern technology, which uses computer software, hardware and firm ware in industries. It is needed to use CNC lathe machine to get more accurate dimensions and irregular shape. So, CNC machines are becoming more and more important in modernized industrialization. There are many conventional lathe machines in our country, Myanmar. To build a new modern developed country, it is required to convert these conventional lathe machines into semi automatic control lathe machine. Developing and changing into semi automatic control lathe machine, there are three required portions, namely, mechanical, electronics and mechatronics. From the mechanical point of view, the design of hydraulic circuit is dramatically needed. The functions of hydraulic circuits for semi automatic control lathe are analyzed in this paper. These consist of changing the tool, working the machining processes and locating the tool in turret. In this research paper, the hydraulic circuit design which can be changed four kinds of tools by using hydraulic motor is made and also constructed. The hydraulic circuit comprises vane pump, hydraulic motor, and two directional control valves for changing the tool; 4/3way valve and 4/2-way valve. The transfer function of each component is derived and the whole system is analyzed in this thesis.
Keywords—Accurate, changing, circuit and vane pump. I. INTRODUCTION
A
lathe is a machine tool for producing cylindrical, conical and flat surfaces. It can be used for drilling and boring holes which may be cylindrical or conical in shape. The basic engine lathe, one of the most widely used machine tools is very versatile when used by a skilled machinist. However, it is not particularly efficient when many identical parts must be machined as rapidly as possible. Numerical control is based on the use of numerical data for directly controlling the position of the operative units of a machine tool in machine operation. Today, a more popular adaptation of the basic process of NC is called Computer Numerical Control or CNC. Computer
Zin Ei Ei Win is with Mechanical Engineering Department, Mandalay Technological University (corresponding author to provide phone: 095-09513-7242; fax: 095-067-404015; e-mail: zineieiwin@ gmail.com). Than Naing Win, Jr., is with Mechanical Engineering Department, Mandalay Technological University (corresponding author to provide phone: 095-09-215-0122; fax: 095-067-404015; e-mail: kaunghtet57@ gmail.com). Seine Lei Win is with Mechanical Engineering Department, Mandalay Technological University (corresponding author to provide phone: 095-09513-7242; fax: 095-067-404015; e-mail: seinelei1@ gmail.com).
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World Academy of Science, Engineering and Technology 42 2008
II. AIM AND OBJECTIVE
To design the hydraulic circuit for CNC lathe machine, the relative components and the working principle of components in this machine should firstly be analyzed. After studying the technologies of components, the basic concept of hydraulic circuit is analyzed for certain and the whole hydraulic circuit is designed. And then the transfer functions of hydraulic circuit are derived for providing MATLAB program.
Basically, there are two types of pump; dynamic and positive displacement pumps. We choose the type of positive for it can change in the small internal leakage that makes the output flow constant. And, positive displace pumps are divided into three types, namely, gear, vane and piston pumps. However, the gear pump is one type of the fixed displacement pumps. Moreover, although the piston pump can be designed both fixed and variable displacement design, it is more accurate for getting the reversible flow through the pump. Therefore, the vane type pump is chosen. On the other hand, it doesn’t need reversible flow and only needs variable type for CNC lathe machine converted from conventional lathe. However, there are two types in vane pump; unbalance and balance. We choose the unbalance type because a balance vane pump can’t be designed as a variable displacement unit. After all, an unbalance vane pump is chosen for hydraulic circuit in CNC lathe machine.
IV. ESSSENTIAL FACTORS IN DESIGNING HYDRAULIC CIRCUIT
VI. EXPERIMENTAL PROCEDURE
The main ambition of this research is to design hydraulic circuit for CNC lathe machine which is more useful than conventional lathe machine. This circuit makes the machine faster, more safety and reliable. The objective of this thesis is to analyze the working principle of components in the hydraulic circuit and to design the hydraulic control system in CNC lathe machine. III. SCOPE OF THESIS
There are three important consideration when designing hydraulic circuit and analyzing. They are as follows: (i) Safety of desired function (ii) Performance of desired function (iii) Efficiency of operation V. DESIGN CONSIDERATION OF HYDRAULIC CIRCUIT At present day, it is required to convert conventional lathe machines that are most widely used in many industrial zones in Myanmar, into semi automatic control lathe machines. Developing and changing into semi automatic control lathe machine, there are three necessary portions, namely, mechanical, mechatronics, and electronics. From the mechanical point of view, it is absolutely needed to design the hydraulic circuit for semi automatic control lathe machines.
Fig. 2 Hydraulic Circuit in CNC Lathe Machine
The diagram of hydraulic circuit in CNC lathe machine at Pyin Oo Lwin (G.T.I) in Myanmar is shown in Fig. 2. In this operation, vane pump is required to supply the pressurized fluid which is controlled to drive hydraulic motor or hydraulic cam type lathe turret by a directional control valve; 4/3 way. The flow being controlled by this valve then passes to hydraulic motor connected to the load. The cylinder piston controls the position of tool changer via a directional control valve; 4/2 way, in order to be clamped or unclamped. In the condition of machining of screwing the work piece, the tool changer tool must be in the clamp
Fig. 1 Conventional Lathe Machine
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World Academy of Science, Engineering and Technology 42 2008
C. Calculation for Cylinder Piston Basing on the head losses of valve and pipes between pump and cylinder piston, we have the force of extension and retraction of cylinder; 11.52 kN and 17.54 kN respectively. And the power of cylinder is 0.7 kW.
position. On the other hand, the tool changer must be in the unclamp position when changing the tools as desired. A. Design Calculation of Vane Pump
D. Calculation for Motor This motor is a type of piston motor which is produced from Lio Shin Company. Its can operate at 80 rpm and its flow rate is 18 L/min (or) 3 × 10-4 m3/s. And its maximum pressure is 20 kgf/cm2 (or) 1933.41kPa. Applying these parameters, the volumetric displacement is 2.1 × 10-4 m3/s and the overall efficiency of motor is 89.43 %. Piston motors typically have an overall efficiency of 85 to 95%. So, the overall efficiency for this system has a range in standard ranges. VII. TRANSFER FUNCTION OF HYDRAULIC CIRCUIT Fig. 3 Measurements of Vane Pump
In this circuit, close loop is clearly used in order to control speed of hydraulic motor and displacement of cylinder piston. It is needed to control to be more accurate. In controlling these devices, the position sensors are widely used to show odd and even numbers of tool changers and to show whether the cylinder is set to be clamped or unclamped. Motor and cylinder don’t operate at the same time. Cylinder only works when changing tools. After changing tools, cylinder is set to be clamped. Therefore, it is necessary to divide this circuit into two portions; namely, portion with cylinder defined by path 1 and position with motor defined by path 2. Also each portion must have its own transfer function. And, this transfer function is firstly derived before modeling and simulating control system of hydraulic circuit. So, the servo system performance is accomplished using block in which each components is represented by a rectangle (block).
Basing on the following equation, TT = (VD _pp) / 2π
(1)
We have the outlet pressure of pump; 2158.14 kPa. ηv = QA/ QT
(2)
Volumetric efficiency; ηv is 96.3 %. B. Determination of Pipe Flow
For path 1,
Fig. 5 Path 1 with Cylinder
To get the transfer function of cylinder piston and valve, G1(s) H(s) =K / [s{1+ (2ζ1s/ωn,1) + (s2/ω2n,1)}]
(3)
When driving the cylinder piston by valve, the power transfer is assumed to be maximum. Fig. 4 Hydraulic Circuit Diagram with Pipe Measurements
So, For deciding laminar or turbulent, we should consider about Reynolds Number which is dimensionless. For both pipes of L1 and L2 in Fig. 4, the flows are laminar.
= 2/3 = 0.67
ωn,1 = [2{1+KW (1-
403
)1/2} / (Lm,1C)]1/2
(4)
World Academy of Science, Engineering and Technology 42 2008
ζ1= 1/4 [2Xm,1 / (1- )]1/2 _[(1+KW) / {1+KW (1- )1/2}] (5) By using above equations, the transfer function of path 1 is obtained. G1(s) H(s) = 2.5 / (0.023s3 + 0.0007s2 +s) By modeling this transfer function, Transfer Function of Valve and Cylinder
Amplifier Gain
Step
For path 1, By simulating the block diagram as shown in Fig (6) with MATLAB program,
Displacement (mm)
Rate Limiter
VIII. RESULT AND DISCUSSION
Scope
Feedback Gain
Fig. 6 Block Diagram for Path 1 Fig. 9 Step Response for Cylinder under Position Control
For path 2,
According to Fig (9), the position of cylinder is controlled by using step response for simulating the model of path 1. As a result, it has a peak at the first operating time due to the step response and it gradually becomes stable. So, the clamp and unclamp system of cylinder is meant to be stable in order to be accurate while operating the tool. For path 2, By simulating the block diagram as shown in Fig (8) with MATLAB program,
Fig. 7 Path 2 with Motor
To get the transfer function of the hydraulic motor, )1/2 / [T1(s) – {Lm,2 / (2RV) T2(s)}]
(6) Angular Speed (rad/s)
G2(s) H(s) = KW (1-
G2 (s) H(s) = 1.15 / (0.013s + 1.74) For having the transfer function of directional control valve, Gv (s) = 1/ [1+ (2ζ2s/ωn,2) + (s2/ω2n,2)]
(7)
= 1 / (0.04s2 + 0.14s+1) By modeling these transfer function, Amplifier Gain
Step
Rate Limiter
Transfer Function of Hydraulic Motor
Transfer Function of Valve
Fig. 10 Closed Loop Step Response for Controlling the Motor Speed
These transfer functions of path 2 are provided with MATLAB program in order to analyze the results. It can be meant that there isn’t any different in curve performance between the two results; simulation and programming with MATLAB.
Scope
Feedback Gain
IX. CONCLUSION Computerized Numerical Control (CNC) machining technology is a mouthful, but it is also fascinating by using computers to create metal parts and products for equipment and machines. In more simple terms, it is high-tech machining
Fig. 8 Block Diagram for Path 2
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World Academy of Science, Engineering and Technology 42 2008
[10] William L. DeRoy: Classification of Positive Displacement Pump, 2000, http://www.eduadv.org/eng/mechtro/hyd.mac [11] Spotts.M.F.: Design of Machine Elements, Maruzen Company LTD, 2000. [12] Ing.grad.A. Schmitt: The Hydraulic Trainer, SIR ISAAC PITMAN & SONS LTD, 2001. [13] John F.Douglas and Janusz M.Gasiorek: Fluid Mechanics, 4th ed. Person Education (Singpore) Pte.Ltd, 2002. [14] H.C.Town: Hydraulic and Pneumatic Operation of Machines, Mac Graw-Hill Co., 2003. [15] Binit Kumar Jha: CNC Porgramming Made Easy, Tota Yodogawa Limited, 2004. [16] Anthony Esposito: Fluid Power With Applications, 6thed. Prentice-Hall of India Private Limited, 2005.
technology. Machining and metalworking have been developed with computer technology. More efficiency output operations with even greater precision resulted from this marriage of machining and computers. In this technology, the hydraulic system is extensively used to control the process and drive the machines. So, most of conventional lathe machine are currently converted to CNC lathe machine. The hydraulic circuit is very important for controlling the hydraulic system and describes the functional structure of the hydraulic system. In this thesis, the main components in hydraulic circuit and its functions have been discussed in chapter three. Some parameters of pump are taken from Camel Pump Company and motor’s, from Lio Shing Company. Basing these parameters, the efficiency of pump is 96% and motor’s is over 89%. The performance of hydraulic circuit is provided with the transfer functions. The hydraulic circuit in this thesis can be divided by two paths; (1) a directional control valve (4/2 way) and a cylinder, and (2) a directional valve (4/3 way) and a piston motor. The main reason for dividing two paths is that these two portions are not operated at the same time. So, each path can be derived to transfer function. From the transfer function, there is a curve for each path. These curves show versus of output and time. And, these transfer functions are provided with MATLAB programming procedure. The main purpose of this thesis is to control cylinder position and especially motor speed. According to these curves, it is found that dynamic responses for components are stable. So, the whole hydraulic circuit is meant to be stable. ACKNOWLEDGMENT This research paper couldn’t have been accomplished without help and guidance from the people who served on the committee of WCSET. Especially, it is worth to express the deepest gratitude to Dr. Khin Maung Aye, Rector, West Yangon Technological University and Dr. Mya Mya Khaing, Associate Professor, Department of Mechanical Engineering, Yangon Technological University. Finally, the author extremely thanks to her friend, Dr. Thin Sandar Oo, Lecturer, Department of Textile Engineering, West Yangon Technological University and all members of her family. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
C.T.Bower: The book of the Lathe, AM.I.Prod.E, London, 1957. Herbert Addison: The Pump User’s Handbook, Mac Graw-Hill Co., 1958. Willian Wolansky and Akers: Modern Hydraulics, A Bell & Howell Information Co, London, 1970. Josheph.P.Messina and Igor J Karassik: Pump Handbook, Mc Graw-Hill Book Company, 1983. Theodore Franklin: Cam Type Turret Lath Machine, Lio Shing Company. 1984, http:// www.turret.com.tw Bumin.F.F.: Conversion of a Conventional General Purpose Lathe to a CNC Lathe, M.S.C Thesis, M.F.T.U, 1985. David Gibbs: An Introduction to CNC Machining, Second Edition, English Language Book Society, Cassel Publisher Ltd, (1987). Thomas E. Ogata R: Types of pump, Engman-Taylor Company.1997, http://www.allpumps.edu.org Genjamin R. Mohan: Hydraulic & Pneumatic Symbols, 1999, http://www.Hyd_Pnue_symbols.html.org
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