Jackson Mark. Machining with Abrasives

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Chapter 9

Impact Abrasive Machining

Yasser M. Ali and Jun Wang

Abstract Impact abrasive machining is a broad class of manufacturing processes

that include surface cleaning, descaling, deburring, sand-blasting, shot-peening,

abrasive water or air jet micromachining, and abrasive waterjet (AWJ) cutting,

milling, turning, and drilling. Starting from the basic principles of ﬂuid and solid

mechanics, this chapter describes the underlying physical processes and mechan-

isms affecting jet formation and material removal, and leads to a practical discus-

sion of factors affecting process performance. Adaptations of the basic cutting

process to operations such as milling, turning, micromachining, surface cleaning,

or surface treatment are explained; and various means for process improvement are

discussed. Throughout, focus is on AWJ cutting as the common generic process,

from which all other processes are treated as special cases. This chapter is suitable

for practicing engineers and researchers, and includes an extensive list of references

to guide further study.

Keywords Impact machining  Abrasives  Micromachining  Abrasive water jets

 Proce ss modeling

9.1 Introduction

Impact abrasive machining uses a mixture of ﬂuid and solid particles that is

accelerated, then impacts the surface of a target workpiece; thus, causing permanent

deformation or material removal. This deﬁnition encompasses a broad ﬁeld of

processes, with similar physical nature:

J. Wang (*)

School of Mechanical and Manufacturing Engineering,

The University of New South Wales, Sydney, NSW 2052, Australia

e-mail: j.wang@unsw.edu.au

M.J. Jackson and J. P. Davim (eds.), Machining with Abrasives,

DOI 10.1007/978-1-4419-7302-3_9,

Springer Science+Business Media, LLC 2011

385

When material removal is unfavourable, it is called wear, while favourable mate-

rial removal is called machining. Wear processes (e.g. damage in pipes carrying

abrasive slurry, damage to turbine blades due to particles in the gas stream, surface

erosion by rainfall, etc) were among the early topics studied in tribology; they

provide basic knowledge about material removal mechanisms that may be valid to

machining [1–4].

In explosive forming and welding processes [5, 6], a ﬂuid-solid mixture is accel-

erated by radial expansion of gases, presenting conditions of extreme impact

velocity and pressure. However, they do not necessarily form a jet:analmost

unidirectional pencil of narrow ﬂow stream.

When a jet removes material from a target surface by impacting at near-normal

angle, the process is called erosion. When impact is at near-tangent angle, it is

called abrasion. Both erosion [7–12] and abrasion [13, 14] may take place during

abrasive jet machining, where the term abrasive refers to the nature of the solid

particles used (being harder than the target material), irrespective of the material

removal mechanisms.

Other abrasive machining processes, such as grinding and polishing [15–17],

involve abrasive particles scratching a surface at relatively low velocity (tens of

m/s), whereas processes studied here involve impacting on the surface at hundreds

of m/s – thus, delivering much greater energy.

Acommonformofimpact abrasive machining is the use of a narrow jet of ﬂuid and

solid-particle mixture, which is called Abrasive Jet Machining. The jet can be

composed of any ﬂuid. However, for economic and environmental reasons, it is

usually a mixture of water and air. Other gases, steam, oils, or super-water are used

in special cases. Water has higher density and is incompressible, compared with air;

hence, it is capable of applying greater impact pressure. Water usually contains some

air, and air usually contains some moisture; when referred to as waterjet or airjet,itis

according to the dominant ﬂuid in the jet. Solid particles can be any material, shape, or

size. However, they should be harder than the target workpiece, so that they remove

material from the target at minimum damage to their own. Particles having irregular

shapes with sharp edges can penetrate and cut more into the target, whereas blunt

particles induce more plastic deformation and compressive residual stresses. The

proportion of liquid to solid in the jet can vary signiﬁcantly. In one extreme, solid

particles are reduced to zero, to have a pure ﬂuid jet. Because of its low mechanical

strength, a pure waterjet can only induce elastic deformation on a solid target.

Prolonged application of a pure jet, or application at near-tangent angle, may result

in fatigue and delamination of a thin surface layer.

There are several manufacturing processes, within the ambit of Abrasive Jet

Machining, that meet the above deﬁnitions and limits, as given in Table 9.1. They

share many physical mechanisms and operational attributes, and can be studied as

special cases of one common process:

386 Y.M. Ali and J. Wang

Surface cleaning, descaling, deburring, sand blasting, etc are operated using

water, air, or steam jets, with or without abrasives, and are usually applied at

oblique impact.

Hydro-honing or shot-peening, uses spher ical cast-iron, hard-steel, or glass

beads as particles, at near-normal impact and relatively low velocity, such that

only plastic deformation – no material removal – is induced.

Abrasive airjet micromachining utilizes rough particles, but at low pressure, and

produces only shallow cuts in the target material.

Abrasive waterjet milling, turning, drilling, etc, as the more general case.

This chapter focuses on AWJ cutting as the most common process, and other

processes, as identiﬁed above, are special cases. We study AWJ in greater detail,

and refer to other processes when applicable. Section 9.2 explains how an abrasive

jet is generated, while Sect. 9.3 studies material removal due to abrasive particles

impact. Sections 9.4 and 9.5 discuss various approaches to improve the process and

various machining operations.

9.2 Generation of Abrasive Jet

Potential energy can be stored in any ﬂuid by compressing it to a high pressure.

However, cost and environmental factors limit the practical options. Oils and other

chemicals are not “green” enough. Steam can be pressurized to 35 MPa, but the

added cost of the boiler is discouraging. In abrasive jet machining, only water and

air seem viable. Air is highly compressible, but moisture condensation is a serious

limitation above 5 MPa. Typical Abrasive Air Jet (AAJ) systems operate ~1 MPa,

and are suitable for micromachining or cleaning applications. Alternatively,

AWJ systems typically operate at pressures up to 400 MPa, and systems up to

600–700 MPa are available [18–21]. They are suitable for deep cutting, even on

hard materials. When pressurized ﬂuid is released through a small opening (oriﬁce),

potential energy is converted into kinetic energy, and a high-speed jet is produced.

Waterjet speeds in excess of 1,000 m/s are available in production.

Table 9.1 Scope of

abrasive jet processes

Water

(super-water,

oils, etc.)

Air (nitrogen,

steam, etc.)

No particles

(oblique impact)

Descaling, paint

stripping,

etc.

Cleaning

Blunt or round particles

(near-normal impact)

Peening Blasting

Irregular sharp particles

(normal or oblique

impact)

Cutting,

machining

Micro-

machining

9 Impact Abrasive Machining 387

Such high pressures are achievable using crankshaft-driven pumps or ﬂuid-driven

intensiﬁers. Yet, capital, maintenance, and operating costs are excessive at ultra-

high pressures, and there is an economic limit on the maximum usable pressure.

Figure 9.1 shows a schem atic of a typical AWJ machine. Naturally, numerous

commercial variations exist, and there are websites [22, 23] and associations

[24, 25] for waterjet systems manufacturers, where product speciﬁcations and

commercial information can be found. This schematic is the general case, treating

pure waterjet and abrasive airjet as special cases of AWJ.

When solid particles are injected into a ﬂuid stream, drag forces result in their

acceleration, and a substantial amount of kine tic energy can be transferred to the

particles. If particle size is relatively large, blunt, and impact velocity is low, it can

only induce on the workpiece residual compressive stresses by plastic deformation,

e.g. in peening processes [26–30]. When particles are small, irregular with sharp

corners, and impact velocity is high, they can penetrate into target material,

removing it, leaving a permanent crater. Typical abrasive particles include garnet,

varieties of sand, corundum (aluminum oxide, Fig. 9.2), silicon carbide, and other

super abrasives such as CBN and synthetic diamond.

The selection of the abrasive material is decided by production econom ics, and

the requirement that particle hardness must be greater than workpiece hardness

(a 1.6 ratio is recommended [31] and 1.25 is absolute minimum [1]) for efﬁcient

material removal. Lower hardness ratios yield excessive particle damage and lower

material removal. Abrasive particle sizes range from tens of micro ns to several

hundred microns, according to a mesh number – or an average characteristic

dimension of the range of particles included. Abrasive particles, their properties,

standards, and suppliers can be found from several associations [32–35].

9.2.1 Pure Fluid Jet

For an ideal potential (incompressible, irrotational, inviscid, and isentropic) ﬂuid

[36], under one-dimensional steady-state ﬂow conditions, energy is conserved, and

; (9.1)

where P is hydrostatic pressure, v is average velocity of the stream, and r is ﬂuid

density. The subscripts i and o refer to inlet and outlet of the ﬂow, respectively.

Since, by assumption, r

¼ r

, P

 P

, and v

 v

,(9.1) reduces to



ﬃﬃﬃﬃﬃﬃﬃﬃ

: (9.2)

This is the upper limit on the maximum velocity of a pure ﬂuid jet as it exits the

nozzle (Fig. 9.3.) For a waterjet, r

 1,000 kg/m

, and for 200  P

 600 MPa,

the maximum jet velocity is 630  v

 1,100 m/s.

388 Y.M. Ali and J. Wang

Fig. 9.1 Schematic of an abrasive waterjet (AWJ) cutting system

9 Impact Abrasive Machining 389

In reality, water is viscous ﬂuid and ﬂow is rotational. Air is also compressible

and its ﬂow is entropic. For viscous ﬂow in a tube, the Reynolds number, Re,is

tube

rvD

: (9.3)

The viscosity of water is m  0.001 Pa.s. For typical well-designed AWJ cutting

heads, the oriﬁce opening in Fig. 9.3 is 0.25–0.5 mm. Inner diameter of the mixing

tube is typi cally ~3 oriﬁce diameter, i.e. 0.75  D  1.5 mm. The mixing tube

length, L, is typically L/D  100. Hence, typical AWJ conditions (10

 Re

 10

) are well into the turbulent ﬂow regime. For turbulent ﬂow, ﬂuid motion is

no longer one-dimensional, nor laminar streamlines, and there is a signiﬁcant

Fig. 9.2 Corundum abrasive particles, nominal size 27 mm

Fig. 9.3 Fluid ﬂow inside an AWJ nozzle

390 Y.M. Ali and J. Wang

amount of energy loses due to vortices and secondary ﬂows. The real exit velocity

of a pure waterjet is less than (9.2). Pressure loses occur at entry and exit across the

oriﬁce, entry and exit across the mixing tube, in the mixing chamber, and due to

friction inside the mixing tube. Th e pressure drop is given by

DP ¼ k

; (9.4)

where k is a loss coefﬁcient that is found experimentally and tabulated in ﬂuid

mechanics textbooks [36]. For well-designed and unworn nozzles, k  0.05, for

each loss component. Adding all the losses, the pressure drop from (9.4)is20% of

initial pressure, P

. The real exit velocity of a pure jet is v

 0.9 v

. This quick

calculation formula matche s experimental measurement within 5% [37].

This estimate is for the average velocity of the jet. Yet, point velocity within the

jet varies. It is maximum at the centerline, and almost zero at the turbulent boundary

layer near the wall. The variation of point velocity, u, as a fraction of maximum

center velocity, U, for turbulent ﬂow, follows a 1/7 power law [36]:

uðrÞ

¼ 1 



; (9.5)

where r is radial distance from center of the tube, 0  r  D/2, and U/v

¼ 8/7.

9.2.2 Abrasive Jet

It is common to consider that abrasive particles are entrained by the ﬂuid stream and

gain acceleration, with negligible initial velocity of the particles and perfectly

smooth spheres inside the mixing tube. The accelerating motion of a sphere relative

to a turbulent viscous ﬂuid has no analytical solution, some asymptotic approxima-

tions [38–41] are valid only duri ng the entry stage or very slow (creeping) laminar

ﬂow [36]. An approximate expression for the drag force, F

, applied by the ﬂuid on

the particle is

¼ C

w; (9.6)

where C

is drag coefﬁcient, d is diameter of the spherical particle, w is relative

velocity between the particle and ﬂuid, and the dot indicates time derivative. The

ﬁrst term on the right-hand side is due to viscous drag between the sphere and ﬂuid

under steady ﬂow motion, while the second term is the inertia component due to

relative acceleration, which is estimated (albeit for ideal potential ﬂow) from the

concept of virtual mass [42–44]. The drag force is also given by

p

w; (9.7)

9 Impact Abrasive Machining 391

where r

is the density of the sphere. Equating (9.6) and (9.7), simplifying, and

solving the differential equation, with initial condition w(0) ¼ v

, gives

wðsÞ

¼ exp

C

2dð2r

þrÞ



; (9.8)

where s is the distanc e along the length of the mixing tube. The relative velocity

decays exponentially with the distance along the mixing tube. At nozzle exit, s ¼ L.

It can be seen that particle exit velocity is always less than v

, but can be

maximized (i.e. w (L) ! 0) by increasing the drag coefﬁcient or mixing tube length,

or by decreasing the size or density of the sphere (but this also reduces the kinetic

energy stored in the particle). Increasing mixing tube length improves exit particle

velocity; but for economic (e.g. nozzle wear and cost ) and operating (e.g. pressure

losses along the tube) reasons, it cannot be made too long. The drag coefﬁcient can

be approximated as [45]:

¼ 0.5 400  Re < 3  10

¼ 0.08 Re ¼ 3  10

¼ 0.000366 Re

0.4275

3  10

< Re  2  10

¼ 0.18 Re > 2  10

where, for a sphere,

sphere

rwd

; w ! v

: (9.9)

Hence, there is a critical value, Re ¼ 3  10

, beyond which C

drops sharply,

and any advantage gained from using higher pump pressure is lost. Increasing water

viscosity, m, reduces turbulent ﬂow and enables more efﬁcient transfer of energy.

It was found experimentally [46] that polymeric additives (e.g. super-water) result

in bigger compact length and better performance of AWJ.

In AWJ, a commonly used abrasive material is garnet, whose density varies

depending on the supplier, but on aver age r

 4 r

. At nozzle exit, s ¼ L  100

D  75 mm for a 0.75 mm nozzle, and from (9.9) and (9.8), it can be found that

1

480d

ðÞ

& v

db0:3: (9.10)

Finally, utilizing (9.2) an approximate expression can be obtained for the

average garnet particle velocity as it exits the nozzle of an AWJ system, i.e.

 0:04

ﬃﬃﬃﬃﬃ

ð1  e

1=480d

Þ & d

ﬃﬃﬃﬃﬃ

b7:4: (9.11)

Real abrasive particles are not spherical. Their irregular shapes result in greater

turbulence and lower drag forces. Some energy is consumed in rigid body rotation

392 Y.M. Ali and J. Wang

of the particles. Further, the above analysis assumes only a single particle. Real

AWJ streams involve numerous particle s, and the energy they absorb reduces

energy available to the ﬂu id. Hence, v

tends to decreases along the length of the

mixing tube. Moreover, drag forces on one particle are lower when it is moving

within the wake of another particle. The rando m motion of particles leads to radial

velocity, frequent inter-particle collisions, and collisions with (and wear of) the

wall of the mixin g tube. Furthermore, inter-particle collisions cause particle frag-

mentation, so that particle size distribution at nozzle exit may be signiﬁcan tly

different from as-supplied [47]. Finally, (9.11) applies to average particle velocity.

From (9.5), ﬂuid velocity varies along the radial direction, and not all particles are

accelerated equally. However, due to random particles sizes and shapes, turbulent

mixing, and inter-particle collisions, those variations even out and it is a good

estimation to consider that abrasive particles exit the nozzle at almost uniform

velocity distribution.

As the abrasive waterjet is released from the nozzle, constraints applied by the

tube are removed, and the jet is free to expand radially at the expense of further

decrease in axial velocity [37]. In AWJ cutting, there is no advantage in placing the

workpiece at large standoff distance from the nozzle. Standoff is only a clearance,

to avoid collision between the workpiece and nozzle, and to allow an escape path

for rebounding abrasive particles. AWJ machine manufacturers recommend that

standoff distance should be maintained 3D, i.e. 2 mm. At such small standoff,

velocity drop after exit from the nozzle is negligible [48].

All the above uncertainties and inefﬁciencies reduce the real particle velocity,

, as it hits the workpiece surface. It was found from the balance of linear

momentum, and experimentally [49], that

1 þ

; (9.12)

where

and

are mass ﬂow rates of the abrasives and waterjet, respectively, and

a is a mixing efﬁciency coefﬁcient. For well-designed systems, a is typically

0.57  a  0.71. AWJ manufacturers recommend an opti mum abrasive ﬂow

rate

 50. Notice that (9.12) applies only to the axial component of particle

velocity. Both the radial velocity component and rigid body rotation are ignored,

because of their lesser effect on material removal.

9.2.3 Design Rules

If pressurized water at 400 MPa is released through a 0.25 mm oriﬁce, expands

inside a 0.75 mm mixing tube and hits a target surface spot ~1 mm diameter, even

assuming no loses inside the nozzle, impact pressure at the surface is about one

order of magnitude less. Such low pressure cannot induce plastic deformation or

material removal, except in very soft materials, e.g. plastics, leather, timber, etc.

9 Impact Abrasive Machining 393

[50] A pure waterjet is not suitable for machining metals, ceramics, or other hard

materials. At a large standoff distance, a waterje t may disintegrate into droplets,

whose impact pressure may be high enough to induce residual stresses, e.g. waterjet

peening [30]. Prolonged exposure to waterjet may cause material failure by fatigue,

and consequent delamination of a thin surface layer [10, 11, 51, 52], e.g. waterjet

cleaning, descaling, paint stripping, etc. [53, 54]

In AWJ and AAJ machining of hard materials, the ﬂuid has no direct effect on

material removal, and the impact pressure by the ﬂuid can be ignored. However,

water (and to lesser extent, air) has an important indirect role in machining: it

Stores and transfers the energy required to accelerate abrasive particles,

Guides the abrasive particles and focuses impacts within a small spot,

Flushes debris and abrasive particles away from the workin g zone, and ensures

that fresh surface material is always exposed,

Provides some lubrication between the particle and workpiece, reduces frictional

heating, and

Provides an effective convection cooling medium, where heat generated during

deformation is immediately extracted from the workpiece. Indeed, this is the

major advantage of AWJ, which is often viewed as a “cold” machining process:

a ﬁrst choice when cutting heat-sensitive materials.

Notwithstanding all the above contributions of the waterjet, Sect. 9.3 below focuses

only on material removal due to impact by the abrasive particle and ignores the

effect of water on the process.

From the above considerations, it seems appropriate to model the average real

abrasive particle velocity at the time of impact on the workpiece surface as

¼ A

ﬃﬃﬃﬃ

1 

BL

ðÞ

þr



()

ﬃﬃﬃﬃﬃﬃﬃﬃﬃ

b10

; (9.13)

where the subscripts p and f refer to the abrasive particles and jet ﬂuid, respectively,

A, B, and C are parameters that depend on the design of a particular nozzle, vary

with nozzle wear [55], and are best determined experimentally for the speciﬁc

conﬁguration, and all other symbols are as deﬁned earlier. In the absence of better

knowledge, for AWJ and for v

within  15%, assume a  0.65, A  0.8,

B  0.25, and C  0.65. For pure airjet or AAJ, due to compressibility of air,

the factor of 2 in (9.2) is reduced to 1.4. A more thorough analysis of AAJ can be

found in [56].

Equation (9.13) captures all the essential features of AWJ generation and the

effect of all major variables:

Increasing pressure, P

, increases particle velocity, v

, to a limit, beyond which a

sharp decline in drag, B, prevents additional velocity gains.

Reducing particle size, d, or density, r

, or increasing mixing tube length, L, can

yield limited increase in particle velocity. When the exponential term in (9.13)

almost vanishes, they become ineffective.

394 Y.M. Ali and J. Wang