Prediction of Tool Fracture in Drilling

inprocess frequency

-multi sensor systems evoluotion in

time domain

domoin

E. Brinksmeier (2); lnstitut fur Fertigungstechnik und Spanende Werkzeugmaschinen, Universitat Hannover Received on January 16,1990

-intelligent

This paper descr ibes a new tool sensor for d r l l l i n g based on the in-process measurement of torque a t the d r i l l shank. An eddy current sensor is used, the output s ignal of which is independent. from the dis tance of the sensor to the d r i l l shank within a range of operation between 0 . 2 - 0 . 5 m. In addition to the measurement of the average torque which indicates the machining parameters, tool fracture can be predicted because a high dynamic s i g n a l can be de- tected short ly before the breakage. Another advantage of t h i s sensor is that its i n s t a l l a t i o n can e a s i l y be per- formed a t nearly every machine tool.

Keywords: Drilling, tool fracture , torque, sensor, eddy current

1. Introduction

The use of modern CNC-controlled machine tools in unmanned shifts requires monitoring and control sy- stems in order to recognize disturbances in the pro- cess and to prevent collisions and damage of tools and workpieces (1). With respect to sensors which produce the basic information for the signal proces- sing, much successful work has been carried out du- ring the last years (1-4). Therefore R+D-work is now mainly focussed on the signal conditioning, modelling and on the development of supervising strategies (fig. 1). This is necessary in order to entirely use the full potential of unmanned machining.

In case of driven tools, however, the developments have not yet reached the high monitoring standard compared with turning. This is true due to the limi- ted accessibility of rotating tools for measuring devices. Therefore, the task to supervise rotating tools can at first be reduced to the question which type of sensor can be used for tool monitoring. The- refore, this paper introduces a new type of sensor which measures the torque of rotating tools; without mechanical contact and in-process.

based systems models

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Fig. 1: Struture of a supervising system (1)

2. Monitorinq of drilling

Since approximately 40 Z of all cutting operations are drilling operations, the monitoring of drills is an important task with a high degree of economical relevance (4). This is true, especially for the use of twist drills which are mostly used in a diameter spectrum between 1 mm and 20 mm. However, the tasks of tool monitoring may be very different dependent on the machine tool, the machining operation and the type and number of workpieces to be machined. For example, the supervision of a machining center with a magazine of more than 100 tools and a large variety

of workpieces to be machined is rather different from the supervision of a multiple-spindle drilling ma- chine.

In spite of the significant improvements in the qua- lity of twist drills and the development of special purpose machine tools for drill manufacturer, many problems related to drill performance still exist: one such problem being the relatively large scatter in tool life. Evaluation criteria for the tool life may be

- tool wear - tool fracture - poor bored hole quality.

In addition the presence of the drill has to be supervised in some applications. This can be achieved by the use of optical or tactile sensors during idling (5). Also uncontrolled and poor chip flow may disturb the drilling operation. Christoffel (6) found out that this is true, if the edge of lands plays an active role in the cutting process.

For the majority of drill monitoring tasks any in- formation on tool wear and criteria for the predic- tion of tool fracture are of major interest. The large variety of wear phenomena occuring at twist drills is shown in fig. 2.

I

Fig. 2: Wear of twist drills

It is evident that the wear characteristics cannot be measured directly in-process. Therefore, only indi- rect measuring methodes can be applied. Basically, the process forces, the power consumption of the driving motor, the drill torque and vibration pheno- mena are qualified measuring quantities for this task.

Various investigations showed that the drill torque is a recommended measuring quantity (6-8). However, a

Annals of the CIRP Vol. X!/l/lSW 97

corrclat ion ket.wci.n the wear And the average torque cannot be found ~n qeneral because a superposition of the individrial YP'I~ charac-tcri:;tirs may a s w ~ l l increase or decrease the average torque. In addition, land wear may qenerate friction induced heat which leads to a radial CJrOXth of the drill and thus to stick slip effects between the land and the borehole surface. This .Gill cause vibrations and chatter. Therefore, the monitoring of the dynamic torque is strongly recommended ( G , 9 ) . The problen is however, to xeasure relevant dynamic quantities and vibrations with a sufficient uide bandwidth. A s the transfer function of the machine exhibits a sharp low pass characteristic, dynamic torque components cannot be measured within driving shafts of the gear box, nei- ther can they be derlved from the motor current. Therefore, torque 2nd feedinq force measurements ha- ve been realized by using force measuring bearings

prepare5 clutches installed to the rain spindle or the drill ( 6 ) . k strong disadvantage of these methods is that the Installation of the sensors interferes w i t h the machine tool desiqn which makes a supplamentary installation rather complica- ted.

An advanced sensor system has been introduced by h'eck and FurbaB (7). They integrated a piezo-electric torque sensor into the tool c!amping system and pro- vided for a wireless transmission of the data. Tn order to measure the drill dynamics, acoustic emis- sion sensors are being used (1,2,3,11,12). lhe basic prob!em xith aco!ist.ic e~ission is, nowever, ro define a qualified measurir:g position and to achieve an adequate signal-to-~oise ratio.

From the staf~:-of-rhe-~i-t. in monitorinq of twist drills, tne fol 1ok:ing coric1iir;ions can be derived: - There is no sensor syster: available which measures

the drill t x r y u e without. interfering with the static and dynamic behaviour of the machining system and the workpiece.

- There is no system which measures the drill torque dynamics up to a kHz-region.

3 . Eddy current torque sensoL

This paper describes the application of an eddy cur- rent sensor tor the measurement of drill torque dy- namics. The measuring position was chosen at the drill shank because this is very close to the cutting region.

During drilling, the shank is loaded mainly by tor- que. This results :n pairs of principal stresses (tensile and conprzssive) under an angle of 45 de- grees to the center line. The stresses have their maximum amount at the shank surface and will intro- duce resultant strains in the drill material. A s

these strains will change the micromagnetic proper- ties of the material, a measurement of torsional stress can be reduced KO the neasurement of magnetic properties at the surface and in the principal stress direction of the twisted shaft.

An eddy current sensor developed by Winterhoff and Heidler (13) can be used €or this task. Actually the sensor in an advanced development of the torque transducer which was first introduced in 1960 (14). The sensor consists of an operating winding and four receiving windings which are fixed synmetrically around the exciting coil (fig. 3 ) .

The coils are in mutual action with the shaft mate- rial which is thus involved into the magnetic circuit via an air gap betreen coils and shaft surface. The receiving coils are switched in an anti-parallel

bridge circuitry in order to fully utilize the per- meability changes in both directions of the principal torsional stresses. Any changes of torque will lead to a mistuning of the measuring bridge. It has to be mentioned, however, that this type of sensor is very sensible with respect to changes of the air gap, or in other words to changes of the distance between the coils and the measuring shaft. This problem was sol- ved by different electronic measures (15) which now allow a change of distance between 0.5 and 1.0 mm by a maximum variation of the measuring signal of only 4 % . Fig. 4 shows the functional block diagram of the sensor system.

The cylindrical sensor design has a diameter of 20 mm which will be reduced in future. The exciting fre- quency is 100 kHz which allows the evaluation of torque dynamics.

I - I

display P I

rectifier

phase shifter

amplitier oscillator W P

excitation I Fig. 4 : Functional block diagram of the sensor system

4 . Calibration tests

At first, calibration tests were necessary in order to find out the sensor sensivity and to evaluate the influence on the drill material. It is widely known that the measuring result of eddy current sensors depends strongly on the material properties to be measured.

For statical calibration purposes a torsional loading fixture was designed and built which allows the pre- loading of drills by a defined torque (9). The sensor was positioned close to the drill shank by a distance of 0.5 mn. Various high speed drills from the same delivery were investigated in the calibration tests. Fig. 5 presents the results of the test drills. The output voltage of the sensor system shows a propor-

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tional dependence on the loading torque and the tor- sional stress respectively. As the drills were taken from the same delivery the differences between the individual output voltages are rather low. The te- sting of drills from other manufacturers lead to very similar results. The level of the output volta- ges may, however, be different due to the magnetic properties of the individual drills.

HSS-twist dr li drll: no.i

0 20 LO

3 : 2 3 L 5 N m 6 torsional s t r e s s T

torque M, . - . . , . , ,

Fig. 5: Calibration curves with HSS drills

5. Experimental results

Further experiments were carried out by investiqating rotating drills during the cutting operation and du- ring idling. A MAHO-milling machine and high speed twist drills (DIN 3 3 8 ) were used for the experiments. The workpiece material was grey cast iron and a quenched and tempered steel. Fig. 6 illustrates the drill clamping and the provisional mounting of the torque sensor. Durinq the basic investigations, an inductive pulse generator was attached to the spind- le for triggering purposes.

.. Fi- . Experimental set-up

Tcterici GG 20 too. HSS - tw:s t drill ,0 10nn DIN 338.

n z SO0 l lmi I

Fig. 7 shows the unfiltered output voltage of the torque sensor during idling and cutting of a blind hole. At first, a repetitive change of the output voltage can be seen, which depends on the tool angu- lar displacement. This change of voltage underlines the sensivity of the sensor which actually measures the circumferential magnetic properties of the drill shank. A correlation was found between the voltage variation and residual surface stresses of the shank. This confirmes that the shank exhibits non-uniform material properties which result from the drill ma- nufacturing process.

The average voltage level is changed during cutting whereas the angular voltage variation remains un- changed. It was proved by further investigations that a change of cutting speed and feed w i l l influence the amount of torque. Therefore, a supervision of the actual machining parameters can be simply realized by averaging the output signal of the torque sensor.

twist drill '99 0 10 mm GIN338

w z - k p e c e 12 C- M o S L V n . 700 Vmin ' 2 018 mm _ _ .

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0 9 18 27 mm36 riJ' or11 ing depth

Fiq. 8: Torque dynamics until tool fracture

However, the major subject of this work was the mea- surement of torque dynamics during the entire opera- ting time of twist drills. The work should answer the question whether tool failure can be predicted by measurement of the drill torque dynamics. A typical result from various investigations is outlined in fig. 8 . A 10 mm twist drill was used for the drilling of blind holes into a quenched and tempered steel. The low pass-filtered signal indicates that there are no process distrubances up to the drilling of the 337th hole.

When drilling is continued high level voltage changes of the sensor output indicate a superposition of both, periodically and stochastically recurring torque portions to the average torque. After drilling of two further holes tool failure happens by fractu- re. Due to the low corner frequency of the analysing electronics, these dynamic portions cannot be attri- buted to the natural vibration frequency of the drill. It is much more supposed that tool wear has proceeded and the drilling operation is disturbed by sticking and break-off phenomena of produced chips. Nevertheless, a prediction of tool failure was safely possible under these machining conditions.

If a high pass filter is used, additional information can be obtained from the sensor signal. Fig. 9 shows a significant change in the higher frequency torque dynamics between the first hole and the 340th hole. Thus it is also possible to predict tool fracture by evaluating the higher frequency content of the drill torque. The presence of torque dynamics may be used to interrupt the machining operation.

' 0 too! angdor displccenci t rod 2n' 5 c,,-,-a

Fig. 7: Measuring signal from the drill shank

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workpmece tool

L2 Cr Ma S L V tw st drill 0 lOirm 0 U 338 n = 700 llmm t = 0 18mm '0 Hz 7 gh p c s s f Ite-

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Fig. 9: Torque dynamics

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Fig. 10: Analysis of torque dynamics In our experiments, torsional vibrations of the drill were measured in a frequency range of 400 - 500 Hz (fig. lo). This is confirmed by investigations from Himburg (16) who found a correlation between the stiffness of the machining system and the torsional vibrations in a range between 100 and 500 H z .

6 . Evaluation Cf-the res.ults and conlusion

It is shown with this work that static and dynamic portions of torsional stresses can be obtained from the shank of a rotating tool by using an eddy current sensor. The sensor signal can be used to monitor the process parameters and to predict the tool fracture. The major advantage of the torque measurements at the tool shank is the proximity to the cutting region. Therefore, the mass of the main spindle and the dam- ping characteristics of the driving unit are of minor importance for the measuring results. A further advantage of the specific sensor is its high fre- quency resolution which enables the user to analyse torque dynamics up to the kHz region. As a matter of fact, this sensor can simply be installed subse- quently to nearly any existing machine tool. As the sensor is not expensive, every tool for example from a multiple drilling unit can be supervised. In addi- tion to the supervision of drills, end mills and threading tools can also be supervised by the use of such sensor (17).

7. References

Tonshoff, H.K., Wulfsberq, J.P., Xals, H.J.J., Koniq, W., van I.uttervelt, C.A., Developments and Trends in Montitoring and Control of Machining Processes, Annals of the CIRP, 3 7

(1988) 2, P . 622-622

Tlusty, J., Andrews, G.C., A Critical Review of Sensors for Unmanned Machining, Annals of the CrRP, 32 (1983) 2, p. 563-572

Hayashi, S.R., Thomas, C.E. Wildes, D.G., Tool Break Detection by Monitoring Yltrasonic Vi- bration, Annals of the CIRP, Val. 37/1/1988

NedeR, C., Himburg, T., Automatisierte Uber- wachung des Bohrens, VDI-Z 128 (1986) 17, p. 651-657

Rogel, E . , Prozehberwachung bei Bearbeitungs- zentren, VDI-2 125 (1983) 18, p. 723-731

Christoffel, K., Werkzeuquberwachung beim Bohren und Frasen, Dr.-Ing. Diss. RWTH Aachen, 1987

Weck, M., FurbaB J . P . , Zerspansensoren fur die Werkzeuguberwachung, 1nd.-Anzeiger 37 (1987), p . 10-15

Schapermeier, E., VerschleiB prozefjbegleitend messen, Werkstatt und Betrieb, 122 (1989) 10, p. 862-866

Brinksmeier, E., Beruhrungslose Drehmomentmes- sung mit maqnetischen Verfahren, Sensor- Maqazin (1989) 2, p. 20-23, (1989) 3 , p. 14-17

Lechler, G., Z u r Werkzeuguberwachung beim Bohren und Drehen, VDI-2 130 (1988) 2, p. 39-41

KOnig, W., Kutzner, K., Schehl, U., Korper- schall als Basis der ProzeRuberwachung, 1nd.- Anzeiger (1989) 11, p. 18-21

Teti, R . , Micheletti, Tool Wear Monitoring through Acoustic Emission, Annals of the CIRP 3 8 (1989) 1, p. 99-102

Winterhoff, H., Heidler, E.A., Beruhrungslose Drehmomentmessung mit Wirbelstromsensoren, Technisches Messen tm 50 (1983). p. 461-466

Dahle, O., Torque Transducer, US-Patent 4135391 (1960)

Winterhoff, H . , Verfahren zur Seruhrungslosen Messung statischer und dynamischer Drehmomente, German Patent DE 3206008 c1 (1983)

Himburg, T., Die Dynamik des Bohrvorgangs als Grundlage eines Uberwachunqskonzepts, Dr.-Ing. Diss. TU Hamburg-Harburg, 1988

ronshoff, H.K., Brinksmeier, E., Verfahren zur Uberwachung des Arbeitens spanabhebender, rotierender Werkzeuge, Patent DE 3642130 A 1 (1989)

Future work will address the development of fast evaluation strategies for the sensor signals and a miniaturization of the sensor design in order to adapt it to small drills.

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