Jackson M.J. Micro and Nanomanufacturing

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Mechanical Micromachining 211

which would indicate that cutting conditions at high speed are stable.

Single chip formations are shown in Fig. 5.7. While the width ob-

served is similar to that for low-speed cutting, Fig. 5.7a is compared

to the high-speed chip form in Fig. 5.7b. The chip length of high-

speed chips is much shorter than of low-speed chips. This could be

because at low speed the chip has a greater time in contact with the

workpiece thereby removing more material, which is reflected in the

increased chip length.

Fig. 5.6. Characteristic chip shape at (a) low and (b) high cutting speeds

Fig. 5.7. Individual chip forms at (a) low and (b) high cutting speeds

212 Micro-and Nanomanufacturing

One of the major differences observed between low- and high-

speed milling is in the spacing of the lamellae. In low speed cutting,

the chip spacing varies by a significant amount. However, at high

cutting speeds the spacing is regular in period. Part of the mecha-

nism of chip removal is the forced alignment of dislocations result-

ing from the impact of the cutting tool against the workpiece. At

high speeds this process is accelerated to an extremely high level as

the strain rate calculations have shown. In fact experiments show

that chip types are similar in different materials such as copper,

brass,

and most notably cancellous bone. This suggests that high

strain rates induce a material removal mechanism independent of the

material

itself.

Figure 5.8 shows cancellous bovine femur machined

at high speed. The closely spaced lamellae seen in metallic chips are

exhibited here and the spacing is at a similar value, around 10 |nm.

This is a most unexpected result when one considers how structur-

ally different these materials are.

Fig. 5.8. Cancellous bovine femur that has been machined at high speed (250,000

rpm)

Mechanical Micromachining 213

5.4.3 Micromachining Results

The results of machining certain metals at the microscale are

compared to the model described for primary chip curl during the

primary stages of metal cutting. It should be noted that all results

presented in Table 5.1 are for metals machined in an oxidizing envi-

ronment. Table 5.1 shows the results for micromachining using a

variety of rake angles. It should be noted that bending of the cutting

tool produces a less acute rake angle when machining takes place.

However, the shear plane angle is increased and larger chips are

produced.

Under the conditions specified, material removal rates were

measured for three important engineering materials. The removal

rates were 30 mm

/min for aluminum, 36 mm

/min for copper, and

48 mmVmin for steel. Removal rates of this magnitude can also be

used for machining molds and dies for use in micro-molding proc-

esses such as injection molding and hot embossing. Unfortunately,

at certain speeds and depth of cut there is a problem with burr for-

mation.

5.4.4 Burr Formation in Micromachining

Burrs in micromachining occur because the chip that is formed is

not removed at the end of the machining path. Burrs are hazardous

and can interfere with the assembly of parts of components and

products. In conventional machining, burrs are simple to remove but

not so at the microscale. Unlike macroscale processes, there are no

handbooks on cutting at the microscale in order to determine optimal

machining conditions. Tool run-out is also a problem at the micro-

scale because of the way that the tool bends when machining. In ini-

tial experiments by Lee, Stirn, and Essel, machining 6061 aluminum

alloy indicate that the cutting edge to workpiece interaction is ap-

proximately D<R, where D is the radial feed and R is the radius of

curvature at the cutting edge. The initial burr is shown in Figure 5.9.

214 Micro-and Nanomanufacturing

Fig. 5.9. Microdrilling burr with inner cap fragment attached. Microdrill diameter

is 100 |iim

Two different tools were used to machine at this scale, 254 |um

and 508 |um. Because smaller tools are used, a much higher spindle

speed is required. Tools are shown in Figure 5.10 and the cutting

parameters are shown in Table 5.2.

Figure 5.11 shows the effect of cutting edge and workpiece inter-

action for a given curvature of radius of the cutting edge. Milled

channels resulting from the experiments conducted by Lee and oth-

ers are shown in Figure 5.12. The experimental observations indi-

cated that exit burrs are larger than the entrance burrs, large tool feed

marks can be seen on the surface of the milled track, with large

depths of cut, chips adhered to the side of the workpiece, built-up

edges are created when cutting time is large, and higher feed rates

are accompanied by large burrs.

Mechanical Micromachining 215

Table 5.2. Microcutting parameters compared with conventional machining pa-

rameters

Handbook

Dia.

32.5

0.254

0.508

DOC

0.127

0.254

0.508

RPM

3183

2653

1768

979

7500

SPM

m/min

100.00

5.98

11.97

chip load

um/tooth

50.000

75.000

130.000

150.000

0.085

0.339

0.085

0.339

0.169

0.677

0.169

0.677

chip load/D|

0.0050

0.0063

0.0072

;

0.0046

0,0003

0.0013

0.0003

0.0013

0.0003

0.0013

0.0003

0.0013

Fig. 5.10. Micrographs of microcutting tools

216 Micro-and Nanomanufacturing

tool

workpiece

flat

cutting

direction

W:*M

cutiJng

direcHwi

workpiece

radius

curvature

of the cutting

edg*

D: radial feed

Fig.

5.11.

Cutting edge-workpiece interaction for different radial feeds at the

given radius of curvature of the cutting edge

Mechanical Micromachining 217

?ff^

Fig. 5.12. Milled slots in 6061 aluminum alloy showing tool marks and exit burrs

218 Micro- and Nanomanufacturing

Gillespie postulated that burrs are classified in the following terms:

Tear burrs; rollover burrs, Poisson burrs, and cut-off burrs. A tear

bear is the result of material tearing loose from the workpiece rather

than shearing. The rollover burr is essentially a chip that is bent

rather than sheared, resulting in a comparatively large burr. This

type of burr is also known as an exit burr. The Poisson burr is one

that results from the material's tendency to bulge at the sides of the

machined cut when it is compressed until permanent plastic defor-

mation takes place. Figure 5.13 shows the tear and rollover burr

formations.

Fig. 5.13. Tear burr and rollover burr formation

A combination of the Poisson burr and the tear burr can form the so-

called top burr or entrance burr along the edge of a machine slot, or

around the periphery of

hole. Figure 5.14 shows such a burr.

Fig. 5.14. Top burr in a micro-drilled hole and micro-machined slot

Mechanical Micromachining 219

Recent results from micromachining stainless steel tended to show

that burr height increases as the feed per tooth and the cutting speed

increases linearly. Figure 5.15 shows the effect clearly.

*10

ffi

0.3

' y

•

ii,i|,

vi,ifv

;

iLjiMmi

•' ? i

•3,2m/ni

•4.8m/min

*6,4m/min

—__

3.2

25-

20-

*•*

N 15-

! tl

1 L

ffl

1 '

{

•

1.3um,f/R<l|

i2.2um,fiR

*3.2um,f/R>1

j |

•-" r i i (

3.2 4.8 6,4 |

Cutting Speed (m

/min)

(a)

ft)

Fig. 5.15. Burr height as a function of feed per tooth and cutting speed

Tool wear also has a significant effect on burr formation. As the

cutting tool wears, the ratio of chip thickness and cutting tool radius

changes significantly. This changes the way that the tool removes

material and consequently, burr height increases. Figure 5.16 shows

the effect of tool wear on burr height.

220 Micro- and Nanomanufacturing

20 40 eO 80 100 120

# of holes

Fig. 5.16. Burr height as a function of tool wear as the number of holes drilled in-

creases

The amount of tool wear that is experienced is measured by looking

at the edge that is worn during machining. Figure 5.17 shows the

magnified image of a cutting tool worn during drilling of material.

Fig. 5.17. Scanning electron micrograph of a fractured cutting tool

The coating of cutting tools to prevent burr formation has already

started to form significant research efforts at different academic in-

stitutions. Machining at such small scales clearly present research

engineers with significant challenges.