MR 3 Milling Process
Unit Process Life Cycle Inventory
Dr. Devi Kalla, Dr. Janet Twomey, and Dr. Michael Overcash
August 19, 2009
Milling Process Summary ................................................................................................... 2
Methodology for Unit Process Life Cycle Inventory Model (uplci) .................................. 4
Milling Process Energy Characteristics .......................................................................... 4
A. Parameters Affecting the Energy Required for Milling ................................................. 7
Milling Energy ................................................................................................................ 8
Milling time for peripheral milling operation ................................................................. 9
Milling time for face milling operation .......................................................................... 9
Idle Energy .................................................................................................................... 13
Basic Energy ................................................................................................................. 14
B. Method of Quantification for Mass Loss ..................................................................... 19
Lci for Material Mass Loss Calculations ...................................................................... 19
Lci for Cutting Fluid Waste Calculations ..................................................................... 19
Lci for Lubricant Oil Waste Calculations ..................................................................... 21
Cutting Tool usage ........................................................................................................ 21
Case Study on Milling ...................................................................................................... 21
Product Details: ............................................................................................................. 22
Cutting Parameters ........................................................................................................ 23
Machining Process: ....................................................................................................... 23
Time, Power and Energy calculations .......................................................................... 23
Lci Material mass loss calculations .............................................................................. 25
Lci for Cutting fluid waste calculations ........................................................................ 26
Summary: .......................................................................................................................... 26
References Cited ............................................................................................................... 26
Appendices ........................................................................................................................ 27
Manufacturers Reference Data ..................................................................................... 27
1
Milling Process Summary
Milling is a frequent unit process in manufacturing as a mass reduction step, in
which a workpiece is fed past a rotating cylindrical tool with multiple cutting edges. The
axis of rotation of the cutting tool is perpendicular to the direction of feed. This
orientation between the tool axis and the feed direction is one feature that distinguishes
milling from drilling. Hence this life cycle heuristic is to establish representative
estimates of the energy and mass loss from the milling unit process in the context of
manufacturing operations for products. The milling unit process life cycle inventory
(uplci) profile is for a high production manufacturing operation, defined as the use of
processes that generally have high automation and are at the medium to high throughput
production compared to all other machines that perform a similar operation. This is
consistent with the life cycle goal of estimating energy use and mass losses representative
of efficient product manufacturing.
Milling is a cutting process in which material is removed by a rotating multiple
tooth cutter typically aided by cutting fluids. In milling the tool progressively generates a
surface by removing chips from a workpiece as it is fed into a rotating tool and these
chips are swept away by the rotation of the cutter. Because both workpiece and cutter can
be moved in more than one direction at the same time, surfaces having almost any
orientation can be machined. The milling process is used to machine external surfaces,
slots, produce flat, contoured, or shaped surfaces using muti-toothed milling cutters or
end mills. This is a versatile process with a high metal removal rate. Consequently, chip
disposal in milling and the effectiveness of cutting fluids are important. An example
milling machine is given in Figure MR3.1, while the milling mechanism is illustrated in
Figure MR3.2.
Figure MR3.3 shows an overview of the developed environmental-based factors
for milling operations. For a given workpiece (illustrated in Figure MR3.2) the life cycle
analysis yields energy use and mass losses as byproducts or wastes.
Figure MR3.1. Computer numerical control (CNC) milling machine with 3-axis control
(Photograph from Haas Automation, Inc. California, USA)
2
Figure MR3.2. Process Schematic of Peripheral Milling (Todd et al., 1994)
Z
Y
Workpiece and
Cutting tool
X
Machine Tool
Milling Process
Product
Environmental factors
Resource Data
Cutting Tool
Cutting Fluid
Energy Consumption
of machine tool
Chips
LCI DATA
Figure MR3.3. LCI data for milling process
3
Methodology for Unit Process Life Cycle Inventory Model (uplci)
In order to assess a manufacturing process efficiently in terms of environmental
impact, the concept of a unit operation is applied. The unit process consists of the inputs,
process, and outputs of an operation. Each unit process is converting material/chemical
inputs into a transformed material/chemical output. The unit process diagram of a milling
process is shown Figure MR3.4.
The transformation of input to output generates five lci characteristics,
a. Input materials
b. Energy required
c. Losses of materials (that may be subsequently recycled or declared waste)
d. Major machine and material variables relating inputs to outputs
e. Resulting characteristics of the output product that often enters the next unit process.
Machine tool, Fixing, Cutting Fluid
Work Piece
Process parameters
Cutting Tools
Cutting Fluid
Energy
Milling
Product with
desired shape
Chips
Noise
Waste Coolant
Scrap
Mist
Figure MR3.4. Input-Output diagram of a milling process
Milling Process Energy Characteristics
Because high production milling is a semi-continuous process. Many of these
automated CNC machines have more than three axes. One of the axes is often designed
as a rotary table to position the work-piece at some specified angle relative to the spindle.
The rotary table permits the cutter to perform milling on four sides of the part. These
machines are classified as horizontal, vertical or universal based on the spindle
orientation. The uplci is based on a representative operational sequence, in which
1) Work setup generally occurs once at the start of a batch in production. The setup
time is composed of the time to setup the machine tool, plan the tool movements,
and install the fixture device into the machine. All drawings and instructions are
consulted, and the resulting program is loaded. Typical setup times are given in
Table MR3.1 (Fridriksson, 1979). The total setup time must be divided by the size
of the batch in order to obtain the setup time per component. The energy
consumed during this setup period is divided by all the parts processed in that
batch and is assumed to be negligible and is discussed in the example below.
2) The power consumption during a batch for positioning or loading each new piece
into the CNC machine, with respect to tool axis is low. Time is required to load
the workpiece into the CNC machine and secure it to the fixture. The load time
can depend on the size, weight, and complexity of the workpiece, as well as the
type of fixture. This is at the level of Basic energy and is labeled Loading.
4
3) Relative movement of the cutting tool and the workpiece occurs without changing
the shape of the part body, referred to as Idle Energy and is labeled Handling.
This is the time required for any tasks that occur during the process cycle that do
not engage the workpiece. This idle time includes the tool approaching and
retracting from the workpiece, tool movements between features, adjusting
machine settings, and changing the tools.
4) Milling of a workpart for desired shape occurs and is labeled Tip Energy. The
time required for the cutting tool to make all necessary cuts in the workpiece for
each operation.
5) The piece is repositioned for subsequent cutting, thus the energy and mass loss
will be repeated. (Idle Energy for Handling and then Tip Energy for milling)
6) When the final shape is attained, the piece is unloaded and typically sent forward
to another manufacturing unit process. This is at the level of Basic Energy and is
labeled Unloading.
Table MR3.1. Set-up times for machining operations (Fridriksson, 1979)
Basic setup time (h)
Additional setup per tool (h)
Machine tool
Horizontal band saw
0.17
---
Manual turret lathe
1.2
0.2
CNC turret lathe
0.5
0.15
Milling machine
1.5
---
Drilling machine
1.0
---
Horizontal-boring machine
1.3
---
Broaching machine
0.6
---
Gear hobbing machine
0.9
---
Grinding machine
0.6
---
Internal grinding machine
0.6
---
Machining center
0.7
0.05
In this representative unit process, the life cycle characteristics can be determined
for milling on a per piece basis or a full piece (with one or more cut) basis. Since this is a
high production process, the start up (at the beginning of a batch or shift) is deemed to be
small and not included. In this uplci, there are three typical power levels that will be used,
Figure MR3.5. Each power level, kw, is the incremental power not the absolute total
power. Thus if electrical measurements are made, the kw during the tip measurement
must have the idle and basic power (kw) values subtracted to obtain this tip power (kw).
Correspondingly, there are times within the milling sequence from which these three
power levels are used, Figure MR3.5. The overall time per piece is referred to as cycle
5
time and is generally consistent in a batch. Each power level (basic, idle and milling) is a
reflection of the use of various components or sub-operations, of the CNC machine,
Figure MR3.6.
Power
Spindle and Coolant
motor Startup
Idle Energy
Tip Energy
Basic Energy
Pmilling
Pidle
Pbasic
tmilling
Time
tidle
tbasic
Figure MR3.5. Determination of power characteristics and energy requirements of
machine tools.
The steps 2), 3), 5), and 6) are estimated as representative values for use in this
unit process lci and energy required of removing material by milling, step 4), is by
measured using specific cutting energy values. The system boundaries are set to include
only the use phase of the machine tool, disregarding production, maintenance and
disposal of the machine. Moreover, the functioning of the manufacturing machines is
isolated, with the influence of the other elements of the manufacturing system, such as
material handling systems, feeding robots, etc. covered in other uplci reports.
The energy consumption of milling is calculated as follows;
Etotal = Pbasic * (tbasic) + Pidle * (tidle ) + Pmilling * (tmilling)
(Basic energy) (Idle energy) (Milling energy)
(1)
6
where power and time are illustrated in Figure MR3.5.
Servo Motors
Fan
Oil pump
Chiller System
Main Spindle
Automatic tool changer
Rapid Axis Movement
Cutting fluid pump
Machine Tool idle power
Machine Tool basic power
Unloaded Motors
Chip Generation Zone
Tip Energy
Numerical control
Lighting
Way lube system
Figure MR3.6. System boundary of the machining process
A. Parameters Affecting the Energy Required for Milling
An approximate importance of the many variables in determining the milling
energy requirements was used to rank parameters from most important to lower
importance as follows:
1. Workpiece Material properties
2. Feed rate
3. Cutting speed
7
4. Cutter diameter
5. Milling time
6. Depth of cut
7. Coolant
8. Part holding fixture
9. Tool wear
10. Geometry and set-up
From this parameter list, only the top 6 were selected for use in this unit process life
cycle with the others having lower influence on energy. Energy required for the overall
milling process is also highly dependent on the time taken for idle and basic operations.
Milling Energy
Milling time (tmilling) and power (Pmilling) must be determined for the milling
energy and are calculated from the more important parameters given above. Milling
process time is used to calculate a part of the energy for this unit process and based on a
milling area (tool in contact with workpiece)
There are two basic types of milling operations, as shown in Figure MR3.7: (a)
peripheral milling and (b) face milling. Milling time varies according to the milling
operation type. In peripheral milling, the axis of the tool is parallel to the surface being
milled, and the operation is performed by cutting edges on the outside periphery of the
cutter. In face milling, the axis of the cutter is perpendicular to the surface being milled,
and machining is performing by cutting edges on both the end and outside periphery of
the cutter.
(a) Peripheral milling
(b) Face milling
Figure MR3.7. Two basic types of milling operation
Actual milling times (tip energy) for these two operations are shown separately.
8
Milling time for peripheral milling operation
For peripheral milling from Figure MR3.8 the actual milling time, is given by the
expression
tmilling = (L+Lc)/fr
(2)
Where L is the length of the workpiece, Lc is the extent of the cutter first contact
with the workpiece and fr is the feed rate (linear speed) of the workpiece (in./min or
mm/min).
From Pythagorean’s Theorem Lc = ( d ( D  d ) )
(3)
Where d is the depth of cut in. (mm), D is the diameter of the cutter in. (mm) and R is the
radius of the cutter in. (mm). If peripheral milling is centered exactly then Lc = D/2.
Side View
Figure MR3.8. Peripheral milling operation
Milling time for face milling operation
For face milling from Figure MR3.9 the actual milling time, is given by the
expression
tmilling = (L+2Lc)/fr
(4)
Where L is the length of the workpiece to be machined, Lc is the extent of the cutter first
contact with the workpiece and fr is the feed rate (linear speed) of the workpiece (in./min
or mm/min).
From Pythagorean’s Theorem Lc = ( w( D  w) )
(5)
Where w is the width of cut in.(mm), D is the diameter of the cutter in. (mm) and R is the
radius of the cutter in. (mm). If face milling is centered exactly then Lc = D/2.
9
Top View
Figure MR 3.9. Face milling operation
The total milling process is illustrated in Figure MR3.10. The cutting speed, V
(m/min), is the speed of the rotating tool at the point of contact on the workpiece. The
rotational speed of the spindle, N, (rev/min) (set on the machine),
N = V/ (π*D).
Where V = cutting speed, mm/min and D = diameter of the cutter, mm. Feed for milling
is the linear distance that a tool advances during one rotation of the work piece or cutting
tool, f (mm/rev). Milling cutters generally have more than one cutting edge – introducing
the concept of tooth loading, or table feed per tooth. Feed per tooth, ft (mm/tooth) is the
thickness of chip material that each cutting edge of a tool removes with one pass. The
feed per tooth is determined from:
ft =V/ (N*No. of teeth)
(6)
Feed (mm/rev) = Feed per tooth (mm/tooth) * No. of teeth/one revolution
(7)
V and ft are estimated from the material properties, Table MR3.2 and Table MR3.3. The
feed rate, fr (mm/min) is the rate at which the cutting tool and the workpiece move in
relation to one another. The feed rate is the product of
fr =ft *N * No. of tooth.
(8)
The volume material removal rate (VRR) for a given width of cut and depth of cut is
VRR = (w)*(d)*(fr), mm3/min
(9)
Where w is the width of cut (mm) and d is the depth of cut (mm).
tmilling= (L+Lc)/fr
(for example for peripheral milling)
10
Figure MR3.10. Schematic top view diagram of milling process
The milling energy is thus E (Joule/mill) = milling time*Pmilling,
E = milling time*(volume removal rate)*(specific cutting energy, Up, W/mm3/sec)
Emilling (J/mill) = ((L+Lc)/fr)*(w*d*fr)*Up =(L+Lc) *w*d*Up = tmilling * Pmilling (10)
So we can see the number of teeth is not needed for the energy of milling.
With a given material to be milled, the specific cutting energy, Up, is given in Table
MR3.2. Then for that material a representative cutting speed, V is selected from Table
MR3.2.
The milling energy is then calculated from equation 10. Thus with only the material to be
milled, the depth of cut, and the width of cut, one can calculate the lci milling energy for
a mill. This then must be added to the idle and basic energies, see below.
Table MR3.2. Average values of energy per unit material removal rate and recommended
speeds and feeds (Erik, 2000; Hoffman, 2001; Joseph, 1989; Kalpakjian, 2008; 9, 10)
Material
Aluminum
Alloys
Magnesium
Alloys
Hardness
[Brinell
hardness
number]
Specific
cutting
energy,
Up
[W/ mm3
per sec]
(Hp/ in3
per min)
Cutting Speed, V
(m/min, ft/min)
30 - 150
0.98 (0.36)
120 -140, 400 - 450
40 - 90
0.49 (0.18)
180- 250, 600 - 800
Feed per
tooth, ft
(mm/tooth,
inch/tooth
0.28 – 0.56,
0.011 - 0.022
0.2 - 0.5,
0.008 - 0.02
Density
(kg/m3)
2712
1770
11
Tungsten
200
6.24(2.3)
10 – 25, 30-70
Copper
80
2.98 (1.1)
30 – 45, 100 - 150
Titanium
80-100
3.26 (1.2)
25 – 30, 80 – 100
Brass
150 - 200
2.25 (0.83)
60 – 90, 200 - 300
Bronze
1.36 (0.50)
45 – 55, 150-180
Malleable iron
1.55 (0.57)
33 – 40, 110-130
Stainless steel
Steel, Low
carbon
Steel, Medium
carbon
100
1.36 (0.5)
30 – 37, 100-120
175-225
1.63 (0.60)
90 – 185, 300-600
225-275
1.95 (0.72)
45 – 140, 150-450
Steel, Hardened
275-325
2.39 (0.88)
15 – 70, 50-225
Cast iron, soft
Cast iron,
medium
150-180
0.81 (0.30)
25 – 33, 80-110
180-220
1.7 (0.63)
18 – 45, 60-150
Cast iron, hard
220-300
2.5 (0.92)
25 – 28, 80-90
Gray cast iron
220-260
1.52 (0.55)
15 – 26, 50-85
Unalloyed steel
110
1.36 (0.5)
48 - 68, 160-220
Unalloyed steel
150
2.2 (0.81)
36 - 45, 120-150
Unalloyed steel
310
2.93 (1.08)
27 - 40, 90-130
Low alloy steel
125-225
2.52 (0.93)
27 - 38, 90-125
Low alloy steel
225-425
3.31 (1.22)
25 - 33, 70-110
High alloy steel
150-300
2.96 (1.09)
15 - 27, 50-75
High alloy steel
Nodular cast
iron
Nodular cast
iron
300-450
4.59 (1.69)
9 - 18, 30-60
160
1.21 (0.45)
33 - 43, 110-140
250
2.10 (0.78)
27 - 45, 90-120
0.025 – 0.08,
0.001-0.003
0.15 – 0.30,
0.006 - 0.012
0.1 – 0.2,
0.004 - 0.008
0.18 – 0.36,
0.007 - 0.014
0.05 – 0.25,
0.002 – 0.010
0.15 – 0.30,
0.006-0.012
0.08 – 0.15,
0.003-0.006
0.01 – 0.18,
0.0005-0.007
0.01 - 0.13,
0.0004-0.005
0.005 – 0.08,
0.0002-0.003
0.2 – 0.4,
0.008-0.016
0.2 – 0.33,
0.007-0.013
0.15 – 0.30,
0.006-0.011
0.25 – 0.46,
0.010-0.018
0.1 – 0.35,
0.004-0.012
0.05 – 0.25,
0.002 – 0.01
0.025 – 0.2,
0.001 – 0.008
0.05 – 0.13,
0.002 – 0.005
0.025 – 0.1,
0.001 – 0.004
0.01 – 0.2,
0.0005 – 0.008
0.005 – 0.07,
0.0002-0.003
0.28 – 0.56,
0.011 - 0.022
0.23 – 0.43,
0.009 – 0.017
19600
8930
4500
7700-8700
8900
6800-7800
7480-8000
7480-8000
7480-8000
7480-8000
6800-7800
6800-7800
6800-7800
6800-7800
7850
7850
7850
7850
7850
7850
7850
6800-7800
6800-7800
12
Table MR3.3 Recommended speeds and feeds for milling plastics (Terry and Erik,
2003)
Material
Thermoplastics
Polyethylene
Polypropylene
TFE fluorocarbon
Butyrate
ABS
Polyamides
Polycarbonate
Acrylics
Polystyrenes, low
and medium impact
Thermosets
Paper
and cotton base
Homopolymers
Fiber glass,
and graphitized
Asbestos base
Cutting Speed, V
m/s
Milling tool per tooth
Feed, ft, mm/tooth Depth of cut, mm
2.5 - 3.8
2.5 - 3.8
3.8 - 5.0
2.5 - 3.8
2.5 - 3.8
2.5 - 3.8
3.8 - 5.0
3.8 - 5.0
2.5 - 3.8
2.5 - 3.8
0.4
0.4
0.1
0.4
0.4
0.4
0.1
0.1
0.4
0.4
3.8
3.8
1.5
3.8
3.8
3.8
1.5
1.5
3.8
3.8
2.0 - 2.5
2.0 - 2.5
2.0 - 2.5
2.0 - 2.5
2.0 - 2.5
2.0 - 2.5
0.12
0.12
0.12
0.12
0.12
0.12
1.5
1.5
1.5
1.5
1.5
1.5
Idle Energy
Energy-consuming peripheral equipments included in idle power (Pidle) are
shown in Figure MR3.6. In the machining praxis it is known as “run-time mode” (Abele
et al., 2005). The average idle power (Pidle) of automated CNC machines is between 1,200
and 15,000 watt*. (* This information is from the CNC manufacturing companies, see
Appendix 1). The handling power characterizes the load case when there is relative
movement of the tool and the work-piece without changing the shape of the body (e.g.
rapid axis movement, spindle motor, coolant, tool changer) - Handling.
The idle time (tidle) is the sum of the handling time (thandling) and the milling time
(calculated above as tmilling, equation 2 or 4), see Figure MR3.5. For CNC milling
machines, the handling times are the air time of cutter moving from home position to
approach point, approach, overtravel, retraction after milling, and traverse, if needed to
other cut in the same work piece. Approximate Handling time will vary from 0.1 to 10
min. We can calculate the idle times and energy as follows.
Idle time = [timehandling + timemilling]
(11)
13
Cutter moves from the home position to the approach point at vertical traverse
rate, VTR (peripheral) or horizontal traverse rate, HTR (face milling) and it can be
defined as the air time1. This distance would be in the range of 10 to 30 mm. During the
milling process, the total travel of the cutter is larger than the length of the workpiece due
to the cutter approach and overtravel distances and this time can be defined as air time2.
The approach and overtravel distances can be assumed to be 5 to 10 mm, enough for the
cutter axis to clear the end of the part. During this time the cutter moves with the constant
feed rate. Thus in actual practice, we add the cutting length to the length of approach and
length of overtravel for the calculation of idle time. These approach and overtravel
distances are at the feed rate, fr. After reaching the overtravel point, the mill retraces back
to an offset position, but at a faster rate called the vertical traverse rate, VTR (peripheral)
or HTR (face).
Time for handling is
Air time1 + Approach/overtravel times + retraction times = thandling
(12)
To this idle time must be added the time to traverse to the next cut (if needed) and
this is (cut spacing)/traverse speed, as given by the CNC manufacturer. The example
given later in this uplci lists such traverse speed data for use in any representative milling
scenarios.
From these calculations the idle energy for a single cut is
E (Joule/cut)idle = [thandling + tmilling]* Pidle
(13)
Thus with just the information used in calculating tmilling, and the representative
idle power (1,200 – 15,000 watts), one can calculate the idle energy for this milling unit
process.
Basic Energy
The basic power of a machine tool is the demand under running conditions in
“stand-by mode”. Energy-consuming peripheral equipment included in basic power are
shown in Figure MR3.6. There is no relative movement between the tool and the workpiece, but all components that accomplish the readiness for operation (e.g. Machine
control unit (MCU), unloaded motors, servo motors, pumps) are still running at no load
power consumption. Most of the automated CNC machine tools are not switched off
when not drilling and have a constant basic power. The average basic power Pbasic of
automated CNC machines is between 800 and 8,000 watt* (* From CNC manufacturing
companies the basic power ranges from 1/8th to 1/4th of the maximum machine power,
(see Manufacturers Reference Data in Appendix). The largest consumer is the hydraulic
power unit. Hydraulic power units are the driving force for motors, which includes chiller
system, way lube system, and unloaded motors.
From Figure MR3.5, the basic time is given by
Tbasic = tload/unload + thandling + tmill
where thandling + tmill = tidle as determined in equation 11.
(14)
14
An exhaustive study of loading and unloading times has been made by
Fridriksson, 1979; it is found that these times can be estimated quite accurately for a
particular machine tool and work-holding device if the weight of the workpiece is known.
Some of Fridriksson, 1979 results are showed in Table MR3.4, which can be used to
estimate machine loading and unloading times. For milling representative work-holding
devices are vise, clamps, magnetic table, parallels, V-blocks and rotary table etc. To these
times must be added the times for cleaning the workholding devices etc.
Thus the energy for loading and unloading is given by
Basic energy, ebasic = [timeload/unload + timeidle ]*Pbasic
(15)
Where timeidle is given in earlier sections and timeload/unload is from Table MR3.4. Pbasic is
in the range of 800 to 8,000 watts.
Thus the uplci user must add some reasonable value from Table MR3.4 for the
load/unload times and can then use the timeidle to determine the Basic energy
Table MR 3.4. Sum of the Loading and Unloading Times (sec) versus Workpiece weight
(Fredriksson, 1979) (load and unload times are assumed equal)
Workpiece Weight
Holding Device
0-0.2
0-0.4
0.2-4.5
0.4-10
4.5-14
10-30
14-27 (kg)
30-60 (lb)
Crane
Angle Plate
27.6
34.9
43.5
72.1
276.5
Between Centers,
with dog
25.6
40.2
57.4
97.8
247.8
15
Between Centers,
no dog
13.5
18.6
24.1
35.3
73.1
Chuck, universal
16.0
23.3
31.9
52.9
--
Chuck, independent
(4 jaws)
34.0
41.3
49.9
70.9
--
Clamp on table
(3 clamps)
28.8
33.9
39.4
58.7
264.6
Collet
10.3
15.4
20.9
--
--
Faceplate
(3 clamps)
31.9
43.3
58.0
82.1
196.2
Fixture, horizontal
(3 screws)
25.8
33.1
41.7
69.4
274.7
16
Fixture vertical
(3 screws)
27.2
38.6
53.3
--
--
Hand-held
Jig
1.4
25.8
6.5
33.1
12.0
41.7
---
---
Magnet table
2.6
5.2
8.4
--
--
Parallels
14.2
19.3
24.8
67.0
354.3
17
Rotary table or
Index plate
(3 clamps)
28.8
36.1
44.7
72.4
277.7
“V” Blocks
25.0
30.1
35.6
77.8
365.1
Vise
13.5
18.6
24.1
39.6
174.2
In summary, the unit process life cycle inventory energy use is given by
Etotal = Pbasic * (tbasic ) + Pidle * (tidle) + Pmilling * (tmilling)
(16)
This follows the power diagram in Figure MR3.5. With only the following information
the unit process life cycle energy for milling can be estimated.
1. material of part being manufactured
18
2. volume material removal rate
3. milling time
4. Table MR3.4
B. Method of Quantification for Mass Loss
The waste streams in milling process, identified with the associated process
performance measures, are depicted in the Figure MR3.11 below.
Milling


Cutting fluid mist
Dust (dry machining)

Chips, worn tools

Spent cutting fluids
Gas/Aerosol
Waste
Stream
Solid
Liquid
Figure MR3.11. Waste Streams in milling process
Lci for Material Mass Loss Calculations
The workpiece material loss after milling a cross sectional area can be specified
as chip mass (ms). Metal chips are accumulated, and cutting fluid is separated from these.
The chip mass (ms) can be calculated by multiplying the volume of material removed
(Vremoval) by the density of the workpiece material ρ.
Density of the material can be attained from the material property list as shown in Table
MR3.2, kg/m3.
Volume of the material removed for a cross sectional area = Vremoval  L * w * d [mm3] (17)
Where
L = Length of the workpiece in mm,
d = depth of cut in mm.
w = width of cut in mm.
Chip mass (ms) = Vremoval * ρ * (1 m3/1 E+09 mm3) [kg]
(18)
Lci for Cutting Fluid Waste Calculations
For milling operations, cutting fluids can be used to allow higher cutting speeds,
to prolong the cutting tool life, and to some extent reduce the tool - work surface friction
during machining. The fluid is used as a coolant and also lubricates the cutting surfaces
and the most common method is referred to as flooding (Wlaschitz and Hoflinger, 2007).
Table MR3.5 shows the recommended cutting fluid for milling operation. Cutting fluid is
constantly recycled within the CNC machine until the properties become inadequate. The
19
dilution fluid (water) is also supplied at regular intervals due to loss through evaporation
and spillage.
Table MR3.5. Cutting fluid recommendations for milling operation
(Hoffman et al., 2001)
Material
Aluminum
Alloy Steels
Milling (most of these
cutting fluids are
aqueous suspensions)
Soluble Oil ( 96 percent
water) (or) Mineral seal
Oil (or) Mineral Oil
10 Percent Lard oil with
90 percent mineral oil.
Brass
Soluble oil (96% water)
Tool steels and Low carbon
Steels
Soluble oil
Copper
Monel Metal
Cast iron
Soluble Oil
Soluble Oil
Dry
Malleable Iron
Soluble Oil
Bronze
Magnesium
Soluble Oil
Mineral seal Oil
The service of a cutting fluid provided to one CNC machine tool for one year was
considered as the functional unit. It is assumed that the number of parts produced per unit
time will not vary depending on the cutting fluid replacement. The milling time
associated with one year of production was based on the schedule of 102 hr of
milling/week for 42 weeks/year from one of the most comprehensive cooling fluid
machining studies (Andres et al., 2008). From (Andres et al., 2008) a single CNC
machine using cutting fluid required an individual pump to circulate the fluid from a 55
gallon (208L) tank to the cutting zone. The 208L/machine is recycled within process until
it is disposed of after two weeks. Assuming cutting fluid is used 204 hr/ 2 weeks, then the
cutting fluid loss is 208L/ (204*60) per minute. Which is 0.017 L/min or about 17 g/min
as the effective loss of cutting fluid due to degradation. The coolant is about 70wt% - 95
wt% water, so at 85wt% water, the coolant oil loss is 15wt% or 2.5 g cutting oil/min.
20
With the machining time for milling a cross sectional area the mass loss of coolant oil can
be calculated.
There is also be a fugitive emissions factor here that could account for aerosol
losses. Wlaschitz and Hoflinger (2007) measured aerosolized loss of cutting fluid from a
rotating machining tool under flooding conditions. For a cutting fluid use of 5,700 g/min,
the aerosol oil loss was about 0.0053 g/min and water loss of 0.1 g/min. Other losses
from spills and carry off (drag-out) on workpieces were not included at this time.
Lci for Lubricant Oil Waste Calculations
Lubricant oil is mainly used for a spindle and a slide way. Minute amount of oil is
infused to the spindle part and the slide way at fixed intervals. From the CNC
manufacturing companies it is found that lubricant oil is replaced only 2-3 times of the
life of the machine. It is assumed that the life of the machine is around 20 years. Since it
is negligible lubricant oil loss is not considered for this study.
Cutting Tool usage
Milling processes often require regular replacement of cutting tools. The tool life
is a time for a newly sharpened tool that cuts satisfactorily before it becomes necessary to
remove it for regrinding or replacement. Worn tools contribute significantly to the waste
in the form of wear particles and a worn tool at the end of tool life. The wear particles
usually are carried away by the cutting fluid. From an environmental perspective the
cutting tools remaining at the end of the tool life are of importance as they are often
disposed off and hence are a burden to the environment. The worn tool can be identified
by the process performance in terms of the cutting forces, energy consumed, and surface
finish. For simplification regrinding of the tools are not considered.
Case Study on Milling
In this report we analyze the detailed energy consumption calculations in milling
process. The machining process is performed on Jeenxi Technology 4-axis CNC machine
(JHV – 1500). The machine specifications are listed below:
Table MR3.6. Specifications of JHV – 1500 CNC Machine
Model
TRAVEL
X axis Travel (mm)
Y axis Travel (mm)
Z axis Travel (mm)
Distance from the table to spindle nose (mm)
TABLE
Table dimensions, mm
Max. load of table (kg)
SPINDLE (rpm)
JHV - 1500
Liner
1500
750
700
120 – 820
1650 x 750
1000
8000
21
Spindle Taper
Spindle Speed (rpm)
Spindle Drive
Spindle Motor (kw)
Spindle Cooling
FEED RATE
Rapid Traverse (X,Y) (m/min), HTR
Rapid Traverse (Z) (m/min), VTR
Cutting Feed rate (mm/min), fr
3 Axes motor output (X, Y, Z) (kw)
A.T.C
Magazine Type
Tool Magazine Capacity (pcs)
Max. Tool Diameter (mm)
Max. Tool Length (mm)
Max. Tool Weight (kg)
Tool Selection
OTHER
Maximum Power Consumption (KW)
Floor Space (L x W x H)
Machine Weight (kg)
BT - 40
8000, 10000
Belt type
7.5 / 11
BT - 40
10000, 12000, 15000
Direct type
7.5 / 11
Oil Cooler
30
24
1 – 15000
4.0 / 4.0 / 7.0
Carosel
16
100 / 150
300
7
Fixed type
Arm
24
80 / 150
300
7
Random
30
4100 x 2640 x 2810 mm
11000
Product Details:
For this example we are assuming an aluminum alloy as the work piece. The work
piece is a rectangular block of dimensions 500 mm x 100 mm x 60 mm (L x H x W). The
objective of the study is to analyze the energy consumption in milling cross section of the
cut (w*d) with a 150 mm diameter cutter with depth of cut of 3mm. The product
dimensions are shown in Figure MR3.12. From the dimensions and the density from
Table MR3.2, the weight of the workpiece is 8.14 kg.
Figure MR3.12. Dimensions of the Work piece
22
Cutting Parameters
The machining conditions and the cutting parameters are listed in Table MR3.7.
Table MR3.7. Cutting Parameters for Example Case
Cutting Conditions
Cutter Diameter (D)
150 mm
Cutting Speed (V), Table MR3.2
120 m/min
Feed per tooth (ft ), Table MR3.2
0.381 mm/tooth
Spindle Speed (N) = V/πD
255 rpm
Number of teeth
10
Feed rate (fr) = ft *N*No. of teeth
971.55 mm/min
Depth of cut (d)
3 mm
Width of cut (w)
60 mm
Volume removal rate (VRR) = w* d *fr
174,879 mm3/min
Rapid Traverse (horizontal, X,Y)
30
(m/min)
Rapid Traverse (vertical, Z) (m/min)
24
Machining Process:
Before milling on the work piece in a CNC machine, it is important to set the coordinate axes of the machine with respect to the work piece. The direction along the
length and breadth are taken as positive X and Y axis respectively. The vertical plane
perpendicular to the work piece is considered as the Z-axis. During the machining
process the tool is considered to be at an offset of 25mm above the work piece. Every
time while milling the tool comes down from a height of 25mm to the approach distance
8mm from the workpiece and after cutting with 8 mm overtravel. After it reaches to the
overtravel distance it goes back to the home position at traverse speed. The feeds and
speed are stated in Table MR3.7.
Time, Power and Energy calculations
The total processing time can be divided into the 3 sub groups of basic time, idle
time, and milling time.
Milling Time:
The time for milling of a cross sectional area of width of cut 60mm and depth of cut 3mm
is determined by
tmilling = (L+2Lc)/fr
(min)
Where L is the length of the workpiece mm, fr is the feed in mm/min, and Lc is the
distance for the first contact by the cutter with the workpiece in mm.
Lc = w( D  w) = 73.48mm
Time for milling a cross section cut will be,
tmilling = (500+(2*73.48))/ 971.55
= 0.67 min/cut = 40 sec/cut
23
Machining Power for each cut,
pmilling = MRR * Specific cutting energy
VRR from Table MR3.7 = 174,879 mm3/min and specific cutting energy, Up, from Table
MR3.2 = 0.98 W/mm3/sec
pmilling = 174,879 * 0.98/60 = 2.86 kW
Tip Energy required per cut is emilling = pmilling * tmilling = 2.86 * 40 = 114.4 kJ/cut
Handling Time:
Time required for the cutter to move from home position to approach point (25mm) is
essentially milling in air. The air time of the rapid traverse speed to approach is
ta1 = 25/ (traverse speed)
ta1 = 25/ 24000 mm/min
= 0.001 min = 0.06 sec (neglect)
After reaching the approach distance 8mm from the workpiece it reaches the workpiece
at feed rate, fr (971.55 mm/min) and after cutting the workpiece cutter travels overtravel
distance of 8mm. When not cutting the workpiece, is the approach plus overtravel
distance,
(approach + overtravel)/fr
ta2 = (16)/971.55 mm/min
= 0.02 min = 1.2sec
Retract time ta3 = (10 + 500 + 10)/24000 = 0.022 min = 1.3 sec
Idle power of the machine can be calculated based on the individual power specifications
of the machine.
Pidle = Pspindle + Pcoolant + Paxis
The assumed values are
Pcoolant = 1 kW (~1.5 hp); Pspindle = 4 kW (~5 hp); Paxis = 5 kW (~7 hp)
(These assumed values are from the CNC manufacturing companies, see Appendix 1)
To convert a horse power rating (HP) to Watts (W) simply multiply the horsepower
rating by 746
Idle power for the process is
Pidle = Pspindle + Pcoolant + Paxis
=4 + 1 + 5
= 10 kW
Total Idle time for cut t idle = ta + tmilling = 1.2 + 40 + 1.3
= 42.5 sec
Total Energy during the idle process is,
eidle = Pspindle * tidle + Pcoolant* tidle + Paxis*tidle
= 10*41.3
= 425 kJ/mill
24
Load/unload Time:
The total basic time can be determined based on the following assumptions for this
example:
 The workholding device used for clamping the workpiece is a simple vise, Table
MR3.4 giving a total time of 24 sec.
 The time required to mount the work piece on the vise manually is thus to be 12
sec.
 After completing the milling process on a single workpiece, the machine is
cleaned using pneumatic cleaners or air blowers. The time required to clean the
machine is assumed to be 0.4 min (25 sec).
 The machined part has to be removed manually from the fixture. The time
required to remove the material from the fixture is assumed to be 12 sec.
Therefore, basic processes time for this study is,
Tb = loading time + cleaning time + unloading time
= 12 + 25 + 12
= 49 sec
Basic power of the machine can be assumed as the 25% of the machine maximum in the
manufacturer specifications. Therefore the power consumed during the basic process is,
Pbasic = 7.5 kW
Energy consumed during this process is,
Ebasic = Pbasic * ttotal
The basic time for the process can be taken as the sum of idle time (which contains
machining time) and load/unload times, i.e.
Tbasic = Tb + tidle
= 49 + 43
= 92 sec
ebasic = 7.5* 92 = 690 kJ per cut
Total Energy required for milling can be determined as,
eprocess = emilling +eidle + ebasic
=114.4 + 413 + 690
= 1,217.4 kJ/ cut
Power required for machine utilization during milling is,
Pmtotal = eprocess / ttotal
= 1,217.4/93 = 13.09 kW.
Lci Material mass loss calculations
Volume of the material removed for a given crossectional area = Vremoval  L * w * d
[mm3]
= 90,000 mm3
Chip mass (ms) = Vremoval * ρ [kg]
ms = 90,000 * 2712 * 10-9
= 0.244 kg/cut
25
Lci for Cutting fluid waste calculations
From (Andres et al., 2008) a single CNC machine using cutting fluid required an
individual pump to circulate the fluid from a 55 gallon (208L) tank to the cutting zone.
The 208L/machine is recycled within process until it is disposed of after two weeks.
Assuming cutting fluid is used 204 hr/ 2 weeks, then the cutting fluid loss is 208L/
(204*60) per minute, which is 0.017 L/min or about 17 g/min. The coolant is about 96
wt% water, so at 96wt% water, the coolant oil loss is 4wt% or 0.68 g cutting oil/min.
Milling time per cut tm = 40 sec
Mass loss of the coolant = 0.68*40/60 = 0.45 g cutting oil/cut
Summary:
This report presented the models, approaches, and measures used to represent the
environmental life cycle of milling unit operations referred to as the unit process life
cycle inventory. The five major environmental-based results are energy consumption,
metal chips removed, cutting fluid, lubricant oil, and cutting tool. With only the
following information the unit process life cycle energy for drilling can be estimated.
1. material of part being manufactured
2. volume material removal rate
3. milling time
4. Table MR3.4
The life cycle of milling is based on a typical high production scenario (on a CNC milling
machine) to reflect industrial manufacturing practices.
References Cited
1. Abele, E.; Anderl, R.; and Birkhofer, H. (2005) Environmentally-friendly product
development, Springer-Verlag London Limited.
2. Clarens, A.; Zimmerman, J.; Keoleian, G.; and Skerlos, S. (2008) Comparison of
Life Cycle Emissions and Energy Consumption for Environmentally adapted
Metalworking Fluid Systems, Environmental Science Technology,
10.1021/es800791z.
3. Dahmus, J.; and Gutowski, T. (2004) An environmental analysis of machining,
Proceedings of IMECE2004, ASME International Mechanical Engineering
Congress and RD&D Expo, November 13-19, Anaheim, California USA.
4. Erik Oberg. (2000) Machinery’s Handbook, 26th Edition, Industrial Press.
5. Fridriksson, L. Non-productive Time in Conventional Metal Cutting, Report No.
3, Design for Manufacturability Program, University of Massachusetts, Amherst,
February 1979.
6. George, F.S; and Ahmad, K. E. (2000) Manufacturing Processes & Materials, 4th
Edition, Society of Manufacturing Engineers.
7. Groover, M.P. (2003) Fundamentals of Modern Manufacturing, Prentice Hall.
8. Hoffman, E.; McCauley, C.; and Iqbal Hussain, M. (2001) Shop reference for
students and apprentice, Industrial Press Inc.
9. http://www.engineeringtoolbox.com/metal-alloys-densities-d_50.html
10. http://www.mapal.us/calculators/milling/CalculatorMilling.htm
11. Joseph R. Davis. (1989) Machining Handbook, Vol. 16, American Society for
Metals international.
26
12. Kalpakjian, S.; and Schmid, S. (2008) Manufacturing Processes for Engineering
Materials, 5th Edition, Prentice Hall.
13. Piacitelli, W.; Sieber, et. al. (2000) Metalworking fluid exposures in small
machine shops: an overview, AIHAJ, 62:356-370.
14. Schrader, G.; and Elshennawy, A. (2000) Manufacturing Processes & Materials,
4th Edition, Society of Manufacturing Engineers.
15. Terry, R.; and Erik, L. (2003) Industrial Plastics: Theory and Applications, 4th
Edition, Cengage Learning.
16. Todd, R.; Allen, D.; and Alting, L. (1994) Manufacturing processes reference
guide, Industrial Press, New York.
17. Wlaschitz, P. and W. Hoflinger. (2007) A new measuring method to detect the
emissions of metal working fluid mist, Journal for Hazardous Materials,
144:736-741.
Appendices
Manufacturers Reference Data
The methodology that has been followed for collecting technical information on
CNC machines has been largely based in the following:
The documentation of the CNC machine and the technical assistances collected
from the manufacturing companies through internet. Several interviews with the
service personnel of the different CNC manufacturing companies have been carried
out. After collecting the information from the different companies it has been put
together in the relevant document that describes the different approaches the different
companies have regarding the technical information on the CNC machines.
Telephone conversations allowed us to learn more about basic power and idle power.
Companies that involved in our telephone conversations are Bridge port, Fadal, Hass
and Jeenxi. These companies’ manufactures different sizes of CNC machines, but this
report shows the lower, mid and highest level of sizes. For our case study we picked
machine at the highest-level.
Specifications
Model Number
Spindle Speed
Spindle Drive
Spindle Motor
Rapid Traverse (X,Y)
Rapid Traverse (Z)
Cutting Feed rate
3 Axes motor
output(X,Y,Z)
JEENXI TECHNOLOGY
JHV – 850
JHV – 1020
JHV – 1500
8000 rpm
8000 rpm
8000 rpm
Belt/Direct type
Belt/Direct
Belt/Direct type
5.5/7.5 kw
7.5/11 kw
7.5/ 11 kw
30 m/min
30 m/min
30 m/min
20 m/min
20 m/min
24 m/min
1 – 15000 mm/min
1 – 15000 mm/min
1 – 15000
mm/min
1.8/ 1.8/ 2.5
1.8/ 1.8/ 2.5
4.0/ 4.0/ 7.0
27
Power Consumption
20 KVA
20KVA
Specifications
Model Number
Spindle Speed
Spindle Drive
VF- 7
7500 rpm
Belt/Direct type
HAAS
VM - 2
12,000 rpm
Inline direct drive
75 ft-lb@1400
250 ft-lb@ 450
20 hp
75 ft-lb@1400
30 hp
MDC
7,500 rpm
Direct speed belt
drive
75 ft-lb@1400
20 hp
3400 lb
600 ipm
600 ipm
500ipm
200 – 250 VAC
380 – 480 VAC
3,400 lb
710 ipm
710 ipm
500 ipm
200 – 250 VAC
380 – 480 VAC
2,500 lb
1,000 ipm
1,000 ipm
833 ipm
200 – 250 VAC
380 – 480 VAC
VMC – 850
8000 rpm
4000/7000 rpm
590.55 ipm
472.44 ipm
236.22 ipm
7.5/ 10 hp
a12, a12, a12
20 KVA
KAFO
VMC – 137
8,000/10,000 rpm
4000/7000 rpm
787.4 ipm
787.40 ipm
393.7 ipm
15/ 20 hp
a22, a22, a30
25 KVA
VMC - 21100
6000/8000 rpm
4000/7000 rpm
393.7 ipm
393.7 ipm
393.7 ipm
15/20 hp
a30, a30, a30
35 KVA
Max Torque
With Gearbox
Spindle motor max
rating
Axis Motor max thrust
Rapids on X-axis
Rapid on Y & Z Axes
Max Cutting
Power
Consumption(min)
Specifications
Model Number
Spindle speed (Belt)
Spindle speed (Gear)
Rapid Traverse (X, Y)
Rapid Traverse (Z)
Cutting feed rate
Spindle drive motor
X,Y,Z axis drive motor
Power consumption
Specifications
Model Number
Spindle Speed(Belted)
Fanuc Motor Power
Heidenhain Motor
Power
Spindle Speed(Directly
coupled)
Fanuc Motor Power
Heidenhain Motor
Power
Rapid Traverse (X,Y)
Rapid Traverse (Z)
Cutting Feed rate
XR 760
9000/15000 rpm
25/25 hp
28/28 hp
BRIDGE PORT
XR 1270 HP
-
40 KVA
XR 1500 HPD
-
15000 rpm
15000 rpm
30 hp
33 hp
40 hp
34 hp
375 – 7500 rpm
(Gear Box)
40 hp
40 hp
1692 ipm
1417 ipm
787 ipm
1417 ipm
1417 ipm
787 ipm
1417 ipm
1417 ipm
787 ipm
28
Power
30 KVA
40 KVA
Specifications
Model Number
VMC 4020
FADAL
VMC 6030
10 - 10,000 rpm
Automatic
Mechanical Vector
Drive
900 ipm
700 ipm
600 ipm
10 hp
80 – 120 psi
10 - 10,000 rpm
Automatic
Mechanical Vector
Drive
400 ipm
400 ipm
400 ipm
14.7 hp
80 – 120 psi
TTC-630
4000 rpm
15/20 KW
2.8 KW
2.8 KW
TTC
TMC 500
6000
5/7 KW
15000 rpm
1 KW
40 hp
XR 1500 HPD
375 – 7500 rpm
(Gear Box)
40 hp
12.6 KW
197 mm/min
630 mm/min
40 KW
1.1 KW
34 hp
1417 ipm
1417 ipm
787 ipm
40 KVA
40 hp
1417 ipm
1417 ipm
787 ipm
40 KVA
Spindle Speed
Spindle Drive
Rapid Traverse (X,Y)
Rapid Traverse (Z)
Cutting Feed rate
Motor Power
Air Pressure Required
Specifications
Model Number
Spindle Speed(Belted)
Spindle Motor Power
X Axis Motor Power
Z Axis Motor Power
Coolant Pump Motor
Power
ATC Motor Power
Rapid Traverse (X,Y)
Rapid Traverse (Z)
Total Driving Power
Hydraulic Pump
40 KVA
VMC 6535
HTX
6000 rpm
Automatic
Electric Vector
Drive
900 ipm
700 ipm
600 ipm
29.5 hp
80 – 100 psi
29