Analysis and estimation of energy consumption for

255
Analysis and estimation of energy consumption for
numerical control machining
Y He1*, F Liu1, T Wu2, F-P Zhong3, and B Peng1
1
State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing, People’s Republic of China
2
Department of Industrial and Systems Engineering, University of Wisconsin–Madison, Madison, Wisconsin, USA
3
Mechanical 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 tasks
as the energy consumption during machining is responsible for a substantial part of the environmental burden in manufacturing industry. Facing the problem, the present paper aims to
analyse the correlation between numerical control (NC) codes and energy-consuming components of machine tools, and to propose a practical method for estimating the energy consumption of NC machining. Each energy-consuming component is respectively estimated by
considering its power characteristics and the parameters extracted from the NC codes, and
then the procedure estimating energy consumption is developed by accounting for the total
energy consumption of the components via the NC program. The developed method is verified
by comparing the estimated energy consumption with the actual measurement results of
machining two test workpieces on two different machine tools, an NC milling machine and an
NC lathe, and is also applied to evaluate the energy consumption of two different NC programs on
the NC milling machine. The results obtained show that the method is efficient and practical, and
can help process planning designers make robust decisions in choosing an effective energyefficient NC program.
Keywords:
1
energy consumption, numerical control machining, machine tools
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 production, manufacturing industry must take responsibility 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
*Corresponding author: State Key Laboratory of Mechanical
Transmission,
Chongqing
People’s Republic of China.
email: heyan@cqu.edu.cn
University,
Chongqing
400030,
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 consumption of machining is motivated by various needs
related to energy efficiency improvement. For characterizing 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 statistical modelling of milling machining efficiency by
using experimental data and response surface
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Y He, F Liu, T Wu, F-P Zhong, and B Peng
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 conditions 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 verification. 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 additional 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 comprehensive and hence the evaluation of energy consumption 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 metalcutting 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 analyses the correlation between the NC codes and the
energy-consuming components of machine tools,
and characterizes the energy consumption of components so as to simplify the estimation of energy consumption. 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 consumption 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 ENERGYCONSUMING COMPONENTS OF MACHINE
TOOLS 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.
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Analysis and estimation of energy consumption for numerical control machining
Table 1
257
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
Stop spindle motor
Stop z/y/z axis feed motor
Turn on coolant pump motor
Stop coolant pump motor
Speed of spindle motor
Rapid movement
Movement at the feed rate
Feed rate of z/y/z axis feed motor
Tool change
M03, M04
M00, M01, M02, M05, M30
M00, M01, M02, M30
M07, M08
M00, M01, M02, M09, M30
x/y/z axis feed
Coolant pump
S
G
Spindle
z/y/z axis feed
F
T
z/y/z axis feed
Tool change system
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 energyconsuming components.
G00
G01, G02, G03
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
3
ENERGY CONSUMPTION ESTIMATION
OF COMPONENTS
The energy consumption of NC machining can be
decomposed into the required energy of the components 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 consumption, 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 workpiece Ec. Hence, Espindle can be estimated by
equation (2)
Z tme
Z tce
Espindle ¼ Em þ Ec ¼
pm dt þ
pc dt ð2Þ
tms
tcs
where pm is the power for enabling the operating
state of the spindle transmission module, pc is the
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 consumption of the spindle motor, and hence pm is the
unloaded power of the spindle motor. Given the spindle rotation speed ns of a machine tool, the unloaded
power of the spindle motor pm is approximately measured 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 ðns Þ
ð3Þ
The 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 cutting 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
ð4Þ
where 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
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Y He, F Liu, T Wu, F-P Zhong, and B Peng
Fig. 1
Energy flow of spindle
where sz, l, B, A, and z denote feed per tooth, depth
of milling, contact length of a milling tool, nonsymmetry 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
Fig. 2
Tool path of a rapid movement
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 controlled 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 ¼
m Z
X
i¼1
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
NC machine tool that the rapid movement is from
point A ðx1 , y1 , z1 Þ to point B ðx2 , y2 , z2 Þ at the rapid
feed speed of vr, and jx2 x1 j ˜ jy2 y1 j ˜ jz2 z1 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 , pry , and prz , the energy estimation
for rapid movement is obtained by equation (8)
Erfeed ðA
! BÞ ¼
ZtB
prx
tA
dt þ
ZtD
tA
pry
dt þ
ZtC
prz dt
tA
ð8Þ
Since each axis feed motor moves at the same
speed vr, equation (8) can also be rewritten as follows
Erfeed ðA ! BÞ ¼ prx þ pry þ prz ðtC tA Þ
þ prx þ pry ðtD tC Þ þ prx ðtB tD Þ
ð9Þ
where
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
tC tA ¼ ðz2 z1 Þ=vr
tD tC ¼ ðy2 z1 Þ=vr
tB tD ¼ ðx2 y1 Þ=vr
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ð10Þ
Analysis and estimation of energy consumption for numerical control machining
3.2.2 Energy estimation of Effeed
Similarly, the energy estimation of Effeed is also dependent 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 , yb Þ and C ðxc , yc Þ as the interpolation point
in one interpolation cycle, the consumed energy of the
two axis feed motors can be described by equation (11)
Effeed ðA ! BÞ ¼
ZtB pfx þ pfy dt
ð11Þ
tA
where pfx
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
ð14Þ
Effeed ðA ! BÞ’ pfx þ pfy ðtB tA Þ
tB tA ¼
vf
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
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 ¼
pos0 posa
numpos ntool
ð17Þ
where pos0, posa, numpos, and ntool are, respectively, 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 constant 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
where
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x2b þ y2b
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 estimation because the cutting force has little effect on
the power of feed motors.
Etool ¼ ptool ttool
and pfy
259
ð15Þ
The energy consumption estimation of coolant pump
motors can be calculated by equation (18)
Ecool ¼ pcool ðtcoe tcos Þ
ð18Þ
where pcool is the power of the coolant pump
motors, which is also a constant value for a specific
machine tool, and ðtcoe tcos Þ represents the running
time of the coolant pump motors which are controlled by M-tags of NC codes.
3.5 Energy consumption estimation of fixed
energy-consuming components
Fig. 3
Line interpolation using numerical increment
interpolation for two-axis feed
Energy consumption of fan motors and servos systems constitutes the fixed energy consumption
for keeping the machine tool in an operational
state. Similarly, the energy consumption of fan
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Y He, F Liu, T Wu, F-P Zhong, and B Peng
motors and servos systems can be estimated with
equation (19)
Efix ¼ pservo þ pfan ðte ts Þ
ð19Þ
where pservo and pfan are the power of the servos
system and fan motors, respectively. ðte ts 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 consumption 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 components controlled by the corresponding tags.
Fig. 4
Step 3: sum up the energy consumption of the components to obtain the total energy consumption of
the machine tool.
Furthermore, Step 2 is classified into several parallel 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.
Procedure of energy consumption estimation
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Analysis and estimation of energy consumption for numerical control machining
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 calculate Efix, in which the running time tfix is estimated 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 calculated with equation (2). The process parameters
can be obtained from NC files or workpiece process 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.
261
respectively; the workpiece type and process parameters 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 components are given by simple measurements on PL700
as shown in Table 4. Based on the detailed information in Table 3, the power parameters in Table 4, and
the equations presented in section 3, the energy consumption of each component for machining the
example workpiece is estimated as shown in Table 5.
In order to compare the estimated energy consumption with the actual one, the example workpiece
In the above steps, the power parameters of components required for estimation can be obtained with
a small number of simple measurements, or from the
machine and component documentations.
Fig. 5
5
Example workpiece
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,
Table 3
Table 2 Workpiece type and process parameters
Parameter
Value
Workpiece
Spindle speed
Feed speed
Cutting depth
Machine tool
C45
2000 r/min
1500 mm/min
0.2 mm
PL700
Detailed information for estimating the energy consumption by parsing NC codes
Detailed information
NC code
Component(s)
Behaviour description
N100 G21
N104 . . . G0 X0 Y0
S2000 M03
N106 . . . Z100 M8; N108 Z3
Fan motor and servos system
Axis feed motor
Spindle motor
z-axis feed motor
Coolant pump motor
z-axis feed motor
x-axis feed motor
y-axis feed motor
x- & y-axis feed motors
z-axis feed motor
Turing on machine tool
Rapid movement to x 0, y 0
Spindle motor running at the speed of 2000 r/min
Rapid movement to z100, and then to z3
Turning on coolant pump motor
Moving to z-0.2 at the feed rate of 300 mm/min
Moving to x170 at the feed rate of 1500 mm/min
Moving to y150 at the feed rate of 1500 mm/min
Moving to x20 y 0 at the feed rate of 1500 mm/min
Moving to z3 at the feed rate of 300 mm/min, and then
rapid movement to z100
Turning off spindle motor
Turning off coolant pump motor
Turning off machine tool
N110 G1 Z-0.2 F300
N112 X170 F1500
N114 Y150
N116 X20 Y0
N118 Z3 F300; N120 G0 Z100
N122 M05
N124 M30
Spindle motor
Coolant pump motor
Fan motor and servos system
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Y He, F Liu, T Wu, F-P Zhong, and B Peng
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
Table 4
Power parameters of energyconsuming components of PL700
Power parameter
Value (W)
pservo þ pfan
pcool
pfx
pfy
prz
pfz
pm
pc
601
340
15
15
770
32
160
100
Table 5
Energy consumption estimation of each component for machining the example workpiece
Consumed
energy
(10–3 kWh)
Energy
parameter
Energy-consuming
component(s)
Efix
Ecool
Effeed
Fan motor þ servos system
Coolant pump motor
x-axis feed motor (feed speed)
y-axis feed motor (feed speed)
z-axis feed motor (feed speed)
z-axis feed motor (rapid movement)
Spindle motor, unloaded
(energy consumption for running
spindle of machine tool)
Spindle motor, machining (energy
consumption for cutting workpiece)
Erfeed
Em
Ec
3.97
2.24
0.06
0.06
0.01
0.26
1.06
0.59
8.25
Total
consumed
energy
(10–3 kWh)
Fig. 6
each component between the estimated value and
the actual value.
Figure 6 shows the comparison of the energy consumption 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 components are less than the actual values, and the estimated 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 process of the machine tools. The consumed energy for
these transitive states is not included in the proposed
estimation method. Additionally, the time for estimating 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
Comparison of energy consumption (percentage of total): (a) the estimated values; (b) the
actual measurement values
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Analysis and estimation of energy consumption for numerical control machining
(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 finishing the hole Ø178, and the step surface.
A similar procedure was used to estimate the
energy consumption of each component for machining 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 estimated percentage for each component is almost
equal to the actual value.
Despite the difference between the estimated
energy consumption and the actual one, the estimated 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 consumption 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 measured 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,
The estimated value
The actual value
9
Table 6
8
7
-3
Energy(10 Kw.h)
263
6
Energy
parameter
5
3
Erfeed
2
Em
1
0
Efix
Ecool
Em
: Fan motor and servos system
Efeed
Em
: The unloaded spindle motor
Ec
Fig. 7
Efeed
Ec
Efix
: Feed motor
Ecool
Total
Ec
: Coolant pump motor
: Machining
Comparison of energy consumption between
the estimated and actual values
Fig. 8
Energy-consuming component(s)
Fan motor þ servos system
x-axis feed motor (feed speed)
z-axis feed motor (feed speed)
x-axis feed motor (rapid movement)
z-axis feed motor (rapid movement)
Spindle motor, unloaded (energy
consumption for running
spindle of machine tool)
Spindle motor, machining (energy
consumption for cutting
workpiece)
Efix
Effeed
4
Energy consumption estimation of each component for machining on C2-6136HK lathe
Total
consumed
energy
(10–3 kWh)
Consumed
energy
(10–3 kWh)
287.51
0.72
17.41
0.10
5.24
345.56
1311.26
1967.80
Workpiece machining on NC lathe
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264
Y He, F Liu, T Wu, F-P Zhong, and B Peng
Fig. 9
10
Comparison of energy consumption (percentage of total) of machining on C2-6136HK lathe:
(a) the estimated values; (b) the actual measurement values
NC1
NC2
Energy(10-3Kw.h)
8
6
4
2
0
The estimated value
Fig. 10
The actual value
Comparison of energy consumption for two
NC files machining the example workpiece
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 contribution 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 components 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 constitutes the total energy consumption of NC machining. The energy consumption of each component
is calculated by multiplying the power by the corresponding time of the operational states of the
corresponding energy-consuming component.
The required parameters for estimation are simplified based on an analysis of the energy consumption characteristics of the components and the
corresponding NC codes.
3. Based on the above correlation and the estimation
method for the components, the procedure proposed for energy consumption estimation is to
sum up the energy consumption of each component 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 automatic technology or statistical methods.
FUNDING
This work was supported by the Fundamental
Research Funds for the Central Universities of
China [grant number CDJZR10110013].
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Analysis and estimation of energy consumption for numerical control machining
Ó Authors 2011
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APPENDIX
Notations
A
B
Ec
Ecool
Efeed
Effeed
Erfeed
Efix
Em
Espindle
Etool
Etotal
fu
Fc
l
m
ns
ntool
numpos
pc
pcool
pfan
pi
pm
pservo
non-symmetry of milling (mm)
contact length of a milling tool (mm)
cutting energy for material removal from
workpiece (kWh)
energy consumption of coolant pump
(kWh)
energy consumption of axis feed
(kWh)
energy estimation of the movement at the
feed rate (kWh)
energy estimation of rapid movement
(kWh)
fixed energy consumption required by the
rest of the components (kWh)
energy consumption for enabling the
operating state of spindle transmission
module (kWh)
energy consumption of spindle (kWh)
energy consumption of tool change system
(kWh)
total energy consumption of NC
machining (kWh)
specific cutting force (N/mm2)
cutting force (N)
depth of milling (mm)
number of axis feed motors
spindle rotation speed (r/min)
rotation speed of the turret (r/min)
number of tool posts in the turret
cutting power for material removal of
workpiece (W)
power of coolant pump motor (W)
power of fan motors (W)
power of the ith-axis feed motor (W)
power for enabling the operating state of
spindle transmission module (W)
power of servos system (W)
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ptool
pfx
pfy
prx
pry
prz
pos0
posa
sz
tA
tB
Y He, F Liu, T Wu, F-P Zhong, and B Peng
power of tool change motor (W)
power of x-axis feed motor for the
movement at the speed of vx (W)
power of y-axis feed motor for the
movement at the speed of vy (W)
power of x-axis feed motor for rapid
movement (W)
power of y-axis feed motor for rapid
movement (W)
power of z-axis feed motor for rapid
movement (W)
initial position of the turret
designed position by NC codes
feed per tooth (mm/tooth)
time at the point A
time at the point B
tC
tce
tcoe
tcos
tcs
tD
te
tfei
tfsi
tme
tms
ts
ttool
vc
vr
z
time at the point C
ending time for cutting
ending time of coolant pump motor
starting time of coolant pump motor
starting time for cutting
time at the point D
ending time of the NC file
starting time of the ith-axis feed motor
ending time of the ith-axis feed motor
ending time for spindle running
starting time for spindle running
starting time of the NC file
turret rotation time
cutting speed (m/s)
rapid feed speed of axis (m/s)
number of teeth of a milling tool
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