temperature has a very important role in machine tool behaviour. Temperature is
the physical property measured by sensing technology.Measurement system
principle can be placed into two categories contact measurement and non-contact
cameras are 24 widely used to measure temperature
distribution on the surface of the machine tool. Body which has temperature non
zero emits IR radiations dpending upon its temprature. This is called
characteristic radiation. This happens because of temperature of of body results
in internal molecules motion. Since the molecule movement represents charge
displacement, electromagnetic radiation (photon particles) is emitted. These
photons travels at the speed of light and behave according to the known optical
principles. They can be deflected,
focused with a lens, or reflected from reflective surfaces
Non-contact temperature measurements show a
high dependency on the emission characteristics and the reflectivity of the analysed surface. The relationship between
reflection and transmission is given by
temperature sensors measure their own temperature. One concludes the temperature
of the body to which the sensor is in contact by supposing or knowing that the
two are in thermal equilibrium, that is, there is no heat transfer between
measurement error sources exist, when assumptions are made.It is very difficult
measure the temprature of surface specially moving surfce by direct contact. It
is wise to be cautuious when perfoming experiments of taking measuments.
Thermocouples 20have two different
metal at theirs sensing ends. Voltage is generated when temperature gradient
occurs between hot sensor element and cold reference junction. Variation in
voltage notice as temperature through seebeck effect. The seebeck effect says
that temperature gradient is linearly proportional to voltage and connected through
coefficient of material used in
Figure 2–7. Thermocouple construction 20
Resistance thermometers are also known as resistance temperature
detectors, or RTDs. They are constructed using
one metal and material property of that material is function of temperature.
The accuracy of resistance thermometers will be high if metal used inside has linear
relation with temperature, such as platinum. By using this linear relationship resistance
of material can be found out and temperature can be measured. 20
of thermo-Mechanical errors of machine tools
There are a lot of different methodologies have been
developed to the model thermo-elastic behaviour of the machine tool in order to
compensate thermal errors. In general, methodologies can be classified into two
Phenomenological model constructs a relationship between
input parameters (e.g. Temperature) and an output value (e.g TCP displacement).
Experiments are carried out at different loads and results with respect to time
are observed by regression model (RM). Other methods like neural networks (NN)
and Fuzzy logic (FL) for compensation also listed in phenomenological models.
Physical modeling approach simulates thermally induce errors
distinguished, in temperature distribution and distortions, in order to
calculate TCP dislocation and enable real-time compensation. All considerations
are based on physical laws.
FEM models and FDM models approaches are part of physical modeling.
of Thermally induce errors:-
A lot of people have presented different methods to reduce
thermal errors, put into net shell these methods can be classified into three
categories according to thesis
temperature fluctuations: for example by cooling or controlled environment
condition as well as minimum heat generation
Reducing thermal sensitivity: reducing the sensitivity
of machine tool structural loop to temperature changes
Compensation of errors: for example by mean of mathematical
the temperature variations:-
Temperature variations can be minimized by reducing the
masses of machine tool structure41,thesis 594, applying cooling to a machine tool, use of oil
shower, through air.
By trying to create even temperature distribution thermal
error can be reduced of machine tool structure. Much lower the temperature
difference will be lower the thermal error present.
The temperature gradient can be reduced by minimizing heat
generated in elements of machine tool. P sekler et al 41 of thesis illustrate thermal error can be reduced by sizing
down the masses of machine tool structure. This usually applies to construct
energy efficient machine tools but also it also helps in reducing the losses
occur in machine tool. With smaller masses less energy is required to move them
result in smaller losses and lower temperature on machine structure.
The most common approach implemented widely in industry is to
apply cooling to machine tool. Some approaches based on try to remove the
excess heat generated in machine tool elements. One of the approaches 40 is
to design special cooling element for the spindle. These cooling tubes try to
make us of Coanda-effect. Working principle of Coanda effect, fluid passage out
from nozzle creating a primary stream. Temperature control of air in a
lithography application is shown in 42. Compensation using oil shower is used in 43 and 44.
Another advantage of using oil shower is that insulation from fluctuations from
in room temperature.
Various methodologies to reduce thermal errors that does not
directly reduce the temperature gradient on machines but modifies it, is practice
of heating and cooling elements. It can be seen during application of
compensation methods to machine (47,48,49). In order to reduce tool center point
displacement key elements of machine tool either can be heated or cooled. For
special cases feed drives are used to for reduction of angular errors on three
of thermal sensitivity:-
Other than temperature gradients approach thermal error of
machine tool can also be reduced by minimizing the sensitivity of elements to
temperature changes. Meaning of this machine tool design in this way that large
deformations do not occur. This can be achieved by applying thermo-symmetrical design to machine tool. In
50, thesis 5946 boundary conditions are applied to headstock
of lathe in such a way that center of axis does not move during the thermal expansion.
Thermal deformation on machine tools 51 present a methodology according to that non-sensitive machine can be design in such way that specific directional
thermal expansion do not affect that workpiece accuracy.
material for compensation of thermal displacement:- (Thermal issues page 782)
Material optimization can be effective in reducing the
thermal errors in machine tool. Alternative materials like carbon fiber
reinforced plastic (CFRP) has negative linear expansion coefficient can be used
to compensate thermal displacement of machine component which have positive
linear expansion coefficient such as aluminum.
Another example thermal distortions due to local temperature
gradients can be reduced by using polymer concrete in machine tool bed. Achieveable
reduction is upto 30%. (thermal
isuue page 783)
Active compensation using adaptronic
devices:- (thermal issue 783)
CFRP structure are used for active compensation of angular
displacement of main spindle of housings and heating of unidirectional carbon fibre
In an adaptronic system, negative thermal expansion of CFRP-
structure compensate the thermal displacement. Thermal sensors, controllers,
and CFRP actuators make possible controlled heating of CFRP laminate by heating filaments and
In general, Thermal displacement can be estimated in two
classes of methods: Direct compensation and indirect compensation. Process
chain of thermal deformation
Direct method uses touch probes to compensate error, for that
machine has to be stopped during an operation to take measurements, the big
drawback of direct methodology, ultimately productivity reduce.
On other hand indirect measurement reduce downtime by active
compensation. The indirect approach uses temperature measurement to calculate
TCP displacement with help of mathematical models. The most common model used for are described
of thermal error compensation based on linear and nonlinear regression:-
Regression model is applied for error compensation, it defines
a relationship between dependent and several independents values. In case of
thermal errors temperature in specific machine tool points are independent
variable and dependent variable are TCP displacement. It is active compensation
method which means without disturbing the machine process errors can be
calculated. The hindrance with indirect compensation is the installation of measuring
system is very costly.
The drawback of RA is selecting positions for temperature
sensors if too many positions are taken it will increase the cost if few than
the accuracy of the solution will be compromised.
based on neural networks:- 8
Using Neural network approach for thermal error compensation
is a common practice. Feedforward networks are used for thermal error
compensation, temperature probes act as input. Neural network approximates the
TCP dislocation relying on the temperature of machine tool. Input and output
layers act as input and output buffer for temperature measurement of machine
and machine thermal errors respectively. Layers in between them are called
hidden layers. The working principle of these to suppress the noise.
Each input is multiplied by the interrelated weight. All of
these weighted inputs are summed up and combined with a threshold to find out
activation level of the neuron.
Compare to ANN and RM a lot of others models are in practice.
In 7, thesis 5946 lumped capacitance method is used to
calculate the temperature distribution of the machine tool. To do that thermal
behavior knowledge will be needed because one must know which parts of machine
can be lumped and how to apply proper boundary conditions for lumped bodies. A
series of temperature is used and TCP dislocation is computed by stress-free
theory and rigid body kinematics.
With advancement in computer field and accessibility of
models, e.g FEM models, new reduction procedure can be developed. Denkena et
al, 42 thermal issues 785
applied FEM to calculate thermal deformation of machine tool in steady state
versus load profile. During operation TCP
displacement are compensated with a linear model comparing temperature
measured on machine tool structure with those computed n steady state. FDEM
approach is endorsed fot real-time compensation of machine tools 141 thermal issues 785.
Unknown boundary condition, simulation-based model, Volumetric TCP
displacement, use of thermal location and components errors as correction
A mixture of FEM and FDM
used in 76 a transient thermal analysis has been
performed using FDM and TCP displacements has been compensated using FEM.