CHAPTER 3 EXPERIMENTAL CONDITIONS AND PROCEDURE...

50

CHAPTER 3

EXPERIMENTAL CONDITIONS AND PROCEDURE

This study was undertaken to perform milling operations on

AISI D2, AISI D3, AISI H13 and AISI P20 steels, using different carbide

cutting tool inserts under various speed-feed combinations. The experiment

was performed under a constant depth of cut. The cutting environments

evaluated in the milling process were: Dry machining, Conventional cooling

(Wet) and Cryogenic cooling (LN2). A cryogenic cooling setup was

developed for supplying liquid nitrogen at the cutting zone. In the present

work, the cutting temperature, cutting force, surface roughness, tool wear,

chip shape and chip morphology are considered, for studying the effect of

cryogenic cooling. The influence of cryogenic cooling using liquid nitrogen

was compared to that of dry and wet machining. This chapter explains

experimental procedure, workpiece materials, cutting tool materials and the

equipment being used.

3.1 WORK MATERIALS

Tool and die steels are high carbon steels, possessing high

hardness, strength and wear resistance. Materials having a hardness of

over 45 HRC (Rockwell hardness C scale), can be classified as

difficult-to-machine materials (Becze and Elbestawi 2002, Poulachon and

Moisan, 2000). The four most widely used die steel materials by the tool and

mould making industry, have been chosen as work materials for this

experimental work. These are High Carbon High Chromium (HC-HCr) type

51

AISI D2 die steel, High Carbon High Chromium (HC-HCr) type AISI D3 die

steel, hot work type AISI H13 die steel and plastic mold type AISI P20 steel.

The workpieces made from these four die steel materials were hardened and

tempered using standard heat treatment procedures before experimentation, to

bring them on par with the state in which they are actually used in the

industry. In this research work, the size of the work materials used was

150mm × 100mm × 50mm. The work materials were tested for their

composition, and the mechanical properties for homogeneity.

3.1.1 AISI D2 Steel

AISI D2 steel is a high-carbon high-chromium tool steel,

designated as a group ‘D’ steel in the AISI classification system, and is the

most highly alloyed cold-work tool steel. Chromium, at a nominal

concentration of 12%, is the major alloying element, but molybdenum,

vanadium, nickel, and manganese may be added in significant amounts. In

view of their high carbon and alloy contents, all the D steels are deep

hardening. They are hardenable by air cooling from austenitizing

temperatures, and they have very low susceptibility to distortion and cracking

(ASM Specialty Handbook, 1995, Roberts and Cary, 1980). The D2 type

HC-HCr tool steels find wide usage as the raw material for blanking and deep

drawing dies, thread rolling & forming dies, burnishing tools, shear and slitter

knives. The chemical composition of the AISI D2 steel is given in Table 3.1,

and its properties are given in Table 3.2.

52

Table 3.1 Composition of the AISI D2 steel

Element Amount (weight %)

Carbon 1.55

Chromium 11.8

Molybdenum 0.8

Manganese 0.4

Silicon 0.3

Vanadium 0.8

Ferrous Balance

Table 3.2 Properties of the AISI D2 steel

Material property ValueHardness (HRC) 62

Density (×103 Kg/m3) 7.7

Poisson ratio 0.30

Tensile strength (MPa) 2940

Yield strength (MPa) 2200

Modulus of elasticity (GPa) 210

Thermal conductivity (W/mK) 20

3.1.2 AISI D3 Steel

AISI D3 steel is an oil hardening, high-carbon, high-chromium tool

steel, designated as a group ‘D’ steel in the AISI classification system, and is

the most highly alloyed cold-work tool steel. This D3 type steel also contains

chromium, and is the major alloying element at a nominal concentration of

12%, which does not contain molybdenum, but contains an addition of

53

tungsten. The oil hardening D3 steel offers the advantage of a better surface

condition, because a combination of lower hardening temperature and liquid

quenching resulting in minimum surface decarburization and scaling. It has an

excellent abrasion/wear resistance, good dimensional stability and high

compressive strength. The high chromium content of these steels provides an

appreciable resistance to staining, after the tools are hardened and polished. It

is widely used for blanking and forming dies, forming rolls, press tools,

punches and bushes. The chemical composition of the AISI D3 steel is given

in Table 3.3, and its properties are given in Table 3.4.

Table 3.3 Composition of the AISI D3 steel

Element Amount (weight %)Carbon 2.1Chromium 11.5Manganese 0.4Silicon 0.3Nickel 0.3Vanadium 1.0Tungsten 1.0Ferrous Balance

Table 3.4 Properties of the AISI D3 steel

Material property ValueHardness(HRC) 53Density (×103 Kg/m3) 7.67Poisson ratio 0.29Tensile strength (MPa) 2770Yield strength (MPa) 1900Modulus of elasticity (GPa) 194Thermal conductivity (W/mK) 20.5

54

3.1.3 AISI H13 Steel

Tool steels for hot work applications, designated as group ‘H’

steels in the AISI classification system, have the capacity to resist softening,

during long or repeated exposures to high temperatures, needed for hot work

or for die-casting other materials. These ‘H’ type hot work tool steels, fall into

three groups, according to the alloying element used to impart high hot

hardness: chromium hot work steels, which contain nominally 5% Cr and

significant amounts of other elements, including silicon, molybdenum, and

vanadium; tungsten hot work steels, and molybdenum hot work steels. The

outstanding characteristics of these tool steels are high toughness and shock

resistance. They have air hardening capability at relatively low austenitizing

temperatures, and exhibit minimum scaling tendency during air cooling.

AISI H13 hot work steel is the best material for hot die works of all kinds,

particularly dies for the extrusion of aluminium and magnesium, as well as

die casting dies, forging dies, mandrels and hot shears, because of its ability to

retain dimensional accuracy and surface characteristics at elevated

temperatures. This material also possesses high resistance to erosion. The

chemical composition of the AISI H13 steel is given in Table 3.5, and its

properties are given in Table 3.6.

Table 3.5 Composition of the AISI H13 steel

Element Amount (weight %)Carbon 0.38Chromium 5.00Molybdenum 1.30Manganese 0.35Silicon 1.00Vanadium 1.00Fe Balance

55

Table 3.6 Properties of the AISI H13 steel

Material property Value

Hardness (HRC) 52


Poisson ratio 0.29




Thermal conductivity (W/mK) 24.6

3.1.4 AISI P20 Steel

Tool steels for plastic injection molding and die casting

applications, designated as group ‘P’ steels in the AISI classification system,

are exposed to less severe wear than metal-working steels, and therefore, have

low carbon content. There are three major groups of ‘P’ type steels: carbon

steel grades used for hubbed cavities, carbon steel grades used for machined

cavities, and stainless steel grades. Plastic moulds for thermoplastics and

thermosets are typically made from cold work tool steel like AISI P20 tool

steels. It has properties like excellent machinability and weldability,

combined with good corrosion resistance and mechanical strength. AISI P20

steel finds application in injection moulds for thermo-plastics, extrusion dies

for thermo-plastics, blow moulds, forming tools, press-brake dies, (possibly

flame hardened or nitrided) and structural components. The chemical

composition of the AISI P20 steel is given in Table 3.7, and its properties are

given in Table 3.8.

56

Table 3.7 Composition of the AISI P20 steel

Element Amount (weight %)

Carbon 0.4

Chromium 1.2

Molybdenum 0.35

Manganese 1.0

Silicon 0.4

Nickel 0.8

sulphur 0.08

Ferrous Balance

Table 3.8 Properties of the AISI P20 steel

Material property Value

Hardness (HRC) 48


Poisson ratio 0.28




Thermal conductivity (W/mK) 20.2

3.2 CUTTING TOOL INSERTS

In this research work, indexable inserts are used as cutting tools.

Three different types of carbide cutting tool inserts have been used in the

research work. They are: i) CVD TiN coated carbide cutting tool insert ii)

PVD TiAlN coated carbide cutting tool insert and iii) Uncoated carbide

57

cutting tool insert. CVD TiN coated carbide cutting tools are used for

AISI D2 and AISI D3 steels; and PVD TiAlN coated carbide cutting tools are

used for AISI H13 steel. The uncoated carbide cutting tool inserts are used for

AISI P20 steel. Figure 3.1(a-c) shows the cutting tool inserts are used in this

experiment. The specifications of the cutting tool inserts are given in

Table 3.9.

(a) (b)

(c)

Figure 3.1 Cutting tool inserts (a) CVD TiN coated carbide insert(b) PVD TiAlN coated carbide insert and (c) Uncoatedcarbide insert

3.3 TOOL HOLDER

The tool holder used for end milling is the WIDIA M680 shoulder

end mill. This tool holder is used in the research work for machining all the

workpiece materials. Figure 3.2 shows the photographic view of the tool

holder. The specification of the tool holder is given below:

58

ISO Designation : 12396920400

Shank diameter : 16 mm

Overall length : 75 mm

Head length : 27 mm

Number of inserts : 2

Figure 3.2 Tool holder used in the experiment

Table 3.9 Cutting tool inserts specifications

Workpiecematerials

SpecificationsCutting

toolmaterial

ISODesignat

ion

Rhombicnose

angle (°C)

Insertlength(mm)

Insertwidth(mm)

InsertThickness

(mm)

Noseradius(mm)

AISI D2cold worktool steel

CVD TiNcoated

tungstencarbide

tool

XDHT090308-TN 450

80 9.67 4.35 3.18 0.8

AISI D3cold worktool steel

CVD TiNcoated

tungstencarbide

tool

XDHT090308-TN 450

80 9.67 4.35 3.18 0.8

AISI H13hot worktool steel

PVDTiAlNcoated

tungstencarbide

tool

XDHT090308 -

HXPA 120

80 9.67 4.35 3.18 0.8

AISI P20plastic

injectionmould steel

Un coatedtungstencarbide

tool

XDHT090308-

THM80 9.67 4.35 3.18 0.8

59

3.4 EXPERIMENTAL CONDITIONS

End milling experiments were carried out on the AISI D2, AISI D3,

AISI H13 and AISI P20 steels, using various tungsten carbide cutting tool

inserts at different speed-feed combinations under dry, wet and cryogenic

machining conditions. The experimental cutting conditions used in the milling

of the AISI D2, AISI D3, AISI H13 and AISI P20 steels are given in

Table 3.10.

Table 3.10 Experimental cutting conditions in the milling of the

AISI D2, AISI D3, AISI H13 and AISI P20 steels

Workpiecematerials

Cutting parameters

Cuttingvelocity(m/min)

Feed rate(mm/tooth)

Depth ofcut (mm)

Cooling methods

AISI D2 coldwork tool steel

75, 100 and125

0.01, 0.015and 0.02

0.5

(i) Dry

(ii) Wet

(iii) LN2 cooling

AISI D3 coldwork tool steel

75, 100 and125

0.01, 0.015and 0.02

0.5

(i) Dry

(ii) Wet

(iii) LN2 cooling

AISI H13 hotwork tool steel

75, 100 and125

0.01, 0.015and 0.02

0.5

(i) Dry

(ii) Wet

(iii) LN2 cooling

AISI P20 plasticinjection mould

steel

75, 100 and125

0.01, 0.015and 0.02

0.5

(i) Dry

(ii) Wet

(iii) LN2 cooling

60

3.5 EQUIPMENTS FOR THE EXPERIMENT

3.5.1 CNC Vertical Machining Centre

End milling experiments were carried out on an ARIX VMC 100

CNC machining centre. The machine specifications are shown in Table 3.11.

Table 3.11 Specifications of the ARIX VMC 100 CNC machining Centre

Description Technical Data1) Working Range

Longitudinal axis (X axis)Cross axis (Y axis)Vertical axis (Z axis)

1000 mm500 mm500 mm

2) Work spindlesTool MountingCentre to table

ISO 4010-450 mm

3) Spindle speed and feed ratesWork spindle speed, directlyprogrammableFeed rates directly programmablealong X, Y and Z axesRapid traverse a long X, Y and Z axes

60 – 5000 rpm

4000 mm/min

4000 mm/min

4) Electrical EquipmentVoltageX,Y,Z servo motorsSpindle motor

220 / 380 V750 W5HP

5) Weights and Space requirementsWeight of machine (including verticalmilling head, circular table, toolchanger, cabin and switch cabinet)Machine dimensionLengthWidthHeight

2700 kg

2700 mm2500mm2300 mm

61

3.5.2 Dynamometer

During the machining tests, the feed force (Fx), radial force (Fy) and

axial force (Fz) were measured using a Kistler type 9257B three component

piezo-electric dynamometer, which was connected to the Kistler type 5070A

charge amplifiers. The force to be measured is introduced via a top plate

(Figure 3.3), and distributed between 3-component force sensors, arranged

between the base and the top plates. Each of the sensors has three pairs of

quartz plates, one sensitive to pressure in the z direction, and the other two to

shear in the x and y directions, respectively. It has high rigidity and hence

high natural frequency that enables very small dynamic changes to be

measured. To obtain and record the cutting force data, data acquisition

software was used. A data acquisition system consists of a personal computer

equipped with an analog to digital converter card RS 232(A/D board) and also

the Dynoware software (Dynoware type 2825A).

Figure 3.3 A three component dynamometer assembly

62

3.5.3 Infrared Pyrometer

The cutting temperature is measured in the tool – chip interfaces, in

different cutting environments, using a non-contact type Infrared pyrometer

(accuracy ±1.0°C), which can measure a temperature range of -50°C to

+1000°C. The cutting temperature is measured by making the Infrared ray

from the Infrared pyrometer to impinge exactly on the tool – chip interfaces

during the machining process, and the maximum temperature attained is

recorded. The Infrared pyrometer is calibrated for its measurement.

3.5.4 Surface Roughness Tester

The surface roughness after end milling was measured, using a

Taylor- Hobson Surtronic 3+ surface roughness tester. This surface roughness

tester combines advanced technology with high precision and its use for the

efficient measurement of the surface finish in the inspection room or

laboratory. The instrument can be handheld on horizontal, vertical and

inclined surfaces, or bench mounted with accessories for batch measurement

or laboratory applications. The pick-up holder is mounted on a slide for

vertical adjustment, and can also be rotated to different measuring positions,

including a right-angled measurement. The surface roughness was measured

at three locations on the machined workpiece. The value of the surface

roughness is the average of the three points taken for each measurement.

Table 3.12 shows the equipment specification.

Table 3.12 Specification of the Taylor-Hobson Surface Roughness tester

Model Surtronic 3+Manufacture UK

Cut-off length 0.25mm, 0.8mm and 2.5mmAccuracy 0.01 m

63

3.5.5 Toolmaker’s Microscope

A toolmaker’s microscope was used to measure the tool wear after

every interval, after every observation point of the run. After different

intervals of machining, the conditions of the tools were monitored and

average flank wear (VB) was measured. Once the VB reached 0.3 mm, the

tool life was considered to be over. The machine specification is listed in

Table 3.13

Table 3.13 Specification of the TM-505 Toolmaker’s Microscope

Model TM-505Manufacture MitutoyoMagnification 30xTable travel (X-axis) 150 mmTable travel (Y-axis) 150 mm

3.5.6 Scanning Electron Microscope

The Scanning Electron Microscope(S-3400N) was used to observe

and capture the image of the tool wear and chip formed at high magnification.

Table 3.15 Specification of the S-3400N Scanning Electron Microscope

Resolution

3.0 nm (at 30 kV, secondary electron image, HighVacuum)10 nm (at 3 kV, secondary electron image, HighVacuum)4.0 nm (at 30 kV, backscattered electron image LowVacuum)

Magnification x 5 x 300,000Accelerating voltage 0.3 30 kVVariable pressure range 6 270 Pa (Through GUI)Specimen size 200 mm diameterSpecimen stages X: 80 mm, Y: 40 mm, Z: 5 35 mm,

T: -20 +90°, R: 360°

64

3.6 CUTTING ENVIRONMENTS

As suggested by researchers, it is desirable for a number of reasons,

to study the environment effects on the cutting process. In this research, the

three different environments studied are, dry machining with no coolant, wet

machining using soluble oil as coolant, and cryogenic machining using LN2 as

a coolant.

3.6.1 Dry Machining

Dry machining is conducted at atmospheric temperature, without

the aid of a coolant to dissipate the heat generated at the tool - chip interface.

3.6.2 Wet Machining

In wet machining, a commercial soluble oil coolant was applied at

the cutting zone, using the flood coolant application method available with the

milling machine. The emulsion cutting fluid was formulated by mixing the

concentrate with water, at a ratio of 1:20 soluble oil. The coolant flow rate

was maintained constantly at 0.6 litres/min during the machining process.

3.6.3 Cryogenic Machining

Cryogenic machining involves the application of a cryogenic fluid

LN2, as a coolant in the machining process. The cryogen could be applied as

an external spray through a nozzle to perform both conductive and convective

cooling of the cutting process, or it could be applied indirectly to cool the

cutting tool through conduction alone. The most efficient cooling system will

be the one that can apply the LN2 to the tool – chip interface, where the heat

generation is large.

65

3.7 CRYOGENIC COOLING SYSTEM

In this research work, a cryogenic cooling system was developed to

supply liquid nitrogen to the tool – chip interfaces. The available liquid

nitrogen storage container (IBP-TA55, Capacity 51.5 litres) is modified for a

pressurized cryogenic coolant flow. The schematic diagram of the cryogenic

cooling set-up is shown Figure. 3.4.

Figure 3.4 Schematic diagram of the cryogenic cooling set-up

3.7.1 Constructional Features of the Cryogenic Cooling System

The developed cryogenic cooling system consists of the following

components:

1. TA55 Liquid nitrogen container

2. Compressor

3. Drier

4. Pressure regulator

5. Pneumatic Hose

66

6. Stainless steel pipes

7. Pressure relief valve

8. Braided Stainless steel hose

9. Nozzle

In this cryogenic cooling system, the TA55 liquid nitrogen

container of 51.5 litres capacity is used. It is made of aluminium, and the

walls of the container are completely sealed, so that the atmospheric

temperature does not affect the fluid inside the container. The container is

closed with a stainless steel cap at the top. Inlet (Ø 6 mm) and outlet

(Ø 4 mm) pipes are inserted in to the container through the cap, which is

made of stainless steel. These pipes are used for taking the compressed air

into the container and transferring the fluid to the cutting zone.

A compressor is placed before the container, which is used to

compress the air to the required pressure. A drier is placed after the

compressor, which removes the moisture from the compressed air before

entering the LN2 container. This will avoid water contamination inside the

container. A pressure relief valve is attached at the inlet pipe by means of a

“T” Joint. A flexible Braided Type stainless steel hose made of very low

thermal conductivity is attached to the nozzle at one end and the stainless

steel pipe at the other end, in order to supply the liquid nitrogen. The hose is

thermally insulated with polymer foam, in order to prevent the fluid being

affected by external heat during the transfer. The nozzle is used to direct the

LN2 at the tool – chip interface. The modified TA55 liquid nitrogen container

cap is shown in Figures 3.5 and 3.6, respectively.

67

Figure 3.5Cryogenic container cap

Figure 3.6 Modified TA55 cryogenic container

3.7.2 Working Principle of the Developed Cryogenic Cooling System

Liquid nitrogen is stored in the cryogenic container (IBP-TA55,

Capacity 51.5 litres). The compressed is supplied by an external compressor

air at a 3 bar pressure. To maintain the consistency of the compressed air

pressure during an experiment, the pressure regulator was used. The

compressed air was dried using a drain filter and passes to the cryogenic

container through an inlet pipe. This compressed air starts to force the fluid

down. The outlet pipe is placed at the bottom of the container. Due to the air

68

pressure inside the container, the liquid nitrogen starts rising through the

outlet pipe, and it is directed to the tool – chip interfaces through a nozzle of

2 mm diameter. The photographic view of the experimental set-up, and the

LN2 delivery nozzle is shown in Figures 3.7 and 3.8, respectively.

Figure 3.7 Experimental set-up for cryogenic cooling

Figure 3.8 LN2 delivery nozzle at the cutting zone

CHAPTER 3 EXPERIMENTAL CONDITIONS AND PROCEDURE...

Documents

Transcript of CHAPTER 3 EXPERIMENTAL CONDITIONS AND PROCEDURE...