Alok Kumar Water Jet Term Paper

8/2/2019 Alok Kumar Water Jet Term Paper

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Introduction

The demand for glass devices has recently increased in the material development, the medical

diagnosis and the environ-mental analysis. The glass devices are usually manufactured using

etching with photolithography. In wet etching of glass, hydro-fluoric acid is used for chemicalreaction. For the sake of safety in the operation and control of the machining rate, the

chemical liquid is diluted and the manufacturing rate is low. An additional cost for the waste

disposal has to be considered for the environmental impact. Dry etching with plasma is also

applied to micro fabrication on the glass surfaces. Although the machining size in the dry

etching is much smaller than that of other processes,the process is performed for a long time

on expensive facilities. Although the machining size in the dry etching is much smaller than

that of other processes,the process is performed for a long time on expensive

facilities.Therefore, alternative processes have been required to improve themanufacturing

cost and environment.An additional cost for the waste disposal has to be considered for the

environmental impact. Dry etching with plasma is also applied to micro fabrication on the

glass surfaces. Although the machining size in the dry etching is much smaller than that of

other processes,the process is performed for a long time on expensive facilities.Therefore,

alternative processes have been required to improve the manufacturing cost and

environment.This study applies abrasive water jet to machining and polishing of glass. The

abrasive water jet processes are originally performed to cut materials with water containing

abrasive grains at a high pressure. The abrasive water jets have also been applied to milling,

drilling, and polishing. Many studies have discussed the removal process and the surface

finish.The abrasive flow process was associated with erosion and the analytical models

proposed for controlling the process.In manufacturing of the glass devices, crack-free surfaces

should be finished without brittle fracture. Erosion of glasses by solid particles has also been

discussed. Because brittle fracture largely depends on the impingement angles of particles, theparticle collision should be controlled at a shallow impingement angle.

Abrasive water jet machining

Machining operation

Machining of micro grooves 20100mm wide 110mm deep is discussed for human cell

operations on the glass chips in thischapter.Fig. 1shows the abrasive water jet machining of

the micro grooves. The diameter of the nozzle is 0.25 mm. CeO2 slurry issupplied with water

by a low-pressure pump. The specifications ofthe operation are shown in Table 1. Themachining area iscontrolled by the V-shaped masks to supply the slurry sufficiently at a

pressure enough to machine.


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Stagnation effect

The process is associated with erosion, in which the surface profiles changes with

deformation, fracture and material removal at collision of the particles. Erosion can be

controlled by the sizes,the velocities, and the impingement angles of the solid particles.The

impingement angle is defined as the angle shown inFig. 2.When the impingement angle is

large, erosion of brittle materials normally is accompanied by brittle fracture. Meanwhile,when small particles collide onto a surface at small impingement angles, the surface profile

changeswithout fracture as erosion of ductile materials. In order to finish a crack-free

surface, the particles should be controlled to collide onto a surface at shallow angles and

move horizontally at high velocities to keep high removal rates with kinetic energies.

Fig. 3 shows CFD analysis of fluid flow around the machining area between the masks

tapered at 45 degrees, where the taper angle is defined as the slope of the sidewall, as shown

inFig. 1.


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Fig. 4 shows the fluid velocity in machining using the masks tapered at 30 degrees. The

stagnation area is smaller than that of Fig. 3(b). Because the size of the stagnation area

changes with the taper angle of the masks, the impingement angles of the abrasive particles

can be controlled by the taper angle.


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Machining test

Fig. 5 shows the machining tests conducted for glass. The tapered masks were set on the

workpiece surface, as shown in Fig. 5(b). The nozzle is mounted on the turret of a NC lathe

to control the traverse motion at a specified feed rate along the exposed area between the

masks. Abrasives are mixed in the mixing tube of the jet nozzle and supplied to the material

at high velocities.

Fig. 6 shows an example of the micro grooves, where the width and the depth of groove are20mm and 2.5mm, respectively. The pictures were taken with a laser confocal microscope

and an AFM.

Fig. 7shows the surface profile along the flow direction on theexposed area. A fine surface is

finished within roughness of 30 nm.Fig. 8shows a magnified picture of the surface in

machining with the masks tapered at 30 degrees. The stagnation area becomes smaller than

that of masks tapered at 45 degrees, as shown inFig. 4.Therefore, the stagnation area is not

large enough to flow the particle horizontally. Consequently, brittle fracture occurs on the

surface due to large impingement angles of the abrasive particles.


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Polishing in micro groove

Polishing operation

When the grooves are machined with the worn tool, the chips are adhered onto the surface.

The fluid polishing is here discussed to finish the micro grooves with the abrasive water jet,

as shown in Fig. 9. The width and the depth of the micro grooves are 175 mm and 20mm,

respectively. The jet nozzle traverses above the grooves to finish the grooves with supplyingthe abrasive slurry.

Stagnation effect

The fluid flow in the groove is compared with that of the flat surface in the CFD analysis.

The fluid velocity is 120 m/s at the exit of the jet nozzle. The feed of the jet nozzle is ignored

in the analysis. Fig. 10(a) shows the fluid velocity in the cross section containing the center

of the nozzle when the abrasive liquid is supplied to a flat surface. The stagnation area underthe nozzle is not large enough to change the vertical flow to the horizontal one. Therefore,

the particles are expected to collide onto the surface at large impingement angles. Fig. 10(b)

shows the fluid velocity in the cross section along the groove when the abrasive liquid is


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supplied to the groove. The stagnation area becomes larger than that ofFig. 10(a).The

abrasive particles are expected to collide onto the surface apart from the nozzle and flow

horizontally. The change in the stagnation area is induced by the sidewall of the groove.

Polishing tests

Fig. 11shows the surface damages after supplying the abrasive slurry to a flat surface and a

micro groove 20mm deep. The pressure of the water pump was 15 MPa and the nominal

fluid velocity at the exit of the nozzle was 90 m/s. 2.5% CeO2 slurry was supplied in a

volume of 800 ml. The nozzle position was adjusted at a height of 1.5 mm from workpiece.


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The polishing tests were conducted for the micro grooves machined in milling.Fig. 12 shows

the change in the surface after polishing with 4000 ml of CeO2. The nozzle was traversed at

a feed rate of 1.5 mm/s. The original surface before polishing, which is finished by the worn

tool, is shown inFig. 12(a). Although the surface finish was 46 nm Ra, the adhered chips and

the cutter traces were observed. The surface finish was improved to be 25 nm after polishing,

as shown inFig. 12(b).

Fig. 13compares the AFM image of the polished surface with that of the original surface.

The polishing performance is verified by removal of the cutter traces.

Conclusion

The abrasive water jet was applied to micro machining and fluid polishing of glass using

stagnation generated under the jet nozzle.In order to finish a crack-free surface, the process

should be controlled so that the abrasive particles flow horizontally and collide onto the

surface at small impingement angles. In machining of the micro groove, the machining area

is controlled by the V-shaped masks on the surface. The jet nozzle is traversed above the

exposed area with supplying the abrasive slurry at a low pressure. The vertical flow from the

jet nozzle changes to horizontal flow around the stagnation area. Then, the abrasive particles

remove the subsurface. The stagnation area can be controlled by the taper angle of the V-

shaped masks. When the taper angle is small, the stagnation area does not become large and

the abrasive particles collide onto the surface at large impingement angles. As a

consequence, brittle fracture occurs on the surface. The taper angle should be large to flow

abrasive particles horizontally. In polishing of the micro groove, the sidewall of the groovespromotes development of the stagnation area and controls the flow direction along the

grooves.


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References

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Alok Kumar Water Jet Term Paper