Jackson M.J. Micro and Nanomanufacturing

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Microgrinding 271

Table 6.5. Some examples of specific abrasive types and their acronyms used in

standard wheel marking system [43, 44].

Abrasive group

Aluminum oxide

Silicon carbide

Cubic boron nitride

Diamond

Abrasive type

Brown fused alumina

White fused alumina

Black silicon carbide

Green silicon carbide

Cubic boron nitride metal coated

Cubic boron nitride microcrystalline

Natural diamond

Synthetic diamond

Synthetic diamond metal coated

Acronym

CBC

CBM

SDC

Boothroyd [45] suggested that the following general guidelines

be used for selection of a grinding wheel:

Choose aluminum oxide for steels and silicon carbide for

carbides and non-ferrous metals.

Choose a hard-grade wheel for soft materials and a soft-

grade wheel for hard materials.

Choose a large grit for soft and ductile materials and a

small grit for hard and brittle materials

Choose a small grit for a good finish and a large grit for

maximum metal removal rate.

Choose a resinoid, rubber, or shellac bond for a good

finish and a vitrified bond for a maximum metal removal

rate.

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Prefix Abrasive

type

Abrasive

grain

Structure

Grade

Bond

type

size

36-L-5-V-23

Manufacturer's

record

Manufacturer's

private marking

to identify wheel

B Resinoid

BF Resinoid reinforced

E Shellac

0 Oxychloride

R Rubber

RF Rubber reinforced

S Silicate

V Vitrified

Soft

ABCDEFGHIJKJJ

Open}*

etc.

(Use optional)

Medium Hard

MNOPQRSTUVWXYZ

Fig. 6.6. Standard marking system for conventional bonded abrasives wheels [44]

Microgrinding 273

D 100-P

- B

1/8

Prefix Abrasive Grit Grade Diamond Bond

type size concentration

modification

(in,)

Cubic boron nitride

A (soft)

Diamond

Z(hard)

100

25 (low)

Resinoid

50 M

Metal

75 V

Vitrified

100(high)

Manufacturer's

symbol

indicate type

diamond

150

1/16

1/4

Absence

depth

symbol indicates

solid diamond

A letter

numeral

combination used

here will indicate

variation from

standard bond

Fig. 6.7. Standard marking system for superabrasive bonded wheels [44]

274 Micro- and Nanomanufacturing

6.2.8 Mounted Grinding Wheels

Mounted points, sometimes known as mounted wheels or grinding

pins,

are commonly used as grinding tools for internal grinding op-

erations. Apart from the above applications, these wheels can be

used as a deburring tool to remove excess material after machining

processes, smoothing out casting risers, fins, and repair welds. They

have various types and shapes to suit different applications (Fig.

6.8(a-c)). Materials used for the tool can be in the form of tungsten

carbide and abrasive. Mounted tools made from tungsten carbide

(Fig. 6.8(b)) are commonly used for smoothing die cavities, cham-

fering corners and forming fillets [44], [46]. Abrasive type mounted

tools include aluminum oxide, silicon carbide, and diamond.

(a) (b)

•MfW^rlPHPPPltlTflTillllT

fcu AM x» *n Ait ui inn «IM nte vra» MI as* MM H> ni s«n •!» »i» *«* wiu wri« *-ia* wi»

(c)

Fig.6.8. (a) Different sizes of diamond-grinding pins that are usually used for in-

ternal grinding operations available both in resinoid and metal bond [47], (b)

Tungsten carbide deburring tools in various shapes ideally suited for fast stock

removal on hard materials [48], (c) Various shapes of aluminum oxide/silicon car-

bide mounted points used together with portable hand grinder for general purpose

grinding and deburring on most ferrous and non-ferrous materials [49, 50]

Figure 6.8 is interesting as it indicates the use of tungsten

carbide (WC) as an abrasive for deburring. WC abrasives are also

Microgrinding 275

used for grinding rubber such as fax machine rollers. This figure

also suggests a use of disc shape grinding wheels.

6.3 Conventional Grinding Processes

One machining process that developed as a precision process (fixed

abrasive) ahead of turning is grinding, which was followed by lap-

ping and polishing which may be considered as a high precision

loose abrasive grinding process. There are various types of conven-

tional grinding operations available today and some of them are

summarized in Fig. 6.9 In general, grinding operations are carried

out on the external and internal surfaces of workpieces by using ver-

tical or horizontal spindle grinding machines.

The development of hardened steel in the latter part of the nine-

teenth century created a need for a machine that was capable of fin-

ishing workpieces, which were as hard as cutting tools. This led to

the development of grinding machines, which over the years were

improved and modified to become the high-precision grinders of to-

day [52].

Surface grinding operations are commonly used for grinding flat

surfaces depending on the workpiece size and shape. Reciprocating

and rotating tables that have an electromagnetic holding surface are

usually employed for holding the workpiece (Fig. 6.10). Cylindrical

and centerless grinding operations are used to grind the outside di-

ameters of round surfaces (Fig. 6.11). These surfaces may be

straight, stepped or tapered. In cylindrical grinding, the work is held

between centers that are rotated at a much lower speed in a direction

opposite to that of the grinding wheel. However, in centerless grind-

ing operations, the workpiece is held between the grinding wheel

and a regulating wheel and a work rest blade. By tilting the rota-

tional axis of the regulating wheel with respect to the grinding

wheel, the workpiece is given a longitudinal force, creating move-

ment or through-feed that is helpful for automation.

276 Micro- and Nanomanufacturing

{»} Herizonlat-spitutte

surface grinding

$<j) Outsld* diamelef

cylindrical grinding

(g) Inside diameter

lorm grinding

(I) €t Merit it grinding

(m) Muitip!«wb*«i*/r»cipr<ic»tJns Ubl«

(q) Cropping (0 C!!cM$

Fig. 6.9. Examples of manufacturing processes using specific grinding operations

[51]

Microgrinding 277

WORK

The vertical spindle surface grinder

with a reciprocating table grinds on the

face of the wheel and is capable of taking

heavy cuts. This is good example of full

immersion grinding in which grinding

wheel is grinding 16 workpiece surfaces

at a time. The main advantage of full im-

mersion grinding are improved surface

roughness and dimensional accuracy.

RKP1ECES

Fig. 6.10. Vertical surface grinder [52]

The fundamental difference between centerless grinding and

most other forms of grinding is the fact that the workpiece is not

firmly held either on a magnetic chuck or between centers or in a

vice,

but rather moves across the rim under the combined action of

the grinding wheel, the regulating wheel and the blade. The grind-

ing pressure is generated dynamically by the difference in velocity

between regulating and grinding wheel.

Grinding wheel

-Wok rest Wader

Ressulatins

Fig. 6.11. One of the special external centerless grinding operations performed is

the grinding of bearing balls. In this process, the grinding wheel is fed against the

roughing wheel. Balls tend to rest on the work rest blade [53]

278 Micro- and Nanomanufacturing

Internal workpiece surfaces can be fine finished by internal

grinding operations. Typical examples of internal grinding are

enlarging a hole or bore size similar to that by boring operations

(Fig. 6. 9 (e and h)) and form grinding of inside diameters (Fig.

6.9 (g)). To machine a bore, the grinding rim attached to the periph-

ery of a supporting core of appropriate dimensions must be at the ex-

tremity of a shaft, and thin enough to permit easy entry into the bore

and long enough to allow machining of the bore over its full depth.

The overhang, which results in the underlying cause of a host of

problems characteristic of internal grinding, leads to an inherent lack

of stability and rigidity.

Slotting, slicing, cropping, and dicing operations can be called as

cut off operations illustrated in Fig. 6.9 (1, o, q ,r), respectively, are

also recognized as conventional grinding operations. In these grind-

ing operations, single or multiple-disks are arranged vertically on a

horizontal spindle to perform simultaneous cutting operations on the

workpiece that are held on a reciprocating table (Fig. 6.9m). The use

of superabrasive cut-off grinding (slicing) wheels has been increasing

substantially over the last 10 years, as harder and more expensive ma-

terials come on the market. These require, in a first machining step, to

be cut rapidly and cheaply to dimensions commensurate with the end

product. One high growth area has been, for instance, the cutting off

to shape of synthetic quartz ingots, which constitute the raw material

required for electronic watches.

A cut-off grinding wheel is essentially a very thin grinding

wheel, whose width usually lies in the vicinity of

mm (0.04 in) or

less.

The rim is mounted on the periphery of a metal disc, usually

made of steel. The primary requirement for such a cut-off wheel is

sufficient freeness of cut. The workpiece should be cut-off without

any substantial pressure build-up at the grinding interface [54].

Creep-feed grinding (Fig. 6.9 (c)) is a new form of grinding op-

eration different from other conventional grinding processes. In

creep-feed grinding, the entire depth of cut (infeed) is completed in

one pass by using a very small feed rate. It is a technique used to

grind a form into a workpiece in a single pass of the grinding wheel.

The workpiece is fed into the revolving grinding wheel, opposite to

the wheel rotation, at a slow, steady table feed rate. The wheel height

is set to the final size and the desired form is generally completed to

Microgrinding 279

size,

tolerance, and surface finish in one pass. It is possible to grind

profiles with depths of cut of 1.0 to 30 mm in one pass using work

speeds from 1.0 to 0.025 m/min [38].

Creep-feed grinding can be competitive with surface grinding,

milling, gear cutting, broaching, and other processes where heavy

stock removal is required. It is very effective where a precise, accu-

rate form is required, and profile accuracy is critical. The key advan-

tages to creep-feed grinding are increased productivity, better di-

mensional part accuracy, and less metallurgical damage to the

workpiece [52].

Double disk grinding (Fig. 6.9 (i)), edging operations on silicon

wafer periphery (Fig. 6.10 (n)) and flattening of silicon ingots (Fig.

6.9 (p)) are also considered as surface grinding operations but here

they involve different types of workpieces, size, and orientation.

Grinding operations are also often being used for removing burrs

and fine finishing of machined threads, and form grinding operations

of grooves on ball screws as shown in (Fig. 6.9 (j) and Fig. 6.9 (k)),

respectively.

Cubic boron nitride grinding wheels were first tested in 1957 on

difficult-to-grind (DTG) hardened tool and die steel cutting tools.

These steels are so hard and abrasion-resistant that they cause rapid

dulling of the conventional aluminum oxide abrasive. Because of

the exceptional hardness of cBN wheels, in tool grinding, tool

dimensions are accurately maintained with minimum downtime

for wheel maintenance. Grinding with cBN wheels improves the

fatigue strength and extends the useful life of the cutting tool by

removing heat from the surface of the workpiece.

6.4 Precision Grinding Processes

6.4.1 Upgrading of a Machining Center into Jig Grinding

for IC Chip Manufacturing

Jig grinding is basically vertical surface grinding process, which can

be condidered to one of the elite percision grinding processes (Fig.

6.12). The need for accurate hole locations in hardened work led to

280 Micro- and Nanomanufacturing

the development of the jig grinder. The name Jig grinding is given to

the process, which was used for grinding jig holes. The final accu-

racy of the drilled hole is largely dependent on the accuracy of the

jig used. This machine was specially designed for finishing the holes

with a high degree of accuracy on drill jig plates.

This precision grinding machine is equipped with a precision

hole grinding facility used to grind hard metals or ceramic work-

pieces. It uses aluminum oxide, diamond, or cubic boron nitride

grinding wheels to grind holes in hardened steels to precise location

and tolerances. It supplements other hole producing machinery. A

typical product is shown in Fig. 6.9 (h)

Often the clamping, the machining, or the hardening operation

would distort the workpiece and alter the hole locations so that they

were no longer accurate. Although the jig grinder was designed pri-

marily for accurately locating holes in hardened workpieces, it has

found wide use for the grinding of contour forms such as radii, tan-

gents,

angles, and flats. The machine's operation centers around a

high-speed air turbine with auxiliary electric rotating head with re-

ciprocating quill. [52]

The performance of any grinding wheel depends on the capacity

and working condition of the jig grinding machine. Jig grinding

wheels are available in various bond types and a wide variety of

styles to suit various jig grinding operations. It is important that the

proper wheel be selected to suit the workpiece material so that the

most efficient grinding can occur. The most important factors to

consider when selecting CBN wheels for jig grinding operations are

the abrasive type, bond, and grit size [45,52].

Continuous-path numerical control (NC) jig grinding requires

grinding abrasives that last a long time, retain their shape, produce

good surface finishes, and maintain size and form without thermal

damage to the workpiece. One factor that is impossible to program is

wheel wear. If

the

wheel loses shape, size, or stock-removal capabil-

ity while making a pass, an inaccurate form will be produced.