Jackson M.J. Micro and Nanomanufacturing

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Microgrinding 281

Fig. 6.12. Moore Jig boring machine: (a) line diagram [55] (b) picture of

the same machine [56]

iJ& • ftgffl

Fig. 6.13. A milling machine converted into a jig grinder with an air turbine at-

tachment [21]. This idea was used for grinding of an IC chip [57]

282 Micro- and Nanomanufacturing

Fig. 6.14. MAHO CNC machining center upgraded for high-speed through the

use of an air turbine [57]

Corner

edge

• a a9Q

Radius bottom Formed angle Radius bottom Radius with

two tangent

angles

Fig. 6.15. Various types of pins available for jig grinding operations

Microgrinding 283

Fig. 6.16. Description of various jig grinding operations [45]: (a) hole or plunge

grinding is the most common method of grinding holes where grinding pin grinds

hole bottom or side surface. Pin movement is controlled by CNC machine; (b) the

wipe grinding mode is generally used for form grinding. The work is fed past the

revolving wheel, which is in a stationary position

Novel techniques were developed by Venkatesh et al. [58-61] to

study formation of ductile streaks during the Jig (Piano) grinding of

glass and Si surface using high-speed air turbine spindle (Figures

6.13 and 6.14). It was found that resinoid diamond wheels gave

more ductile streaks than metal-bonded wheels but better form accu-

racy was obtained with the latter (Figs. 6.15 and 6.16). Ductile

streaks were obtained more easily with Pyrex rather than with BK 7

glass thus necessitating very little time for polishing. Table 6.6

shows the time required to polish Pyrex glass when using a variety

of polishing reagents.

Results indicate that the surface roughness of precision-ground

Si sample improves with lower feed except at finest depth of cut of 5

|Jm where higher feed rates improves the finish. Ductile streaks also

appear at higher feed rates (Fig. 6.17).

284 Micro- and Nanomanufacturing

(a) (b)

Fig. 6.17. SEM pictures of ground Si the surface consist of (a) microfractures

and (b) grinding streaks [62]

6.4.2 Critical Depth of Cut

As can be seen in the Fig. 6.18, there are also various shapes of

wheel end faces such as pointed, round, flat, slight conical, etc. The

advantage here is that using a mounted wheel whose end face is in

slight conical shape, critical depth of cut in grinding can be deter-

mined. Figure 6.15 indicates a diamond pin of 5 mm diameter that

was used to grind Pyrex glass. The tapered diamond pin resulted in

circular areas having fractured and partial ductile surfaces (Fig.

6.18(b)). From diamond pin and grinding geometry, the wheel depth

of cut can be seen as the highest at the center of the track and gradu-

ally decreased to the shoulder. As a result, one grinding pass consti-

tutes at least two tracks, one with fracture and the other with partial

ductility. In fact, Fig. 6.18 indicates the experimental observation on

grinding of Pyrex glass with 64 ]im grit diamond pin. Actual cone

angle of end surface was approximated to be 179.4°. Grinding con-

ditions were 39 m/s cutting speed, 2.5 mm/min feed rate, and 10 |um

maximum depth of

cut.

In Fig. 6.18(a), y' represents the depth of cut

at which transition from brittle to ductile grinding occurs and thus

the corresponding depth of cut will be critical value. With the aid of

optical microscope and integrated image analysis software, the width

of the tracks can be measured Fig. 6.18(c) whereas y' was found to

be 7 |um.

Microgrinding 285

Fig. 6.18. (a) Exaggerated conicity of grinding wheel (actual cone angle will be

about 179.4°). (b) Plan view of ground surface whose diameter is less than wheel

diameter, because depth of cut is less than y. The central part despite being frac-

tured is transparent and the outer part is translucent despite the occurrence of par-

tial ductile streaks, (c) Enlargement of triangular area O'B'A in (a) enables esti-

mation of critical depth of cut (y') [62]

286 Micro- and Nanomanufacturing

6.4.3 Precision Grinding with Electrolytic in-Process

Dressing (ELID)

Grinding with superabrasive wheels is an excellent way to produce a

precise surface finish on hard and brittle materials. To achieve this,

superabrasive diamond grits need higher bonding strengths than

metal bonded and resinoid bonded wheels can offer. However, tru-

ing and dressing of the wheels are a major problem as they tend to

glaze because of wheel loading. These problems can be avoided by

dressing periodically but this interupted action makes the grinding

process very tedious and time consuming. A Japanese research

group has introduced an effective technique to overcome the poor

self-dressing properties of metal bonds, especially cast iron bonds, in

the presence of aqueous lubricants. Ohmori and Nakagawa [63]

called the method electrolytic in-process dressing (ELID). The basic

concept of grinding with ELID is illustrated in Fig 6.19. It uses an

electro-chemical method to remove the metal bonds and properly

expose the diamond particles, thereby maintaining the high effi-

ciency of the grinding operation. The basic ELID system consists of

a metal or cast iron bonded diamond grinding wheel, an electrode

(copper or graphite), a power supply, and an electrolyte as shown in

Fig. 6.19.

A. System construction B. Electrode detail

Fig. 6.19. Basic ELID system shows the essential requirement of a cast iron bond

for the grinding wheel to function using the grinding fluid as an electrolyte [63]

Microgrinding 287

The power supply for ELID is used to control the dressing

current, voltage, and pulse width of the dressing process. The metal

bonded wheel is made into the positive pole through the application

of a brush smoothly contacting the wheel shaft and the electrode is

made into negative pole. In the small clearance between the positive

and negative pole (0.1 to 0.3 mm), electrolysis occurs through the

supply of

the

grinding fluid and an electrical current.

There is an important feature on ELID grinding that ELID grind-

ing makes oxide hydroxide (insulation) layer on surface of ELID

wheel by electrolysis. The oxide hydroxide layer has a lower electro-

lytic conductivity.

(a)After truing (b)Begining of dressing (c)After dressing

Diamond

particle

Insulating

layer

Removed

oxide layer

• •

(e)End of grinding (d)During grinding

Fig. 6.20. Schematic illustration of ELID grinding principle [64]

Figure 6.20 describes the mechanism of ELID grinding of metal

bonded diamond wheel. After truing (a), the grains and bonding ma-

terial of the wheel surface are flattened. The trued wheel needs to be

288 Micro- and Nanomanufacturing

electrically pre-dressed to protrude the grains on the wheel surface.

When pre-dressing starts (b), the bonding material breaks away from

the grinding wheel and an insulating layer composed of the oxidized

bonding material is formed on the wheel surface (c). This insulating

layer reduces the electrical conductivity of the wheel surface and

prevents excessive breakdown of the bonding material from the

wheel. As grinding begins, (d), diamond grains wear out and the

layer also become worn, (e). As a result, the electrical conductivity

of the wheel surface increases and the electrolytic dressing restarts

with the breakdown of bonding material from grinding wheel. This

cycle is repeated during the grinding process to achieve stable grind-

ing [63,65,66]. ELID now becomes the most efficient method for

dressing metal bonded grinding wheels continuously that eliminates

the wheel loading and glazing problems during the grinding process

[67].

It has been reported that surface roughness (Ra) with the ELID

process can be achieved as low as 0.33 nm on BK7 glass and silicon

when using ultrafine #3000000 grit metallic bond wheel [63].

The applications of ELID are numerous. It has been successfully

used for processes such as surface grinding, cylindrical grinding, in-

ternal grinding, and centerless grinding. Some others are in abrasive

cut-off of ceramics [68], mirror surface grinding of silicon wafers

[63],

small-hole machining of ceramic materials [69], sawing of

steel, polymer, sapphire and glass [70], precision machining of

CVC-SiC reflection mirrors [69] and mirror internal cylindrical

grinding on steels and alumina components [67].

6.4.4 Partial Ductile Mode Grinding

To transform a rough piece of glass into a crystal-clear optical com-

ponent, opticians grind and then polish the piece. Traditionally, opti-

cians use small abrasive particles such as aluminum oxide mixed in

water-based slurry to gradually remove imperfections and shape a

rough piece of glass into the optical shape that is needed to precisely

direct light rays from a distant point. The grinding process leaves

tiny cracks just below the surface as well as small crater-like imper-

fections that make the glass piece opaque and prone to fracture. To

remove the damaged layer and produce a clean optic, the piece goes

Microgrinding 289

through extensive finishing or polishing. "Polishing is the most time-

consuming and expensive part of the optics process", states Ruck-

mann [71].

Research performed by Venkatesh and Van Ligten to generate

aspheric surfaces by a novel technique is described in detail. In this

technique, an aspheric surface was generated on a 4 axis CNC ma-

chining center initially for glass, and subsequently for silicon and

germanium [72 - 74]. Preformed blanks were obtained from the

supplier and aspherics were generated using a diamond cup. When

the grit size was changed from 63 \im to 20 |im, an increase in

grinding streaks was observed with Si and Ge materials (Fig.6.21).

To reduce tool costs, resinoid bonded wheels were tried out with

remarkable success. With Ge in particular almost 90% ductile grind-

ing was observed (Fig. 6.22) with a wheel that was partly worn out

to flatten the diamond grains.

Fig.. 6.21. (a) and (b) Grinding streaks on Si and Ge. (c) Almost 90% ductile

grinding on Ge [75]

Grinding and polishing of industrial aspheric lenses was carried

out by Venkatesh and Zhong [75] on an aspheric profile generator.

The results indicate that semi-ductile grinding works well for oph-

thalmic industries. Polishing time was reduced by almost 50%. (Fig.

6.23), which give maximum ductile streaks for both spherical and

aspheric lenses.

290 Micro- and Nanomanufacturing

Polishing time min

Fig. 6.22 Roughness values of polished aspheric glass moulds versus polishing

time [75]

Inspired by Konig and Sinhoff s model referred to in reference

[12],

the following explanation is stated:

The resinoid wheel by virtue of its design is slightly elastic and

with a compliant bond causes nascent diamond grits to protrude

slightly and cut with a fine depth of cut that results in large amount

of grinding streaks. These streaks are much more abundant in Ge

than in Si and glass. Though all are brittle materials, their brittle-

ness varies. Si and Ge become increasing ductile above 60% of the

absolute melting temperature, i.e., about 450°C for Ge and 750°C

for Si. An explanation of formation of ductile bands, along with

fractured bands, is shown in a modified form of Konig and Sinhoff s

model in Fig. 6.23.