Optimized Chipping Processes with a New Mechatronic Tool System

Application of Strain Gauge Sensors and Piezoelectric Actuators

Franz Haas

, Elke Nuspl

and Manfred Pauritsch

Department of Automation Technology, FH CAMPUS 02, Körblergasse 126, A-8010 Graz, Austria

TCM International Tool Consulting & Management GmbH, Technologiepark 3, A-8510 Stainz, Austria

Keywords: Chipping Technology, Mechatronic Tool System, Condition Monitoring, Piezoelectric Actuators.

Abstract: The paper introduces a new measurement device which allows collecting the chipping force in three

directions and additionally the process temperature. The apparatus consists of a specially designed tool

holder with integrated strain gauges, the electronic measuring equipment and the evaluation software.

During the first period a new device for research and development purposes has been designed, experiments

on innovative materials are presented including the comparison with simulation results. The next step of the

project incorporates the product development based on the previous experience with targets accuracy, cost

effectiveness, reliability and the possibility of active damping of tool vibrations. To achieve the last

mentioned demand a piezoelectric actuator is integrated into the tool shank and works against the vibration

forces.

1 INTRODUCTION

Chipping technologies (e.g. turning, milling or

grinding) are still of high importance in the field of

industrial production. For instance both combustion

engine and electrically driven automobiles need

many parts that have to be produced or finished by

chipping processes because of accuracy and cost

effectiveness. Some of the current developments in

this field are listed below:

 Increasing number of parts made of innovative

materials (e.g. titanium, composites) for

lightweight constructions;

 High temperatures caused by hard machining

and dry processing;

 General demand of optimized cutting

parameters and less vibration;

 Hybrid machine tools characterized by

integration of several techniques (e.g. turning

and laser heat treatment or milling and friction

stir welding).

Therefore the demand to integrate mechatronics

into the tool holders is evident. Preconditions are the

robust and simple sensor and/or actuator application

at acceptable prices.

2 STATE OF THE ART

This chapter gives a summary of useful basics in the

area of test methods and represents an overview of

new cutting tools and cutting materials.

Concerning the measurement of cutting forces

two different principles can be divided. Most of the

laboratory devices use a dynamometer based on the

piezoelectric effect. The sensor delivers electric

charges that are transported to a charge amplifier

through an insulated cable. The amplifier converts

the charges into an output voltage (Kistler, 2009).

As standard a special platform with four sensors is

used to enable a multi-component measurement of

forces and torques. In the case of chipping the

orthogonal components F

, F

and the drive

torque M

at the main spindle are of special interest.

Turning as a process with a rotating workpiece

and tool inserts of defined geometry represents the

typical example to measure the orthogonal process

force components.

Strain gauge and piezoelectric force

measurement devices are presented in (Audy, 2006)

with a detailed discussion and classification of the

error sources. It is pointed out that a two-component

metal machining dynamometer equipped with strain

gauges doesn´t have satisfying long term stability

without any compensation because of temperature

614

Haas F., Nuspl E. and Pauritsch M..

Optimized Chipping Processes with a New Mechatronic Tool System - Application of Strain Gauge Sensors and Piezoelectric Actuators.

DOI: 10.5220/0004044006140617

In Proceedings of the 9th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2012), pages 614-617

ISBN: 978-989-8565-21-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

effects on the strain gauges.

A small three-component dynamometer with

circular holes is described in (Tani, 1983). The

linearity of output is excellent without hysteresis.

The calibration of all three directions can be carried

out with low interaction between measurement

directions.

3 LABORATORY DEVICE

3.1 Mechanics and Sensors

The mechatronic tool is constructed similar to a

standard tool holder consisting of a shank and the

cutting part with the carbide metal inserts. In

comparison with standard tool holders the fixation of

modules with different insert geometries is enabled.

The basic measurement principle is very simple

and can also be utilized in low cost products (Haas,

2011). The tool shank has the function of a beam in

bending as the result of cutting and feed force

components (see Fig. 1).

Figure 1: Dynamometer with strain gauges.

The very small deflections cause output signals

of each bridge circuit with four strain gauges to

compensate for temperature effects. Concerning the

third axis the deformation near a rectangular hole is

picked up by additional strain gauges, but the signal

is small in relation to the others and calibration is

much more difficult.

The FEM analysis (Fig. 2) of the mechatronic

tool is helpful to estimate the strain values at the

virtual model to select the suitable strain gauge

types.

Another benefit of simulation is the optimization

of stiffness to ensure that distortion und stresses are

below the limits.

The force components F

, F

and F

can be

calculated according to the equations (1) to (3). The

specific force values (kc1.1, kf1.1, kp1.1) are

Figure 2: X-component of strain (F

= 1200 N).

multiplied with the width (b) and thickness (h) of the

chips. The exponent (m

c-1) considers the decreasing

amount of the specific forces with higher values of

(h) in the case of cutting force.

= k

c1.1

b h

()

(1)

= k

f1.1

b h

(



)

(2)

= k

1.1

b h

()

(3)

To complete the test equipment it is necessary to

measure the temperature at the deformation zone

simultaneously. In this case a dynamic test shows

the direct consequence of tool wear with regard to

the process forces and to the thermal load that tool

insert and work piece have to withstand. Especially

thin and lightweight parts are sensitive to higher

temperatures, distortion and failure costs may be the

results. Dry processing and hard machining accept

higher temperatures to reduce the costs for cooling

lubricants and to avoid environmental pollution.

These machining strategies require thermal test

results to detect the speed limits.

Therefore an infrared temperature sensor with a

range of 200°C to 1000°C is integrated into the

measuring and evaluation concept. The temperature

level is able to be influenced by location and

intensity of heat sources, the heat conductance value,

the dimensions of heat dissipation parts, time

interval of direct contact and at last external cooling.

3.2 Electronics

The interface electronics fulfils functions that

exceed commercially available equipment. Four

voltage signals can be processed as a multichannel

measurement, three bridge circuit values from the

dynamometer and the temperature sensor output.

The main item on the printed board (see Fig. 3)

is the analogue digital converter with a sampling rate

of 70 kHz and a resolution of 24 bit.

In relation to commonly used boards for strain gauge

bridges the new design enables the highly dynamic

digitalization of the analogue signals.

As electronic protective method a high frequency

OptimizedChippingProcesseswithaNewMechatronicToolSystem-ApplicationofStrainGaugeSensorsand

PiezoelectricActuators

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Figure 3: Interface electronics, PCB-layout.

noise filter and an over-voltage protection are

integrated per channel. The measuring board

consists of a USB connector to communicate with

the PC. By default the communication is done by a

WLAN module with 2.4 GHz transmission

frequency, therefore it is also possible to access the

device worldwide per internet. This feature ensures a

high reliability of the remote data transfer even

under rough conditions on the factory floor. In

addition an external analysis or remote maintenance

is possible.

In the case of turning operation tests can also be

triggered by indication at the main spindle. A

microcontroller is responsible for internal

measurement control and communication.

Configuration and status information are displayed

on a small screen.

3.3 Calibration and Errors

During calibration exactly weighted loads are

applied to the dynamometer. As consequence each

mass causes deformation and defined output

voltages at the bridge circuits. One calibration curve

per direction must be calculated as a result of output

voltages, the applied masses and calibration setup

dimensions. Possible device errors are caused by

long term and temperature drift, nonlinearity of

strain gauge signals. As basic requirements of

measurement calibration has been finished and

comparisons with other systems and with calculated

force results has been done

4 MEASUREMENT RESULTS

The new device offers benefits for a lot of

applications, for example research and development

as well as large-scale production or education. The

independent comparison between measuring results

and chipping process simulation illustrates a

sufficient compliance.

4.1 Turning Process

First straight turning measurements are presented.

To achieve reliable results system calibration should

be performed before data collection.

Figure 4 shows force values and the process

temperature for hardened steel machining, which

becomes more and more important for economic and

sustainable production. In this case the dominant

factors are heat production, thermal resistance of

inserts and efficient cooling.

As a result of cutting depth, cutting speed and

feed velocity, the temperature rises up to about

260°C as a constant condition (see Fig. 4 at the right

axis).

Figure 4: Straight turning of hardened steel (F

f,.

T).

The following diagram (Fig. 5) introduces results

of stainless steel machining.

Figure 5: Straight turning of stainless steel.

One of the promising materials of lightweight

constructions is titanium, but it is difficult to be

machined with chipping processes. Figure 6 shows

the increase of the measurement values after the tool

starts with machining. The alternating temperature

profile is caused by discontinuous chip formation.

ICINCO2012-9thInternationalConferenceonInformaticsinControl,AutomationandRobotics

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Figure 6: Cutting force, feed force and temperature,

straight turning of titanium alloy with carbide inserts.

Infrared camera pictures and videos have been

shot to validate the temperature results and to get an

impression of the whole thermal field (Fig. 7).

Figure 7: Infrared picture of titanium machining.

It should be pointed out that satisfying

correlation between the thermal radiation result (see

Fig. 6) and the infrared method could be achieved

(maximum temperature of about 250°C).

4.2 Simulation

Optimization of chipping processes based on

measurements should be extended to simulation. The

target of this project (Blaha, 2010) is the FEM

simulation of turning operations for three materials

(C45, X2CrNiMo17-13-2 and Ti6Al4V).

As the most important simulation assessment criteria

cutting forces have to be analysed and compared

with measurements mentioned above.

The required tribologically parameters have to be

determined by special tests (Horwatitsch, 2007). The

comparison of measurement versus simulation has

shown a satisfying compliance in the case of

numerical integration of the local cutting forces

along the comma-shaped chip (Blaha, 2010).

Depending on the material the deviation varies

between 2 and 12 percent considering the cutting

force results.

5 CONCLUSIONS

The triangle relationship between workpiece, cutting

tool and machine tool finally determines the

economic production and always requires new

solutions for each particular case, which can only be

found as a result of chipping tests and process

monitoring.

The paper presents a measuring device for

laboratory purposes as well as a mechatronic tool for

industrial production. The idea of the laboratory

device is to pick up all necessary process data for

optimization purposes. In the case of the

mechatronic product low cost and robust design are

of higher importance. This principle is not limited to

specific manufacturing techniques, it is generally

applicable and represents a significant module to

fulfil the targets of intelligent production.

REFERENCES

Audy J., 2006. An appraisal of techniques and equipment

for cutting force measurement, In Journal of Zhejiang

University, p. 1781-1789.

Blaha, J., Horwatitsch, D., (et al.), (2010). Endbericht

F&E Headquarter Strategy, TCM International Tool

Consulting & Management GmbH, p. 7-28.

Haas, F., Nuspl, E., 2011. Präzises, schnelles Messgerät

für mobiles Condition Monitoring von Zerspa

nungsprozessen, In AALE 2011, p. 161-169.

Horwatitsch, D. (et al.), (2007). Ermittlung der Reibung

zur Verwendung in einer FE-basierten Prozess-

simulation eines Zerspanungsprozesses, In

Proceedings ÖTG Symposium 2007, p. 45-52.

Kistler (Ed.), 2009. Präzise Messsysteme für die

spanabhebende Fertigung, p. 7-9.

Tani, Y., Hatamura, Y., Nagao, T., (1983). Development

of Small Three-component Dynamometer for Cutting

Force Measurement, p. 657-658.

OptimizedChippingProcesseswithaNewMechatronicToolSystem-ApplicationofStrainGaugeSensorsand

PiezoelectricActuators

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