Jackson M.J. Micro and Nanomanufacturing

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Mechanical Micromachining 251

increased to

1,000,000

rpm then the orders of magnitude are in-

creased further still.

The experimental results and observations provide an interesting

view of machining a variety of metals at the microscale. When one

considers the approximations made in the derivation of the chip curl

model, the experimentally measured results compare well with the

calculated chip curl. This indicates that cutting tool bending con-

tributes significantly to initial chip curl prior to any significant fric-

tional interactions on the rake face of the cutting tool. The proposed

model describes the initial stages of chip curl quite well. If the de-

scription of chip curl is accurate, then continuous chip formation at

the microscale needs to be re-investigated. If one considers the

movement of the cutting tool (Fig. 5.4), from point A toward point

we expect the shear plane to oscillate between AC and HC de-

pending on the amount of energy required to move the built-up edge

into the segment of the subsequent chip. The cycle begins again

when accumulated metals are deposited to the edge of the cutting

tool, then to the subsequent segment of the chip produced during

machining. Under the conditions specified, material removal rates

were measured for three important engineering materials. The re-

moval rates were 30 mm

/min for aluminum alloys, 36 mm

/min for

copper alloys, and 48 mm

/min for steel.

The development of high-speed spindles using air turbine tech-

nology clearly plays an important role in the development of very

high strain rate machining applications. These applications will have

a significant impact on the automotive, mold and die tool, and aero-

space sectors, in addition to the medical device industry for machin-

ing bone prior to prosthetic implantation and of

course,

the manufac-

ture of micro-and nanofluidic devices.

5.9 Conclusions

The equations of metal cutting can be applied in the high-speed

micro-scale environment. The nomographs of Merchant and Zlatin

[3] can be applied confirming that future calculations can be com-

pared to these well-constructed charts. High strain rates change the

252 Micro- and Nanomanufacturing

mechanism of chip formation, thereby altering the shape of the chip.

Also,

high strain rates appear to provide less dependence on material

properties in determining chip formation and shape. A model of

chip curl at the microscale has been developed and agrees well with

experimental data. It appears that the bending of the cutting tool

contributes significantly to the primary chip prior to significant fric-

tional interactions on the rake face of the cutting tool. It is shown

that primary chip curl is initiated by the amount of material depos-

ited onto the cutting tool that manifests itself as a wedge angle that

controls the amount of material pushed into the base of the segment

of the chip between oscillations of the primary shear plane. Further

studies on chip formation at the atomic scale are needed to develop

nanomanufacturing processing methods for metals and other engi-

neering materials. High strain rate machining of structural steels can

be achieved but only with increases in spindle speeds. The future

development of this technique lies in the ability to rotate cutting

tools at extremely high spindle speeds.

5.10 Future Developments

The development of micromachining processes is focused on the

construction of high-speed spindles and stable machine tools. In ad-

dition to the basic construction of the machine tool

itself,

the devel-

opment of tool coatings on tools other than milling cutters will be of

paramount importance.

The construction of reconfigurable micromachining centers will

also form the basis of combining a number of different micro-

machining processes into one unit that will reduce manufacturing

costs.

In this way, "top down" and "bottom up" micro-and nano-

manufacturing processes could be realized. Therefore, the develop-

ment of hybrid micro-and nanomachine tools may be a step forward

in the right direction.

Mechanical Micromachining 253

5.11 Problems

Explain how microfluidic devices work.

Why are microfluidic devices manufactured?

What are the challenges facing traditional silicon micro-

machining processes when producing complex-shaped mi-

cro-and nanofluidic channels?

What is a lab-on-a-chip?

What is the Coanda effect?

6. How are mixing channels machined using micromilling

tools?

Describe the method of modeling initial chip curl.

8. What effect does micro-tool bending have on initial chip

curl?

9. Describe the observation on chip formation in micromachin-

ing.

10.

Describe the developments in meso-micro-machine tools and

what characteristics do they have compared with conven-

tional machine tools?

11.

Why are high-speed air turbine spindles useful for micro-

machining?

12.

Describe the different ways that machine tools are designed.

13.

Why do burrs form and how can they be eliminated?

14.

Explain why burr formation affects the mating of microparts.

References

Gravesen P, Branebjerg J, and Jensen OS,

1993,

Microfluidics - A Re-

view, Journal of Micromechanics and Microengineering, 3, p.p.

168-182.

Kanjakar K, Cui J, and Jackson MJ, 2004, Design and Analysis of

High-Speed Spindles For Nano-Machining Applications Using a

Computational Fluid Dynamics Approach, Proceedings of the

ASME South Eastern Region XI Technical Journal, 3, 1,

p.p.

1.1 -

1.8.

254 Micro- and Nanomanufacturing

Shaw MC, 1996, Metal Cutting Principles, Oxford Science Publica-

tions - Series on Advanced Manufacturing, Clarendon Press, Uni-

versity of Oxford, p.p. 18-46.

Bowden FP and Tabor D, 2001, The Friction & Lubrication Of Solids,

Oxford Science Publications, Clarendon Press, University of Ox-

ford, p.p. 168,327.

Doyle ED, Home JG, and Tabor D, 1979, Frictional interactions be-

tween chip and rake face in continuous chip formation, Proc. Roy.

Soc,

of London, A366, 173-183.

6. Jawahir IS, and Zhang JP, 1995, An analysis of chip formation, chip

curl and development, and chip breaking in orthogonal machining,

Trans.

North Amer.

Manuf.

Res. Inst. - Soc.

Manuf.

Eng., 23, 109

-114.

Kim CJ, Bono M, and Ni J, 2002, Experimental Analysis of chip for-

mation in micro-milling, Trans. North Amer.

Manuf.

Res. Inst.-

Soc.

Manuf.

Eng., 30, 247 - 254.

8. Komanduri R, Chandrasekaran N, and Raff LM, 2001, Molecular dy-

namics simulation of the nanometric cutting of silicon, Philosophi-

cal Magazine, B81, 1989-2019.

9. Luo X, Cheng K, Guo X, and Holt R

2003,

An investigation into the

mechanics of nanometric cutting and the development of its test

bed, Int. J. Prod. Res., 41, 1449-1465.

6. Microgrinding

6.1 Introduction

Microgrinding is an aspect of advanced manufacturing technology

that has been growing rapidly and playing an important role in

manufacturing [1]. Precision levels in machining can be classified

into four categories [2]:

i. Normal machining

ii.

Precision machining

iii.

High-precision machining

iv. Ultra-high precision machining

Figure 6.1 shows the progress of achievable machining accuracy

in each category as a function of time (years). Notable events that

led to a rapid surge toward high precision are [3]:

Taylor 1906 - 26 High-speed steel tools

Schroeter 1939 Tungsten/ titanium carbide tools

Merchant 1945 Mechanics of machining

Gilbert 1960 Economics of machining

For example, terms used for precision machining 30 years ago

have become normal machining today. As the machining accuracy

increases, energy processes become more relevant for the future ma-

chining processes to produce extremely accurate machining compo-

nents.

These processes include laser beam machining (LBM), elec-

256 Micro- and Nanomanufacturing

tron beam machining (EBM), and focused ion (FIB) beam that do

not require sharp tools to remove materials. Choice of materials and

machining processes are directly affected by the machining achiev-

able accuracy since different processes provide different range of

tolerance bands. Taniguchi's Table 6.1 [2] classifies components

into mechanical, electronic, and optical system according to preci-

sion levels and tolerance limits.

For instance, optical lenses and mirrors are manufactured to pre-

cise and ultraprecise machining levels. Most of these applications

require a crack-free surface [5]. Generally, hard and brittle materials

such as glass, silicon and germanium are commonly used for making

these products [6]. Traditionally, optical glasses require grinding and

finishing by a polishing process in order to remove the damage

caused by the previous operation and to obtain a mirror-like surface

[7].

The conventional ground surface of glass results in a matt finish

due to brittle fracture during removal process and this surface needs

to be polished to less than 20 nm

max

to reduce absorption and scat-

tering of light on the glass surface [8]. However, advances in preci-

sion machining of brittle materials have led to the discovery of a

"ductile regime" of operation in which material removal is by plastic

deformation. Fracture mechanics predicts that even brittle solids can

be machined by the action of plastic flow, as is the case in metal,

leaving crack free surfaces when the removal process is performed

at less than a critical depth of cut [9]. This means that under certain

controlled conditions, it is possible to machine brittle materials such

as ceramics and glass using single-point diamond tools so that mate-

rial is removed by plastic flow, leaving a smooth and crack-free sur-

face.

It has been reported that almost 100% ductile mode machining is

possible when machining hard materials using a well defined ge-

ometry of single-point single-crystal diamond tools on a rigid ultra-

precision turning machine [12]. Ductile regimes are realized on a

machined component that exhibits a mirror-like finish with

nanometric roughness, with crack free smooth surfaces and continu-

ous ribbon chip generation during turning

[5,13].

Although ductile

mode cutting can be achieved through the application of this ad-

vanced technology, the rapid tool wear continues to present prob-

lems [14,15]. In order to overcome these problems, multi-point cut-

Microgrinding 257

ting (grinding) therefore becomes more economical especially when

machining hard and brittle materials

[4,14].

Ultraprecision surface

grinding with electrolytic in-process dressing (ELID) provides in-

process dressing of the wheel [8,16,17], achieving almost 100% duc-

tile regime machining with a finish surface without the need for sub-

sequent polishing when grinding optical glasses and silicon-based

materials. With conventional grinding machines, less than 90% duc-

tile mode grinding is achievable because of lack of in-process dress-

ing and therefore requires subsequent polishing [12].

Time

Fig. 6.1. Modified Taniguchi chart [2] of achievable machining accuracies as a

function of time with insertion of landmark events that led to an upsurge in ma-

chining activities

258 Micro- and Nanomanufacturing

Table 6.1. Modified Taniguchi table of various

ent achievable tolerance bands with surface

products categorized in differ-

finish [4,10,11]

Precision

levels

Normal

machin-

ing

Precision

machin-

ing

Ultrapre-

cision

machin-

ing

Toler-

ance

band

200

jam

50 jam

5 um

0.5

0.05

0.005

urn

Mechanical

Normal domes-

tic appliances

and automotive

fittings etc.

General purpose

mechanical parts

for typewriters,

engines etc.

Mechanical

watch parts, ma-

chine tool bear-

ings,

gears, ball-

screws, rotaiy

compressor

parts.

Ball and roller

bearings, preci-

sion drawn wire,

hydraulic servo-

valves, aerostatic

bearings, ink-jet

nozzles, aerody-

namic gyro bear-

ings

Gauge blocks,

diamond inden-

tor tip radius,

microtome cutter

edge radius.

Ultraprecision

parts (plane,

ball, roller,

thread), Shape

(3-D)

Electronic

General purpose

electrical parts,

e.g., Switches,

motors and con-

nectors.

Transistors, di-

odes,

magnetic

heads for tape

recorders.

Electrical relays,

resistors, con-

densers, silicon

wafers, TV color

masks.

Magnetic scales,

CCD,

Quartz os-

cillators, mag-

netic memory

bubbles, magne-

tron, 1C line

width, thin film

pressure trans-

ducers, thermal

printer heads,

thin film head

discs.

IC memories,

electronic video

discs,

LSI.

VLSI,

super-lattice thin

films.

Optical

Camera, tele-

scope and

binocular

bodies.

Camera shut-

ters,

lens

holders for

cameras and

microscopes.

Lenses,

prisms, opti-

cal fibre and

connectors

(multi mode)

Precision

lenses, opti-

cal scales, IC

exposure

masks (photo

x- ray), laser

polygon mir-

rors,

x- ray

mirrors, elas-

tic deflection

mirrors,

monomode

optical fibre

and connec-

tors.

Optical flats,

precision

Fresnel

lenses, opti-

cal diffrac-

tion gratings,

optical video

discs.

LUtrapreci-

sion diffrac-

tion gratings.

Surface

finish R

Shaping

12.5-16

Milling

6.3

0.8

Reaming

3.2-0.8

Turning

6.3

0.4

Drilling

6.3-0.8

Boring

6.3

0.4

Laser

6.3

0.8

ECM

3.2-0.2

Grinding

3.2-0.1

ELID

0.6 - 0.2

Honing

0.8 -0.1

Super

finishing

0.2

0.025

Laping

0.4

0.05

ISO

Toler-

ance

IT 10

IT 9

IT 5

IT 6

IT 9

IT 6

IT 5

IT 3 -

IT 5

IT 3 -

IT 4

IT 1 -

IT 2

Microgrinding 259

Several models have been put forward to explain ductile mode

theory in real machining processes. Scattergood and his colleagues

[18,

19, 20] proposed a critical depth of cut and a feed rate concept

for ultraprecision machining as shown in Fig. 6.2.

Chip cross section

(per workpiece rev)

Uncut

shoulder

Microfracture

damage zone

Cut surface plane

Tool center

Damage transition line

Fig. 6.2 Scattergood's model of ultraprecision machining showing 3-D view of

diamond tool while cutting material and a cross-sectional view of the tool and

workpiece [10]

260 Micro- and Nanomanufacturing

An initial model was developed based on indentation fracture

mechanics analysis [9]. According to Scattergood, fracture initiation

plays a central role for ductile-regime machining. A critical penetra-

tion depth,

for fracture initiation was derived by Scattergood and

Blake [18] and is:

(6.1)

Where K

is fracture toughness, H is hardness and E is elastic

modulus. P is a factor that will depend upon geometry and process

conditions such as tool rake angle and coolant.

A round nosed diamond tool moves through the workpiece as

shown in Fig. 6.2. Figure 6.2 shows a projection of the tool perpen-

dicular to the cutting direction. Using the critical depth concept,

fracture damage will initiate at the effective cutting depth d

= d

as shown in Fig. 6.2) and will propagate to an average depth y

shown. If the damage does not continue below the cut surface plane,

ductile regime conditions are achieved. The cross feed /determines

the position of d

along the tool nose. Larger values of/move d

closer to the tool centerline. It is important to note that when ductile

regime conditions are achieved material removal still occurs by frac-

ture.

The model proposed was verified by interrupted tests and the

following relationship was obtained [10]:

Z - f

(6.2)

where R is the tool nose radius and the other parameters are defined

as shown in Fig. 6.2. For a typical example using [100] Ge single

crystal, it was found that d

= 130 nm and>'

= 1300 nm when using

equation 6.2 with a tool having a nose radius of 3.175mm and -30°

rake angle.