CIRP Annals - Manufacturing Technology 60 (2011) 355–358

Contents lists available at ScienceDirect

CIRP Annals - Manufacturing Technology

journal homepage: http: / /ees.elsevier.com/cirp/default .asp

Abrasive water jet machining of glass with stagnation effect

T. Matsumura *, T. Muramatsu, S. Fueki

Department of Mechanical Engineering, Tokyo Denki University, Tokyo, Japan

Submitted by T. Hoshi (1), Toyohashi, Japan

A R T I C L E I N F O

Keywords:

Waterjet machining

Glass

Micromachining

A B S T R A C T

Abrasive water jet processes of glass are presented for crack-free machining of micro grooves and fluid

polishing of micro channels with CFD analysis. In machining of the micro grooves, the abrasive is supplied

to flow through intended machining area using the tapered masks. Stagnation under the jet and the

horizontal flow on the machining area are controlled to generate crack-free surfaces by the mask shape.

The same effect can be applied to polishing of the micro channels pre-machined by milling. Stagnation

controlled by the inner wall of the channel changes the flow direction while keeping high fluid velocities.

� 2011 CIRP.

1. Introduction

The demand for glass devices has recently increased in thematerial development, the medical diagnosis and the environ-mental analysis. The glass devices are usually manufactured usingetching with photolithography. In wet etching of glass, hydro-fluoric acid is used for chemical reaction. For the sake of safety inthe operation and control of the machining rate, the chemicalliquid is diluted and the manufacturing rate is low. An additionalcost for the waste disposal has to be considered for theenvironmental impact. Dry etching with plasma is also appliedto micro fabrication on the glass surfaces. Although the machiningsize 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.

This study applies abrasive water jet to machining andpolishing of glass. The abrasive water jet processes are originallyperformed to cut materials with water containing abrasive grainsat a high pressure [1]. The abrasive water jets have also beenapplied to milling [2,3], drilling, and polishing [4,5]. Many studieshave discussed the removal process and the surface finish [6–9].The abrasive flow process was associated with erosion [10,11] andthe analytical models proposed for controlling the process [12,13].In manufacturing of the glass devices, crack-free surfaces should befinished without brittle fracture. Erosion of glasses by solidparticles has also been discussed [14,15]. Because brittle fracturelargely depends on the impingement angles of particles, theparticle collision should be controlled at a shallow impingementangle.

The paper discusses control of abrasive flow using thestagnation in the abrasive water jet processes for machining andpolishing of micro grooves. The stagnation area under the jetnozzle is evaluated using computational fluid dynamics and

* Corresponding author.

E-mail address: tmatsumu@cck.dendai.ac.jp (T. Matsumura).

0007-8506/$ – see front matter � 2011 CIRP.

doi:10.1016/j.cirp.2011.03.118

associated with the surface finishing. The effect of the stagnationarea is verified in the machining tests.

2. Abrasive water jet machining

2.1. Machining operation

Machining of micro grooves 20–100 mm wide 1–10 mm deep isdiscussed for human cell operations on the glass chips in thischapter. Fig. 1 shows the abrasive water jet machining of the microgrooves. 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. The machining area iscontrolled by the V-shaped masks to supply the slurry sufficientlyat a pressure enough to machine.

2.2. Stagnation effect

The process is associated with erosion, in which the surfaceprofiles changes with deformation, fracture and material removalat 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 in Fig. 2.When the impingement angle is large, erosion of brittle materialsnormally is accompanied by brittle fracture. Meanwhile, whensmall particles collide onto a surface at small impingement angles,the surface profile changes without fracture as erosion of ductilematerials. In order to finish a crack-free surface, the particlesshould be controlled to collide onto a surface at shallow angles andmove horizontally at high velocities to keep high removal rateswith kinetic energies.

Fig. 3 shows CFD analysis of fluid flow around the machiningarea between the masks tapered at 45 degrees, where the taperangle is defined as the slope of the sidewall, as shown in Fig. 1. Thefluid velocity at the exit of the jet nozzle is 180 m/s. The feed of thenozzle is ignored in the analysis. A time-dependent analysis wasconducted in explicit scheme using the commercial codePHOENICS based on finite volume method [16]. Turbulent flow

[()TD$FIG]

Fig. 1. Micro machining in abrasive water jet process.

Table 1Specifications.

Pump Triplex

plunger

Abrasive

slurry

Cerium oxide

(CeO2)

Capacity 15.1 ml/min Particle size 300–500 nm

Maximum pressure 35 MPa Density 2.5% (diluted

at jet nozzle)

Nozzle diameter 0.25 mm Workpiece Crown glass

(72% SiO2,

18% K2CO2,

10%CaCO2)

Mask Tungsten

carbide

[()TD$FIG]

Fig. 2. Particle collision in erosion.

[()TD$FIG]

Fig. 4. Abrasive water flow with masks tapered at 30 degrees in CFD conditions:

fluid velocity at the jet nozzle, 180 m/s; width of groove, 20 mm; taper angle of

masks, 30 degrees.

T. Matsumura et al. / CIRP Annals - Manufacturing Technology 60 (2011) 355–358356

is expressed by k–e model and free boundary between liquid andair is formulated in SEM (Scalar Equation Method) scheme [17].Fig. 3(a) shows the interface between liquid and air. The liquidflows onto the exposed surface vertically and then spreadshorizontally along the machining area between the masks.Fig. 3(b) shows the fluid velocity in the cross section containing

[()TD$FIG]

Fig. 3. Abrasive water flow in CFD conditions: fluid velocity at the jet nozzle, 180 m/

s; width of groove, 20 mm; taper angle of masks, 45 degrees. (a) Boundary between

air and the abrasive liquid, (b) flow velocity distribution.

the center of the exposed area. When the liquid flows verticallywith respect to the surface, the flow is accompanied by stagnationunder the jet nozzle. Although the pressures are high in thestagnation area, the abrasive particles flow at low velocities.Therefore, brittle fracture does not occur in the stagnation areaunder the jet nozzle. Then, the vertical flow from the jet nozzle ischanged to the horizontal one outside of stagnation. The liquidflows at around 150 m/s, which are high enough velocities toremove the material.

Fig. 4 shows the fluid velocity in machining using the maskstapered at 30 degrees. The stagnation area is smaller than that ofFig. 3(b). Because the size of the stagnation area changes with thetaper angle of the masks, the impingement angles of the abrasiveparticles can be controlled by the taper angle.

2.3. Machining tests

Fig. 5 shows the machining tests conducted for glass. Thetapered masks were set on the workpiece surface, as shown inFig. 5(b). The nozzle is mounted on the turret of a NC lathe tocontrol the traverse motion at a specified feed rate along theexposed area between the masks. Abrasives are mixed in themixing tube of the jet nozzle and supplied to the material at highvelocities.

Fig. 6 shows an example of the micro grooves, where the widthand the depth of groove are 20 mm and 2.5 mm, respectively. Thepictures were taken with a laser confocal microscope and an AFM.The nominal fluid velocity at the exit of the nozzle was 180 m/s.Cerium oxide dispersion was diluted to be in a concentration of2.5%. 800 ml of the colloidal liquid was supplied from the slurrytank to the jet nozzle in a test. The taper angle of the masks was 45degrees. The machining is verified in observation of a crack-free

[()TD$FIG]

Fig. 5. Machining test. (a) Set up, (b) masks.

[()TD$FIG]

Fig. 6. Micro groove machined. (a) Microphotograph, (b) AFM image.

[()TD$FIG]

Fig. 8. Surface finish in machining with mask tapered at 30 degrees.[()TD$FIG]

Fig. 9. Polishing of micro groove with abrasive water jet.

[()TD$FIG]

T. Matsumura et al. / CIRP Annals - Manufacturing Technology 60 (2011) 355–358 357

surface. Fig. 7 shows the surface profile along the flow direction onthe exposed area. A fine surface is finished within roughness of30 nm.

Fig. 8 shows a magnified picture of the surface in machiningwith the masks tapered at 30 degrees. The stagnation area becomessmaller than that of masks tapered at 45 degrees, as shown in Fig. 4.Therefore, the stagnation area is not large enough to flow theparticle horizontally. Consequently, brittle fracture occurs on thesurface due to large impingement angles of the abrasive particles.

3. Polishing in micro groove

3.1. Polishing operation

The micro grooves, which are used for the micro channels of themicro TASs, can be machined on the glass plates without brittlefracture in millings with ball end mills [18]. Although the surface isfinished around 40 nm Ra, the cutter trace is left on the surface.When the grooves are machined with the worn tool, the chips areadhered onto the surface. The fluid polishing is here discussed tofinish the micro grooves with the abrasive water jet, as shown inFig. 9. The width and the depth of the micro grooves are 175 mmand 20 mm, respectively. The jet nozzle traverses above thegrooves to finish the grooves with supplying the abrasive slurry.

3.2. Stagnation effect

The fluid flow in the groove is compared with that of the flatsurface in the CFD analysis. The fluid velocity is 120 m/s at the exitof 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 containingthe center of the nozzle when the abrasive liquid is supplied to aflat surface. The stagnation area under the nozzle is not largeenough to change the vertical flow to the horizontal one. Therefore,the particles are expected to collide onto the surface at largeimpingement angles. Fig. 10(b) shows the fluid velocity in the crosssection along the groove when the abrasive liquid is supplied to thegroove. The stagnation area becomes larger than that of Fig. 10(a).The abrasive particles are expected to collide onto the surface apartfrom the nozzle and flow horizontally. The change in thestagnation area is induced by the sidewall of the groove. Thesidewall promotes development of the stagnation area withcontrolling the flow direction, as the tapers of the V-shaped

[()TD$FIG]

Fig. 7. Surface profile.

masks work in the machining of the micro grooves discussed in theprevious chapter.

The arrows in Fig. 10 designate the velocity vectors. In theabrasive flow on the flat surface, the fluid velocity is low becausethe fluid flow spreads to all directions on the surface. Meanwhile,when the abrasive liquid is supplied to the groove, the flowdirection is restricted by the sidewall of the groove. Thus, thesidewall keeps the flow velocity high with a large stagnation area.Consequently, the abrasive particle flows fast enough to polish thesurface sufficiently.

3.3. Polishing tests

Fig. 11 shows the surface damages after supplying the abrasiveslurry to a flat surface and a micro groove 20 mm deep. Thepressure of the water pump was 15 MPa and the nominal fluidvelocity at the exit of the nozzle was 90 m/s. 2.5% CeO2 slurry wassupplied in a volume of 800 ml. The nozzle position was adjusted ata height of 1.5 mm from workpiece. Brittle fracture was observed

Fig. 10. Distribution of fluid velocity in CFD fluid velocity at jet nozzle, 120 m/s. (a)

Flat plate, (b) groove (20 mm deep).

[()TD$FIG]

Fig. 13. Change in surface finish. (a) Original, (b) polished.

[()TD$FIG]

Fig. 11. Brittle fracture on surface finishes. (a) Flat plate, (b) groove (20 mm deep).

[()TD$FIG]

Fig. 12. Improvement of surface finish in polishing. (a) Original, (b) polished.

T. Matsumura et al. / CIRP Annals - Manufacturing Technology 60 (2011) 355–358358

on the flat surface; while a crack-free surface was finished in thegroove. In order to finish a crack-free surface, the abrasive particleshould be flown horizontally on the machining surface. As may beseen from Fig. 10(a), the vertical flow does not change to horizontalone because of the small stagnation area. Therefore, brittle fractureon the flat surface is induced by large impingement angles of theabrasive particles around the stagnation area. When the abrasiveliquid is supplied to the surface in the groove, the sidewall of thegroove promotes the size of the stagnation area. The abrasiveparticles flow horizontally at high flow velocities outside of thestagnation area. As a consequence, a crack-free surface can befinished with removing the cutter traces in the groove.

The polishing tests were conducted for the micro groovesmachined in milling. Fig. 12 shows the change in the surface afterpolishing with 4000 ml of CeO2. The nozzle was traversed at a feedrate of 1.5 mm/s. The original surface before polishing, which isfinished by the worn tool, is shown in Fig. 12(a). Although thesurface finish was 46 nm Ra, the adhered chips and the cuttertraces were observed. The surface finish was improved to be 25 nmafter polishing, as shown in Fig. 12(b). Fig. 13 compares the AFMimage of the polished surface with that of the original surface. Thepolishing performance is verified by removal of the cutter traces.

4. Conclusion

The abrasive water jet was applied to micro machining and fluidpolishing of glass using stagnation generated under the jet nozzle.In order to finish a crack-free surface, the process should becontrolled so that the abrasive particles flow horizontally andcollide onto the surface at small impingement angles.

In machining of the micro groove, the machining area iscontrolled by the V-shaped masks on the surface. The jet nozzle istraversed above the exposed area with supplying the abrasiveslurry at a low pressure. The vertical flow from the jet nozzlechanges to horizontal flow around the stagnation area. Then, theabrasive particles remove the subsurface. The stagnation area canbe controlled by the taper angle of the V-shaped masks. When thetaper angle is small, the stagnation area does not become large andthe abrasive particles collide onto the surface at large impingementangles. As a consequence, brittle fracture occurs on the surface. Thetaper 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 flowdirection along the grooves. The flow velocity is also high enoughto polish the surface by restriction of flow direction. When theabrasive slurry is supplied to a flat surface, brittle fracture isinduced by collision of the particles at large impingement anglesbecause of a small stagnation area. The polishing performance ispoor because the abrasive flow spreads to all direction at lowvelocities.

References

[1] Hoogstrate AM, Luttervelt CA (1997) Opportunities in Abrasive Water-JetMachining. Annals of the CIRP 46(2):697–714.

[2] Hashish M (1989) An Investigation of Milling with Abrasive-Waterjets. Trans-actions of ASME Journal of Engineering for Industry 111(2):158–166.

[3] Axinte DA, Srinivasu DS, Billingham J, Cooper M (2010) Geometrical Modellingof Abrasive Waterjet Footprints: A Study for 908 Jet Impact Angle. Annals of theCIRP 59(1):341–346.

[4] Hashish M, Bothell D (1993) Diamond Polishing with Abrasive Suspension Jets.Proceedings of 7th American Water Jet Conference, 793–800.

[5] Tricard M, Kordonski WI, Shorey AB (2006) Magnetorheological Jet Finishing ofConformal, Freeform and Steep Concave Optics. Annals of the CIRP 55(1):309–312.

[6] Siores E, Wong WCK, Chen L, Wager JG (1996) Enhancing Abrasive WaterjetCutting of Ceramics by Head Oscillation Techniques. Annals of the ClRP45(1):327–330.

[7] Hoogstrate AM, Susuzlu T, Karpuschewski B (2006) High Performance Cuttingwith Abrasive Waterjets Beyond 400 MPa. Annals of the CIRP 55(1):339–342.

[8] Chao J, Zhou G, Leu MC, Geskin E (1995) Characteristics of Abrasive WaterjetGenerated Surfaces and Effects of Cutting Parameters and Structure Vibration.Transactions of ASME Journal of Engineering for Industry 117(4):516–525.

[9] Hashish M (1991) Optimization Factors in Abrasive Waterjet Machining.Transactions of ASME Journal of Engineering for Industry 113(1):29–37.

[10] Wilkins RJ, Graham EE (1993) An Erosion Model for Waterjet Cutting. Transac-tions of ASME Journal of Engineering for Industry 115(1):57–61.

[11] Finnie I (1995) Some Reflection on the Past and Future of Erosion. Wear 186–187:1–10.

[12] Williams RE, Rajurkar KP (1992) Stochastic Modeling and Analysis of AbrasiveFlow Machining. Transactions of ASME Journal of Engineering for Industry114(1):74–81.

[13] EITobgy M, Ng E-G, Elbestawi MA (2005) Modelling of Abrasive WaterjetMachining: A New Approach. Annals of the CIRP 54(1):285–288.

[14] Buijs M, Pasmans JMM (1995) Erosion of Glass by Alumina Particles. Transi-tions and Exponents Wear 184:61–65.

[15] Ballout Y, Mathis JA, Talia JE (1996) Solid Particle Erosion Mechanism in Glass.Wear 196:263–269.

[16] Suhas V, Patankar. (1980) Numerical Heat Transfer and Fluid Flow. HemispherePublishing Corporation.

[17] Jun L, Spalding D (1988) Numerical Simulation of Flow with Moving Interface.PHC PhysicoChemical Hydrodynamics 10(5–6):625–637.

[18] Matsumura T, Ono T (2008) Cutting process of glass with inclined ball endmills. Journal of Material Processing Technology 200:356–363.