ANALYSIS OF TOOL WEAR: TURNING WITH IN SITU ACETYLENE

By

GREGORY D. SUSIL

A THESIS PRESENTED IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR GRADUATION WITH

MAGNA CUM LAUDE HONORS

FOR A BACHELORS DEGREE IN

MECHANICAL ENGINEERING

UNIVERSITY OF FLORIDA

2011

Page | 2

Analysis of tool wear: turning with in situ acetylene

Gregory D. Susil1, Nicolas Argibay1, Carson Ingley, Mark E. Gallrein1, Tony L. Schmitz1, W.G.

Sawyer1, and Gerald Bourne2

1 Department of Mechanical and Aerospace Engineering 2 Department of Materials Science and Engineering

University of Florida, Gainesville, FL 32611 USA

Abstract

Recent research suggests that the use of acetylene gas in high-temperature metal cutting

operations may result in the deposition of a carbonaceous solid lubricant and a reduction in tool

wear. This paper compares the tool wear of an uncoated carbide insert while turning low carbon

steel in lab air to the tool wear of the same turning process while operating with in situ acetylene

gas deposition with a nitrogen cover gas. The objective was to determine if the acetylene gas

would form deposits of solid carbon on the cutting edge, effectively reducing the tool wear by

inhibiting carbon diffusion out of the tool and/or by depositing the solid lubricant. Orthogonal

cutting conditions were simulated on a lathe using two different cutting procedures. Tool wear

was measured qualitatively by optical micrographs of the cutting edge and quantitatively by

plotting the spindle power as a function of tool passes (an indirect measurement of tool wear) for

all cutting arrangements. Though the cutting methods examined did not demonstrate a clear

reduction of tool wear, interesting interactions were observed and are discussed. This research

provides valuable insight for the continuation of in situ lubrication of metal cutting operations

with carbonaceous feed gases.

Keywords

Tool, Wear, Turning, Machining, Acetylene, Gas, Deposition

Page | 3

1. Introduction

Tool wear is an important factor affecting the productivity and profitability of metal cutting

processes. For this reason, it is beneficial to research methods that reduce tool wear.

Traditionally, cutting fluids are introduced to the system in order to decrease wear rates. Cutting

fluids must be filtered and monitored before introducing them to the system. They also must be

disposed of properly once they are deemed unusable. Cutting fluids can reduce wear rates, but

also incur a financial and environment burden on the manufacturing process.

This work used a new approach to reduce tool wear. For two sets of cutting tests, the wear

rates were measured while first turning in lab air and then turning with in situ acetylene gas

(C2H2). The acetylene gas was accompanied by a nitrogen cover gas (N2) in order to avoid

oxidation of the carbon. The tool was an uncoated carbide insert and the work piece was low

carbon steel. The wear rate was measured qualitatively by optical micrographs and quantitatively

by plotting the spindle power as a function of tool passes. Comparisons of the tool wear rates in

lab air to those while exposed to acetylene gas are presented.

1.1. Tool Wear

Tool wear depends on many parameters including the tool material and geometry, the work

piece material, the cutting conditions, the coolant (if applicable), and the machining process

(turning, milling, drilling, etc.) [1]. When machining a steel work piece with a carbide tool, a

primary wear mechanism is embrittlement due to the diffusion of carbon from the carbide insert

into the chip [2]. This diffusion rate increases with cutting speed due to the subsequent

temperature rise. Tool wear is also affected by abrasion and adhesion [1].

The wear regions shown in Figure 1.1 are typically clearly visible. These wear regions grow

as material is removed from the work piece. As they grow, they can affect the process stability,

chip generation, sharpness of the cutting edge, and part dimensions. The wear rate of the tool

describes how fast these regions grow as the cutting process is executed. The cutting process

must be paused and the tool replaced once the wear regions render the tool unusable. The time

taken to replace the worn out tool, as well as the cost of the tool/insert itself, increases the cost of

the manufacturing process. The reduction of tool wear leads to improved efficiency and

profitability of the manufacturing process.

Page | 4

Figure 1.1: Regions of tool wear. Note that crater wear (left) occurs on the rake face, while flank

wear (bottom) occurs on the relief face.

Monitoring the spindle power provides another effective method to indicate tool wear. As the

tool wears, the forces between the tool and the work piece grow. As they grow, there is more

resistance to the rotation of the work piece. As there is more resistance to rotation, there must be

more power supplied to the spindle in order to maintain the selected spindle speed. Thus, spindle

power is a function of tool wear; spindle power increases as tool wear increases.

1.2. Previous Research

Previous research has shown that the deposition of solid carbon on the tool-work piece

interface during milling can lead to lower forces and reduce tool wear. Figure 1.2 displays the

variation in flank wear width (FWW) with volume of material removed when using a carbide

insert to machine 1018 steel. Two results are presented: one trend in air and one when using

acetylene gas (in these tests the gas was ignited to produce a carbon-rich flame). Using the flame,

solid carbon was deposited on the tool during the milling process. This deposition acted as a

solid lubricant, reducing the cutting forces and slowing the growth of flank wear. It was

Continuous Chip

Tool

Work Piece

Rake Face

Relief Face

Region of

Crater Wear

Region of

Flank Wear

Tool

Cutting Edge

Page | 5

concluded that the carbon deposition reduced the cutting forces and the tool wear rate [3]. In

order to further explore the use of acetylene gas in reducing tool wear, this paper examines the

effect of the gas on tool wear rate during a turning operation.

Figure 1.2: Milling with ignited acetylene gas resulted in a minor reduction in average flank

wear width (FWW) [3].

0 20 40 60 80 100 120 1400

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Volume removed (cm3)

FW

W (

mm

)

Average

With Carbon

Page | 6

2. Objectives

The purpose of these experiments was to determine if the introduction of acetylene gas to a

turning operation would reduce the tool wear rate for uncoated carbide inserts. Therefore, the

following objectives were to be accomplished:

• complete orthogonal cutting tests using a lathe and tube work piece;

• establish a stable cutting process;

• measure tool wear while machining in air (no other lubricant);

• measure tool wear while machining in a low flow of nitrogen-shielded acetylene gas;

• vary cutting arrangements to optimize the exposure of the rake face to acetylene gas; and

• compare the wear rates between machining in air and in acetylene gas.

By achieving these objectives, a complete picture of the wear characteristics could be

determined. With the wear rates of the different arrangements characterized, they could be

compared and conclusions could be drawn in order to determine what affect, if any, the acetylene

gas had on the system.

3. Experimental Setup

Two turning procedures were studied in this research. They were designed to analyze

different tool wear rates and to see acetylene’s influence on each. Turning Method A was

designed to establish a standard wear rate that could be referred to as studies continued. In this

method, the tool was in constant contact with the work piece and only exposed to the gas at the

beginning and end of each pass. Turning Method B was designed to allow for more exposure of

the cutting edge to the acetylene gas through intermittent contact with the work piece. The two

arrangements allowed for a comparison of results and a more in depth view of the effects of in

situ acetylene while turning. The arrangements are discussed in detail in the following sections.

Page | 7

3.1. Turning Method A

Turning Method A was designed to establish a baseline tool wear rate. This tool wear rate

was influential on developing further arrangements. The spindle speed was set at 1800 rpm and

the tool feed rate was set at 150 mm/min. There was a single cutting pass per specimen. The tool

came into contact with the work piece and was fed at a constant feed rate for 14 mm without

stopping; this generated a continuous, spring-like chip. The entire pass took approximately five

seconds. Turning Method A was a very aggressive operation, resulting in high forces on the tool.

3.2. Turning Method B

Turning Method B was designed to allow for more exposure of the cutting edge to the

acetylene gas during the turning pass. The spindle speed was set at 1800 rpm and the tool feed

rate was set at 180 mm/min. This turning arrangement consisted of periodic cutting for one

revolution of the work piece. The tool then backed off 1 mm and came back into contact with the

work piece for another rotation. This process continued for the length of the work piece,

breaking the chips along the way, and allowing for intermittent exposure of the cutting edge.

This was an aggressive cut as well, though the forces were not as high due to smaller chip

thicknesses.

3.3. Gas Delivery

The gas was delivered to the system by the concentric tube shown in Figure 3.1. When

introducing the gas to the turning process, the nitrogen flowed at about 3.5 LPM and the

acetylene was flowed at about 0.5 LPM. The cloud of nitrogen (N2) surrounding the acetylene

(C2H2) prevented the oxidation of the carbon molecules in the acetylene. The flow rates were

maintained such that the acetylene gas did not ignite while turning. Figure 3.2 shows how the

nozzle was set up during the turning operations. The nozzle was fastened in a way that kept the

tip close to the cutting edge. Thus, the flow was always pointing directly at the cutting edge

whenever it moved. The gas was in place to interact with the system at any possible time.

Page | 8

Figure 3.1: The top photograph shows the actual nozzle with an inlet tube for acetylene and an

inlet tube for nitrogen. The bottom image [3] is an illustration of how the gases exit the

concentric tube. The arrows indicate the nitrogen and acetylene flows.

Figure 3.2: The left photograph shows how the gas was delivered (Turning Method A is

pictured). The right photograph shows a closer view.

Nitrogen

Flow

Acetylene

Flow

Work Piece

Continuous

Chip

Nozzle

Page | 9

3.4. Data Acquisition

Two methods were used in order to measure the tool wear rates. Optical micrographs were

collected using a digital microscope. The setup is shown in Figure 3.3. Images were taken from

above (looking down onto the rake face) because the flank wear grew very slowly and was not

the primary wear region. Wear on the rake face eventually led to instability, rendering the tool

unusable.

Figure 3.3: The microscope was placed above the carbide insert in order to capture images of

the wear regions that developed on the tool.

The other method used to measure the tool wear rate was spindle power. As previously

discussed, the spindle power increases as the tool wear increases. The spindle current was

measured to produce a 0-10 V analog output. This voltage was proportional to 0-5 hp. A

National Instruments USB data acquisition system was used to sample the voltage at a rate of 10

kHz; data points were saved every 0.01 seconds (each recorded data point was the average of

100 measured values). For each recorded data point, the standard deviation of the one hundred

data points was also calculated and recorded. The average spindle power for each pass was used

to track the tool wear.

Microscope Carbide

Insert

Work Piece

Page | 10

4. Results

The results are presented as they were accumulated for each cutting arrangement. Results

from Method A and Method B are both presented in the discussion that follows. Optical

micrographs are shown, displaying the wear associated with the completion of a wear test. Plots

of the spindle power are also given. The plots show the average spindle power recorded from a

single pass. Figure 4.1 shows data collected from a single pass of both turning arrangements. It

was this data that was averaged for each pass in order to compile the plots discussed later in this

section.

Figure 4.1: The average power for the pass was taken from when the cut began until its

completion. This is the average that is plotted per pass in the following sections. One thing to

notice about Method B is that the spindle power increases as the turning operation continues.

The initial period of ramp before cutting corresponds to the spindle’s acceleration to to the

commanded speed.

time (s)

Page | 11

4.1. Results from Method A

The results from the spindle power acquisition are displayed in Figure 4.2. While turning in

this arrangement, the average horsepower was scattered. Though there was higher fluctuation of

spindle power in the tests using acetylene, there is no clear effect on the system. Both

arrangements experienced similar highs and similar lows with no obvious pattern.

The optical micrographs are shown in Figure 4.3. The flank wear width (FWW) was minimal

and indistinguishable in the cutting operations, so the rake face wear is shown. While turning in

acetylene, the wear region had colorful patterns, but there was no noticeable difference in the

amount of wear. The stability of the system was the same while turning both in air and in

acetylene.

Figure 4.2: The standard deviations displayed are the average of the standard deviations of all of

the data points collected for each pass. There is no clear trend in the spindle power readings.

Page | 12

Figure 4.3: Turning in acetylene (left) resulted in a more colorful wear region than turning in lab

air (right). However, the amount of wear was similar.

4.2. Results from Method B

The results from the spindle power acquisition are displayed in Figure 4.4. There was a clear

trend while operating in this arrangement. Upon the introduction of acetylene, the spindle power

increased and maintained a higher value for the remainder of the test. This was the opposite

effect of what was expected. The introduction of the gas actually increased spindle power. This

brought up a question. Was the acetylene reacting with the system in some way that caused

increase spindle power? Or, was it the combination of acetylene and nitrogen that was causing

the change? To answer this question, tests were run with only nitrogen gas. The results are

displayed in Figure 4.5. When only nitrogen was introduced to the system, the spindle power

behaved as it did in air.

The optical micrographs are shown in Figure 4.6. The images agreed with the spindle power

readings. There was higher tool wear when the acetylene gas was introduced. Not only was the

tool wear rate measurably higher, the chip dynamics were also altered by the acetylene. While

turning in lab air, chip formation was continuous and there was negligible, if any, built up edge.

The chip formation was also continuous while turning with acetylene, however, there was

considerable built up edge. This was confirmed as an effect from the acetylene when the test was

run only in nitrogen. When the acetylene was removed, the built up edge was eliminated.

In acetylene In lab air

Page | 13

Figure 4.4: Once again, the standard deviations displayed are the average of the standard

deviations of all of the data points collected for each pass. All tests started at about the same

spindle power. When acetylene was introduced, the spindle power became progressively higher,

in contrast to that in lab air. The power remained high for the remainder of the test in acetylene.

Page | 14

Figure 4.5: When only nitrogen was applied to the turning process, the spindle power behaved

as it did when in lab air. Only when acetylene was introduced to the system did the spindle

power increase.

Figure 4.6: When the adhered chip was removed, it revealed a greater amount of tool wear on

the cutting edge when acetylene was introduced to the turning operation.

In lab air In acetylene with adhered chip (left)

and with the chip removed (right)

Page | 15

4.3. Carburization of Work Piece while Turning Method B

From the results discussed in regards to Turning Method B, it was hypothesized that the

acetylene gas was actually hardening the work piece. As the work piece was sheared and the new

surface exposed, it was at very high temperatures. The temperatures were high enough for the

acetylene to react with the surface of the work piece. The carbon in the acetylene gas could

actually diffuse into the work piece, effectively hardening the newly formed surface that would

soon be cut by the next movement of the tool. If this was the case, the hardening of the work

piece would be the cause of the higher tool wear rates and higher spindle power.

To determine if this surface hardening was actually occurring, a Vickers Hardness test was

performed at a 10 μm maximum depth of indentation. Figure 4.7 displays the specimen that was

analyzed. From the hardness test, it was found that the surface turned in the presence of

acetylene was harder. In air, it was calculated that HVair ≈ 200 MPa. In acetylene, it was

calculated that HVacetylene ≈ 400 MPa. The surface was hardened by absorbing the carbon from

the acetylene gas.

Figure 4.7: The machined surface tested harder when it was turned in acetylene.

4.4. Summary of Results

The introduction of acetylene gas to Turning Method A had no measurable effect on the

amount of tool wear. However, the introduction of acetylene had a significant impact on Turning

Method B. The acetylene gas carburized the surface of the work piece, leading to higher tool

wear rates and higher spindle power.

Surface where

the hardness

test was

performed

Specimen after

turning test

was completed

Page | 16

5. Conclusions

From the results found from Turing Method A, it was concluded that acetylene does not have

an impact on a system when the rake face is never exposed. In this arrangement, the cutting edge

was always in contact with the work piece, and the depth of cut was large enough that surface

features of the work piece do not play a part. The acetylene gas never has a chance to interact

with the system in a way that would affect the tool wear rate.

On the contrary, results found from Turning Method B imply that acetylene gas had a

significant impact on the tool wear rate when the cutting edge was intermittently exposed. The

work piece’s newly turned surface was at high enough temperatures for the acetylene to harden

it. Carbon diffused into the surface of the work piece, resulting in more tool wear. This

interaction was interesting, though it was the opposite effect of what was anticipated.

5.1. Future Work

This study lead to some intriguing conclusions and turning tests with in situ acetylene gas are

warranted. It would be beneficial to use Raman spectroscopy to analyze the surface of the tool.

This analysis would indicate if there was any deposition of solid carbon on the tool. Deposition

of solid carbon on the uncoated carbide inserts could lead to further studies in the reduction of

tool wear. If a work piece material was selected that did not allow carburization of the surface,

then the tool wear rates could be slowed.

This study provided an unexpected result that also deserves further research. This is the

carburization of the newly turned surface. While turning low carbon steel with the introduction

of acetylene, the surface was hardened to about twice that of when the same material was turned

in air. Methods should be researched that apply acetylene to a machining operation in order to

achieve case hardening.

Page | 17

6. References

1. Nouari, M., Molinari, A., Experimental verification of a diffusion tool wear model using

a 42CrMo4 steel with an uncoated cemented tungsten carbide at various cutting speeds,

Wear 259 (2005) 1151-1159.

2. Tlusty, J. Manufacturing Processes and Equipment, Prentice Hall, NJ (1999).

3. Gallrein, M.E., In-situ Acetylene Gas Deposition for Reduced Tool Wear in Machining,

Honors Thesis, University of Florida (2010).

4. Argibay, N., Keith, J.H., Krick, B.A., Hahn, D.W., Bourne, G.R., Sawyer, W.G. High-

temperature vapor phase lubrication using carbonaceous gases, Tribology Letters 40

(2009) 3-9.