Handbook of Advanced Ceramics Machining Episode 9 pdf

– Full potential of ELID grinding can be achieved only after 24 min ofgrinding when the grinding force stabilizes at a low value.. Traverse and plunge ELID grinding for evaluating the ef

9.4.5 ELID Grinding of Ceramics on Vertical Grinding Center

(Bandyopadhyay et al., 1997)

The experiments were carried out on a vertical machining center The siliconnitride workpieces were clamped to a vice firmly fixed onto the base of astrain gauge dynamometer The dynamometer was clamped onto themachining center table and the reciprocating grinding operation wasperformed

A direct-current pulse generator was used as a power supply The wave voltage was of 60–90 V with a peak current of 16–24 A The pulsewidth was adjusted to 4 msec on-time and off-time Different values for thedepth of cut and for the width of cut were explored Material removalrates of 250 mm3=min up to 8000 mm3=min were obtained A comparisonbetween the results obtained after an ordinary grinding operation and theresults obtained after an ELID grinding operation was carried out

square-A modified ELID dressing procedure was also studied The modifiedELID dressing was performed in two stages: (a) at 90 V for 30 min; theinsulating oxide layer was mechanically removed by an aluminum oxidestick of #400 grit size at 300 rpm; (b) another dressing stage at 90 V for

30 min (Figure 9.20, Figure 9.21, Figure 9.22, and Figure 9.23) The sions of the study were as follows (see also Figure 9.24):

0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

– ELID grinding can achieve high material removal rates.

– ELID grinding is recommended for rigidity machine tools and rigidity workpieces

low-– The grinding force increases continuously during ordinary grinding

Effect of feed rate on surface roughness (From Ohmori, H., Takahashi, I., and Bandyopadhyay, B.P.,

J Mat Proc Technol, 57, 272, 1996 With permission.)

Volume of material removed, 3 mm 3

V = 1200 m/min

f = 5000 mm/min DOC = 0.05 mm WOC = 2 mm CIFB-D wheel #170 (avg 80 µm) Material: silicon nitride

FIGURE 9.24

Relationship between the volume of material removed and the grinding force for: (a) tional grinding (From Bandyopadhyay, B.P., Ohmori, H., and Takahashi, I., J Mat Proc Technol,

conven-66, 18, 1997 With permission.)

FIGURE 9.24 (continued)

(b) ELID grinding after ELID dressing; (c) ELID grinding after modified ELID dressing.

– Grinding force is lesser during ELID grinding when compared withconventional grinding; this effect became more visible after 18 min ofgrinding.

– An increase in the voltage value during ELID grinding will reduce thevalue of the grinding force even more; this effect became more visibleafter 18 min of grinding

– Full potential of ELID grinding can be achieved only after 24 min ofgrinding when the grinding force stabilizes at a low value

– ELID and conventional grinding produced almost the same surfacefinish in rough grinding

– The grinding force was constant and low after the modified ELIDdressing procedure was applied to the wheel

9.4.6 ELID Grinding of Bearing Steels (Qian et al., 2000)

In this research, ELID as a superfinishing technique for steel bearing ponents was studied The experiments were conducted in both traverseand plunge in feed modes The wheels utilized were cast-iron-bondedusing CBN as abrasive The experiments were carried out on a commoncylindrical grinder with a 3.7 kW motor spindle The negative electrode wasmade out of stainless steel A Noritake AFG-M grinding fluid diluted to 1:50

com-at a rcom-ate of 20–30 L=min was used as electrolytic fluid A direct-current pulsegenerator was used as a power supply The square-wave voltage was of60–150 V with a peak current of 100 A The pulse width was adjusted to

4 msec on-time and off-time Three types of experiments were conducted:

1 Traverse and plunge ELID grinding for evaluating the effects of the

grinding wheel mesh size and grinding method on surface

rough-ness and wavirough-ness value

2 Traverse grinding with different mesh size wheels to assess the

influence of the mesh size of the wheel over the surface finish and

material removal rate

3 ELID grinding with #4000 mesh size wheel was compared with

honing and electric finish methods

The conclusions were as follows:

– ELID grinding as compared to conventional grinding offered a bettersurface finish (Ra ¼ 0.02 mm for #4000 wheel)

– Plunge ELID grinding outputted poorer surface finish than traverseELID grinding, especially for coarser grit abrasive wheels (seeFigure 9.25)

– Waviness of ground surface improves with the increase in wheel meshsize (Figure 9.26)

Depth of cut : 1 −2 µm Spark-out : 10 Grinding time : 260 sec

ELID (M band)

No ELID (M band) ELID (H band)

No ELID (H band) Traverse mode grinding

No ELID (H band) Plunge mode grinding

– ELID process is stable for traverse mode grinding and not so stable forplunge mode grinding (see Figure 9.27).

– Roundness of the ground surface increases with the mesh size of theused wheel

– Out-of-roundness can be negatively affected by the stiffness of themachine tool, not only as a result of the grinding operation

– ELID traverse grinding tends to offer a more promising potential thanplunge mode grinding

– A number of three to four spark-out passes will improve the precision(waviness and roundness) of the ground surface.

– Effects of grinding parameters on surface roughness for ELID grindingand conventional grinding are comparable

– Increased depth of cut and increased traverse rate worsen the surfacefinish

– ELID grinding offers better results than both honing and electricpolishing (see Figure 9.28)

– Smooth ELID ground surfaces have poorer high band waviness thanhoned ones, explained by the tool performance because of the smallercontact zone

– ELID grinding technique induces compressive surface stress of about150–400 MPa, with a smaller peak than that outputted by a honingprocess of 600–800 MPa

– The depth of the compressive layer produced by ELID operation(10 mm) is half the one produced by a honing operation (15–20 mm).– The cycle time of ELID grinding is twice as large as the one thatcharacterizes the honing and the electric polishing operations; but betterroughness and higher removal rate achieved through ELID grinding by

a coarser wheel and higher traverse speed prove ELID grinding morecost-effective for small batch production situations

9.4.7 ELID Grinding of Ceramic Coatings (Zhang et al., 2001a)

Ceramic coatings include a large group of subspecies, such as CVD-SiC,plasma spray deposited aluminum oxide, and plasma spray deposited

FIGURE 9.28

Surface waviness with different processes.

chromium oxide The efficient machining and the required quality of theseceramic coatings have not been mastered yet In this research, a comparativestudy of diamond grinding of ceramic coatings on a vertical grinder wasdone Two types of dressing procedures as applied to cast-iron-bondeddiamond wheel, with #4000 mesh size—alumina rotary dressing and ELIDdressing and grinding—were compared The conclusions can be synthe-sized as follows:

– There is a critical current value for each electrolytic dressing system;when the current is smaller the thickness of the insulating oxide layerincreases with the value of the current; otherwise, it decreases.– Thickness and depth of oxide layer largely depends on the coolant type.– A small increase in the wheel diameter or thickness after electrolyticdressing is noticed, conversely as in the rotary and other mechanicalmethods of dressing

– Roughness for the rotary dressing decreases sharper than ELID method

in the first 3 min; after 3 min, the roughness decreased constantly forELID grinding but showed a wavelike model for the dressing method.– The wear of abrasive grains will produce an instability of the grindingperformance for dressing technique, while it remains constant for ELIDtechnique

– Surface roughness depends on material properties in both methods

– All ceramic coatings except sintered SiC presented a better roughnessafter ELID dressing than after rotary dressing

– Plasma spray deposited chromium oxide is difficult to grind to anextremely fine roughness

– For both dressing methods, the micrographs prove that the materialremoval mechanism presents both brittle-fracture and ductile modes;for the ELID dressing, more ductile mode than the brittle-fracture modewas present, except for sintered SiC; whereas for the rotary dressingmore brittle-fracture mode than the ductile mode was present

– Ductile-mode grinding can be implemented even on a common grinder,

by controlling the wheel topography

– For the ELID method, the interaction between the abrasive grain andthe workpiece surface is accomplished through a spring-dampersystem (because of the existence of the oxide layer), whereas for rotarydressing, the contact is rigid and stiff (see Figure 9.29); oxide layerabsorbs vibrations and reduces the actual exposed cutting edge of theabrasive grain

9.4.8 ELID Ultraprecision Grinding of Aspheric Mirror (Moriyasu et al.,

2000; see also Figure 9.30)

The quality of soft x-ray silicon carbide mirrors influences the performance

of modern optical systems To accomplish the high precision of these

aspheric mirrors, an ELID grinding system was employed A #1000 cast-irongrinding wheel was mechanically trued and predressed using electricalmethods The workpiece surfaces were concave spherical with a curvature

of 2 m After grinding, the form was measured and the data were comparedwith the planned data by the mean of the least squares method A form error

Wheel in

conventional

grinding

Wheel in conventional grinding

Damper, c

Work

Abrasive Spring, k

Electric supply

Measuring instrument

Metal-bonded grinding wheel

Workpiece

Coolant

Electrode

CNC grinding machine

was calculated and compensation data were generated Accordingly, a newform was ground This procedure was applied five times to exponentiallydecrease the errors from 2.6 to 0.38 mm Analyzing the repeatability of thesystem, the achieved data are considerably accurate.

9.4.9 ELID Grinding of Microspherical Lenses (Ohmori et al., 2001)

The ductile-mode grinding of brittle optical parts is studied in this research.The implementation of ductile-mode cutting requires expensive items likeultraprecision vibration-free rigid machine tools, high-resolution feedingmotion control, submicron grit wheels, clean work environment, and so

on The conclusions of this study are as follows:

– ELID grinding is stable, efficient, and economical

– Low grinding speed, insufficient work-wheel fixture, instability onusing ultrafine abrasive and small-size wheels, difficulty in achievingprecise and efficient truing and dressing of the wheels, trouble inobtaining precise and effective fixtures are some problems connectedwith implementing the ductile or semiductile-mode grinding of microoptical components

– Coarse grit size wheels (#325) do not show any difference in the finalroughness when ELID is applied

– #4000 wheels, however, showed a better surface finish when ELID wasemployed

– ELID high-precision grinding of microspherical wheels with cupwheels (ELID-CG grinding) has the potential to achieve high sphericalaccuracy and finish Ra20 nm

– ELID-CG grinding can be successfully used to fabricate microsphericallenses with a more stable process, higher efficiency, and better surfacequality than conventional grinding

9.4.10 ELID Grinding of Large Optical Glass Substrates (Grobsky

and Johnson, 1998)

The ability of ELID grinding using fine mesh superabrasive wheels to duce spectacular finishes (Ra4–6 nm) on brittle surfaces of BK-7 glass, silicon,and fused silica was proved For some applications, ELID grinding elimin-ated polishing or lapping operations In this research, ELID grinding wasemployed to grind optical components of 150–250 mm diameter

pro-9.4.11 ELID Precision Internal Grinding (Qian et al., 2000, 2001)

Few researches have reported on mirror-surface internal grinding because

of the limitation of the abrasive grit size applicable to nonmetallic bond

grinding wheels A novel method to carry out ELID grinding of internalcylindrical surfaces on an ordinary grinding tool, named interval ELID, ispresented (see Figure 9.31) The wheel is dressed at intervals and theabrasive grains remain protruding After predressing operation, the insu-lating oxide layer is 30 mm thick, increasing, therefore, the external diameter

of the grinding wheel The current characteristics for interval ELID grindingare shown in Figure 9.32 In internal grinding, the abrasive wear occurs

Workpiece Coolant

CIB-D wheel

Brush ELID power source

Pipe dressing electrode Chuck

FIGURE 9.31

Schematic of interval ELID grinding (Qian et al., 2000b)

ELID II grinding

1 1

#170 CIFB-D wheel φ 30 ⫻ L20 Brass pipe electrode, φ 32 ⫻ L40

rapidly because of the smaller diameter of the wheel The conclusions of thestudy are presented briefly here:

– Because of the limitation of the wheel diameter, the wheel speed can beadjusted only within a small range; the effect of speed on work results

– Internal mirror-surface finish was possible for pieces made of bearingsteel, alumina Later studies (Qian et al., 2001) added some newconclusions that are presented here

– Pipe dressing electrodes are superior to other shape electrodes

– Another ELID technique, ELID III, is introduced (see Figure 9.33)

– It is possible to achieve mirror-like surface with an appropriate coarsegrinding wheel; the roughness obtained after ELID III grinding with

#2000 grit size is almost the same with the roughness obtained afterELID with #4000 with cBN

– A combination of ELID II and ELID III can be used for finishingprocesses especially when small diameters are applicable

– Both rough and finish grinding can be achieved on the same machinetool using ELID II and ELID III procedures

Workpiece

MRB-cBN wheel

CIB-cBN wheel FIGURE 9.33

ELID II and ELID III internal grinding processes (From Qian, J., Ohmori, H., and Li, W., Int J Mach Tools Manuf, 41, 193, 2001 With permission.)

9.4.12 ELID Grinding of Hard Steels (Stephenson et al., 2001)

Hardened bearing steels like M50 was ultraprecision ground to produce anoptical quality surface (Ra<10 nm), using a 76 mm cBN grit and 500 mm depth

of cut The final surface finish is enhanced by the burnishing action of the worncBN grits ELID technique was employed to improve the surface finish bymaintaining the protrusion and sharpness of the cBN grits and avoid thepullout of the carbides in the secondary finishing zone phenomenon

Another grinding technique used to minimize microcracking, surfaceburn, and phase transformation is low stress grinding (LSG) However,special demands of machine tool stiffness, low and controllable vibrationstate, low wheel speed, and frequent wheel dressing operations should beaddressed in this case The LSG is characterized by lower removal rates,lower grinding ratios, and significant increasing of production costs,while some localized surface damage and surface finish in the range of Ra100–200 nm are to be obtained

Other studies (Onchi et al., 1995) reported an Ra30 nm roughness aftergrinding SAE52100 with a porous cBN wheel, but with relatively lowremoval rates and very fine cBN wheels, or up to Ra60 nm (Puthanangady

et al., 1995) after superfinishing hardened steel pieces with #500 grit sizefused alumina stones

This study employed cBN grinding wheels with the grit size of 30 mm forroughing, 2 mm for medium finish, and 0.7 mm for final mirror-like finish Thefollowing conditions were used: 100 mm diameter D151 electroplated diamondwheel at 3000 rpm, traverse rate of 5 mm per rev and an n-feed of 1–4 mm perpass The following parameters were used for the current work such as: voltage

60 V, peak current 10 A, on-time 6 ms, off-time 2 ms, and square pulse wave.Some of the most important conclusions of the study are presented here(see also Figure 9.34 and Figure 9.35):

. A repeatable surface finish less than Ra10 nm can be ultraprecision

grounded with 75 mm cBN grit and 500 mm depth of cut

250 200 150 100 50 0