application of abrasive water jet machining in … · available abrasive water jet machine (omax...

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

IIT Guwahati, Assam, India

535-1

APPLICATION OF ABRASIVE WATER JET MACHINING IN

FABRICATING MICRO TOOLS FOR EDM FOR PRODUCING ARRAY

OF SQUARE HOLES

Vijay Kumar Pal1*, S.K. Choudhury2

1*Ph.D. Scholar, Indian Institute of Technology Kanpur, Kanpur, 208016, Email: [email protected]

2Professor, Indian Institute of Technology Kanpur, Kanpur, 208016, Email: [email protected]

Abstract

In the field of Abrasive Water Jet (AWJ) machining, the current focus is on the fabrication of complex three-

dimensional features. The current work is aimed at using this strategy to manufacture micro-tools for machining

arrays of square holes using Electric Discharge Machining (EDM). Based on selected parameters of AWJ

process(step over, traverse speed, path strategy),micro tools were fabricated on brass sheet of 6 mm

thickness.The performance of such fabricatedtool was investigated by performing experiments on EDM

machine to make arrays of square blind holes( texture ) on stainless steel and Ti-6Al-4V alloy sheet. The depth

of texture was measured through 3D profilometer of FOV (2x) and objective (5x) embedded with vision 64

software.The textured square holes were obtained and its depth was achieved in the range of 10 to 60 microns

and thecorresponding depth obtained of the texture is less on titanium. Discrepancy between the tool dimension

(square) and work piece (textured) was found because of the tool wear on the electrode. Key words: AWJM, Texturing, EDM, Electrode (Tool)

1 Introduction

Micro-machining and miniaturization of products

has become the need of the industry. Currently in

commercial applications micro-machining is mostly

performed by non-conventional machining processes

like lasers, electro-discharge machining (EDM) and

chemical etching [1]. Now-a day’s texturing on the

surfaces is major interest of research because surface

texturing is an effective means to improve the

tribological properties [2]. It is mostly performed by

Cutting, Burnishing, Laser machining, Electron beam

writing, Lithographic methods, Thermal reflows,

Focused ion beam machining etc. Tool design and

mask fabrication are the major obstacles to perform

texturing by non-conventional machining processes as

EDM/ECM, lithography etc. because these masks

and micro-tools are costly and very less

sources/companies are available to fabricate

them.Abrasive Water Jet Machining (AWJM) is a

state- of –the- art non-traditional machining process

making use of high pressure water converted to high

velocity jet mixed with abrasives with an ability to cut

various materials ranging from soft material like

plastics, rubber, wood etc. to hard materials like

titanium and inconel. Originally AWJM technique

was only used for linear cutting and shape cutting of

difficult to cut materials [4]. Now, researchers have

also started experimenting on generating blind

features using AWJM. The literature study reveals

that the geometrical features, like channels (single

slot) and pockets (closed loop path) can be fabricated

using AWJ milling.

In the present study, a novel path strategy is

introduced to fabricate the micro-tool of different

shapes (here, array of square spikes) using the concept

of multi-pass linear traverse cutting. Here, the

distance between the two parallel passes was kept

more than the nozzle diameter [5].

The objective of the present work is to fabricate

micro-tools (electrode) for EDM process on brass

sheet by AWJM process. Primary experiments were

carried out to find out suitable range of process

parameters, based on these selected parameters, tools

were fabricated. EDM is one of the advantageous

methods to provide shape on very hard material.The

performance of such fabricated tool by AWJM

process wasshown by performing experiments on

EDM machine to make texture (arrays of square

holes) on stainless steel and Ti-6Al-4V sheet.

2 Methodology

2.1 Experiments to fabricate micro-tool

Experiments were performed using commercially

available abrasive water jet machine (OMAX Corp.).

Table 1 shows the specifications of the machine.In

AWJMprocess, highly pressurised water is mixed

APPLICATION OF ABRASIVE WATER JET MACHINING IN FABRICATING MICRO TOOLS FOR EDM FOR PRODUCING

with abrasive particles in a mixing tube

abrasive water jet is focused through a

before making an impact on the selected area on the

work material.

Table 1 Machine specification

Maximum traverse speed 4572mm/min

Jet impingement angle

Orifice diameter

Abrasive flow rate 0.226 kg/min

Mixing tube diameter

Mixing tube length

Maximum working

pressure

Figure 1 AWJM process (nozzle and mixing

chamber)

In this work brass sheet of 6 mm thickness is

considered as material. Test samples

before subjecting to AWJM in order

flatness of the resulting tool. Fig. 2 shows the surface

roughness profile of work piece.

Figure 2 Surface roughness profile of

brass sheet

2.2 Path strategy used to fabricate (square

pattern) shaped tool


ARRAY OF SQUARE HOLES

with abrasive particles in a mixing tube (Fig. 1). The

abrasive water jet is focused through a focusing tube

before making an impact on the selected area on the

Table 1 Machine specification

4572mm/min

900

0.33 mm

0.226 kg/min

0.762 mm

101.6 mm

45 kpsi

1 AWJM process (nozzle and mixing

brass sheet of 6 mm thickness is

Test samples were grinded

to improve the

. Fig. 2 shows the surface

2 Surface roughness profile of grinded

cate (square

A novel path strategy is introduced to fabricate

micro-pillars by AWJM process. The resulting

machined surface is to be used as a tool in the EDM

process. The path strategy is derived from the work of

(Pal and Chaudhury, 2014) and briefly

the help of Fig.3. Here machining was carried by

keeping the distance between two successive passes

(also known as step-over (SO))

diameter of jet such that there was no superimposition

of the parallel passes of the jet. By this manner some

material in the form of strip was retained between two

consecutive passes during machining, as shown

first and second raster paths (Fig. 3). Both the raster

paths cross each other perpendicularly

It can be clearly seen that, the two crossed raster

paths generate thearrays of square on the same area.

The cross motions of nozzle along these raster

leave some material in between their consecutive

passes which results in the square pillars

implicit that the height of these square lands is

essentially the depth of the slots created due to metal

removal during the AWJ passes.

Figure3 Path strategy to fabricate

square pins)

Based on the present strategy, four different tools

were fabricated with different geometric

configurations by varying process parameters

associated with the AWJM. The quantitative features

of these tools are listed in Table 2.and the tools are

shown in Fig. 4. If one changes the step over distance,

the number density of the resulting features changes.

Similarly by varying the traverse speed, the depth of

cut (hence height of the features) can be varied.


535-2

is introduced to fabricate

pillars by AWJM process. The resulting

machined surface is to be used as a tool in the EDM

is derived from the work of

and briefly explained with

achining was carried by

distance between two successive passes

more than the

diameter of jet such that there was no superimposition

of the parallel passes of the jet. By this manner some

trip was retained between two

, as shown in the

). Both the raster

ly.

It can be clearly seen that, the two crossed raster

on the same area.

these raster paths

in between their consecutive

the square pillars. It is

implicit that the height of these square lands is

the slots created due to metal

Path strategy to fabricate tool (Array of

four different tools

with different geometric

configurations by varying process parameters

associated with the AWJM. The quantitative features

nd the tools are

. If one changes the step over distance,

he resulting features changes.

Similarly by varying the traverse speed, the depth of

cut (hence height of the features) can be varied.



535-3

Figure 4 Fabricated tools by AWJM

Table 2 Parameters considered for AWJM

T1 T2 T3 T4

Step over

(mm)

1.1 1.3 1.3 1.5

Traverse

speed

(mm/min)

3500 3500 2500 2500

Area (mm2) 15x25 15x25 15x25 25x25

3 Performance of Textured Tool in

Electric Discharge Machining

The surfaces machined using the AWJM processes

were used as tool (electrode) in the Electric Discharge

Machine (EDM). Since these surfaces could not be

directly fitted into an EDM, they were brazed at one

end of the copper cylinders so that they could be

inserted into the tool holder. This arrangement is

pictorially presented in Fig.5.The performance of

tools thus fabricated was investigated in an EDM

setup. The process parameters for this machining were

taken based on micro features machining and

presented in Table 3.

Figure5 EDM set up for texturing

Table 3 EDM parameters in present work

S.No. Parameter Value

1 Current 5 Amps

2 Voltage 75 volts

3 Pulse on time 150 micro sec

4 Duty Factor 72%

The experiments were conducted on Ti-6Al-4V

alloy and stainless steel sheets of 1 mm thickness.

This work aims to achieve textured arrays of square

holes on sheets. All the four tools developed in the

previous section were used here to machine these

materials.

Figure 6 Textured images by various tools

The responses of texturing tools were characterized

by the depth and geometry of the texture obtained on

these sheets. The images of the surfaces machined

using the four tools enlisted in Table 2 are shown in

Fig.6. The numbering is in accordance with the tool

numbers in the Table 2. For the first two images (Fig.


ARRAY OF SQUARE HOLES

535-4

6), the work piece is stainless steel while for the later

two titanium alloy work piece was used. Tool 1 (T1)

had a smaller step over distance than tool 2 (T2). This

resulted in smaller fins on the tool and subsequently

the features produced during EDM. This is apparent

from the Fig.6. Similarly, tool 3 had smaller step over

than tool 4, thus producing smaller features on the

corresponding work piece. It can also be observed

from these figures that the shapes produced on these

sheets correspond to the texturing on the tool.

Figure 7 Enlarge view of texure obtained by Tool

(T4)

Enlarged image of one of the surfaces machined

(corresponding to tool 4) is shown in the Fig. 7. The

measurement shows that the produced features are of

an approximate dimension of 700 µm x 700 µm. The

image of the corresponding tool is shown in Fig.

8.The area of the top surface of each square fin in 600

µm x 600 µm. This discrepancy between the tool

dimension and work piece can be explained as

follows: As shown in Fig. 8 (a), the tool produced by

AWJM has taper along the height of the fins. During

EDM, the tool also undergoes some wear. Because of

these, the tool area responsible for ED machining

increases in time and produces features larger than the

initial tool dimensions.

Figure 8 Images of fresh and wear tool

This image of the tool wear is shown in Fig 8. Fig 8

(a) shows a new tool and Fig 8(b) shows the

corresponding worn out tool. Because of this

taperness in the tool, the resulting feature is also

expected to be tapered along the depth direction. This

is confirmed by the optical image of the machined

work piece surface as shown in Fig. 9.The depth of

texture was measured through 3D optical profilometer

of FOV (2x) and objective (5x) embedded with vision

64 software.

Figure 9 (3D) Optical image of the square hole

texture

Since titanium alloy is difficult to machine

compared to stainless steel surface, therefore

corresponding depth obtained after machining

(texturing) was found less on titanium. This fact is

also reflected by larger tool wear while machining the

titanium alloy surface. These comparisons can be

justified since the same machining parameters were

used while working on all these surfaces and tools.

Based on these arguments, the following trends are

expected.

• Tool wear should be more while machining

titanium surface (compared to stainless steel

surface).

• Because of larger tool wear, the ratio of

obtained textured profile area to the tool

surface area should be larger on titanium.

• Depth of features should be less on titanium.

• For same operating parameters and machined

surfaces, fractional wear should decrease

with tool cross section.

Quantitatively, machining of stainless steel using tool

cross sections 200x200 µm and 400x400 µm give

features of dimensions 290x290 µm and 480x480 µm

respectively.

Similarly, tools of dimensions 400x400 µm and

600x600 µm produced features of cross sections

450x450 µm and 700x700 µm, accordingly, while

machining titanium surface.

Depth of cut on titanium was found in the range 10-20

µm which was quite less than stainless steel

machining (50-60 µm). This proves that stainless steel

is easier to machine.



535-5

Thus, all the expected trends are also obtained

experimentally.

4 Conclusions

From the advancement of technology, the capability

of AWJM process can be extended to fabricating

micro-tool (electrode) for EDM. Present work shows

the path strategy to fabricate tool (arrays of square

pin) on brass sheet. Performance of tools were

analysed by conducting experiments on stainless steel

and Ti-6Al-4V sheet. Based on the observations of

present investigation following conclusions can be

drawn:

• Smaller step-over produces the smaller fin (top

area) e.g Tool 1 (T1) has a smaller step over

distance than tool 2 (T2). This result in smaller

fins on the tool and subsequently the features

produced during EDM.

• This discrepancy between the tool dimension and

work piece was found because of taperness of

tool. The tool produced by AWJM has taper

along the height of the fins.

• Because of taperness in the tool, the resulting

textured feature on the surface was also found

tapered along the depth direction.

• Titanium alloy is difficult to machine compared

to stainless steel surface therefore, corresponding

depth (texture) obtained of the features is less on

titanium

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application of abrasive water jet machining in … · available abrasive water jet machine (omax...

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