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

Guwahati, Assam, India

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HSS TOOL WEAR MECHANISM IN MACHINING OF HTBP BASED

COMPOSITE PROPELLANT GRAIN

Kishore Kumar Katikani1,VanapalliSrinivasaRohit 2,Anne Venu Gopal3*

and V.V. Rao4

1Scientist-D,NSTL,DRDO,Visakhapatnam-530027,Email: b4ukishorekk@rediffmail.com

2 Research Scholar, NIT Warangal -506004, Email: vsrohit@gmail.com 3* Professor, NIT Warangal -506004, Email: venu@nitw.ac.in

4 Director,SPRITE,ASL,DRDO,Hyderabad -500058 Email:vemanavrao2005@gmail.com

Abstract

Thrust characteristics of Solid propellant Rocket Motor (SRM) depend on the surface area of the propellant grain

exposed to initial ignition. For controlled combustion of propellant, contours and slots of initial ignition surface on

propellant grain are generated by turn milling operation. Metallic Aluminum powder is the fuel and Ammonium

Perchlorate (NH4ClO4) is the oxidizer in majority of HTPB (Hydroxyl Terminated Polybutadiene) based composite

propellants. Since the propellant being highly inflammable due to these sensitive ingredients, basic understanding of

machining process is very crucial for safety. The present paper focuses on the mechanism of tool wear of custom

made HSS inserts used in machining of propellant grain.

Eight cutting elements used in machining of propellant grain were examined to study the tool wear pattern and

predominant wear mechanism. Flank and rake surface were analyzed to determine the tool wear phenomena.

Microstructure of the machined surface of grain was determined to understand dispersion pattern of the ingredients

in composite propellant. K2 Chemical wear was found to be predominant. Understanding the wear mechanism helps

in development of improved insert or coatings on present insert in machining the solid propellant rocket motor. Keywords:HSS insert, Chemical wear, Propellant machining, and Solid rocket motors

1. Introduction

Understanding the science underlying machining of new

materials is an important aspect of metal cutting

research. HTPB(Hydroxyl Terminated Polybutadiene)

based solid propellant material has properties similar to

elastomers but is chemically reactive and highly

inflammable.Lewis. M.A has studied End milling of

elastomers using woodworking tools, along with

cryogenic cooling [1]. Use of induction heated tools at

low speed machining of elastomers has been studied by

Luo et al [2,3]. Use of cryogenic cooling and induction

heated tools is not possible options in this case due to

problem of rejection by contamination of propellant and

fire/explosion hazard due to heat sensitivity of

propellant respectively. Hence, a conventional

machining process using custom-made HSS inserts was

developed. The present paper focuses on establishing

tool wear mechanism for HSS inserts being used in

machining of Solid propellant Rocket Motor (SRM)

grain.

The thrust characteristics of a rocket motor depend

on the grain surface area exposed to initial ignition.

Generally solid propellant grains are vacuum casted

withAmmonium Perchlorate as oxidizer and metallic

Aluminium as fuel ingredients. The grain is casted with

central hole in a thermally insulated capsule supported

by metallic/composite casing and cured in oven to

achieve desired mechanical properties. The cured strand

of solid propellant material without casing is referred to

as propellant grain. To optimize the thrust

characteristics and to tailor the ballistic requirements,

the propellant grain is configured with secondary slots

in addition to precast central hole. These secondary slots

are remotely machined by CNC vertical turn milling

machine using custom build hollow side and face

milling cutter carrying HSS conical inserts, with

integrated safety systems to the machine.

2. Methods Experiments were conducted with a special purpose

CNC Vertical Turn Milling machine (Make: BECO,

Model: Special Purpose VTM-15kW) using a turbine

shaped cutter (Patent titled “A milling cutting tool”

bearing Application No.3023/DEL/2013,dated:10 Oct

2013) as shown in Figure 1. Custom made four conical

HSS TOOL WEAR MECHANISM IN MACHINING OF HTBP BASED COMPOSITE PROPELLANT GRAIN

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HSS inserts (Figure 2) are integrated to the cutter.

Because of the explosive nature of the material,

experiments were remotely monitored. The cutter is

connected to a chip evacuation and collection system

through hollow arbor (not shown in figure) for enhanced

safety. Safe machining parameters were established by

monitoring temperature at the cutting zone to be less

than 50oC. Temperature was measured using an infrared

thermometer. Machining parameters and environmental

conditions at which the experiments were conducted are

shown in Table 1.

Figure 1 Experimental set-up of CNC vertical turn

mill center

Figure 2 Custom made HSSconical cutting inserts

Work material was analyzed for composition of

Oxidizer and the metallic fuel ingredients and their

dispersion in the composite matrix using a microscope

with an image Analyzer (Olympus GX71, 200x

Magnification). Mechanical properties were determined

to get comprehensive knowledge of the material

characteristics. INSTRON UTM machine (model: 5500)

was used to measure tensile strength, Modulus of

elasticity, and percentage elongation as per IS 3400

standards. Density was measured using Gravimetric

method. Hardness was measured using Shore’s hardness

tester (Make: Mitutoyo, Model: MLR322). Machined

surface, as cured surface, and propellant chips were

analyzed under microscope to analyze the phenomenon

of chip formation and investigate the reasons of tool

wear. Tool wear progression was analyzed by observing

the flank surface at equal intervals. Vision inspection

system (Figure 3) was used to analyze tool wear pattern,

along the rake and flank surface. Cutting was continued

till appreciable wear was visible on the insert.

Table1: Machining parameters & environmental factors

Figure 3Vision Inspection System

Machining Parameters

Cutting speed 100m/min

Rotary table feed 0.42m/min

Depth of cut 3mm

Machining environment

factors

Machine bay Ambience

Temperature

28 -32° C

Relative Humidity 75-85%

Coolant Dry machining

Vacuum at the cutting

edge

240 mm Hg below

Atm

Propellant grain

HSS hollow

Insert

Hollow arbor

Turbine

shaped cutter

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

Guwahati, Assam, India

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3. Results

3.1 Work material Phase/volume analysis The contents of the work piece in as cured condition are

found out using a microscope image with image

analyzer. Finding out volume fraction of the oxidizer,

fuel and other ingredients helps in analyzing the

possible reasons for tool wear. A sample image of the

analysis is shown in Figure 4. The average of three

samples was found out to be 67.86% of oxidizer (Table

2).Table 3 shows mechanical properties of the

propellant material.

Figure 4 Volume fraction analysis of work piece

material (a) Image from microscope (50x) (b) Image

analysis, pink showing Ammonium Perchlorate with

binder and green shows Aluminium (as cured

condition)

Table 2 Volume fractionof workpiece material

Table 3 Mechanical properties of propellant grain material

Tensile strength 600 kN/m2

Young's Modulus 4000 kN/m2

% elongation 30

Density 1.17–1.21×103kg/m3

3.2 Analysis of Machined surface

Machined surface was compared with as cured surface

to get an idea of surface transformation. De-bonding of

Ammonium Perchlorate particles from the machined

surface was found (Figure 5(c)). The cross section of

chip was found to still have the particles intact as shown

in Figure 6(a).

Figure 5 (a) Unmachined (b) Machined surface (c) Voids

and debonded particles (Magnification 100x)

Figure 6 (a) chip cross section (b) Ammonium Perchlorate

(AP) particles collected along with the chip (Magnification

100x)

3.3 Tool wear

Figure 7 shows the progression of tool wear on flank

surface. Figure 7(a) shows formation of local corrosion

lines/dendrites. Figure 7(b) shows further dispersion by

growth of corrosion lines in the contact area close to the

cutting edge. Figure 7(c) shows formation of corrosion

pits from the corrosion lines. The tool surface can be

seen to have eroded unevenly. Figure 7(d) shows

joining of local corrosion lines and corrosion pits into

corrosion/pitting patch. Figure 7(e) shows deepening of

corrosion patches. Figure 7(f) shows growth of pitting

patches into large pitting volumes. Figure 7(g) shows

the final worn out area of tool by corrosion action.

3.4 Analysis of Tool wear pattern using Visual

Inspection system

Tools were inspected after their end of life to measure

flank and its wear pattern. The wear pattern observed

under system is converted into a CAD drawing that can

be used to find wear area and also maximum wear in

horizontal and vertical direction (Figure 8). Figure 8(a)

and (c) show the magnified images of the tool flank and

Sample

No

Ammonium

Perchlorate

(ρ=1.95)

Binder &

metallic

fuel (ρ=2.4)

1 71.51 28.49

2 67.21 32.79

3 64.86 35.14

Average 67.86 32.14

Coarse AP

particles

fine AP

particles

a b

Embeded AP

under binder

void

De-bonded

AP particles

Void

a b

c

a b

HSS TOOL WEAR MECHANISM IN MACHINING OF HTBP BASED COMPOSITE PROPELLANT GRAIN

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rake respectively. Figure 8(b) and (d) show the profiles

generated from the magnified images. These have been

used to calculate maximum wear in both X and Y

direction. The average of maximum tool wear is

presented in Table 4.

Figure 7 Tool wear progression on flank surface

(Magnification 100x)

4. Discussions Normal mechanisms of HSS tool wear were not found

[5] while machining propellant grain. As the workpiece

material being soft (65-70 on Shore ‘A’ scale), there is

no abrasive wear on HSS insert. Visual inspection of the

tool shows no traces of adhesion of machined material.

As can be seen from the images and wear

measurements, the wear was spread in a large area with

visible corrosion pits and patches. Due to lack of large

electrode potential difference between the tool and

workpiece material, the possibility of electrochemical

wear can be ruled out.Chemical (corrosive) wear can be

the predominant wear mechanism. This can be

attributed to presence of strong oxidizer in large

quantities (67.86%) in work piece and chips and

presence of moisture(RH 85%) in the surrounding

environment (Table 1). Perchlorate in presence of

humidity in atmosphere reduces Fe present in HSS

according to the following reaction [4]:

( )4 4 2 42NH ClO 4H O 4Fe 4Fe OH NH Cl+ + → +

This kind of wear can be prevented by a suitable inert

coating on tool or machining in inert environment.

Possibility of adsorption of inert gas by workpiece

material prevents application of inert gas in cutting

zone.

Figure 8 Tool wear pattern of rake and flank surface

Table 4 Average of maximum wear of tool

Hx, max

(in mm)

Hy, max

(in mm)

Rake surface 2.82 2.8

Flank surface 2.48 0.55

a

b

d

c

a b c d

g f e

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

Guwahati, Assam, India

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5. Conclusions The predominant wear in HSS insert of turbine cutter

while machining of HTPB based composite propellant

grain with AP as oxidizer is Chemical wear. The pitting

corrosion on rake and flank face of the HSS insert

confirms the same. This wear is further accelerated in

presence of high RH values at the cutting zone and

application of air stream as vacuum near cutting edge.

The knowledge of possible tool wear mechanism

enables in designing a suitable inert coating for the tool

and in selection of new tool materials. The method used

to measure wear pattern provides an easier method to

determine various parameters of flank and crater wear,

including crater depth up to some extent.

6. Acknowledgements The authors express their sincere thanks The General

Manager, DRDO, Jagdalpur and Director, NSTL for the

support of facility extended and equipments.

7. References 1. Lewis, M. A. (2002). End milling of

elastomers.(Doctoral dissertation, NC State

University)

2. Luo, J. (2005). Machining of Elastomers (Doctoral

dissertation, The University of Michigan).

3. Luo, J., Ding, H., & Shih, A. J. (2005). Induction-

Heated Tool Machining of Elastomers—Part 2:

Chip Morphology, Cutting Forces, and Machined

Surfaces. Machining science and technology, 9(4),

567-588.

4. Prinz, H., &Strehblow, H. H. (1998). Investigations

on pitting corrosion of iron in perchlorate

electrolytes. Corrosion science, 40(10), 1671-1683.

5. Trent, E. M., & Wright, P. K. (2000). Metal cutting.

Butterworth-Heinemann.