he2012

Analysis and estimation of energy consumption fornumerical control machiningY He1*, F Liu1, T Wu2, F-P Zhong3, and B Peng1

1State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing, People’s Republic of China2Department of Industrial and Systems Engineering, University of Wisconsin–Madison, Madison, Wisconsin, USA3Mechanical Engineering Department, Chongqing Industry Polytechnic College, Chongqing, People’s Republic of China

The manuscript was received on 16 December 2010 and was accepted after revision for publication on 30 June 2011.

DOI: 10.1177/0954405411417673

Abstract: Understanding and estimating the energy consumed by machining are essential tasksas the energy consumption during machining is responsible for a substantial part of the envi-ronmental burden in manufacturing industry. Facing the problem, the present paper aims toanalyse the correlation between numerical control (NC) codes and energy-consuming compo-nents of machine tools, and to propose a practical method for estimating the energy consump-tion of NC machining. Each energy-consuming component is respectively estimated byconsidering its power characteristics and the parameters extracted from the NC codes, andthen the procedure estimating energy consumption is developed by accounting for the totalenergy consumption of the components via the NC program. The developed method is verifiedby comparing the estimated energy consumption with the actual measurement results ofmachining two test workpieces on two different machine tools, an NC milling machine and anNC lathe, and is also applied to evaluate the energy consumption of two different NC programs onthe NC milling machine. The results obtained show that the method is efficient and practical, andcan help process planning designers make robust decisions in choosing an effective energy-efficient NC program.

Keywords: energy consumption, numerical control machining, machine tools

1 INTRODUCTION

Owing to the link between carbon dioxide and global

warming, reduction of carbon dioxide emissions

is currently top of the global agenda. Since carbon

dioxide emissions are directly related to energy pro-

duction, manufacturing industry must take responsi-

bility and strive to adopt more energy-efficient

techniques [1]. Machining is one of the fundamental

manufacturing technologies and its material-removal

characteristics inherently make it wasteful in the use

of energy [2]. Understanding and characterizing

energy consumption is the first step towards reducing

the energy consumption of machine tools and their

machining processes [3].

The increasing interest in attempts to explore new

ways of analysing and modelling energy consump-

tion of machining is motivated by various needs

related to energy efficiency improvement. For char-

acterizing energy losses of the spindle system of

machine tools, Liu et al. [4] modelled energy flow

from the input of electric motors, through kinematic

chains, to the output of the tool–chip interface. Some

literature addresses this issue for specific machining

processes. Draganescu et al. [5] proposed the statis-

tical modelling of milling machining efficiency by

using experimental data and response surface

*Corresponding author: State Key Laboratory of Mechanical

Transmission, Chongqing University, Chongqing 400030,

People’s Republic of China.

email: [email protected]

255

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at UNIV OF PITTSBURGH on December 12, 2014pib.sagepub.comDownloaded from

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methodology. Pfefferkorn et al. [6] addressed the

problem of the minimum thermal energy required

in thermally assisted machining by defining some

efficiency metrics with regard to energy flow of the

machining processes. The selection of turning condi-

tions for minimizing energy footprints was studied by

Rajemi et al. [7], who modelled an optimal tool-life

with minimum energy requirement for turning a

machined part.

Numerical control (NC) is a means of controlling

the movements of machine tools by directly inserting

coded instructions into the system in the form of

numbers and letters [8]. It is common knowledge

that NC machining plays an important role in the

metal-cutting industry. Owing to its material-removal

characteristics, metal cutting consumes a large

amount of energy associated not only with removal

of the cut material, but also operation of the machine

tool. Kordonowy [9] performed a large number of

experiments related to the energy consumption of

different machine tools for NC machining and its ver-

ification. Further research by Gutowski and Dahmus

[2, 10] reported that the energy requirements of

machining are not constant as assumed in many

life-cycle analysis tools, and that the process rate is

the most important variable for evaluating the energy

consumption of machining. Shi et al. [11] proposed a

power balance equation for the spindle system of NC

machine tools based on the analysis of energy flow

characteristics.

The relationship between energy requirements

and operational parameters has been formulated in

order to model or evaluate the energy consumption of

NC machining. Hu et al. [12] characterized the addi-

tional load energy losses of a spindle system in NC

machining by modelling the relationships between

the losses and some cutting parameters including

spindle speed, cutting torque, and cutting force.

Avram and Xirouchakis [13] presented a methodology

for evaluating the variable energy consumption of a

machine tool system based on a formula of various

types of torque associated with energy consumption,

such as friction torque. In this research, an accurate

analysis and evaluation of energy consumption for

NC machining was performed. However, the energy

requirements of machining processes are compre-

hensive and hence the evaluation of energy con-

sumption is very complex, involving too many

parameters related to machining processes, some of

which are barely satisfied.

The present work was motivated by the need to

develop a practical estimation method for the

energy consumption of NC machining for metal-

cutting industries, especially for small- and

medium-sized enterprises in China where the

traditional manual NC programming is commonly

used to perform part production. The method analy-

ses the correlation between the NC codes and the

energy-consuming components of machine tools,

and characterizes the energy consumption of compo-

nents so as to simplify the estimation of energy con-

sumption. To show the efficiency of the method, two

test workpieces on two different NC machine tools,

an NC milling machine and an NC lathe, were

machined to compare the estimated energy con-

sumption and the actual measurement results, and

the application of the method was also demonstrated

with two different NC files on the NC milling

machine.

The remainder of this paper is organized as follows.

Section 2 introduces the correlation between the

energy consumption of machine tools and NC

codes. Section 3 presents the estimation method of

energy-consuming components. Section 4 develops

the procedure of energy consumption estimation of

NC machining. Finally, in section 5, case studies are

made to illustrate the efficiency of the proposed

method in estimating the energy consumption of

NC machining.

2 CORRELATION BETWEEN ENERGY-

CONSUMING COMPONENTS OF MACHINETOOLS AND NC CODES

The energy-consuming components of NC machine

tools generally consist of spindle, axis feed, servos

system, tool change system, and other auxiliary

equipment such as coolant pump and fans [14]. The

energy consumption of these components can be

classified into a fixed part and a variable part. The

former is the basic and constant energy consumption

during machining processes such as that required by

the fan motor and servos system, which enable the

machine tool to run; the variable part encompasses

the required energy consumption that depends on

the operation behaviours of the machine tool [15].

NC codes are composed of a sequence of directions

for controlling the operation behaviours of NC

machine tools, and consist primarily of G-codes,

M-codes, T-codes, S-codes, and F-codes [16].

Table 1 lists the detailed operation behaviours of

energy-consuming components controlled by the

tags of NC codes.

The variable energy consumption of NC machining

is generated according to the various operation

behaviours of the energy-consuming components

listed in Table 1. Apart from these components,

other energy-consuming components generate a

fixed energy consumption which is conceived as

constants.

256 Y He, F Liu, T Wu, F-P Zhong, and B Peng




The detailed tags for operation behaviours of

energy-consuming components are easily obtained

from the specification of NC machine tools. Based

on the detailed tags, NC codes are interpreted into

the corresponding operation behaviours of energy-

consuming components.

3 ENERGY CONSUMPTION ESTIMATION

OF COMPONENTS

The energy consumption of NC machining can be

decomposed into the required energy of the compo-

nents including spindle, axis feed, coolant pump, tool

change system, and other components that consume

a fixed amount of energy. Consequently, the total

energy consumption can be estimated as a sum of

the energy consumption of each component

Etotal ¼ Espindle þ Efeed þ Etool þ Ecool þ Efix ð1Þ

where Etotal is the total energy consumption of NC

machining. Espindle, Efeed, Etool, Ecool, and Efix are the

energy consumption of spindle, axis feed, tool change

system, coolant pump, and the fixed energy con-

sumption, respectively.

3.1 Energy consumption estimation of spindle

Energy consumption of the spindle is related mainly

to material removal from the workpiece. The energy

flow from a motor to a tool or a workpiece is shown

briefly in Fig. 1.

As shown in Fig. 1, the energy consumption of the

spindle Espindle can be subdivided into the energy

requirements for enabling the operating state of

the spindle transmission module Em and the energy

requirements for material removal from the work-

piece Ec. Hence, Espindle can be estimated by

equation (2)

Espindle ¼ Em þ Ec ¼Z tme

tms

pm � dtþZ tce

tcs

pc � dt ð2Þ

where pm is the power for enabling the operating

state of the spindle transmission module, pc is the

power for material removal from the workpiece, tms

and tme are respectively the starting time and the

ending time for spindle running, and tcs and tce are

respectively the starting time and the ending time for

cutting.

3.1.1 Estimation of Em

Em is conceived as the energy input of the spindle

motor under the condition that Ec is equal to zero.

Em is simplified to be the unloaded energy consump-

tion of the spindle motor, and hence pm is the

unloaded power of the spindle motor. Given the spin-

dle rotation speed ns of a machine tool, the unloaded

power of the spindle motor pm is approximately mea-

sured as a constant at the given speed in reference [4].

Therefore, pm is a function of the spindle rotation

speed ns as follows

pm ¼ f ðnsÞ ð3ÞThe simple statistical measurement approach is

used to acquire the unloaded power pm at different

spindle rotation speeds ns for the specific machine

tool. Also, the spindle rotation speed ns and the

other time parameters are easily obtained through

tags S and M in NC files.

3.1.2 Estimation of Ec

Ec can be estimated by equation (2), in which the cut-

ting power pc and the cutting time parameters must

be satisfied. The cutting time is calculated based on

the tool path and the cutting speed vc, both of which

are derived from NC files. The cutting power pc can be

written as equation (4) [17]

pc ¼ Fc � vc ð4Þwhere Fc is the cutting force.

Fc is theoretically expressed as a function of the

related cutting parameters. For milling, Fc can be

written as equation (5) [5]

Fc ¼ f ðvc, sz, l,B,A, zÞ ð5Þ

Table 1 Detailed operation behaviours of energy-consuming components controlled by the tags

NC tag Energy-consuming component Operation behaviour Tag examples of FANUC

M Spindle Turn on spindle motor M03, M04Stop spindle motor M00, M01, M02, M05, M30

x/y/z axis feed Stop z/y/z axis feed motor M00, M01, M02, M30Coolant pump Turn on coolant pump motor M07, M08

Stop coolant pump motor M00, M01, M02, M09, M30S Spindle Speed of spindle motorG z/y/z axis feed Rapid movement G00

Movement at the feed rate G01, G02, G03F z/y/z axis feed Feed rate of z/y/z axis feed motorT Tool change system Tool change

Analysis and estimation of energy consumption for numerical control machining 257




where sz, l, B, A, and z denote feed per tooth, depth

of milling, contact length of a milling tool, non-

symmetry of milling, and the number of teeth of a

milling tool, respectively.

Since the required parameters in equation (5) are

too complex to be satisfied in practice, the specific

cutting force fu is used to simplify the cutting force

estimation. Based on it, the cutting force Fc is given by

equation (6)

Fc ¼ fu � B � l ð6Þ

3.2 Energy consumption estimation of axis feed

Axis feed consumes energy to move the working

table or the cutting tool at a given feed speed.

Generally, the number of feed motors equipped on

an NC machine tool equals the number of NC con-

trolled axes. For example, the three-axis NC machine

tool is equipped with three axis feed motors including

x-axis feed motor, y-axis feed motor, and z-axis

feed motor. Supposing m is the number of axis

feed motors, the consumed energy of axis feed

is calculated as

Efeed ¼Xmi¼1

Z tfei

tfsi

pi � dt ð7Þ

where pi, tfei, and tfsi are, respectively, the power,

the starting time, and the ending time of the ith-axis

feed motor.

As shown in Table 1, axis feed is performed

with two different regularities including the rapid

movement and the movement at the feed rate.

Hence the required energy of axis feed is classified

into the energy estimation of rapid movement Erfeed

and the energy estimation of movement at the

feed rate Effeed.

3.2.1 Energy estimation of Erfeed

The required energy of rapid movement Erfeed is

dependent on the tool path generated by the axis

feed motors, the rapid movement time of each axis,

and the rapid feed speed. Supposing for the three-axis

NC machine tool that the rapid movement is from

point A ðx1, y1, z1Þ to point B ðx2, y2, z2Þ at the rapid

feed speed of vr, and x2 � x1j j˜ y2 � y1j j˜ z2 � z1j j.The tool path is shown in Fig. 2.

First, the three axis feed motors move with the

speed vr through the tool path from A to C; then,

the z-axis feed motor stops and the x-axis and y-axis

feed motors continue to move through the tool path

from C to D; finally, the next tool path is from D to B

with only the x-axis motor running.

Assuming the power of each axis feed motor is

denoted with prx, pr

y, and prz, the energy estimation

for rapid movement is obtained by equation (8)

ErfeedðA! BÞ ¼

ZtB

tA

prx � dtþ

ZtD

tA

pry � dtþ

ZtC

tA

prz � dt

ð8Þ

Since each axis feed motor moves at the same

speed vr, equation (8) can also be rewritten as follows

ErfeedðA! BÞ ¼ pr

x þ pry þ pr

z

� �tC � tAð Þ

þ prx þ pr

y

� �tD � tCð Þ þ pr

x tB � tDð Þ

ð9Þ

where

tC � tA ¼ z2 � z1ð Þ=vr

tD � tC ¼ y2 � z1ð Þ=vr

tB � tD ¼ x2 � y1ð Þ=vr

ð10Þ

Fig. 1 Energy flow of spindle

Fig. 2 Tool path of a rapid movement





3.2.2 Energy estimation of Effeed

Similarly, the energy estimation of Effeed is also depen-

dent on the tool path, the movement time of each axis,

and the feed speed, which are controlled by servo

interpolation. Numerical increment interpolation is

often utilized in modern NC machine tools. The line

interpolation in numerical increment interpolation for

two-axis feed is given as an example in Fig. 3 [16].

Given the line interpolation between A (0,0) and

B ðxb, ybÞ and C ðxc, ycÞ as the interpolation point

in one interpolation cycle, the consumed energy of the

two axis feed motors can be described by equation (11)

EffeedðA! BÞ ¼

ZtB

tA

pfx þ pf

y

� �� dt ð11Þ

where pfx and pf

y are the power of the two feed motors

at the speed of vx and vy, respectively, given by

vx ¼ vf � cos a ð12Þ

vy ¼ vf � sin a ð13Þ

Ignoring the acceleration and deceleration of feed

speeds, the resultant vector of feed speed vf is defined

as a constant from point A to B. Thus the speed

of each axis feed motor also keeps constant according

to equations (12) and (13), which means that feed axis

motors have the same movement time. Therefore,

equation (11) can be rewritten as

EffeedðA! BÞ’ pf

x þ pfy

� �� tB � tAð Þ ð14Þ

where

tB � tA ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix2b þ y2b

qvf

ð15Þ

The required speed parameters in equations (10)

and (15) can be derived from the NC codes. The

unloaded power of the feed motors can be used as

the input parameters for energy consumption esti-

mation because the cutting force has little effect on

the power of feed motors.

3.3 Energy consumption estimation of

tool changes

Energy consumption of the tool change system

results primarily from rotating the tool turret for

changing tools. The tool change motor rotates the

turret to the specific post designated by NC codes,

and the energy consumption estimation is calculated

as follows

Etool ¼ ptool � ttool ð16Þ

where ptool is the power of the tool change motor

and ttool is the turret rotation time, which is written as

equation (17) [18]

ttool ¼pos0 � posa

numpos � ntoolð17Þ

where pos0, posa, numpos, and ntool are, respec-

tively, the initial position of the turret, the

designed position by NC codes, the number of

tool posts in the turret, and the rotation speed of

the turret.

The power of the tool change motor ptool is a con-

stant value for a specific machine tool, and it is

obtained referring to the specification documents of

machine tools.

3.4 Energy consumption estimation of

coolant pump

The energy consumption estimation of coolant pump

motors can be calculated by equation (18)

Ecool ¼ pcool � tcoe � tcosð Þ ð18Þ

where pcool is the power of the coolant pump

motors, which is also a constant value for a specific

machine tool, and tcoe � tcosð Þ represents the running

time of the coolant pump motors which are con-

trolled by M-tags of NC codes.

3.5 Energy consumption estimation of fixedenergy-consuming components

Energy consumption of fan motors and servos sys-

tems constitutes the fixed energy consumption

for keeping the machine tool in an operational

state. Similarly, the energy consumption of fanFig. 3 Line interpolation using numerical increment

interpolation for two-axis feed





motors and servos systems can be estimated with

equation (19)

Efix ¼ pservo þ pfan

� �� te � tsð Þ ð19Þ

where pservo and pfan are the power of the servos

system and fan motors, respectively. te � tsð Þ denotes

the running time of the machine tool throughout the

entire NC file.

4 PROCEDURE OF ENERGY CONSUMPTION

ESTIMATION

Figure 4 depicts the procedure of energy consump-

tion estimation that includes the following three

steps.

Step 1: parse NC files to extract the tags for identifying

the energy-consuming components.

Step 2: estimate energy consumption of the compo-

nents controlled by the corresponding tags.

Step 3: sum up the energy consumption of the com-

ponents to obtain the total energy consumption of

the machine tool.

Furthermore, Step 2 is classified into several paral-

lel sub-steps as shown in Fig. 4, the details of which

are as follows.

Sub-step 2.1: tag ‘S’ marks the spindle speed ns which

is used as the input parameter to acquire the

power pm. Since tag ‘M’ controls the turning on/

off state of spindle motors, the spindle’s running

time is calculated by identifying the turning on

tags and stopping ones. Equation (2) is used to

estimate energy Em based on the spindle power

pm and the running time tms and tme.

Sub-step 2.2: tag ‘M’ also marks the turning on/off

state of coolant pumps, so the running time tcos

and tcoe of the coolant pump motors are similarly

estimated for one of the spindles. According to

equation (18), the running time tcos and tcoe, and

the acquired coolant pump power pcool are used to

calculate Ecool.

Fig. 4 Procedure of energy consumption estimation





Sub-step 2.3: both tags ‘G’ and ‘F’ are used to estimate

the energy consumption of axis feed. If there is no

tag ‘F’, equation (9) is used to calculate rapid feed

energy Erfeed; otherwise, equation (14) is employed

to calculate feed energy Effeed.

Sub-step 2.4: tag ‘T’ is specific to calculate the tool

energy Etool according to equation (16) with the

acquired power ptool and the running time ttool

estimated by equation (17).

Sub-step 2.5: as the fixed energy consumption Efix is

the basic element for the whole running process of

the machine tool, equation (19) is adopted to cal-

culate Efix, in which the running time tfix is esti-

mated including all the NC tags’ running time, and

the acquired power pfix is the sum of power of

servos and fan motors.

Sub-step 2.6: additionally, the cutting energy Ec is cal-

culated with equation (2). The process parameters

can be obtained from NC files or workpiece pro-

cess documents to estimate the required cutting

power pc by equations (4) and (6), and the cutting

times tcs and tce are estimated based on tag ‘G’ and

‘F’ and tool paths.

In the above steps, the power parameters of com-

ponents required for estimation can be obtained with

a small number of simple measurements, or from the

machine and component documentations.

5 CASE STUDIES

To verify the efficiency of energy consumption

estimation, experiments were conducted on a PL700

vertical-milling machine centre, which was made by

Chengdu Precise CNC Machine Tool of China. For the

machine centre, one example workpiece with milling

is considered as shown in Fig. 5. The width and depth

of the area to be machined are 10 mm and 0.2 mm,

respectively; the workpiece type and process param-

eters are listed in Table 2.

According to the machining requirements and

process parameters, NC codes are programmed to

machine the workpiece on the PL700 machine

centre. The detailed information for estimating

energy consumption is parsed based on the NC

codes shown in Table 3.

The power parameters of energy-consuming com-

ponents are given by simple measurements on PL700

as shown in Table 4. Based on the detailed informa-

tion in Table 3, the power parameters in Table 4, and

the equations presented in section 3, the energy con-

sumption of each component for machining the

example workpiece is estimated as shown in Table 5.

In order to compare the estimated energy con-

sumption with the actual one, the example workpiece

Table 3 Detailed information for estimating the energy consumption by parsing NC codes

NC code

Detailed information

Component(s) Behaviour description

N100 G21 Fan motor and servos system Turing on machine toolN104 . . . G0 X0 Y0 Axis feed motor Rapid movement to x 0, y 0S2000 M03 Spindle motor Spindle motor running at the speed of 2000 r/minN106 . . . Z100 M8; N108 Z3 z-axis feed motor Rapid movement to z100, and then to z3

Coolant pump motor Turning on coolant pump motorN110 G1 Z-0.2 F300 z-axis feed motor Moving to z-0.2 at the feed rate of 300 mm/minN112 X170 F1500 x-axis feed motor Moving to x170 at the feed rate of 1500 mm/minN114 Y150 y-axis feed motor Moving to y150 at the feed rate of 1500 mm/minN116 X20 Y0 x- & y-axis feed motors Moving to x20 y 0 at the feed rate of 1500 mm/minN118 Z3 F300; N120 G0 Z100 z-axis feed motor Moving to z3 at the feed rate of 300 mm/min, and then

rapid movement to z100N122 M05 Spindle motor Turning off spindle motor

Coolant pump motor Turning off coolant pump motorN124 M30 Fan motor and servos system Turning off machine tool

Fig. 5 Example workpiece

Table 2 Workpiece type and process parameters

Parameter Value

Workpiece C45Spindle speed 2000 r/minFeed speed 1500 mm/minCutting depth 0.2 mmMachine tool PL700





was machined on the PL700 machining centre with

the programmed NC codes. The energy consumption

of the machining process was measured by power

measurement devices, and the consumed energy of

each component was also separated. Figures 6 and 7

present a comparison of energy consumption for

each component between the estimated value and

the actual value.

Figure 6 shows the comparison of the energy con-

sumption percentage of each component. It is seen

that the estimated percentage for each component is

almost equal to the actual one. For both estimated

and actual values in this case, the maximum energy

consumption is generated by the fan motors and

servos, which accounts for about 48 per cent of the

total energy consumption. About 27 per cent of the

total energy is consumed by the coolant motor, which

is the secondary maximum energy consumption for

machining the example workpiece. The unloaded

energy consumption of the spindle motor cannot be

ignored due to energy consumption of about 13 per

cent, while the energy consumption for the axis feed

motor is the lowest. Relatively, the energy for cutting

the material, also known as the specific energy,

accounts for only 7 per cent of the total energy

consumption.

Figure 7 illustrates further comparison between the

estimated energy consumption and the actual. All

estimated energy consumption values of the compo-

nents are less than the actual values, and the esti-

mated value of the total energy consumption is

about 9.3 per cent less than the actual one. There

are several reasons for the comparison result. For

example, in the actual machining process, there are

some transitive states when changing the operation

status of the machine tools such as the start-up pro-

cess of the machine tools. The consumed energy for

these transitive states is not included in the proposed

estimation method. Additionally, the time for esti-

mating energy consumption is shorter than the

actual machining time because of the variable

speed processes of NC machine tools.

Similar experiments were also performed on a

C2-6136HK machine, which is an NC lathe

Fig. 6 Comparison of energy consumption (percentage of total): (a) the estimated values; (b) theactual measurement values

Table 5 Energy consumption estimation of each com-

ponent for machining the example workpiece

Energyparameter

Energy-consumingcomponent(s)

Consumedenergy(10–3 kWh)

Efix Fan motor þ servos system 3.97Ecool Coolant pump motor 2.24Ef

feed x-axis feed motor (feed speed) 0.06y-axis feed motor (feed speed) 0.06z-axis feed motor (feed speed) 0.01

Erfeed z-axis feed motor (rapid movement) 0.26

Em Spindle motor, unloaded(energy consumption for runningspindle of machine tool)

1.06

Ec Spindle motor, machining (energyconsumption for cutting workpiece)

0.59

Totalconsumedenergy(10–3 kWh)

8.25

Table 4 Power parameters of energy-

consuming components of PL700

Power parameter Value (W)

pservo þ pfan 601pcool 340pf

x 15pf

y 15pr

z 770pf

z 32pm 160pc 100





(Chongqing Machine Tool, China). The workpiece,

a part of a hobbing machine, is shown in Fig. 8. The

experimental machining scheme included processing

the end surface at the left, rough machining and fin-

ishing the hole Ø178, and the step surface.

A similar procedure was used to estimate the

energy consumption of each component for machin-

ing the example workpiece, as shown in Table 6.

The actual energy consumption was measured to

compare with the estimated one for each component

as shown in Fig. 9. This also showed that the esti-

mated percentage for each component is almost

equal to the actual value.

Despite the difference between the estimated

energy consumption and the actual one, the esti-

mated values are still a valuable reference to evaluate

the energy consumption of machining. One of the

applications for the energy consumption estimation

is to evaluate different NC files for machining the

same workpiece. Figure 10 illustrates the consumed

energy of two different blocks of NC files, NC1 and

NC2, for machining the example workpiece on the NC

milling machine. The two bars on the left of Fig. 10

depict the estimated energy values of the two blocks

of NC codes, and show that the estimated energy con-

sumption of the NC1 file is lower than that of NC2. In

order to prove the results derived from the estimated

energy, the actual energy consumption was also mea-

sured by machining the example workpiece with the

two NC files. The actual measured energy is depicted

in the two bars on the right of Fig. 10, which also show

that the consumed energy of block NC1 is lower.

Therefore, under the same machining requirements,

Fig. 8 Workpiece machining on NC lathe

0

1

2

3

4

5

6

7

8

9

: Machining

Efeed

EcEm

EcoolEfix

Ene

rgy(

10-3

Kw

.h)

The estimated value The actual value

Efix Ecool Em EcEfeed

Total

: Fan motor and servos system : Coolant pump motor: Feed motor

: The unloaded spindle motor

Fig. 7 Comparison of energy consumption betweenthe estimated and actual values

Table 6 Energy consumption estimation of each com-

ponent for machining on C2-6136HK lathe

Energyparameter Energy-consuming component(s)

Consumedenergy(10–3 kWh)

Efix Fan motor þ servos system 287.51Ef

feed x-axis feed motor (feed speed) 0.72z-axis feed motor (feed speed) 17.41

Erfeed x-axis feed motor (rapid movement) 0.10

z-axis feed motor (rapid movement) 5.24Em Spindle motor, unloaded (energy

consumption for runningspindle of machine tool)

345.56

Ec Spindle motor, machining (energyconsumption for cuttingworkpiece)

1311.26

Totalconsumedenergy(10–3 kWh)

1967.80





use of the first NC file to machine the workpiece is

preferable from the energy-saving point of view.

6 CONCLUSIONS

This paper has presented a method to estimate the

energy consumption of NC machining. The contribu-

tion of this work is to provide a practical tool to

predict or evaluate the detailed energy consumption

of NC machining by considering the correlation

between NC codes and the energy-consuming com-

ponents of machine tools, and simplifying the energy

consumption estimation of components based on an

analysis of their energy consumption characteristics.

The following procedures should be noted for the

method.

1. The energy consumption of NC machining

depends highly on the operational states of

energy-consuming components controlled by NC

codes. The correlation between energy-consuming

components of machine tools and NC codes is

analysed to identify the corresponding operation

behaviours of energy-consuming components.

2. Energy consumption of the components consti-

tutes the total energy consumption of NC machin-

ing. The energy consumption of each component

is calculated by multiplying the power by the cor-

responding time of the operational states of the

corresponding energy-consuming component.

The required parameters for estimation are simpli-

fied based on an analysis of the energy consump-

tion characteristics of the components and the

corresponding NC codes.

3. Based on the above correlation and the estimation

method for the components, the procedure pro-

posed for energy consumption estimation is to

sum up the energy consumption of each compo-

nent controlled by the corresponding NC files.

Experiments were performed for machining an

example workpiece in an NC milling machine

centre and an NC lathe. The estimated value of

energy consumption was compared with the actual

measured value to verify the energy consumption

estimation of NC machining. Although the estimated

values do not exactly equal the measured ones, the

estimations proved to be valuable reference data to

help NC code designers make decisions regarding

energy-efficient NC programs.

One limitation of the method is the requirement for

power parameters for the specific machine tool.

Future work will be directed towards developing a

power parameter database of machine tools by auto-

matic technology or statistical methods.

FUNDING

This work was supported by the Fundamental

Research Funds for the Central Universities of

China [grant number CDJZR10110013].

0

2

4

6

8

10

The actual value

Ene

rgy(

10-3

Kw

.h)

The estimated value

NC1 NC2

Fig. 10 Comparison of energy consumption for twoNC files machining the example workpiece

Fig. 9 Comparison of energy consumption (percentage of total) of machining on C2-6136HK lathe:(a) the estimated values; (b) the actual measurement values





� Authors 2011

REFERENCES

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APPENDIX

Notations

A non-symmetry of milling (mm)

B contact length of a milling tool (mm)

Ec cutting energy for material removal from

workpiece (kWh)

Ecool energy consumption of coolant pump

(kWh)

Efeed energy consumption of axis feed

(kWh)

Effeed energy estimation of the movement at the

feed rate (kWh)

Erfeed energy estimation of rapid movement

(kWh)

Efix fixed energy consumption required by the

rest of the components (kWh)

Em energy consumption for enabling the

operating state of spindle transmission

module (kWh)

Espindle energy consumption of spindle (kWh)

Etool energy consumption of tool change system

(kWh)

Etotal total energy consumption of NC

machining (kWh)

fu specific cutting force (N/mm2)

Fc cutting force (N)

l depth of milling (mm)

m number of axis feed motors

ns spindle rotation speed (r/min)

ntool rotation speed of the turret (r/min)

numpos number of tool posts in the turret

pc cutting power for material removal of

workpiece (W)

pcool power of coolant pump motor (W)

pfan power of fan motors (W)

pi power of the ith-axis feed motor (W)

pm power for enabling the operating state of

spindle transmission module (W)

pservo power of servos system (W)





ptool power of tool change motor (W)

pfx power of x-axis feed motor for the

movement at the speed of vx (W)

pfy power of y-axis feed motor for the

movement at the speed of vy (W)

prx power of x-axis feed motor for rapid

movement (W)

pry power of y-axis feed motor for rapid

movement (W)

prz power of z-axis feed motor for rapid

movement (W)

pos0 initial position of the turret

posa designed position by NC codes

sz feed per tooth (mm/tooth)

tA time at the point A

tB time at the point B

tC time at the point C

tce ending time for cutting

tcoe ending time of coolant pump motor

tcos starting time of coolant pump motor

tcs starting time for cutting

tD time at the point D

te ending time of the NC file

tfeistarting time of the ith-axis feed motor

tfsi ending time of the ith-axis feed motor

tme ending time for spindle running

tms starting time for spindle running

ts starting time of the NC file

ttool turret rotation time

vc cutting speed (m/s)

vr rapid feed speed of axis (m/s)

z number of teeth of a milling tool





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