Jackson M.J. Micro and Nanomanufacturing

Подождите немного. Документ загружается.

512 Micro- and Nanomanufacturing

Fig. 9.15a. Me-Scope measured mode shape data at a frequency of 232 Hz

DISPLACEMENT

STEP=1

SUB = 114

rmoruz.s

t*K =.404780

JUL 21 2003

10:21:25

Fig. 9.15b. Finite element generated Ansys model of mode shape data at a fre-

quency of222 Hz

Micromachining Using Pulsed Water Droplets 513

Fig. 9.16a. Me-Scope measured mode shape data at a frequency of 1820 Hz

MSPLACEMm

3IEP=1

SUB

=351

MC =.1506Si

JUL 21 2003

10:32:41

Fig. 9.16b. Finite element generated Ansys model of mode shape data at a fre-

quency of 1794 Hz

514 Micro-and Nanomanufacturing

The results from ANSYS above 1794 Hz did not coincide with

what was measured and therefore no comparison was made. How-

ever, measured frequencies above 1820 Hz are shown because they

are useful for future design revisions to the structure. Axial re-

sponses, as well as transverse responses, from the measured data

were used to compare to the finite element results.

Torsional data was ignored since they were not recorded using

the accelerometer and the Me-Scope measurement software. It can

be seen from the percent difference that the results from ANSYS

have a natural frequency that resembles the measured results. How-

ever, they do not converge exactly instead the results oscillate about

the measured data. The measured mode shape data using Me-Scope

software at a frequency of 125 Hz shows an axial deflection for the

spindle frame. However, the spindle itself remains stationary. The

ANSYS model shows bending in the spindle sub-frame. The finite

element models show bending modes in the structural bars.

The measured mode shape at 232 Hz illustrates how the structure

cancels out oscillations that are transmitted through the spindle. It is

apparent from the measured mode shape, as well as the finite ele-

ment model, how various structural members are out of phase,

which prevents any displacement of the spindle from its equilibrium

position thus achieving a preferred effect for micromachining.

The measured data vaguely coincides with the finite element

model once the frequencies reach approximately 1800 Hz, as illus-

trated from the images from Me-Scope at 1820 Hz and the finite

element results at 1794 Hz. For this reason the finite element results

have been omitted above 1820 Hz. This may be due to inadequate

modeling of the structural connections, but most likely to Bernoulli

beam theory not being applicable at these frequencies.

The only characteristic that is common to both of the models is

the restricted oscillation of the spindle. It can readily be seen from

Figures 9.17-9.19 that there is virtually zero oscillation in the spin-

dle at most of the measured frequencies, which is accompanied by

the finite element model as well. The reason for omitting results

above 4460 Hz is because the F.R.F. from the measured data was not

clear, thereby resembling noise, which is not useful for an adequate

conclusion to be made. This is because the impact hammer method

of exciting a structure is limited to approximately 4000 Hz.

Micromachining Using Pulsed Water Droplets 515

Fig. 9.17. Me-Scope measured mode shape data at a frequency of 2890 Hz

Fig. 9.18. Me-Scope measured mode shape data at a frequency of 3400 Hz

516 Micro- and Nanomanufacturing

Fig. 9.19. Me-Scope measured mode shape data at a frequency of 4460 Hz

However, the frequencies used in this design exercise are those

commonly encountered during micromachining processes. It ap-

pears that further improvements can be made by using passive

damping systems on subsequent designs of meso machine tools

(mMT) used for micromachining processes.

9.10 Conclusions

The computer model developed for pulsed water drop micromachin-

ing simulates impacts from an 0.8 mm diameter nozzle and deter-

mines the extent of crack growth. If an estimate of "visible" crack

size is made, then the machining threshold velocity for a given liq-

uid impact velocity can be predicted. The simple machining thresh-

old model of liquid impact produced accurate M.T.V.s (300 impacts)

with good agreement with experimentally obtained values. The

lower impact end of the damage threshold curve was the most diffi-

cult to model as it depends on the size of the largest crack and the

Micromachining Using Pulsed Water Droplets 517

minimum crack size that is detectable by eye via an optical micro-

scope. This was apparent when an opaque material was investi-

gated.

With silicon, the model underestimated the experimental single

impact threshold velocity. The model showed that the M.T.V. (300

impacts) is a reliable estimate of the A.M.T.V. The variation in the

absolute machining threshold velocity with crack size was investi-

gated and was seen to increase when the initial crack length was re-

duced. When the crack became longer, the A.M.T.V. became al-

most independent of the length of the crack. This is an agreement

with experimentally obtained data, where the change in the M.T.V.

(300 impacts) with surface roughness is lower than the effect on

fracture stress. This is due to the brevity of the Rayleigh wave pulse

both spatially and temporally, and the fact that the damaging

Rayleigh wave decreases rapidly with depth. When breaking sam-

ples,

the whole piece is quasi-statically loaded and, therefore, larger

cracks are exploited in reducing strength and resulting in a loss of

material. Pulsed water drop micro machining appears to be a new

method to add to the newly developing field of environmentally

conscious manufacturing processes.

It should be pointed out that the pulsed water drop machining

process has not been compared to the traditional continuous water

drop machining processes. Therefore, one cannot compare the ef-

fectiveness of the new method compared to these techniques other

than in terms of the quality of cut and material removal rates.

The finite element model predictions of the pulsed water drop

machining center compare well with measured data at low frequen-

cies.

Owing to this fact, the finite element model may be used for

future design improvements to the structure. It can be seen from the

experimental and the measurement results that multiple constraints

on the spindle enhance the ability of the structure to resist excitation.

One possible reason for the structure's oscillation is probably due to

the lack of passive damping. Therefore, it is recommended that im-

provements be made to improve passive damping of oscillations.

The results presented show that modifications can be made to a

seemingly stiff machine tool structure. Improvements made to en-

hance passive damping through the use of different materials that

will enable the tetrahedral framework to be adapted to produce a

518 Micro- and Nanomanufacturing

meso machine tool (mMT) structure that will possibly allow pulsed

water drop micromachining processes to become mass manufactur-

ing processes.

9.11 Problems

What is abrasive water jet machining?

State the problems associated with water jet machining.

Describe the theory of pulsed liquid impact.

Describe and illustrate the nature of damage caused by water

drop impacts.

What is the machining damage threshold?

6. Explain how Rayleigh waves affect the removal of material

in pulsed water drop machining.

What is lateral jetting and how does it affect the removal of

material?

8. How are machining threshold curves generated?

9. Comment on material removal rates and how they are af-

fected by the shape and size of water drops.

10.

Explain how machine tools are designed for pulsed water

drop machining of ceramic materials.

References

Hashish M, and Hilleke M., (1999), Water Drop Machining of

Composites and Ceramics, in Machining of Ceramics and Compos-

ites,

Chapter 13, p.p. 427-482, Marcel Dekker, New York.

Jackson MJ and Field JE, (2000) Modelling Liquid Impact Frac-

ture Thresholds in Brittle Materials, Journal of the Institute of Mate-

rials,

Minerals, and Mining - British Ceramic Transactions ,99, 1-

13.

Bowden FP and Brunton JH (1961), The deformation of solids by

liquid impact at supersonic speeds, Proc. Roy. Soc. Lond.,

A263,

433-450.

Steverding B, and Lehnigk SH, (1969), Dynamic thresholds for

crack propagation, Int. J. Fract. Mech., 5, 369-370.

Micromachining Using Pulsed Water Droplets 519

Miller GF, and Pursey H, (1956), On the partition of energy be-

tween elastic waves in a semi-infinite solid, Proc. Roy. Soc. Lond.,

A233,

55-69.

6. Jackson MJ, and Field JE, (1999), Liquid impact erosion of sin-

gle-crystal magnesium oxide, Wear, 233-235, 39-50.

Obara T, Bourne NK, and Field JE (1995), Liquid drop impact on

liquid and solid surfaces, Wear, 186-187, 338-344.

8. Hartranft RJ, and Sih GC (1973): in Methods of analysis and So-

lutions of crack problems (ed. G.C. Sih), 179-238, Noordhoff Inter-

national Publishing.

9. Kolsky H (1953), "Stress waves in solids", Clarendon Press, Ox-

ford, England.

10.

Lord Rayleigh (1885), On waves propagated along the plane sur-

face of an elastic solid, Proc. Lond. Math. Soc, 17, 4-11.

11.

Swain MV, and Hagan JT (1980), Rayleigh wave interaction

with, and the extension of, microcracks, J. Mat. Sci., 15, 387-404.

12.

Freund LB (1972), J. Mech. Phys. Solids, 20, 129-140.

13.

Broberg KB (1960), The propagation of a brittle crack, Arkiv.

Fur Fysik., 18, 159-192.

14.

Kerkhoff F, and Richter H (1969): in 2

Int.

Conf.

"Fracture",

463-473,

Brighton, England.

15.

Dulaney EN, and Brace WF (1960), J. Appl. Phys., 31,

2233-

2236.

16.

Berry JP (1960), J. Mech. Phys. Solids, 8,194-216.

17.

Roberts DK, and Wells AA (1954), Engineering, 178,

820-821.

18.

Evans AG, in "Treatise on Materials Science and Technology",

Erosion (ed. CM. Preece), 1979, 16, 1-67, Academic Press, New

York.

19.

Sih GC: "Handbook of stress intensity factors", 1973, Lehigh

University Press, Pennsylvania, U.S.A.

20.

Hand RJ, Field JE, and Townsend D, (1991), The use of liquid

water drops to simulate angled drop impact, J. Appl. Phys., 70,

7111-7118.

21.

Inman DJ, Engineering Vibration^ Prentice Hall, Upper Saddle

River, New Jersey, 2001.

22.

Cook RD, Finite Element Modeling For Stress Analysis^ John

Wiley & Sons Inc., New York, 1995.

10.

Diamond Nanogrinding

10.1 Introduction

Nanoscale fabrication requires a substrate made from an engineering

material to be truly flat so that "bottom-up" nanofabrication tech-

niques such as lithographically induced self-assembly and soft li-

thography can be used to deposit nanofeatures. The coating of pie-

zoelectric materials with sub-micron size diamond particles has

enabled the production of truly flat substrates so that nanofeatures

can be created on engineering materials using a new manufacturing

process known as "piezoelectric nanogrinding". The principle of the

process relies on applying an electric current to the diamond coated

piezoelectric material that causes the material to strain. When the

diamond-coated piezoelectric material is placed in close proximity

to the substrate, the diamonds remove extremely small fragments of

the substrate when the electric current is applied to the material. The

magnitude of the applied current controls the material removal rate.

The process can be used to process biomedical materials especially

in the production of nanoscale ducts and channels in micro-and nan-

ofluidic devices. To achieve the generation of truly flat surfaces, the

process must be executed within a specially constructed vibration

dampening space frame. The chapter describes the principle of the

process of nanogrinding using coated piezoelectric materials, and

correlates the wear of diamonds with stresses induced into the dia-

monds when an electric current is applied to the piezoelectric in or-

der to remove very small amounts of

material.

The removal of mate-

rial can also be performed using a porous tool with abrasive

materials embedded in them such as diamonds that increases the ma-

522 Micro-and Nanomanufacturing

terial removal rate as long as the porous tool is engineered in such a

way that the loss of abrasive fragments is eliminated. This is

achieved by laser assisted dressing, by engineering the bond of the

porous tool to resist wear, and by laser assisted microstructural

modification of

the

surface of

the

porous tool. The chapter describes

how the bonds in porous tools are engineered to minimize abrasive

grain loss and how vitrified bonding bridges can be processed using

a laser to form extremely sharp nanoscale cutting wedges. The po-

rous nanogrinding tool can be bonded to a piezoelectric material so

that it can be used in the piezoelectric nanogrinding process.

10.2 Piezoelectric Nanogrinding

The piezoelectric nanogrinding process is a process that relies on us-

ing a nickel-coated ceramic material with microscale diamond parti-

cles bonded to it that are cubo-octahedral in shape to machine nano-

scale features in a variety of workpiece materials. The diamonds are

bonded to the piezoelectric material by gaseous deposition, laser

cladding, or directly bonding a porous tool to the material via an ad-

hesive paste. The process is executed by applying a known sinusoi-

dal frequency to the piezoelectric crystal in order to achieve a de-

sired oscillatory displacement. Rapid vibration of the crystal will

allow material removal rates to be increased, thus making it a nano-

manufacturing process. The nanogrinding process is accompanied

by wear of the diamond grains, and the rate of this wear plays an

important role in determining the efficiency of the nanogrinding

process and the quality of the nanomachined surface. Wear mecha-

nisms in nanogrinding processes appear to be similar to that of sin-

gle-point cutting tools, the only difference being the size of swarf

particles generated. Fig. 10.1 shows the arrangement of grinding

swarf and abrasive grains that have been lost during the nanogrind-

ing process.