Design of Hydraulic Circuit for CNC Lathe Machine Converted from ...

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; 4/3- 

way 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. 

A 

Design of Hydraulic Circuit for CNC Lathe 

Machine Converted from Conventional Lathe 

Machine 

I. INTRODUCTION 

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: 095-09- 

513-7242; 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: 095-09- 

513-7242; fax: 095-067-404015; e-mail: seinelei1@ gmail.com). 

World Academy of Science, Engineering and Technology 18 2008 

Zin Ei Ei Win, Than Naing Win, Jr., and Seine Lei Winn 

401 

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.

II. AIM AND OBJECTIVE 

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 

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. 

IV. ESSSENTIAL FACTORS IN DESIGNING HYDRAULIC CIRCUIT 

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. 1 Conventional Lathe Machine 


402 

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. 

VI. EXPERIMENTAL PROCEDURE 

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

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 

Fig. 3 Measurements of Vane Pump 

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 

Fig. 4 Hydraulic Circuit Diagram with Pipe Measurements 

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. 


403 

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. 

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 m 3 /s. And its maximum pressure is 20 

kgf/cm 2 (or) 1933.41kPa. Applying these parameters, the 

volumetric displacement is 2.1 × 10 -4 m 3 /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 

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). 

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) + (s 2 /ω 2 n,1)}] (3) 

When driving the cylinder piston by valve, the power transfer 

is assumed to be maximum. 

So, = 2/3 = 0.67 

ωn,1 = [2{1+KW (1- )1/2 } / (Lm,1C)] 1/2 (4)

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 / (0.023s 3 + 0.0007s 2 +s) 

By modeling this transfer function, 

Step 

For path 2, 

Fig. 6 Block Diagram for Path 1 

Fig. 7 Path 2 with Motor 

To get the transfer function of the hydraulic motor, 

G2(s) H(s) = KW (1- )1/2 / [T1(s) – {Lm,2 / (2RV) T2(s)}] (6) 

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) + (s 2 /ω 2 n,2)] (7) 

= 1 / (0.04s 2 + 0.14s+1) 

By modeling these transfer function, 

Step 

Rate Limiter 

Rate Limiter 

Amplifier 

Gain 

Amplifier 

Gain 

Feedback Gain 

Transfer Function of 

Hydraulic Motor 


Valve 

Feedback Gain 

Fig. 8 Block Diagram for Path 2 



Valve and Cylinder 

Scope 

Scope 

404 

VIII. RESULT AND DISCUSSION 

For path 1, 

By simulating the block diagram as shown in Fig (6) with 

MATLAB program, 

Displacement (mm) 

Fig. 9 Step Response for Cylinder under Position Control 

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, 

Angular Speed (rad/s) 

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. 

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

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. 

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