Monitoring of Grinding Wheel Wear

E. Brinksmeier (2), F. Werner; IFW, University of Hannover Received on January 17,1992

A new, optical sensor has been developed in order to monitor the cutting capacity of grinding wheels. This sensor is based on the principle of triangulation. For the investigation of the grinding wheel topography even during the grinding process the sensor has been mounted at the opposite side oi the working area of an external grinding machine. The output signals of the sensor are processed by a digital data acquisition system controiling the position of the sensor and calculating different parameters to describe the state of wheel wear. These parameters take various aspects of macroscopic and microscopic wheel wear into consideration. Some results which have been obtained using a corundum grinding wheel are presented . These results were achieved during the rotation of the wheel at cutting speed.

Keywords: Cylindrical Grinding, Data Acquisition. Monitoring, Optical Measurement. Wear

Introduction

The quality of ground workpieces depends to a great extent on the grin- ding wheel topography, which changes during the tool life time between two dressing cycles. In most cases dressing cycles have to be carried out without any information about the actual wheel wear /T685/. In com- mon, grinding wheels are dressed without reaching their end of tool life in order to prevent workpiece damages like e.g. workpiece burning.

Grinding wheel topography plays an important role in characterization of grinding processes. Therefore it is necessary to develop a method for identifying the wheel surface condition. If the efficiency of this method is good enough for measuring grinding wheel wear in process the results might be used for investigations of grinding mechanism as we!l as for adaptive control applications for grinding processes.

Established methods of measurement

In the last decades there were many attempts to identify the cutting per- formance of grinding wheels in order to correlate the results with machined workpiece quality. All measuring methods for characterization of the grin- ding wheel surface can be classified in static and dynamic measuring me- thods. The most important static measuring methods are the taperprint /Nak73/ and the stylus method /Op72,K676/, which measure all grains and cutting edges on the wheel surface.

Using the taperprint method the wheel surface is printed on paper. A polished spherical plastic ring is supported in bearings. Between the ring and grinding wheel white paper and a strip of thin carbon paper is placed. During rotation of the grinding wheel, the grinding wheel surface is printed on the white paper. During evaluation, each darkened area on the paper surface is regarded as a static cutting edge. From the print made this way the number of grains per unit area can be obtained. Repeated prints of the same area at varied depths of indentation give the topography of invidual grains.

Another way to obtain information about the cutting surface of a grinding wheel is to trace its cutting profile along the circumference. Each high point will he regarded as a cutting edge. An average value over traced length gives the number of static cutting edges per unit length. The principle of such a measuring system is based on the technology of surface roughness measurement.

Both measuring methods have the disadvantage of insufficient accuracy in the depth of grinding wheel surface. A coxnparison of results from dif- ferent authors while investigating the topography of the same grinding wheel shows a significant discrepancy /Ver77/. Another disadvnatage of both methods is the extreme time consumption for measuring and calcu- lating the topographic parameters. Furthermore the analysis of all cut- ting edges for characterization of cutting performance of grinding wheel didn't lead to the desired result. Due to the relative motions of wheel and workpiece in the contact area, only a few static cutting edges in the wheel surface will be active during the grinding process. These are tiie so called active grains. Although several attempts have been undertaken it was not possible to calculate exactly the number of active grains for a normal grinding situation on basis of grinding conditions and the distribu- tion of static grains /Ver77/. For this reason the number of active grains has been measured directly with e.g. the thermo-couple method /Pek57/ or other dynamic methods. All these difficulties have led to the fact that these measuring methods have no significance for M industrial application today.

Non contacting dynamic method

In order to identify the cutting capacity of a grinding wheel during the grinding process a new sensor has been developed. This optical sensor is

based on the principle of triangulation. Figure 1 shows the basic elements

I :An',',",::: displocement of beam on PSD

fig. 1: which are a laser diode with 32 mW continuous wave (c.w.) power, a position sensitive detector (PSD) with amplifier and two lenses. The la- ser diode emitts monocromatic laser light of 830 nm wavelength, focused by lense Ll on the grinding wheel surface. The scattered reflected light is collected by lense LZ and focused on the PSD. If the distance to the sensor changes, the position of the reflected and focused light on the PSD also changes. The PSD gives two output signals allowing to calculate the displacement of beam on the PSD. The left part of figure 1 shows the triangulation sensor device. The sensor's housing is made of a solid metal block and three plates. The housing contains all necessary elements inclu- ding the amplifier for the PSD with a frequency range from 0 to 1.5 MHz. The front plate of the sensor has special apertures for the emission of the measuring beam and compressed air. This protective system is provided to avoid penetration of coolant and dirt like e.g. small grinding chips into the sensors housing /Go88/.

Basic principle of the triangulation sensor

PC - Bus

RS 232

PC

Rg. 2: Sensor mounting and signal flow

The sensor is installed at the opposite side of the working area of a cylindri- cal surface grinder. Figure 2 gives an overview about the sensor mounting and signal flow. An IBM-PC controls data acquisition and the position of the triangulation sensor. The PC is connected with a CNC control unit via serial interface. The CNC sends data about steps and direction to the stepper drivers in dependence on the commands of the PC. Steppers

Annals of the CIRP Vol. 41/1/1992 373

with ba!! screws convert these informations into an axial movement with a resolution of about 10 microns per step. Due to this concept the PC can move the triangulation seiisor in direction of the tool spindle and radial to the grinding wheel surface.

Data about grinding wheel topography are recorded in several traces du- ring a complete measuring cycle. For this task the sensor is positioned in a predefined distance to the wheel surface at the selected trace. Then data acquisition is released by a reference signal of an angle encoder and the output signals of the triangulation sensor are further processed by the P C via .&,‘D-converter.

Resolution and accuracy of the triangulation sensor

The obtainable accuracy of the triangulation sensor depends both on the resolution of the PSD and on the actual laser beam diameter a t the qrin- ding wheel surface. The resoiution of the PSD doesn‘t set limits and offers a resolution better than 1 micron depending on the signal to noise ratio. So the focused beam diameter is the limiting element. Figure 3 gives a basic idea of the influence of the beam diameter on the measured value.

hpht intenslty tunctlon wheel ‘.-.a&;9d laser beam

scattering light

lense \

resolution

fig. 3: Cut-08 of h e r beam

The laser beam strikes on the grinding wheel topography. The weighting result of the light intensity distribution with the illuminated wheel topo- graphy is projected at the PSD. The mean value of the projected distribu- tion gives the measuring value. This described process causes a low-pass filtering of the surface roughness independent on the surface speed but just dependent on the function of the light intensity distribution. Since the gaussian intensity function is known, the cut-off wave length can be calculated and is equal to the laser beam diameter 2w illuminating the wheel surface.

In order to determine the exact obtainable spatial resolution of the trian- gulation sensor the laser beam focus w a s measured with a special develo- ped device within the measuring range from -0.8 to 0.8 mm of the sensor /WerSla/. The smallest beam diameter of 20 microns is achieved at the focus location, a t the ends of the measuring range the beam expands to roughly 50 microns. Based on this result and taking into consideration the cut-off frequency of the PSD amplifier the developed triangulation sensor is suitable to measure surface roughnesses up to a cutting speed of 30 mjs.

Ve.maz $ ‘ 2 W ’ fG (1)

Parameters for characterization of the grinding wheel surface

In common the end of tool life time is reached when a required workpiece quality is no longer given. Possible qualities could be both dimensional tolerances and workpiece surface roughness or integrity. Basing on these requirements i t is necessary to make a difference between macroscopic and microscopic wheel wear.

Types of macroscopic wheel wear are: - profile devitation like e.g. radial or edge wheel wear - radial run out - waviness

Types of microscopic wheel wear are: - grain breakage - grain pullout - flat wear - wheel loading

Macroscop ic parameters The waviness oi grinding wheel surface is important up to a cut-off fre- quency of 1 kHz in cylindrical surface grinding jTe34/. Depending on

the cuttiny spred this c u t 4 frequency corresponds to a minimum wave length of about 30 ... 60 mrn. The low-pass quality of the laser beam has no effect on waves with this length. Thereiore this waviness can be mea- sured w*ilh an accuracy of 2 or 3 microns. The spectrum of waviness is calculated from the output signals of the sensor with the Fast Fourier Transform. For this purpose the output signals are low-pass filtered with a cut-off frequency of 1 kHa. Data acquisition is triggered by an angle encoder with 1021 data samples per grinding wheel revolution. Using this device a periodic waviness signal is guaranteed and spectral leakage effects are avoided :Rays/. Each data sample is correlated with a point of refe- rence at the grinding wheel surface independent from the cutting speed. Therefore waviness parameters can easily be evaluated from the frequency spectrum. For instance, amount and phase of radial run out is determined by the fourier coefficients oi the first waveform.

In order to characterize the confidence level of the proposed macroscopic parameters the repeatability of these parameters has been investigated. For this purpose the grinding wheel surface was measured with the trian- gulation sensor at I1 traces equally distributed along the grinding wheei width. The number of replications per trace was 100. During the measu- rements were carried out the surface speed was the same as cutting speed. Figure 4 shows the results for waviness, radial run out and ellipticity. The results are drawn with their minimum and maximum value for each trace. The maximum discrepancy is 0.5 micron for radial run out and ellipticity and less than 1 micron for waviness, which is the root mean square va- lue of radial run out, ellipticity and the waveforms ti1 number 32. The point of interest is the large variation of waviness along the width of grin- ding wheel in contrast to former investigations /We 8 5 j . Furthermore the large amounts of waviness and radial run out in fig. 4 are not typical. The reason is that the measured results were obtained from a grinding wheel surface, which had reached the end of tool life time due to chatter marks on the workpiece surface.

Microscopic p a r a m e t e r s Since grains and pores are randomly distributed on the surface of grinding tools, it is necessary to describe the cutting capacity of these tools with statistical parameters. With regard to the use of a data processing system the bearing ratio curve ( b ) and it’s special parameters /DINSO/ are a convenient way to pay attention to this aspect. In this case the b.r.c. is calculated from surface roughness profiles recorded along the grinding wheel circumference. The point of interest is the variation in slope of Ab- bott’s b.r.c.. For this purpose, a new German standard has been developed with special parameters for describing the shape of curve. These parame-

64.0

P 60.0

VI VI 9)

0

.S 56.0

52.0

48.0 0 2 4 6

width of grinding wheel bS

.- 3 18.0

._ ; fim .- 13.5

\

3 9.0 0

C

2 4.5

e 0.0

- 0 ‘0 .-

0 2 4 6 8 mrn 10 width of grinding wheel b,

g w g conditions: grinding wheel : EKW 80K5 V62 meosurement:

triongulotion sensor

100 rneos. per trace

: 0; = 2.0 mm’/mm s : vc = 30 m/s 1 1 traces : V i = 500 rnmJ/mm

workpiece : 100 Cr 6 hardened ‘u= 30 m/s @ I W

fig. 4: Repeatability of macroscopic parameters

ters divide the total peak-to-valley height of a surface profile into three portions. The three portions of the curve correspond to three heights, as follows: RL, the kernel or core roughness, Rph, the reduced peak height, and Rv&, the reduced valley depth. The division of the b.r.c. into three separate portions is illustrated in Figure .5.

For the calculation of these three portions a straight line tangent of 40 %

374

A.corcximot on of the beormg r c h c u r / ? ni'h 3 strc,g.iI :me I , 7---

0 u .g 1.25 t

9, - keriel roughness 8 ~ reauced peek roqnness ker7e: ?rolile 2:;- recuced galley -3ugnness

Uql - smcilesl Fro1 le zecr fig ocr-lon 31 Ihr

M q 2 -oigges:pr3tile tear na car'lon ct 'he kernel protde

\ \

0; - 0.5

.'

fig. 5: Pummetcrs of hearing ratio cnrue

0.00 7

0 150 300 450 mm3/mrn 75( volume of metoi removal V,

g-g grincing wneel : EKW 80K5 V62 rnecsurement~

bearing ratio (b.r.) is moved from the top of the curve along ir's slope to a position of smallest gradient. A t this position the straight line tangent is fixed. The lengthening of this line to the intersection points of 0 to 100 % b.r. defines two parameters BIR~ and R i ~ 2 whose height difference is Rk. Subsequently, Rp& and R u k , are evaluated ail in the same way. At first the area bounded with 0 % b.r., the b.r.c. and the height line of 1 1 ~ 1 is calculated. As a next step, this area is replaced by a triangle of the same area. The height of the triangle gives the parameter R,k. Rp*. is said to describe the fraction of the profile height contained in the peaks of the profile, R,,k describes the one of the valleys, and Rk describes the central region of the profile which is neither peaks nor valleys. In this paper the sun1 of the three heights Ri, R,i and R,,r is defined as a new parameter, the reduced roughness Roe,

= R,s + Rr + Rvr (21

In contrast to R,, the value of which is determined by singIe grains and single pores, Roe, gives a statistical information about the whole surface trace.

The effects of grinding wheel wear on the parameters of the b.r.c. is sche- matically shown in Figure 6. Various types of grinding wheel topography after dressing and after microscopic wear and their b.r.c.'s are plotted. The b.r.c. after dressing is approximately normally distributed leading

grinding wheel topography after bearing ratio curve

1

O 2o LR&2909*8%,F'W00 fig. 6: Diflerent types of wheel wear (schematic f igure)

to similar values for R,,k and R+ If the peak and valley distribution of the grinding wheel topography changes, influenced by grain wear or wheel loading, the value of R G ~ ~ decreases. This process is accompanied by a changeof the relation between Rpk and R.r dependingon the type of wheel wear. /Sa87/ found the same effects on the b.r.c. while investigating the topography of honinq tools, as mentioned above.

The repeatability of microscopic parameters was investigated under the same grinding conditions as for the macroscopic parameters. For the cal- culation of the microscopic parameters, the high-pass filtered signal with a cut-off frequency of 20 kHs is used. For a single measurement 25000 data samples randomly distributed along the circumference of the gnn- ding wheel are converted. This number of samples corresponds to a trace length of roughly 150 mm according to 10 % of the whole circumference. An intensity check is carried out before the sampled data are further pro- cessed. This concept guarantees just valid data not falsified by tight re-

flection eriors. The results are plotted in figure ?. For each trnce the mean value and thc range of 33.9 '% confidence level from the Student t-distribution are drawn.

110

fim 135

2 4 6 0 C -

1 co

95 u 4"- = ,

% - I -

F I I

0 2 4 3 9 nrr ;o ,width of grrncinq weel jS

gr.lo;lc concit'ors. $r,'Cing *bee* : -I<W eCK5 '162 meisurement:

: C i = 2.5 mm'/mrn s : vc = 30 T/S 1 1 traces : V i = 500 mrJ/mm I C O rneas. oer trace

mrkciece I00 Cr E hordened 'M = 30 m / s O m

triGngufotion sensor

~

fig. 7: Repeatabiiity of microscopic psmmeters

Results

In fiqure 8, 'wrkpiece surface roughnesses obtained while grinding harde- ned ball bearins steel with various metal removal rates are shown as a function of volume of metal removal.

After reaching certain volumes of metal removal the condition of grinding wheel surface was measured with the triangulation sensor. In figure 9: the reduced roughness RG.. and the reduced peak height R,k are compared for various Q'w corresponding to the workpiece roughnesses of figure 8. For all metal removal rates the reduced roughness increases about 20 microns or more during the grinding process. lrievertheless the level of radial wheel wear wah measured below 5 microns for dl rates. A s can be seen from R,i, versus V'w. the larger is Q'w the more non-uniform is the course of

fig. 8: Workpiece roughness as a function of V h and qw

R,1; as a function of V'w. Furthermore it is remarkable that the values measured for R G ~ . at the beginning of grinding cycles are not the same although equal dressing conditions were used. Further investigations have to explain this discrepancy. With regard to the results achieved in former investigations /Werglb/ the parameters of the b.r.c. seem to be suitable to describe the roughness of the grinding wheel surface. However, the results obtained from these parameters have no direct correlation with the machined workpiece roughness. A reason for bad correlation might be that workpiece roughness is measured traversely, while on the other hand the tool roughness is taken parallelly to cutting direction. Attention has to be paid to this fact in further investigations.

Figure 10 shows macroscopic wear parameters versus volume of metal removal. The point of interest is the large amount of radial run out at the beginning of grinding process. Although the corundum grinding wheel wad dressed with small infeed rates it wasn't possible to reduce the amplitude of radial run out to less than 2.5 microns. The waviness on the grinding wheel surface measured after dressing is caused both from random changes

375

References:

grinding wheel : EKW 60K5 V62 measurement: triongdation sensor : ‘ J ~ = 30 m/s

: q = 6 0 1 1 :races

0 150 300 450 mm3/mm 750 1

750 I 0 150 300 450 mmJ/mm g*g conditions: volume of metal rernovoi V;

grinding wheel : EKW 80K5 V62 : vc = 30 m/s : Q = 6 0

spork out time 10 s dressinq : multi-point

measurement: trionguiation sensor 1 1 t roces v,, = 30 m/s

Q l-a workpiece : 100 Cr 6 hardened

fig. 9: RG<, and R,s 8s a function of V& and QL

in wheel material and from the rest of unbalancy. The increase of waviness during grinding process is mainly effected by non-uniform wear. /Jii78/ found the same result while investigating the influence of cutting forces on chattet vibrations in internal grinding.

As can be seen from fig. 10, the courses of waviness and radial run out are depend to a great extent on the rate of metal removal. For the small rate of 0.5 mm3/mm s the amount of radial run out keeps the level of 2.5

2 o chatter 10; 1.0

- radial run out ‘ ._ U

0 150 300 450 mm3/mm 750 g-g conditions: volume of metol rernovoi V,

workpiece : 100 Cr 6 hordened 0 IFW

fig. 10: Waviness and radial run out as a function of V,’,, and Qi,,

microns. Although this grinding cycle was started with the largest amo- unt of waviness the macroscopic wear stabilized at V’w=300 mm3/mm. While grinding with larger metal removal rates the macroscopic wear be- comes unsteady at V’~r=500 mm3/mm and V’w=750 mm’/mm leading to chatter marks on the workpiece surface.

Conclusion

The subject of this paper is the analysis of topographic data obtained from grinding wheels with an optical sensor. For the examples given, the state of wheel wear could be monitored with macroscopic and microscopic parameters while rotating the wheel with cutting speed. While the macros- copic parameters are suitable to describe the stability of the process, the grain wear of the grinding tool can be classified with statiscal parameters. However, the results obtained from these parameters have no direct cor- relation with the machined workpiece roughness. An explanation for poor correlation might be that the workpiece roughness is measured traversely, while on the other hand the tool roughness is taken parallelly to cutting direction. Attention has to be paid to this fact in further investigations.

Kenngriinen Kc, R,i, R ,>i . M R ~ , h l ~ 2 zur Reschreibung des Materialantcils iin Rnuheits?roSI. DI4 4776, Xlai 1990.

Goschruch, H: Verfahren und Vorric!itung zur Erfassurig von Schleifscheiben. P-38-27-75?.2, (1988j.

Jirgenhake, 8.: Gntersuchungen zur Technologie des innnen- rundschleifens. Dr.-inq. Dissertation. TLi Hannover 1978.

K&ig W., W. Lortz-r Three (dimensional measurement of the grindins wheel surface evaluation and effect on cutting beha- viour. Annals of CIRP 35 i l376) 1. pp. 197-203.

.Vakayama. h’.: Taper print method for the measurement of grinding wheel surface. Buli. Japan SOC. of Prec. Engg. 7 il973). 2. pp. 59-60,

Opitz , H., W. Xainig, G. Werner.: Kinematics and mechanics in grinding with regard :o :he machining process. New Deve- lopment in Grinding. Prod. int. Grinding Conf. Pittsburgh 1972. pp. 259-263.

Pehlinik, J . : E.rmittlung von geometrischen und physikalischen Kenngr6Oen f i r die Grundlagenforschung des Schleifens. Dr.- Ing. Dissertation, TH Aachen 1957.

Rabiner, L.R. und Goid, B.: Theory And Application Of Digi- tal Signal Processing. Eng!ewood Cliffs, (1975).

Salje‘ E., Y. uon See: Process-Optimization in Honing. Annals of CIRP 36 (1987) 1, pp. 235.239.

Tdlbiacher. E: Konstruktion yon Dampfern und deren Ein- satz an Rundschleifrnaschinen. Reihe 11: Schwingungstechnik Xr.80, VDI-Verlag, Diisseidorf, (1986).

T&doj$, H.K. und Zinngrebe, M.; Konstantkraftschleifen si- chert den Einsatz keramisch gebundener CBX-Schleifscheiben. Industrieanzeigcr 89. (1985) 107, S.40-41.

T&shofl, H.K. und Chen, Y.: Schwingunqen und Welligkeiten beim Schleifen. VDI-Z 131. (1989) 10, S.35-41.

Vederk, J. : Final report concerning CIRP cooperative work on the charakterization of grinding wheel topography. Annals of the CIRP 2 (19771, pp. 385-395.

Wech, Y.: MeOtechnische Untersuchung und Beurteilung. Werkzeugmaschinen Bd. 4, VDI-Verlag, Dke ldor f , (1985).

[Wergla] Werner, P.: Vermessung der Strahlgeoinetrie eines Laserstrahls mit Hilfe eines mechanischen Choppers. tm-Technisches Mes- sen 3, (1991).

Werner, F.: Sensortechnik und KenngrijDen zur Beschreibung des Einsatzverhaltens von Schleifscheiben. Sensor ’91, Niirn- berg, Band 3.

[WerSlbj

376