Download Grinding in metallic materials – Study the influence

Grinding in metallic materials – Study the influence of cutting velocity
Isabel Maria Abreu Madeira de Faria
October 2007
Abstract. Metal cutting processes have an important role among other fabrication
processes, due to capacity to generate almost any complex geometry with good tolerances and
surface finishing. The machining processes such as turning, milling, drilling and others
distinguish by their industrial and economic importance. This interest has motivated many
research and development on these processes. Unfortunately, some of these processes have
been paid less attention in teaching and investigation on universities, like grinding. The main
goal of this investigation consists in analyze the influence of some parameters, such as cutting
velocity in grinding forces. It was studied other parameters like depth of cut, cutting mode and
the material influence in the process. The experimental work was carried out with a specially
designed a numerical control grinder with forces instrumentation by the author. This equipment
was developed and install on IMTT Lab (Industrial Machine tools Training Laboratory) of STM
(Secção de Tecnologia Mecânica). It was developed a program based in LabView for control
and monitoring of the process. The experimental tests allow identifying the principal impact of
selected parameters. It is important the determination of velocity value which minimize grinding
forces. This aspect has relative importance on grinding of slim components. And another issue,
like the influence of the workpiece material on forces evolution and its related problems, was
also cover by the present work.
Keywords. Grinding, Cutting velocity, Machine tool, Numerical control, Forces
measurement
Introduction. Grinding is the most common word when we think in abrasive metal
cutting. Nowadays, we could live on Stone Age if the pre-historic men didn’t find a way to
sharpen their tools, scratching on the stone. Four thousand years ago, Egyptians discovered
the grinding process in metals, and it was when metallurgy begun. Since that, many
developments have been done on machine tool design and its components. On XIX century
appear the first machine tools powered by steam. Later, steam was replaced by electric energy.
Though, it was during the two World Wars that occurs a considerably progress on machine
tools. At the present time, grinding is responsible for 20 to 25% of expenses in machining
operations on industrialize countries. One of the main problems with industrial application of
metal cutting is related with the difficulty to optimize the principal parameters. It is usually the
manufacture quality change with the material, quantity mechanic and geometric characteristics
of cutting tools, but the most important, with some parameters such as cutting velocity or chip
thickness. The aim of this work is to evaluate the influence of the cutting velocity in grinding
processes. It is an objective too develop an equipment to experiments, that allow realize cutting
experiments and monitorship grinding forces.
Theoretical background. Grinding processes have the objective of reducing the
roughness or improving the surface quality by removing the marks left by other operations such
as drilling, turning or milling, using the abrasive grains. Forces measurement and grinding
power is an efficient way to identify the interaction wheel-material. Forces developed during the
process, can be separated on tangential and normal components (refer to figure 1). The force
tangent to the wheel-work contact, when multiplied by wheel velocity and a constant,
determines the power consumed by the operation. The tangential and normal forces are related
by the friction coefficient; therefore, in production grinding, in which the normal force is almost
unknown (without a dynamometer of some type), the tangential force can be calculated from
power measurements and used as an alternative for the normal force.
FQ
FP
Figure 1 – Forces diagram
There are two types of velocity in this process: work velocity v (gives by the table move
velocity in plain grinding) and grinding wheel velocity V. When both are on the same way, the
process is called as down-cutting and the surface quality is improved relatively to up-cutting,
lowering tool wear and improving grinding rates. Grinding forces are lower and consequently the
specific energy requested [1]. For the same conditions, the increase of grinding wheel velocity
leads to a reduction of mechanic strength on grains. At the same time, the temperature
becomes higher, because more heat is generated.
Experimental background. This section, presents the experimental develop that were
necessary to study cutting velocity influence on grinding forces.
Materials and cutting tool. The cutting tool used in this process is a grinding wheel.
These wheels are cutting tools whose body has a revolution form, and are composed by
abrasive grains of hard and brittle substances, connected by another substance called bond.
Gaps between grains and without bond are pores.
Grinding wheel selected to conduct the experiments, had the following characteristics
code 4A46Q4VB7 and had 13 mm of diameter. The mean of this code is the following:
4 – Fabricant symbol
A – Abrasive type (aluminum oxide)
46 – Grain size (medium)
Q – Hardness (relatively hard)
4 – Structure (compact)
VB – Bond type (V – vitreous; B – resinous)
7 – Wheel identification for manufacturer
The materials of specimens were AISI 1045 CD steel and AA6009-T6 aluminum alloy.
The main characteristics are represented on the Table 1 and Table 2:
AISI 1045 CD
Chemical Components
Component
Main Properties
%
Density (g/cc)
7.85
C
0.42 – 0.5
Vickers Hardness
188
Fe
98.51 – 98.98
Yield Strength (MPa)
530
Mn
0.6 – 0.9
Elongation (%)
12
P
Max 0.04
Elasticity modulus (GPa)
205
S
Max 0.05
Poisson coefficient
0.29
Table 1 – Steel properties
AA 6009-T6 Aluminum Alloy
Chemical Components
Main Properties
Component
%
Density (g/cc)
2.71
Al
95.7 – 98.7
Vickers Hardness
102
Cu
0.15 – 0.6
Yield Strength (MPa)
320
Mg
0.4 – 0.8
Ultimate Strength (MPa)
340
Si
0.6 - 1
Elongation (%)
12
Fe
Max 0.5
Elasticity modulus (GPa)
69
Others
Max 0.15
Poisson coefficient
0.33
Table 2 – Aluminum properties
Experimental apparatus. The experimental apparatus of this work consists on a plain
grinder with numerical control. This machine tool is also designed to measure the grinding
forces, allowing the assessment cutting velocity influence, and also to assure the necessary
accuracy of the different experimental grinding parameters. The apparatus is structured on
three different groups: the machine tool, the numerical control software, and the measuring
system.
The initial idea to the structure of the plain grinder was building two modulus: a vertical
component and a horizontal component, a machine with two defined axis. The horizontal
component should allow the movement in xx axis. For that purpose it was used step motor
which was fundamental to provide the horizontal movement. This step motor was tested using a
computer program based on programming language LabView 8.0. The next step was building
the body of the structure.
The main body of the machine-tool in C structure has been building in heavy material to
confer more stability. It was added some servo-mechanisms, ventilators, the bi-dimensional
charge sensor. The result of this development is represented in figure 2.
Rotation velocity 6000 to 31000 rpm
Sensibility x and y on charge
X axis course 100mm
sensor 10mV/V
Y axis course 150mm
Step motors actuation
X movement accuracy 0.01 mm
Power 12W
Y movement accuracy 0.01 mm
Feeding Monophase
X axis velocity 50 to 1200 mm/min
Control Open Loop
Y axis velocity 50 to 1200 mm/min
Control software LabView
Evaluate forces capability
Maxim charge on X axis 100kg
Maxim charge on Y axis 150kg
Figure 2 – Machine-tool for plain grinding
To set the machine tool in motion, it was necessary to create a digital circuit that
reproduces the right sequence to boost the internal coils of step motors. For this proposal, it
was necessary test them with a bread board and create a software to control them.
However, it is necessary a circuit that receives the control logic from the computer and
turn on or off each coil. To execute this function, it was chosen an octal high voltage, high
current Darlington transistor array, the ULN 2803 (refer to figure 3). The eight NPN Darlington
connected transistors in this family of arrays are ideally suited for interfacing between low logic
level digital circuitry, such as TTL (transistor-transistor logic) and the higher current/voltage
requirements of lamps, relays, printer hammers or other similar loads for a broad range of
computer, industrial, and consumer applications. All devices feature open–collector outputs and
free wheeling clamp diodes for transient suppression.
Figure 3 – ULN 2803 Darlington transistor
The device that was use to data acquisition was NI-6023E PCI from National
Instruments manufacturer. This is ideal for applications ranging from continuous high-speed
data logging to control applications to high-voltage signal or sensor measurements when used
with NI signal conditioning. This DAQ (data acquisition) device includes the following features
and technologies:
• 16 analog inputs at up to 200 kS/s, 12 or 16-bit resolution
• Up to 2 analog outputs at 10 kS/s, 12 or 16-bit resolution
• 8 digital I/O lines (TTL);
• Two 24-bit counter/timers
• Digital triggering
• 4 analog input signal ranges
Creation phases and final setting of this digital circuit are represented on figure 4.
a)
b)
c)
Figure 4 – a) Printed circuit; b) Digital circuit; c) Final setting of digital circuit
Measure of the process consists essentially on observation of grinding forces with
software help. For this purpose, the bi-dimensional charge sensor is fundamental and is used in
mechanical strength measure. This sensor gives normal and tangential forces. Its function uses
two Wheatstone bridge made of four strain gauges each one. The strain gauge is a transducer,
which converts measure size through a physical effect of a change in conditions. One of the
Wheatstone bridge measures normal force, and the other bridge measures tangential
component of grinding forces. This sensor has been designed for the expected range of
grinding forces. The final scheme of data acquisition is depicted on figure 5.
Figure 5 – Representative scheme of measuring
Experimental plan. The experimental work was planned to accomplish the main objectives of
this work. It consists in make six experiments for each grinding mode in each cutting velocity.
The vertical advance is executed for each two passages. Data acquired by the equipment
correspond to grinding forces (cutting and penetration forces). Table 3 traduces the
experimental plan.
Case
Material
Cutting
velocity
Passages
Lubrification
(m/min)
Up
Grinding
(CCA)
Down
Grinding
(CSA)
Aluminum
264
AA 1100
539
-
814
Steel
1089
AISI 1045
1363
6
Don’t
Penetration
between
passages
(µm)
Horizontal
table
velocity
(mm/min)
10
400
Table 3 – Experimental plan
Results and Discussion. Cutting velocity has direct influence on cutting power, on
thermal losses in chips, in cutting tool and in grinding forces. The main goal of this work is
evaluate how cutting velocity influences grinding forces. It is also study, how cutting mode has
influence in grinding process, and a comparison between different materials, by evaluating the
grinding forces.
Influence of cutting velocity. The figures 6 and 7 represent the evolution of grinding
forces in function of cutting velocity. The present test in steel does not distinguish cutting mode.
log F (N)
3
2
1
200
A
B
C
1000 D
1100
log v (m/min)
Figure 6 – Evolution of cutting force in function of cutting velocity
log F (N)
5
4
3
2
1
200
A
B
C
1000
D
log v (m/min)
1100
Figure 7 – Evolution of normal force in function of cutting velocity
As noticed, normal forces are higher than tangential forces. On the first stage, occurs a
reduction of forces, as predicted in literature, with the increase of velocity. At the contrary of
tangential, normal force continue to decrease until a cutting velocity about 800 m/min. After this
minim normal force suffer a sudden increase, which is explained by the generated heat and by
radial impact, influencing normal forces and less the tangential forces, because radial impact
occurs in radial direction.
Influence of the grinding mode. The following figures represent the evolution of grinding forces
as a function of cutting velocity, for each cutting mode.
log F (N)
Down-grinding
Up-grinding
A
B
C
log v (m/min)
D
Figure 8 – Evolution of cutting force in function of cutting velocity in both grinding modes
log F (N)
Down-grinding
Up-grinding
log v (m/min)
A
B
C
D
Figure 9 – Evolution of normal force in function of cutting velocity in both grinding modes
As it is predicted, grinding mode doesn’t have much influence in grinding forces.
However, grinding forces are a little bit smaller in down-grinding than up-grinding. The similarity
of both forces can be explained by the difference between cutting velocity and working velocity.
Actually working velocity is almost insignificant compared with cutting velocity ( V >> v w ).
Influence of the workpiece material. The following figures represent the evolution of
grinding forces in function of cutting velocity, for two different workpiece materials.
log F (N)
Aluminum
Steel
A
B
C
D
log v (m/min)
Figure 10 – Evolution of cutting force in function of cutting velocity for both materials
log F (N)
Aluminum
Steel
log v (m/min)
A
B
C
D
Figure 11 – Evolution of normal force in function of cutting velocity for both materials
It was expected that grinding forces in aluminium will be smaller, however, they are
much higher than grinding steel. In spite of comportment are similar, the magnitude of
aluminium tangential force are above of steel tangential force. What can justify so drastically
increase is the tendency to gather in pores of grinding wheel, enlarged with mentioned radial
impact. The same factors increase normal forces.
Conclusions. The experimental results agree with theoretical research, however with
some justified variants. It was noticed that grinding forces changes with a complex behaviour
due to interdependence of grinding parameters. Even so, it is possible to find an optimal cutting
velocity that minimizes drastically the grinding forces. The material of specimens has also an
important role. For a harder material, higher is the tendency to gather in pores of grinding
wheel. This, traduces on a drastically increase of normal forces. Variations found on
theoretically predicted and experimental results can be justified not only by vibration on the
developed machine tool, but also by radial impact of the wheel as it contact with the specimen,
and the temperature increase, result from the heating of cutting zone for absence of a cutting
fluid.
References.
[1] Tawakoli Taghi, “High efficiency deep grinding”, VDI – Verlag GmbH, London
(1993);
[2] Shaw M. C., “Principles of Abrasive Processing”, Clarendon Press, Oxford, (1996);
[3] Sebenta de Tecnologia Mecânica I – Associação de estudantes do Instituto Superior
Técnico, Secção de Folhas 1993/1994;
[4] Malkin, S., B.S., M.S., Sc.D., Grinding Technology – Theory and Applications of
Machining with Abrasives, Elli Horwood Limited, Chichester, (1989).