Acoustic emission in dressing of grinding wheels: AE intensity, dressing energy, and quantification of dressing sharpness and increase in diamond wear-flat size

Accepted Manuscript Acoustic emission in dressing of grinding wheels: AE intensity, dressing energy, and quantification of dressing sharpness and increase in diamond wear-flat size Jeffrey Badger, Stuart Murphy, Garret E. O'Donnell PII:

S0890-6955(17)30161-X

DOI:

10.1016/j.ijmachtools.2017.11.007

Reference:

MTM 3307

To appear in:

International Journal of Machine Tools and Manufacture

Received Date: 17 July 2017 Revised Date:

3 November 2017

Accepted Date: 8 November 2017

Please cite this article as: J. Badger, S. Murphy, G.E. O'Donnell, Acoustic emission in dressing of grinding wheels: AE intensity, dressing energy, and quantification of dressing sharpness and increase in diamond wear-flat size, International Journal of Machine Tools and Manufacture (2017), doi: 10.1016/ j.ijmachtools.2017.11.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Acoustic emission in dressing of grinding wheels: AE intensity, dressing energy, and quantification of dressing sharpness and increase in diamond wear-flat size

Jeffrey Badgera, Stuart Murphyb, Garret E. O’Donnellc a

Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) & Advanced Materials BioEngineering Research Centre (AMBER) Trinity College Dublin, Dublin 2, Ireland.

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b

The Grinding Doc, San Antonio, Texas, USA

Jeffrey A. Badger, Ph.D. is an expert in grinding. He works independently as “The Grinding Doc”, a consultant in grinding. He continues to do research in the field and has strong ties to Trinity College, Dublin, where he did

[email protected].

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his Ph.D. His is widely published in the academic journals and in CIRP. Contact: +1-512-934-1857 //

Stuart Murphy received his Ph.D. from Trinity College Dublin in 2015. His research areas are in the field of grinding including eccentricity, loading, process monitoring via acoustic emission, force and power measurements. Contact: [email protected].

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Dr. Garret O’Donnell is a University lecturer in the Department of Mechanical and Manufacturing Engineering in Trinity College Dublin and an associate member of CIRP. Garret's research activity in Trinity College Dublin is based on advancing manufacturing technologies that underpin sectors such as biomedical,

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automotive, aerospace, and ICT. His principal research area is the process monitoring of machining processes with goals of understanding the fundamental science, developing new sensor concepts, signal processing

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algorithms, intelligent decision making strategies and adaptronic control systems for high performance cutting processes. Contact: [email protected].

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Acoustic emission in dressing of grinding wheels: AE intensity, dressing energy, and quantification of dressing sharpness and increase in diamond wear-flat size

Abstract

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Although significant work has been done on the application of acoustic emission (AE) to grinding and to dressing of grinding wheels, several fundamental AE relationships between have not been established. These are: 1) the relationship between dressing energy and the measured AE signal; 2) how different diamond/grit contact modes (fracture, plastic deformation, rubbing, etc.) affect AE energy; and 3) how this can be used to

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quantify dressing efficiency, wheel sharpness and wear-induced changes in diamond shape. This paper describes an investigation into these fundamental concepts, with quantification of the relationship between AE intensity

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and dressing energy and the influence of different diamond/grit contact modes. A new parameter is introduced, the specific acoustic-emission dressing energy, which can be used to quantify dressing efficiency and wheel sharpness. Finally, the use of the AE intensity in evaluating diamond wear is explored, allowing the operator to know the size of the wear flat and when changes are necessary to avoid workpiece burn. Experimental work validates these concepts and practical recommendations are given on its application in industry.

1. Introduction

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Keywords: Acoustic emission; grinding; dressing; monitoring.

Acoustic emission (AE) has been applied to various aspects in grinding, most notably in contact and collision detection. Oliveira et al. [1] used AE to determine the wheel shape and Jayakumar et al. [2] reported on its use

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in contact detection when fine-grinding lenses. It has also been used to assess wheel imbalance and wheel and part roundness, as reported by Xue et al. [3]. Aguiar et al. [4] used AE to detect thermal damage, where severe

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burn coincided with a significant increase in the AE RMS value. Sutowski and Plichta [5] used the AE RMS value to evaluate grinding-wheel wear progression and also to detect grinding burn on the machined surface. Hundt et al. [6] gave the sources of AE energy as elastic impact, bond fracture, grain fracture, indentation cracks and friction and showed different frequencies for bond fracture and grain fracture (Fig. 1). Mokbel and Maksoud [7] found lower AE amplitude when grinding with sharper wheels. In dressing, Lee et al. [8] used AE to build a map of the circumferential wheel topography. Inasaki and Okamura [9] took a more microscopic view of AE in dressing and found that AE intensity increased with dressing depth and dressing lead and correlated this empirically with grinding power and workpiece surface finish. Oliveira et al. [1] showed an increase in the AE

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ACCEPTED MANUSCRIPT intensity with dressing depth and highlighted the challenges associated with sensor positioning. Kim et al. [10] also used the AE RMS signal to determine the optimum dressing depth. In spite of this, several fundamental relationships are not understood when using AE in dressing, namely: 1) how repeated diamond/grit contact and its possible associated interactions – grit/bond fracture, plastic deformation, elastic impact and friction – affect AE intensity; 2) the correlation between AE intensity and

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dressing energy; 3) whether AE can be used to predict dressing efficiency; 4) whether AE can be used to

quantify wheel sharpness; and 5) whether AE can be used to evaluate changes in dressing-diamond geometry,

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which can be detrimental to grinding performance.

Fig. 1. Sources of acoustic emission in dressing, adapted from Hundt et al. [6].

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Therefore, an investigation was made into these fundamental concepts. The relationships between dressing forces, dressing energy and AE intensity was established. Then, how the contact mechanisms between the diamond tool and the grit affect AE intensity was explored. The relationship between AE intensity and dressing

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energy was quantified experimentally and this was used to predict wheel sharpness and, in turn, grinding energies. It was then investigated whether AE could be used to measure the increasing diamond flat width with

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wheel wear, which is a major problem in industry that contributes to unstable grinding operations and increased risk of grinding burn.

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Background

In dressing of conventional-abrasive and cubic-boron-nitride grinding wheels, diamond dressers are used in the form of stationary tools (single-point, clusters, logs and blade tools), plunge diamond rolls and diamond traverse discs. The resulting sharpness of the grinding wheel depends greatly on dressing parameters. Numerous studies have shown that more aggressive dressing conditions create a sharper wheel, resulting in lower grinding forces

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and temperatures [11]. One important parameter is the number of times a grit comes in contact with the diamond dresser, referred to in rotary dressing as the collision number [12] and in stationary dressing as the overlap

Ud = bd / sd,

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Ratio, Ud [13], with typical values ranging from Ud =2 to Ud =8 [14]. It is calculated by:

(1)

where bd is the width of the diamond wear spot and sd is the dressing lead, given by sd = vtr / ω

(2)

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where vtr is the diamond traverse velocity and ωs is the rotational wheel velocity in revolutions per second. Malkin and Murray [15] showed that repeated contact with the diamond dresser resulted in plastic deformation of the hard Al2O3 grits. Rowe et al. [16] discussed the increase in the wear spot with repeated dressings and gave a guideline of bd<0.6 mm. This increase in diamond width results in a larger number of collisions, resulting in dulling of the abrasive grits and a detrimental increase in specific energies during

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grinding.

Malkin and Murray also found that specific dressing energies correlated with grinding specific energies. Considering that other researchers have found that AE signals change with dressing conditions [8 ,9, 10], it

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should be possible to quantify dressing efficiency and diamond condition via acoustic emission and use that to predict grinding power, heat generation, temperatures and the risk of thermal damage. Experimental set-up

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3.

Dressing and grinding investigations were undertaken on a Jones and Shipman 540 surface grinder (ds,i=200 mm, ωs=2880 RPM). The dressing diamond was mounted in a holder affixed to a Kistler, three-axis piezoelectric dynamometer which measured forces in three directions at 10,000 samples/second. A Kistler 8152B AE sensor (50 to 400 kHz) was mounted on the diamond dresser toolholder 45 mm from the point of wheel/dresser contact (Fig. 2). Data was sampled at 1 MHz. with a time-constant of 1.2 ms, and AE intensity was quantified by the RMS amplitude, as given by Kwak and Ha [17] and Liao [18]. Dressing in-feed was done on both sides of the wheel.

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ACCEPTED MANUSCRIPT Two grinding wheels were used: a vitrified-bond, monocrystalline, aluminum-oxide, 60-mesh, J-grade wheel

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(32A60JVBE) and a vitrified-bond, 50%-ceramic-grit (Norton SG®), 60-mesh, J-grade wheel (5SG60JVS).

Fig. 2. Schematic diagram of AE and force-sensor set-up used in dressing investigations.

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Dressing was performed with single-point diamonds with flat widths of bd=0.3 mm, 0.7 mm and 1.0 mm and with CVD logs with a near-constant flat width of bd=0.7 mm, at depths of ad=10 µm, 20 µm and 30 µm over a

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range of traverse velocities, giving overlap ratios ranging from Ud=0.4 to Ud=50. (CVD logs are synthetic diamonds produced via chemical-vapor deposition giving a shape that looks like a match stick. As the diamond wears, the 0.7 mm X 0.7 mm cross-section stays more uniform that single-point diamonds, giving a more consistent dressing width.) Electron-microscope photos were taken of worn diamonds and EDX analysis was performed on embedded material. Results

4.1 AE intensity

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4.

The first step was to determine the effect of residual noise originating from the grinding wheel, spindle bearings, coolant and other possible sources. The AE intensity was measured with the wheel and coolant on, and

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then during dressing. The result is shown in Fig. 3, indicating that residual noise is negligible.

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Fig. 3. Raw AE intensity showing the signal during wheel idling and during dressing.

In addition, a Fourier transform (FFT) was performed on several signals to determine if certain frequencies might be dominant and if they could be associated with a particular energy source. The results are shown in Fig.

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4. The frequencies will be explored in greater detail in the discussion.

0.40

d) Inasaki dressing f) Inasaki grinding e) Babel, Koshy & Weiss grinding

Y : 0.3148

X: 1.839e+005

Y : 0.2636 estimated grit-collision frequency, fc

X: 1.437e+005 X: 9.826e+004 Y : 0.2256 X: 2.011e+005 Y : 0.212 Y : 0.2058

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magnitude

0.20

b) overlap = Ud = 1 magnitude (a): 0.001

a) wheel idle

0.02

c) overlap = Ud = 20

X: 1.839e+005 Y : 0.01884

X: 4.763e+004 Y : 0.01785

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X: 4.589e+004 Y : 0.01523

X: 9.835e+004 Y: 0.01385

X: 1.438e+005 Y : 0.01362

0.01

0.00

0

100

200 300 frequency (kHz.)

400

Fig. 4. Fourier transform of AE signals.

Next, a new, CVD dressing log was used to dress the two wheels at three depths (ad=10, 20 and 30 µm) with a constant traverse velocity, giving sd=0.2 mm/rev. The results are given in Fig. 5, showing increasing AE intensity with increasing depth. The acoustic emission response does not increase proportionally with depth,

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ACCEPTED MANUSCRIPT indicating that there is a “size effect” similar to grinding [19]. More importantly, the worn diamond – when the initial, rough topography of a CVD diamond wears down to a single 0.7 mm X 0.7 mm flat – showed a larger AE intensity in both cases. This indicates that subsequent hits of the dressing diamond against the alreadydressed portion of the wheel, where previous researchers have found a greater degree of rubbing and plastic

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deformation [15], are a significant source of AE energy. 4.2 AE energy and dressing energy The dressing power, Pd, can be calculated from: Pd = FT · vs

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where FT is the tangential dressing force and vs is the wheel velocity [15].

(3)

The volumetric removal rate of abrasive media (bond, grit and porosity) can be defined as the volumetric grit

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removal rate (GRR), Qd, given by: Qd = ad · π · ds· vtr

(4)

Similar to grinding, the specific grit-removal rate per mm width, Q’d, can be written as: Q’d = ad ·vs.

(5)

Qd = Q’d ·sd.

(6)

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Therefore, where Qd can also be written as

The specific dressing energy, ed, is the energy required to remove 1 mm3 of abrasive media according to: (7)

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e d = P d / Qd

2.0

1.6 1.4

(d) 3SG, ceramic Al 2O3, worn diamond

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iae, AE signal Intensity (V)

1.8

1.2

(b) 32A Al 2O3, worn diamond

1.0 0.8 0.6

(a) 32A Al 2O3, new diamond

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(c) 3SG, ceramic Al 2O3, new diamond

0.2 0.0

0

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10

15

20

25

30

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ad, depth of dress (µ µ m)

Fig. 5. AE intensity vs. dressing depth of dress.

Malkin and Murray [15] found a direct correlation between specific dressing energy and grinding specific energy, where an increase in the dressing energy correlated with an increase in grinding specific energy. Therefore, if it would be possible to determine the relationship between dressing power and AE intensity, it 6

ACCEPTED MANUSCRIPT should be possible to correlate the AE intensity with the grinding specific energy, a parameter which is useful in predicting grinding temperature and thermal damage [11, 19].

2.0 y = 0.0135x + 0.0208 R² = 0.8608

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1.2 1.0 0.8 0.6

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1.4

(a) 32A Al2O3, new diamond (b) 32A Al2O3, worn diamond (c) 3SG, ceramic Al2O3, new diamond (d) 3SG, ceramic Al2O3, worn diamond

0.4 0.2 0.0 0

20

40

60

80

Pd, dressing power (Watts)

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120

140

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iae, AE signal Intensity (V)

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Fig. 6. AE intensity vs. measured dressing power.

Fig. 6 shows the AE intensity vs. the measured dressing power, Pd. It can be seen that there is a direct correlation between the dressing power and AE intensity regardless of dressing parameters, diamond shape and even grit-type and wheel hardness. Also, a line-fit gives a near-zero intercept, indicating that the AE intensity is

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directly proportional to dressing power according to:

iae = PFae · Pd

(8)

where PFae is the AE power factor. In the set-up here, the value is PFae=13.8 millivolts/Watt (1/PFae=72.5

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Watts/volt).

Therefore, it appears that there is a proportional fraction of dressing power which is picked up by the AE

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sensor regardless of the nature of the energy source. 4.3 AE intensity and overlap ratio Next, an investigation was made into how the AE intensity varies with the grit removal rate using the singlepoint diamond. Fig. 7 shows the AE intensity vs. the grit removal rate for tests done with a fixed depth of cut (ad=0.025 mm) and an increasing diamond-traverse velocity (vtr=0.3 to 38.4 mm/s) for three different diamond widths (bd=0.3, 0.7 and 1.0 mm). In addition, the converted dressing power (Pd,c=iae/PFae from Eq. 8) is given on the secondary axis. Here, it can be seen that a larger GRR produces a larger AE intensity. It can also be seen that the relationship is not linear and that, for a given GRR (and the same value of sd), a wider diamond

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ACCEPTED MANUSCRIPT produces a greater AE intensity, indicating that the “finishing” portion of the diamond is a not-insignificant

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source of AE energy.

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3.5

250

bd=0.7 mm 150

2.0 1.5

100

bd=0.3 mm

y = 0.2007x0.3529 R² = 0.9644

1.0

y = 0.4509x0.2996 R² = 0.9367

0.5

0

100

200

50

y = 0.6968x0.2388 R² = 0.9829

bd=0.3 mm, Ud<1.0

0.0

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iae, AE signal Intensity (V)

200

2.5

300

400

Qd, Grit Removal Rate

Pd,c , converted dressing power (Watts)

bd=1.0 mm

3.0

0 600

500

(mm3/s)

Fig. 7. AE intensity vs. grit-removal rate.

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To investigate this further, the AE intensity was evaluated with respect to overlap ratio. Fig. 8 shows a

decrease in AE intensity with overlap ratio. This does not imply, however, that larger overlap ratios yield lower

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AE intensities. In this case, the larger overlap ratio was achieved by a slower diamond traverse velocity and, in turn, a lower grit removal rate. If we take the case where bd=1.0 mm, the value of iae at Ud=20 (iae~1.6 volts) is only slightly lower than the value of iae at Ud=10 (iae~1.9 volts), even though the grit-removal rate was cut in half. Therefore, it appears that the “finishing” passes of the diamond are contributing significantly to the AE intensity and that a more comprehensive model of AE energy and dressing parameters is needed.

bd=0.3 mm, Ud<1.0

3.0 2.5 2.0 1.5

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iae, AE signal Intensity (V)

3.5

1.0

y = 3.3035x-0.239 R² = 0.9836

250

y = 2.9706x-0.299 R² = 0.9372

200

y = 1.3046x-0.351 R² = 0.9654

150

bd=1.0 mm 100

bd=0.7 mm

50

0.5

bd=0.3 mm

0

0.0

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10

15

20

25

30

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Pd,c , converted dressing power (Watts)

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4.0

40

45

50

Ud , Overlap Ratio

Fig. 8. AE intensity vs. overlap ratio.

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4.4 Specific AE dressing energy Now that it has been shown that the specific dressing energy is proportional to the AE intensity, we can define the specific AE dressing energy as: eae = iae/ Qd

(9)

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which is the AE intensity per volumetric grit removal rate. This is the acoustic emission energy picked up by the sensor per mm3/s of grinding wheel removed by the diamond via dressing and is analogous to the specific energy in grinding. Fig. 9 shows the specific AE dressing energy plotted vs. overlap ratio. In addition, the

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converted specific AE dressing energy, ed,ae, using the constant PFae, is plotted on the secondary axis. 7

bd=1.0 mm

0.09

bd=0.7 mm

6

0.08

bd=0.3 mm

0.07

bd=0.3 mm, Ud<1.0

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(Ud<1.0 not used in curve fit)

0.05 0.04 0.03 0.02 0.01

y = 0.005493x0.712224 R² = 0.987577

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0.06

4 3 2

ed,ae, converted specific AE dressing energy (J/mm3)

eae, specific AE dressing energy (V/mm3/s)

0.10

1

ed,ae at Ud=1.0 = 0.40 J/mm3

0.00 0

5

10

15

20

25

30

35

40

45

0

50

Ud, Overlap Ratio

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Fig. 9. Specific AE dressing energy vs. overlap ratio.

It can be seen that the relationship is now independent of diamond width, with a strong correlation through all data points across a broad range of dressing parameters. Considering the significant increase in AE energy as overlap ratio increased, we can divide the contacts into their respective contact number, ic, with the first contact

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(ic=1) being the “cutting contact”, where the majority of grit removal takes place via grit/bond fracture, and subsequent contacts (ic=2,3…n, where n=Ud) being “finishing contacts” in the finishing portion of the diamond,

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and then determine the relative contribution of each contact to the total AE intensity. A curve-fit gives the relationship: eae = 0.005493 · Ud0.71224

(10)

Some important observations can be made. First, at Ud=1, the value of eae appears to be the same regardless of diamond width, traverse velocity or grit-removal rate. Second, eae continues to increase even at very large (25+) values of overlap ratio. This indicates that there is a repeated, energy-producing contact with the diamond even after 25 “finishing passes”, which is likely to be rubbing or plastic deformation without any additional grit removal [15]. Third, if we substitute in Ud=1.0 into Eq. 9, this gives eae=5.49 millivolts/mm3/s and a

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ACCEPTED MANUSCRIPT corresponding specific AE dressing energy of ed,ae=0.40 J/mm3. This is almost identical to the minimum singlepoint dressing energy found in experiments by Malkin and Murray [15] (ed =0.394 J/mm3).

4.5 Cutting and finishing specific energy contributions

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It can be seen in Fig. 9 that specific dressing energy determined via acoustic emission continues to increase even after 20 “finishing passes” in the finishing portion of the diamond. Therefore, we can use this curve to determine the contribution of each diamond contact, ic=1 to ic=Ud, considering that ed,ae= Σ ed,ae,ic

(11)

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by subtracting out the value of ed,ae,ic-1. The results are shown in Fig. 10, in blue.

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0.50

9

from Inasaki

0.40

8

0.35

7

range of data points

0.30

6

extrapolated

0.25

5

0.20

4

from Fig. 5

0.15

3

0.10

2

0.05

1

eae,ic specific AE dressing energy at contact (mV/mm3/s)

0

0.00 0

10

20

30

40

ic, dressingcontact contact number number ic, dressing

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ed,ae,ic, specific AE dressing energy at contact (J/mm3) contribution (J/mm3)

0.45

10

50

Fig. 10. Specific dressing energy contribution vs. diamond dresser contact number.

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From the figure it can be seen that dressing energy continues to be emitted even at very high values of contact number. In addition, this value reaches a near-steady-state at around ic=30.

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To corroborate this phenomenon, data was examined from work by Inasaki and Okamura [9], where the AE intensity was measured with increasing dressing lead and diamond flat approximated from the figure given in the article. Here a similar trend is seen, with subsequent hits still showing a significant AE energy contribution. 5.

Discussion

The results indicate that the AE intensity is directly proportional to dressing power. Considering that Malkin

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found dressing energy is proportional to grinding energy, it should be possible to use AE to predict grinding power and evaluate wheel sharpness. Inasaki and Okamura [9] measured AE during dressing and grinding, power during grinding and workpiece surface finish after grinding at different dressing depths and dressing leads. If the grinding power is converted to grinding specific energy and the AE intensity is converted to

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converted specific AE dressing energy, the results shown in Fig. 11 are obtained. Here it can be seen that an increase in the AE intensity correlated well with both specific grinding energy and

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surface finish.

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600

0.30

550 0.25

500 450

0.20

400 0.15 350

Fig. 8; cyl plunge; WA60JV; wp: SU J2; ds=405 mm; vs=22.9 m/s; vw=570 mm/s; sd=40 µm/rev; dw=50 mm.

300 0

2

4

6

8

10

12

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eae, specific AE dressing energy (mV/mm 3/s)

Ra, surface roughness (µ µ m)

0.35

y = 299.33x0.2494 R² = 0.9649

0.10

16

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e, grinding specific energy (J/mm 3)

650

Fig. 11. AE and grinding energies, adapted from [9].

In a production environment, equipment manufacturers could place the AE sensor in a fixed position and

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determine experimentally the power factor simply by measuring the dressing power and the AE intensity for several different dressing parameters. Using this relationship, the sharpness of the grinding wheel could be determined. In the data here, values of eae from 0.005 to 0.010 V/mm3/s indicate a sharp grinding wheel, with

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values of eae>0.010 V/mm3/s indicate an increasingly dull wheel.

In addition, some interesting observations can be made regarding specific dressing energies for sharp wheels and dull wheels. Malkin and Murray’s [15] lowest value of specific dressing energy for stationary dressers was ed=0.4 J/mm3, which corresponded to the sharpest dressing conditions, giving low specific grinding energies (e=45 J/mm3). Therefore, it appears that there is a narrow range of minimum fracture energy that appears to be

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independent of grit size or grit type. Malkin and Murray’s tests used a 60-mesh, I-grade, standard-structure (8), monocrystalline grit, whereas the tests here used either (a) a very similar wheel (60-mesh instead of 46, also monocrystalline, J-grade instead of I grade, also standard structure) or (b) a much tougher grit (30% Norton

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SG), but with other similar wheel properties (60-mesh, J-grade, standard structure). In all cases, the tests yielded strikingly consistent minimum specific dressing energies in the region of ed~0.4 J/mm3. This indicates that,

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similar to Malkin’s minimum energy for chip-formation in grinding of e=13.8 J/mm3 [11], there appears to be a minimum fracture energy in dressing of ed~0.4 J/mm3, and values in this range indicate a sharp-dressed wheel and values much larger than this indicate a dull-dressed wheel. Of course, additional tests with a wider range of grit-types, grit sizes, bond formulations wheel speeds and diamond types would be necessary to confirm this, in addition to rotary dressing, but the initial findings look promising, particularly since AE could rapidly measure a range of conditions without the need for numerous measurements of dressing force or dressing power. This concept could also be used to minimize temperatures during “dressing in”, where a specific form is put on a new wheel with rapid dressing, since dressing diamonds are very sensitive to high temperatures. When a wheel is

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ACCEPTED MANUSCRIPT new, parameters during the dressing-in portion should be chosen to give the minimum specific energy in order to minimize dressing temperatures and shorten dressing times. The dressing of aluminum-oxide and CBN wheels with diamond tools suffers from several transient conditions that undermine the achievement of steady, repeatable results. Diamond wear creates a changing dressing condition that leads to greater heat generation and risk of grinding burn, with the operator either: a)

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adjusting dressing parameters in an attempt to alleviate this changing condition, or b) not changing dressing parameters and suffering through this period of increased heat generation until the diamond is either rotated (for single point) or changed. Both of these create an increased risk of thermal damage.

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In an effort to mitigate this problem, the diamond tool is often held at a 10º to 15º angle. Ideally, once the diamond develops a significantly large wear-spot, the diamond is rotated, exposing a “fresh cutting edge”. In practice, however, diamond rotation is at the discretion of the operator. In most cases, the operator has no simple

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method available to measure how this wear spot is growing. Therefore, some diamonds are rotated and some diamonds are not, without any regime as to when to rotate.

Furthermore, even diamonds that are diligently rotated still suffer from a changing diamond contact width. A wear flat develops; the diamond is rotated, decreasing the diamond contact width (and overlap ratio); a new wear spot develops; the diamond is rotated again; a new wear spot develops; the diamond is rotated again,

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perhaps to a fresh “cutting edge” or perhaps to a pre-existing wear spot. The result is an ever-changing diamond contact width and, consequently, a changing wheel sharpness. Now that it has been established that the AE intensity is proportional to the width of the wear spot, with

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increasing overlap ratios associated with a wider diamond giving increasingly greater AE intensity strengths, the AE intensity could be used to measure the width of the diamond and determine when diamond rotation is

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necessary.

Using the constants obtained in Fig. 10, we can see that an increase in diamond width of 50%, moving from a value of Ud=2.0 to Ud =3.0, would give an increase in the AE intensity of 39% for the data here and 34% for the Inasaki data. This information could be used to either: 1) rotate the diamond, decreasing the wear-spot width; or 2) increase the diamond traverse velocity, which would decrease the Overlap Ratio back to Ud=2.0. The first case carries the problem of not knowing the width of the wear spot of the “fresh” diamond contact point. However, if the AE intensity was larger than the value at Ud=2.0, the diamond could be automatically rotated again until this value was found. If it is smaller, the wheel could be dressed several times until a wear spot developed as the AE intensity reached the desired value. In both cases, the desired value would be found before

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ACCEPTED MANUSCRIPT grinding proceeded, avoiding the risk of grinding burn. The second case may be more feasible, assuming it is within the limits of the machine. In all cases, the CNC controls could measure the AE intensity and calculate the dressing energy. When it increases by, say, more than 20%, a warning is given to the operator that the dressing is becoming duller. Or, the CNC program could automatically increase the traverse velocity of the dresser. Fig. 12 shows the AE intensity for the data taken above, for three different grit removal rates at the three

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different diamond widths. Here the increase in AE intensity is obvious in all cases.

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3.5

i ae, AE signal Intensity (V)

3.0 2.5 2.0

Qd=400 mm3/s 1.5

Qd=250 mm3/s Qd=100 mm3/s

1.0

0.0 0.0

0.2

0.4

0.6

0.8

bd , diamond bd, diamond wearwidth spot(mm) width (mm)

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0.5

1.0

1.2

Fig. 12. Increase in AE intensity with diamond wear.

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Fig. 10 shows that repeated diamond hits, even at values of Ud>30, continue to contribute to the AE intensity. This could be due to elastic contact or plastic contact between the grit and the diamond. To investigate

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this, the wear spot in a worn single-point diamond was analyzed under an electron microscope (Fig. 13). Fissures were visible on the worn surface, with backscatter showing white material embedded in the fissures. EDX analysis revealed that this material was composed almost entirely of aluminum. Klocke and Linke [20, 21] found high flash temperatures and localized melting of aluminum-oxide at the surface, along with maximum bulk temperatures as high as 800°C, which is above the graphitization temperature for diamond. Therefore, it appears that the high temperatures are leading to either localized melting

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or plastic deformation of the abrasive. The AE measurements here, with increasing AE intensity even after Ud>20, and the SEM photo showing embedded aluminum both support the conclusion that localized plastic flow is occurring and that dressing temperatures were high enough to cause significant softening or even localized

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melting of the aluminum-oxide.

1 mm

0.4 mm

Fig. 13. Embedded aluminum in a diamond dresser.

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ACCEPTED MANUSCRIPT Fig 4. shows that the dominant AE frequencies were in region of 25 to 300 kHz. This was true for both (b) Ud=1 and (c) Ud=20, and the relative amplitudes of the various frequencies did not change significantly. An FFT of the wheel while idling (c) showed much smaller values and different and somewhat higher frequencies, indicating that the peaks during dressing are likely coming from the grit/diamond contact. In addition, FFT frequencies from previous researchers are given – (d) from Inasaki [9] for both dressing and grinding, and (e)

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from Babel et. al. [22] for dressing. Although specific frequencies did not match, the general range of frequencies for all tests was consistent. Finally, in an effort to determine if frequencies may be coming from the energy source or simply from the frequency of collisions with the diamond, an estimate was made of the

fc = bd ·vs· C

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collision frequency, fc, for Ud =1, considering that

(12)

where C is the cutting-point density in points/mm2. If we pull an estimate of C=6.2 points/mm2 based on the

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measured values from Shaw [23] for the same grit size and wheel grade, we obtain fc=130.8 kHz., which falls into the range of frequency values measured, further adding to the ambiguity of the results. Clearly, a dedicated study would be needed to determine the frequency of the diamond/grit interactions and corresponding energysource frequencies. Finally, some researchers have found a spike in the AE signal in the entry and exit [22], colloquially known as the “Batman ears” profile. This was not seen here. However, the tests here were done on

Conclusions

Acoustic-emission intensity during dressing is proportional to dressing power.

•

This proportionality appears to be true regardless of dressing parameters, abrasive grit type, or the AE

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energy source, be it grit/bond fracture, plastic deformation, rubbing, or other possible sources.

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A new concept was introduced, the specific AE dressing energy, in volts/mm3/s, which is proportional

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dressing, whereas the spikes were seen for AE measurements during grinding.

to the specific dressing energy via the AE power factor. Once the power factor is determined, AE can be used to determine specific dressing energy.

•

Because specific dressing energy is proportional to grinding specific energy, once the AE power

factor is determined, AE intensity can be used to quantify dressing sharpness. •

Minimum dressing energies for overlap ratios of 1.0, where grit and bond fracture dominate, appear to be in the region of 0.4 J/mm3 for a variety of grit-types and grit sizes.

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Diamond-grit contact can be divided into the “cutting pass” and subsequent “finishing passes” in the finishing portion of the diamond. A significant AE intensity during finishing passes indicates that that diamond/grit rubbing and/or plastic deformation is a significant source of AE energy.

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AE intensity can be used to measure the width of the diamond wear spot and determine when diamond rotation or an increase in dressing traverse velocity is necessary. Plastic deformation appears to be occurring during the “finishing passes” in the finishing portion of

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the diamond, leading to grit dulling and in some cases localized melting or softening of the aluminumoxide, which can be seen by the embedding of aluminum in wear-fissures in the diamond.

AE during dressing is a valuable tool for quantifying dressing sharpness and for circumventing

7.

Acknowledgements

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problems associated with increasing diamond wear.

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Funding for this project was provided by The Grinding Doc Consulting and Element Six under the auspices of the IRCSET Enterprise Partnership framework, and in part by a research grant from Science Foundation Ireland

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(SFI) under Grant Number SFI/12/RC/2278.

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References

1.

Oliveira J. F. G., Dornfeld D.A., Winter B., 1994. "Dimensional Characterization of Grinding Wheel Through Acoustic Emission," Annals of the CIRP, Volume 43(1), p. 291–294.

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Jayakumar T., Mukhopadhyay C.K., Venugopal S., Mannan S.L., Raj Baldev., 2005. "A review of the

Materials Processing Technology, Volume 159, Issue 1, p. 48-61. 3.

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Aguiar P., Serni P., Dotto F., Bianchi E., 1994. "In-Process Grinding Monitoring Through Acoustic Emission," ACBM, XXVIII(1), p. 295-298.

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Sutowski P., Plichta S., 2006. "An investigation of the grinding wheel wear with the use of root-mean-

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square value of acoustic emission," Archives of Civil and Mechanical Engineering, Volume 6, Issue 1, p. 87-98. 6.

Hundt W., Leuenberger D., Rehsteiner F., Gygax P., 1994. "An Approach to Monitoring of the Grinding Process Using Acoustic Emission (AE) Technique," CIRP Annals - Manufacturing Technology, Volume 43, Issue 1, p. 295-298.

Mokbel A. A., Maksoud T.M.A, 2000. “Monitoring of the condition of diamond grinding wheels using

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acoustic emission technique”, Journal of Materials Processing Technology, Volume 101, Issues 1–3, p.292297.

Lee D.E., Hwang I., Valente C.M.O., Oliveira J.F.G., Dornfeld D.A., 2008. "Precision manufacturing

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process monitoring with acoustic emission," International Journal of Machine Tools and Manufacture,

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Volume 46, Issue 2, p. 176-188.

Inasaki I., Okamura K., 1985. "Monitoring of Dressing and Grinding Processes with Acoustic Emission Signals," CIRP Annals - Manufacturing Technology, Volume 34, Issue 1, p. 277-280.

10. Kim H.Y, Kim S.R, Ahn J.H, Kim S.H, 2001. "Process monitoring of centerless grinding using acoustic emission," Journal of Materials Processing Technology, Volume 111, Issues 1–3, p. 273-278. 11. Malkin S., Guo C., 2008. Grinding Technology: Theory and Applications of Machining with Abrasives, Second edition, Industrial Press Inc. 12. Brinksmeier E., Çinar M., 1995. "Characterization of Dressing Processes by Determination of the Collision Number of the Abrasive Grits," CIRP Annals - Manufacturing Technology, Volume 44, Issue 1, p. 299-304.

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ACCEPTED MANUSCRIPT 13. Linke, B., 2008. “Dressing process model for vitrified bonded grinding wheels”, CIRP Annals – Manufacturing Technology, Volume 57, Issue 1, p. 345-348. 14. Graf, W., 2010. Handbook Cylindrical Grinding, Winterthur Schleiftechnik AG, Switzerland. 15. Malkin S., Murray T., 1977. "Comparison of Single Point and Rotary Dressing of Grinding Wheels", Proc. Fifth North American Metalworking Research Conference, p. 278 – 283.

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16. Rowe B., Marinescu I., Ohmori H., Dimitrov B., 2013. "Tribology of Abrasive Machining Processes," Second edition, W. Andrew.

17. Kwak J-S., Ha M-K., 2004. "Neural network approach for diagnosis of grinding operation by acoustic

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emission and power signals," Journal of Materials Processing Technology, Volume 147, Issue 1, p. 65-71. 18. Liao, W. T., 2010. "Feature extraction and selection from acoustic emission signals with an application in grinding wheel condition monitoring," Engineering Applications of Artificial Intelligence, Volume 23, Issue

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1, p. 74-84.

19. Rowe B., 2009. Principles of Modern Grinding Technology, ed. W. Andrew. 20. Klocke F., Linke B., 2008. "Mechanisms in the generation of grinding wheel topography by dressing," Production Engineering, Volume 2, Issue 2, p. 157-163.

21. Linke B., Klocke F., 2010. "Temperatures and wear mechanisms in dressing of vitrified bonded grinding

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wheels," International Journal of Machine Tools and Manufacture, Volume 50, Issue 6, p. 552-558. 22. Babel, R., Koshy, P., Weiss, M., 2013. “Acoustic emission spikes at workpiece edges in grinding: Origin and applications,” International Journal of Machine Tools & Manufacture, Volume 64, p. 96–101.

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23. Shaw, M.C., 1996. Principles of Abrasive Processing, Oxford University Press, Oxford.

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Nomenclature µm mm points/mm2 mm mm V/mm3/s joules/mm3 joules/mm3 joules/mm3 collisions/s Newtons volts

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Watts Watts millivolts/Watt mm3/m·s mm3/s µm mm/revolution overlap ratio m/s mm/s revolutions/s

dressing depth diamond wear spot width cutting point density wheel diameter new wheel diameter specific AE dressing energy specific dressing energy converted specific AE dressing energy specific AE dressing energy at contact ic collision frequency tangential force AE signal intensity dressing contact number dressing power converted dressing power AE power factor specific grit removal rate volumetric grit removal rate surface roughness dressing lead

wheel velocity diamond traverse velocity wheel rotational velocity

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ad bd C ds ds,i eae ed ed,ae ed,ae,ic fc FT iae ic Pd Pd,c PFae Q’d Qd Ra sd Ud vs vtr ωs

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Highlights

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Acoustic emission (AE) was measured for diamond dressing of aluminum-oxide grinding wheels. Measured AE intensity was proportional to dressing power. The diamond was divided into the “cutting portion” and “finishing portion”. The finishing AE signal was significant, even after 20+ diamond “finishing contacts”. Since dressing energy correlates with grinding energy, quantifying AE by the specific AE dressing energy is a measure of dressing sharpness. AE can be used to quantify diamond wear.

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• • •