Jackson Mark. Machining with Abrasives

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insertion loss of 0.07 dB on average. Alcohol, propylene glycol diamond and

water-based alumina suspensions produced acceptable insertion losses, which are

twice of that for silica suspension. Water-based diamond suspension resulted in the

highest insertion loss, with the maximum value higher than 0.3 dB. The results show

that suspensions have a signiﬁcant effect on the geometrical quality and the optical

performance of the polished connectors. Using silica suspension gave the best

geometrical quality, including the ﬁnest ﬁber and ferrule surface ﬁnish, the smallest

ﬁber height and the comparable radius of curvature and apex offset, and hence the

best optical performance of the acceptable return loss and the signiﬁcantly small

insertion loss. Alumina suspension did not offer an advantage over alcohol in terms

of both geometrical and optical qualities. Diamond suspensions did not provide

better results either in terms of geometrical parameters and optical losses.

Better optical performance of a ﬁber-optic connector results from the well-

polished connector end face. This is because better ﬁber and ferrule surface ﬁnish,

smaller ﬁber height and apex offset, and reasonable radius of curvature of polished

connect end faces could provide a better physical contact between the two mated

connectors for optical communication.

100

200

300

400

500

600

ABCDE

Linear Apex Offset (mm)

Suspension Type

Fig. 8.23 Effect of

suspension on the linear apex

offset of polished ﬁbres

0.2

0.4

0.6

0.8

ABCDE

Return Loss

Insertion Loss

Return Loss (dB)

Insertion Loss (dB)

Suspension Type

Fig. 8.24 Effect of

suspension on optic

performance

8 Polishing Using Flexible Abrasive Tools and Loose Abrasives 365

In summary, with the judicious selection of polishing parameters and protocol,

the efﬁciency for polishing physical contact connectors can be improved at least

30%. The best polishing process involves only two-step polishing with a total cycle

time of 1 min. To achieve return loss of < 45 dB and insertion loss of <0.3 dB, the

polished ﬁber and ferrul e surface much have a roughness, R

, of smaller than 50 nm,

a ﬁber height or undercut of 25 or 75 nm, a radius of curvature ranging from 10 to

75 mm, a linear apex offset of smaller than 150 mm and an angular apex offset of

smaller than 0.2



8.3 Polishing with Free Abrasives

8.3.1 Polishing of Microbores Using Liquid Suspended

Abrasive Flow

Device miniaturization is one of the frontier technologies in the twenty-ﬁrst century,

which has brought about completely different ways in which human beings interact

with the physical world. The miniaturization of devices associated with a number of

ﬁelds demands for the production of micro-scale mechanical components with

manufactured features ranging from several to several hundred microns with tight

tolerances in a broad range of engineering materials. These ﬁelds mainly include

optics, electronics, medicine, bio-technology, communications and avionics.

The rapidly growing needs in making products from macroscopic to nanoscopic

scales from photonic, microelectronic and biomedical industries echoes the inter-

national research direction in modern manufacturing. Micromachining technologies

have attracted great attention of research in the fabric ation of miniaturized industrial

products that are extensively applied into our society [34–45].

In the making of the miniaturized products, a common challenge is from the

fabrication of microbores of diameters ranging from several tens to several hundreds

of microns. The microbore are a typical feature in various products, such as ﬂuidic

ﬁlters, grids, bio-medical ﬁlters, ink-jet printer nozzles, fuel injection nozzles,

optical ferrules, high-pressure oriﬁces, standard defects for testing materials, micro-

pipettes, pneumatic sensors and manipulators, guides for wire-bonders and spinning

nozzles, and fuel injection nozzles [34]. Great research efforts have been directed

towards developing the micromachining technologies for microbores in the past two

decades. Among them, micro electrical discharge machining (EDM) [40–42] and

laser machining [43–45] have been widely used. Other technologies for such

applications include microcutting [39, 46], ultrasonic machining [47], microforming

and micromolding [48], micro electro-chemical-machining (ECM) [34] and

micropunching [49]. There exist certain limitations in those technologies in terms

of capability of handling various materials and surface ﬁnish of micro bores. For

example, micro-EDM is only capable for making holes on conductive or semi-

conductive materials. Laser machining has limitation to obtain microholes with

366 H. Huang et al.

large (length/diameter) aspect ratios. For major ity of the technologies mentioned

above, bore surface quality is not satisfactory and a ﬁnal ﬁnishing process is still

required to meet a stringent quality requirement on microbores.

The challenges for polishing microbores are obvious. Bores are so small that

conventional polishing tools cannot be manipulated inside the holes, so does the

characterization of machined surface quality. Consequently, non-conventional

methods for tooling, machining and char acterization must all be developed. Polish-

ing using a slurry ﬂow is one of the useful methods for polishing microbores [50–52 ].

In this section, we introduce an abrasive ﬂow technology developed for polishing

microbores. The bore diameter investigated varied from 260 to 500 mm and the

length-diameter ratio ranged from 25 to 50. Both metal and ceramic materials

were investigated. Three-dimensional characterization methods involving stylus

proﬁlometry and optical interferometry were also employed to investigate the

effects of ﬂow parameters on inner wall surface roughness and topography.

8.3.1.1 Experimental Appar atus and Worki ng Principle

The experimental apparatus consists of a motor-driven hydraulic pump that is

capable of generating pressure up to 40 MPa, a direction control valve, a hybrid

cylinder, a work unit, two pressure gauges and a slurry tray. In this work, hydraulic

oil 32 is pressurized in the drive power unit and supplied to the hybrid cylinder

through a manually operated directional control valve, which pushes abrasive slurry

going through a microbore to be polished. As illustrated in Fig. 8.25, the workpiece

is precisely aligned in the work unit. The pre-ﬁlled abrasive slurry is forced to ﬂow

through the microbore, which is deﬁned as one polishing pass. When one polishing

pass is ﬁnished, the used slurry ﬂows to a tray and fresh slurry is added to the

slurry portion of a hybrid cylinder for the next pass of polishing. O-rings are doubly

inserted between the two ends of the workpiece in order to avoid leaking of slurry

under high ﬂow pressure.

Abrasive ﬂow polishing is in fact a slurry ﬂow in turbulent ﬂow regime through a

microhole [53]. The critical Reynolds number for turbulent ﬂow was 2,300 in this

Fig. 8.25 Illustration of slurry ﬂowing route

8 Polishing Using Flexible Abrasive Tools and Loose Abrasives 367

case, which was calculated using the equation [54]: R

¼UD/n, where R

is the

Reynolds number, U the mean velocity in m/s, n the kinematic viscosity in m

/s and

D the hydraulic diameter in meters. In the apparatus developed , U varied from 12 to

29 m/s, n was kept at 1.1 10

6

/s, and D ranged from 260 to 500 mm. So the

Reynolds number varied from 8,390 to 9,090, much higher than the critical number

of 2,300 for turbulent ﬂow.

The test materials selected include 304 stainless steel, S45C steel and partially

stabilized zirconia. All the specimens have a cylindr ical shape with a leng th of

13.6 mm and outer diameter of 6 mm. Diameters of bores are 500 mm for stainless

steel specimens, 400 and 500 mm for S45C and 260 mm for zirconia. The bores in

specimens made of stainless steel and steels were drilled, but the holes in zirconia

specimens were formed by micro injection molding. Polishing conditions are

summarized in Table 8.6. The pressure in abrasive ﬂow polishing was kept constant

at 10 MPa. Polishing slurry used alumina abrasives of grit size of 17.5 mm with a

concentration of 3.44 vol.%. Flow speeds applied were 19.3 m/s for the 500 mm

holes, 25 m/s for the 400 mm holes and 35.5 m/s for the 260 mm holes. Polishing

passes of 5, 10, 15 and 20 were used.

8.3.1.2 Surface Characteristics and Roughness of Polished Bores

Figure 8.26 show the inner wall surfaces for the stainl ess steel specimens.

Figure 8.26a is the typical surface produced by drilling as the drilling marks are

observed and the surface has long grooves parallel to the drilling direction . When

pressurized abrasive slurry passes the hole, the plastic groove s are worn awa y due

to impact of abrasive particles. The drilling marks are gradually diminished when

the polishing pass is increased, as shown in Fig. 8.26b–d.

Figure 8.27 shows the inner wall surface changes of S45C steel with bores of

500 mm in diameter. Similar to stainless steel specimens, the bores were ﬁrst drilled.

As shown in Fig. 8.27a, the drilled surface is rough, but the drilling grooves are not

apparent. The surface ﬁnish after ﬁve polishing passes appears to be improved

Table 8.6 Polishing

conditions

Workpiece (13.6 mm in length,

6 mm in outer diameter)

304 stainless steel, 500 mm

inner diameter

S45C steel, 400 and

500 mm inner diameters

Zirconia, 260 mm inner

diameter

Abrasive ﬂow Alumina of grit size of

17.5 mm

Concentration: 3.44 vol.%

Pressure: 10 MPa

Solution: water

368 H. Huang et al.

signiﬁcantly, as can be seen in Fig. 8.27b. After that, the surface ﬁnish improves

continuously and gradually with the progress of polishing, as shown in Fig. 8.27c, d.

For comparison, the inner wall surfaces of S45C steel with bores of 400 mmin

diameter is shown in Fig. 8.28. The initial surface of inner wall of the bore in

Fig. 8.26 White light interferometer images of inner wall surfaces of 304 stainless steel

specimen (500 mm in diameter) (a) prior to and after different abrasive polishing of (b)5,(c)10

and (d)15 passes

Fig. 8.27 White light interferometer images of inner wall surfaces of S45C steel specimen

(500 mm in diameter) (a) prior to and after different abrasive polishing of (b)5,(c) 10 and

(d) 15 passes

8 Polishing Using Flexible Abrasive Tools and Loose Abrasives 369

Fig. 8.28a appears to be discontinuous and rough, similar to that of the 500 mm inner

wall in Fig. 8.27a. Figure 8.28b–d show that the wall surface ﬁnish is gradually

improved with the increasing polishing pass. However, the roughness improvement

is slower than that in the polishing of microbores with greater diameters.

Figure 8.29 shows the inner wall surfaces of zirconia specimens with bores of

260 mm. In Fig. 8.29a, the powder-injection moulded bore surface after sintering

exhibits to have defects, such as porosity. Pores are randomly distributed in the

zirconia material. Apparently, porosity would contribute to the surface roughness

measured. It is difﬁcult to remove the pores via polishing. As shown in Fig. 8.29b, a

relatively large pore of about 20 mm in diameter is observed on the wall surface

after polished ﬁve passes. However, there are no pores observed on the bore

surfaces polished 10 and 20 passes, as shown in Fig. 8.29c and d. It is also clear

that an increase in polishing pass results in a much smoother surfaces.

Values of surface roughness of stainless and S45C steels with bores of 500 mmin

diameter are plotted against polishing pass in Fig. 8.30. There is a signiﬁcant

difference in their initial surface roughness, with the S45C steel having a much

rougher surface than the stainless steel after drilling. After ﬁve polishing passes,

their surfaces become quite similar in terms of roughness. Further polishing

improves the surface ﬁnish, but with very limited improvement. It is apparent

that at the late stage of polishing, the trends of surface ﬁnish improvement for the

two materials become similar. The surface roughness of stainless steel bores drops

from 1.58 to 0.48 mm, with 70% of improvement after 20 polishing passes.

However, the roughness of S45C steel bores reduces from 4.02 to 0.78 mm after

20 passes, with a more signiﬁcantly improvement rate.

Fig. 8.28 White light interferometer images of inner wall surfaces of S45C steel specimen

(400 mm in diameter) (a) prior to and after different abrasive polishing of (b)5,(c) 10 and

(d) 15 passes

370 H. Huang et al.

The values of surface roughness of the S45C steel bores of 400 and 500 mmin

diameters are compared and shown in Fig. 8.31, together with the roughness values

of zirconia bores. It is seen that the starting surface roughness for the larger S45C

bore is much greater than that of the smaller bore. However, after polishing the

surface of the larger bore is more signiﬁcantly improved than the smaller bore with

the progress of polishing. This suggests that the bore diameter plays important role

during polishing. In the polishing processes studied, both the ﬂow pressure and the

ﬂow volume per pass were kept constant, leading to different abrasive ﬂow speeds

of 19.3 and 25 m/s for 500 and 400 mm bores, respectively. Though the average

Fig. 8.29 White light interferometer images of inner wall surfaces of zirconia specimen (260 mm

in diameter) (a) prior to and after different abrasive polishing of (b)5,(c) 10 and (d) 20 passes

0 5 10 15 20

Stainless steel

S45C steel

Roughness, R

(

Flow Pass (times)

Fig. 8.30 Average surface

roughness plotted as a

function of polishing pass for

different materials. Bore

diameter ¼500 mm

8 Polishing Using Flexible Abrasive Tools and Loose Abrasives 371

velocity was greater for the smaller bore, however, its velocity gradient dropped

more sign iﬁcantly. In other words, the abrasive ﬂow velocity near the wall surface

in the smaller bore could still be lower. This means that the polishing of a smaller

bores is more difﬁcult. The improvement of surface roughness of zirconia bores is

signiﬁcant too, decreasing from 3.5 to 0.5 mm after 20 polishing passes. The

decreasing rate of roughn ess is similar to that of the S45C of 500 mm bore though

its bore diameter is only 260 mm. This suggests that a different material removal

rate existing in S45C steel and zirconia, with the later more easily to be removed in

the abrasive ﬂow polishing process.

The results shown in Figs. 8.30 and 8.31 indicate that abrasive ﬂow polishing can

signiﬁcantly improve the inner wall surfaces of stainless steel, S45C steel and

zirconia bores of less 500 mm inner diameters. The surface roughness for all the

materials investigated is reduced with the progress of polishing, yielding at least

60% improvement in surface ﬁnish. The roughn ess decreasing rate appears to be

relevant to both bore size and mater ial.

Based on the results from the topographic analyses in Figs. 8.26–8.29, it is clear

that the material removal in the polishing of stainless steel, S45C steel and zirconia

is mainly due to abra sion. The abrasive ﬂow polishing process developed can be

considered as a lapping operation where abrasive slurry becomes a self-forming

grinding stone as it proceeds through the inner wall of the microbores [54–56]. The

maximum load that can be applied to the abrasive slurry is the cross sectional area

of the restricting inner area multiplied by the applied pressure. Some of this force is

consumed in pushing the abrasive slurry onto the inner wall and in getting the slurry

ﬂow through the inner wall (i.e. friction). The remaining force is applied to the

cutting grains of abrasives at the interface between the abrasive slurry and the inner

wall surfaces. The abrasive ﬂow polishing ﬁnishes surfaces by forcing abrasive

slurry through microbores with high pressure. The pressurized slurry interacts with

the inner wall surface of the microbores, thus eroding bore material gradually.

During polishing, the abrasive particles suspended in the slurry ﬁrst erode away the

sharp spots on the wall surface, thus yielding a more uniform, reliable and smooth

surface. The material removal is thus gentle and uniform, like honing the inner wall

-50 510152025

S45C, 500

S45C, 400

ZrO

, 260

Roughness, R

(mm)

Flow Pass (times)

Fig. 8.31 Average surface

roughness plotted as a

function of polishing pass for

different bore diameters

372 H. Huang et al.

surfaces. Bores of various materials, such as soft steel, stainless steel and ceramics,

can be successfully polished using this technology.

In summary, the abrasive ﬂow polishing demonstrates to be a feasible technique

for polishing microbores made of stainless steel, mild steel and ceramic material.

It can be certainly used to polish microbores of 260 mm or larger. The surface

roughness of the polished microbores decreases with the increase in pass of slurry

ﬂow, but the improveme nt is gradually slowed down. The polishi ng efﬁciency

appears to be relevant to bore material and bore diameter, but less dependent on

their initial surface status.

8.3.2 Polishing of Free-Form Component with Free Abrasives

In manufacturing industries, numerous components of free forms that need to be

machined and polished. Those components normally have a complex shape, which

is not well deﬁned. There are stringent quality requirement on surface ﬁnish,

however, almost no demand in terms of geometric accuracies. A copper plated

egg component is a typical component in this category. The copper plated eggs are

formed using an electroplating technique, which ﬁrst forms a layer of copper with a

thickness of 1–2 mm surrounding real egg shells. After forming, abrasive belt

polishing and bufﬁng are required prior to the ﬁnal gold plating. In the production,

the polishing/bufﬁng processes of copper eggs are manually performed. A highly

skilled worker can only polish 60 components or buff 30 components each day.

In addition, as the belt polishing and bufﬁng requires very skillful operators, the

maintaining of consist quality is of great challenge. The automation of the grinding

and bufﬁng processes has thus become signiﬁcant.

In this section, we report a study of the ﬁnishing processes using tumbling

polishing with free abrasives for free-f orm copper components. The study focuses

on the development of tumbling methods, which includes a detailed investigation of

the effects of polishing variables, such as mill type, abrasive type and mesh size and

the rotation speed, on the polishing efﬁciency and surface ﬁnish.

8.3.2.1 Tumbling Method and Apparatus

Tumbling has been extensively used in mineral processing for reﬁning mines and

particles. The process involves numerous collisions among the mines or particles

charged in a tumbling container or tumbler, which reﬁnes or breaks the mines and

particles. In the polishing process that uses the tumbling method, abrasives are

charged into a container together with components to be machined. The collisions

between the abra sives and the components will erode the unnecessary material of

the components away and polish the component surface. Apparently, this method

can handle workpieces with irregular shapes. The energy level of collisions can be

controlled by adjusting the tumbling conditions.

8 Polishing Using Flexible Abrasive Tools and Loose Abrasives 373

The tumbling equipment used here include laboratory ball mills, which is

slightly different from the conventional one in the mining industry. Smaller and

softer (plastic) containers are used to avoid the damage caused by the collision

between the components and container wall. Three types of tumbling mills are used

for polishing free-form copper eggs, which are described as follows.

The schematic illustration of a typical tumbling mill is shown in Fig. 8.32. The

mill consists of two rollers and a cylindrical container. The roller A is driven by an

electric motor with adjustable rotational speeds and the roller B is free for rotation

along the axis. The container C is freely put on the gap between the two rollers and

it is driven to rotate by the roller A. The free-fo rm components and abrasives

charged in the container thus rotate or move alongside their own trajectories, being

dependent on the rotation speed of the container. The workpiece material can be

removed by micro-cutting, micro-ploughing, micro-grooving or fatigue cracking,

which is determined by the component material and energy level of collision due to

the relative movements of the components and abrasives, resulting in different

polishing effect.

A planetary mill is another type of tumbling mill. It involves the rotational

movement of a container about two separate parallel axes, analogou s to the motion

of the earth around the sun. As illustrated in Fig. 8.33, the planetary container that is

charged with the components and abrasives rotates about the point A, at the same

time, also rotates about the axis O of itself. This results in a complex movement to

both component and abrasives. As a very high rotational speed of the container can

be operated, the energy level of the planetary mill is much higher than that of the

tumbling mill. Thus the planetary mill can be used for the applications that requires

relatively larger material removal.

The vibratory mill has the simplest structure for tumbling. As shown in Fig. 8.34,

a container charged with components and abrasives as is loaded on a vibrating

platform, which is sinusoidally driven by a slider-crank mechanism with an electric

motor. The components and the abrasives also vibrate inside the container. The

relative motions between them produce the polishing effect. Polishing efﬁciency

and production capacity can be controlled by adjusting the frequency and amplitude

of the moving platform.

copper egg

abrasives

Fig. 8.32 Illustration of

tumbling method for polishing

free-form component

374 H. Huang et al.