^ . 1 ^ L

AN INVESTIGATION OF INTERMITTENT VERSUS CONTINUOUS

CUTTING METHODS OF TOOL LIFE TESTING

by

JAMES LEO THOA^S, B . S . in I . E .

A THESIS

IN

INDUSTRIAL ENGINEERING

Submitted to the Graduate Faculty of Texas Technological College

in Pa r t i a l Fulfil lment of the Requirements for

the Degree of

MASTER OF SCIENCE IN

INDUSTRIAL ENGINEERING

Approved

Accepted

August, 1969

r ^ ACKNOWLEDGMENTS

I a m deeply indebted to Dr. B. K, Lamber t for his di rect ion

of this thes is and to the other m e m b e r s of my commit tee , Dr . C. L.

Burford, Dr . P . Kc Koh, and P ro fes so r W. D SandeL I wish to

thank the depar tment technicians C. D. Mittan, J . L. Gibbs, and

Co E, Hipp for the i r a s s i s t ance in setting up the exper imenta l equip-

ment .

I am also indebted to the Texas Tech Industr ia l Engineering

Depar tment for the i r sponsorship of this t he s i s .

I v^ish to express my appreciat ion to my wife and to my paren ts

without w^hose constant patience and understanding this r e s e a r c h

would not have been completed.

11

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ii

LIST OF TABLES v

LIST OF ILLUSTRATIONS vii

Chapter

I. PURPOSE AND SCOPE 1

Introduction 1

Purpose 4

L i t e ra tu re Survey 4

II. EXPERIMENTAL DESIGN, EQUIPMENT,

AND PROCEDURE 30

Genera l Considerat ions 30

Equipment 32

Design of the Exper iment 37

Exper imenta l P rocedure 42

III. ANALYSIS OF FLANK WEAR AND FORCE DATA 46 Flank Wear Data for Intermit tent and

Continuous Methods 47

Fo rce Data for Intermit tent and Continuous Methods 63

i i i

IV

Chapter Page

Tool Life Data and Economic Analysis 79

IV. CONCLUSIONS AND RECOMMENDATIONS

FOR FURTHER RESEARCH 90

Flank Wear Data 90

Fo rce Data 92

Tool Life Values and Economic Considerat ions 94

A r e a s for Fu r the r Resea rch 95

LIST OF REFERENCES 98

APPENDIXES

A. Dynamometer Calibrat ion Curves 102

B. Exper imenta l Data 104

LIST OF TABLES

Table Page

1. Cutting Tool Mater ia ls and Charac te r i s t i c s . . . 20

2. List of Equipment and Use 32

3. In termit tent Method t - T e s t s for Flank Wear Between Opposite Cutting Edges 49

4. Continuous Method t - T e s t s for Flank Wear Between Opposite Cutting Edges 51

5. t ' T e s t s for Flank Wear Between Intermit tent and Continuous Testing Methods 59

6. Intermit tent Method t - T e s t s for Cutting Fo rces Between Opposite Cutting Edges 64

7. Intermit tent Method t - T e s t s for Longitudinal F o r c e s Between Opposite Cutting Edges . . . 66

8. Continuous Method t - T e s t s for Cutting Fo rces Between Opposite Cutting Edges 68

9. Continuous Method t - T e s t s for Longitudinal F o r c e s Between Opposite Cutting Edges . . . 69

10. t ' - T e s t s for Cutting Fo rces Between Inter-mit tent and Continuous Testing Methods 75

11. t ' - T e s t s for Longitudinal F o r c e s Between Intermit tent and Continuous Testing Methods 76

12. Summary of Tool Life ve r sus Cutting Speed Curves 83

v

VI

Table Page

13. Minimum Cost Economic Analysis of In termit tent and Continuous Test ing Methods 87

14. Maximum Product ion Rate Economic Analysis of Intermit tent and Con-tinuous Testing Methods 87

15. Intermit tent Testing Method 105

16. Continuous Testing Method 108

LIST O F ILLUSTRATIONS

F i g u r e P a g e

1. F o r c e s Act ing on a Single Po in t Tool

Dur ing a Turn ing O p e r a t i o n 7

2. Types of W e a r Resu l t ing in Tool F a i l u r e 10

3. Typ i ca l Tool Life v s . Cutt ing Speed C u r v e . 12

4. Re l a t i onsh ip of H a r d n e s s and T e n s i l e S t r eng th to Workp iece and Tool M a t e r i a l s 15

5. S t anda rd Tool G e o m e t r y N o m e n c l a t u r e 18

6. F l a n k W e a r v s . Cut Length (or T ime) for

V a r i o u s Cutt ing S p e e d s - C a r b i d e Tools . . . . 24

7. , T h r e e Di raens iona l La the D y n a m o m e t e r 35

8. Mean F l a n k W e a r - T i m e C u r v e s for I n t e r m i t t e n t Method 5 3

9. Mean F l a n k W e a r - T i m e Curve for Cont inuous Method 54

10. Mean F lank W e a r v s . Cutt ing Speed for In t e r 171 it t en t Method 56

11. Mean F l a n k W e a r v s . Cutt ing Speed for I n t e r m i t t e n t Method 57

12. Di f fe rence Be tween InteriTiittent and Cont inuous Method F l ank W e a r 62

13. Mean Cutt ing F o r c e v s . T i m e for I n t e r m i t t e n t Method 71

v i i

V l l l

Figure Page

14. Mean Cutting F o r c e v s . Time for Continuous Method 72

15. Mean Longitudinal Force vs . Time for In termit tent Method 73

16. Mean Longitudinal Fo rce vs . Time for Continuous Method 74

17. Tool Life Curves for Intermit tent Testing Method 81

18. Tool Life Curves for Continuous Testing

Method 82

19. Cal ibrat ion Curve for Cutting Fo rce , F . . . . 103

20. Cal ibrat ion Curve for Longitudinal F o r c e , F. 103

CHAPTER I

PURPOSE AND SCOPE

Introduction

Tool life studies play a vital role in determining the optimuin

cutting conditions for economical metal removal operations. As

more is known about the metal cutting process , conservative hand-

book values for the machining variables traditionally used by industry

may gradually be replaced with values based on economical consider-

ations. Only continuous research in this area and the application of

the resul ts , whether it be directly or by handbook revisions, will en-

able manufacturing organizations to operate economically under the

ever increasing production requirements of today.

It must be recognized, however, that each research study

determines tool life values for a particular set or sets of workpiece

mater ia ls , tools, machining variables, etc. , and even under the

same carefully controlled experimental conditions spreads in tool

life values of as much as 10:1 have been found (1). Many explanations

have been offered for this wide dispersion such as tool differences,

mater ia l differences, method of holding the work, length to diameter

ratio, etc. , but as of yet no one has completely answered this dilema (1) 1

The one factor which does not vary in most experimental pro-

cedures is the manner in which tool life measurements are taken.

Regardless of the tool life criterion chosen, the conventional ex-

perimental procedure is:

1, Determine the tool life criterion to be used,

2, Begin cutting with the test tool for a short period

of time, usually one-half to one minute,

3, Remove the tool from the work and take the necessary

measurements,

4, Continue cutting with the same tool for the next

specified time period,,

5, Remove the tool again from the work and take the

required measurements,

6, Repeat this procedure with the same tool until

the end of the test (2).

Lambert (3) and Sowinski (4) have cast doubt on this method

of tool life testing. They show that different tool life values are ob-

tained when using the conventional, or intermittent, raethod and

when using an alternate, or continuous, method of tool life testing.

The continuous method allows the tool to cut without interruption

for a longer period of time with only one measurement taken at the

end of the period.

These two papers , however, are limited in scope. Lambert

compares only one continuous cutting time length, 4 minutes, with

the same time length using the intermittent method and the cutting

stopped at 0. 5, 1. 0, 1. 5, 2. 0, 3. 0, and 4. 0 minutes. Sowinski

tests to find a carbide insert which will res is t chipping during an

intermittent cut for a particular metal type, V-57 iron-base high-

temperature alloy. Both recognized and established variability be-

tween the two methods, but neither investigated the nature of this

variability; i. e. , if there is a critical cutting speed - cutting time

combination whereby it makes no difference v/hich testing method

is used.

Another problem is encountered when applying the results of 0

tool life tests to disposable carbide inser ts . It is normally assumed

that tool life values obtained for one cutting edge will remain con-

stant for all cutting edges on the same insert . Leon (5) has shown

this assuraption to be incorrect for successive edges of an insert

when using flank wear as the tool life criterion. More specifically,

flank wear values of the first and fourth edges differed significantly,

at the five per cent level, at every one of the three speed levels

tested. At the highest and lowest speed levels, Leon also showed that

the first and third edges had significantly different flank wear values.

These results indicated a "cumulative effect" on flank wear as suc-

cessive edges were tested.

Leon did not, however, consider the possibi l i ty of using only

the two opposite edges of each inse r t in order to offset this "cumu-

lative effect. "

Purpose

It was the purpose of this r e s e a r c h to invest igate:

1. The effect of cutting speed on tool life values

obtained when using an in termit tent and a con-

tinuous method of tool life tes t ing,

2. The effect of cutting speed on tool life values

obtained using opposite cutting edges of a carbide

inse r t ,

3. The rela t ionship existing between speeds for

opposite cutting edges and test ing methods to

de termine if a c r i t ica l speed- t ime condition

ex i s t s ,

4. The effect of any variabi l i ty in tool life values

between edges and methods on economic models

of the cutting operat ion.

L i te ra tu re Survey

The meta l cutting p roces s has been under careful investigation

p r i m a r i l y in the las t sixty yea r s because of the work of F . W. Taylor

in the ea r ly 1900's (6). Many re la t ionships , formulas , e t c . , have

been developed which a t tempt to verify by empi r i ca l means what

physical ly happens in the p r o c e s s of rennioving of meta l f rom a work-

piece by a cutting tool .

Probably one of the bes t ways to evaluate the efficiency of a

me ta l cutting operat ion is to re la te the "ease of machining" to the

different conditions which effect the p r o c e s s . The cominonly used

t e r m to exp res s this re lat ionship is "machinabili ty ra t ing. " This

rat ing is normal ly expressed as a percentage and is determined by

comparing how the workpiece ma te r i a l under considerat ion machines

with r e spec t to a s tandard material^__Sj Le.el^ given a rating

of 100 per c^nt,

/

In evaluating the machinabil i ty rating of a metal , the followin

c r i t e r i a may be considered:

1. Magnitude of the cutting forces on the tool,

2. Quality of the workpiece surface finish,

3. Life of the cutting tool between resharpen ings ,

under s tandardized conditions,

4. F o r m and size of chips,

5. Power consumption of the machine,

6. Cutting t e m p e r a t u r e s ,

7. Rate of cutting under a s tandard force,

8. Rate of nietal removal (7,8),

8

curve is obtained which may be utilized to convert in-process re-

corder readings to force values.

Surface finish is also an important consideration but is not of-

ten taken into account in machining calculations. The machining

operation is set up using previously determined values of the metal

cutting variables and the resulting surface finish is noted. If the

finish is not acceptable, the process variables are changed until the

finish meets specifications. Normally, surface finish will innprove

with increased cutting speed, decreased feed, decreased depth of

cut, increased workpiece temperature, and improved friction con-

ditions between the work and tool (7, 9).

Tool life is the primary machinability factor controlling the

cost of a cutting operation, and for this reason most machinability

ratings of workpiece materials are based on tool life values only.

Tool life is a term in the metal removal industry which has many

definitions. A coiTiiTion definition is the time for a tool to go from a

"sharp" condition to a condition considered to be "dull" (10). Often

tool life is specified in terms of equivalent cutting speed, i .e . , the

cutting speed at which a standard value of cutting time, such as 60

minutes, is obtained under a given set of cutting conditions (7, 11).

A tool ceases to cut efficiently and reaches the end of its use-

ful life because of:

). Flank wear - - Caused by abrasion or wear on the

flank below the cutting edge;

2. C r a t e r wear - - Caused by the moving chip wear ing

a cup in the tool face in back of the cutting edge; which

gradual ly grows l a rge r and finally causes the cutting

edge to crumble ;

3. Chipping - - Caused by the breaking out of smal l chips

from the face or flank at the cutting edge; usually due

to mechanica l or t he rma l shock on br i t t le tool m a t e r i a l s ;

4 . Various combinations of the above (11, 12).

These types of fa i lures a re shown graphically in Figure 2. Often

included as a type of tool failure is the complete breakdown of the cut-

ting edge. This type of fai lure, however, is not a common occur-

rence with proper ly designed and proper ly selected tools .

Tool life is dependent on many factors , some kno"wn and some

sti l l unknown. Basical ly, there a re four types of var iab les known

to affect tool life. They a r e :

1. Machining var iables - - Feed, speed, and depth of cut;

2. Workpiece var iables -- Phys ica l and chemical p rope r t i e s ;

3. Tool var iab les - - Tool ma te r i a l and geometry;

4. Cutting condition var iables - - Cutting fluids, ambient

t empera tu re of workpiece, and the frequency or length

of cut (3, 9).

10 10

C r a t e r Wear

Wear Land Width

Flank Wear

Chipping

Fig . 2. Types of Wear Resulting in Tool Fai lure ,

11

The manner in which these var iab les effect tool life, the common

methods to exper imenta l ly de termine tool life, and the economical

cons idera t ions of tool life will be discussed in the remainder of this

sect ion.

Relat ionship of Tool Life to Machining Variables

The th ree p r i m a r y machining var iables a r e :

1. Cutting speed - - The velocity at which the workpiece

moves past the tool or at which the tool moves past

the workpiece; expressed in feet per minute (fpm);

2. Feed - - The distance the tool advances longitudinally

or into the workpiece for each revolution of the work-

piece; expressed in inches per revolution (ipr);

3. Depth of cut - - The normal distance fronni the original

cutting surface to the freshly cut surface; expressed in

inches .

Of the three var iab les listed above, cutting speed influences

tool life by the g rea tes t amount (9, 10, 11). So great is this effect

that Taylor developed the following equation:

y T^ = C

where :

V = Cutting Speed,

T = Tool life,

12

n = Dimensionless constant depending on tool and v/ork-

piece va r i ab l e s ,

C = Dimensionless constant depending also on tool and

workpiece va r i ab le s ; numerical ly equal to the cutting

speed giving one minute tool life (6).

Upon examining this equation, it is obvious that as speed in-

c r e a s e s tool life d e c r e a s e s in an exponential manner . In fact, when

plotted on log-log graph paper , tool life ve r sus cutting speed values

obtained under no rma l operating conditions will plot approxiinately

as a s t ra ight l ine. A typical curve is shown in Figure 3.

Of the two remaining machining va r i ab les . Cook states that feed

has more effect on tool life than depth of cut (10).

Mater ia l : Steel Tool: Tungsten carbide Depth of cut: 1/16 inch Feed: 0. 062 ipr

50 100

Tool Life (min)

F ig . 3, Typical Tool Life v s . Cutti]\r, Speed Curve, log-log scale (15).

13

If these two variables are incorporated into a tool life equation, the

resulting expression is:

V T^ d^ f^ = C

where:

V, T, n, C = Defined as before,

d = Depth of cut,

f = Feed,

X, y = Dimensionless constants again depending on work-

piece and tool variables (7, 11).

Relationship of Tool Life to Workpiece Variables

It would be very convenient to be able to relate tool life directly

with known properties of the Vv'^ orkpiece material. Many researchers

have spent countless hours in performing tool life tests for a particular

workpiece materia], and consequently there are several sets of tab-

ularized data available on this subject (7, 11, 13, 14, 15).

The Tool Engineers Handbook lists the following six material

variables as having an effect on tool life:

1. Hardness,

2. Tensile strength,

3. Chemical composition,

4. Microstructure,

14

5. Degree of cold work,

6. St ra in hardenabi l i ty (7).

Hardness and tensi le s t rength have been shown to have a rough

co r re l a t ion to cutting speed for a given tool life and may be used

to es t imate (roughly) the cutting speed to use for a specific r aa te r i a l .

This re la t ionship is shown in Figure 4 for different workpiece and

tool m a t e r i a l s .

Exact re la t ionships between the chemical composition of the

workpiece and tool life have not as yet been de termined. However,

the genera l effect of alloying elements on tool life is known. For

example , the addition of silicon (0.0 - 2.0%) or nickel (0.0 - 5.0%)

to cer ta in types of s teels dec reases tool life values , whereas the ad-

dition of sulphur (0. 0 - 0 . 3%) or phosphorus (0. 00 - 0. 15%) may in-

c r e a s e tool life (7). Of course , these effects could vary consider-

ably if the l imi ts shown a re exceeded.

Workpiece m i c r o s t r u c t u r e has also been shown to have a cor-

re la t ion with the life of a cutting tool (7, 8, 11, 16, 17). In general ,

any consti tuents in the workpiece m i c r o s t r u c t u r e which a r e ha rder

than the tool m a t e r i a l tend to decrease tool life. For exainple, hard

insoluble pa r t i c l e s of a luminum oxide in the workpiece s t ruc tu re

dec r ea se tool life because of their abras ive action on the tool face.

Conversely , some types of softer par t i c les such as manganese sul-

fide in s tee l and lead in s teel and b r a s s have a very beneficial effect

15

Workpiece Key

0) 11

o CO

o

0)

m

H

U

3000 2000

{ " -I- ^ + 4-

(1)

(2) (3)

(4) (5)

(6) (7)

- Magnesium and a l loys .

- B r a s s andbronze . - Aluminum and a l loys ,

p las t i c s , - Cast i ron. - Carbon steel ,

cas t s tee l . - Alloy s tee l . - Stainless s teel ,

Monel me ta l s .

Cast Tool Mater ia l

High Speed Steel

50 100 150 200 300 400 600

Brinel l Hardness (Log scale) 4- + 1 1

20,000 50,000 100,000 200,000

Tensile Strength (Log Scale)

Fig. 4. Relationship of Hardness and Tensi le Strength to Workpiece and Tool Alriterials (7, 11).

16

on tool life. It is believed these particles decrease the tendency for

localized pressure welding to take place at the tool-chip or tool-work-

piece interface, thereby creating better machined surfaces and longer

tool life (8).

Cold working the workpiece raaterial is often beneficial to tool

life depending on the amount of carbon found in the metal. In low

carbon steels, up to 0. 3%, cold working improves tool life by lower-

ing the friction encountered in machining; however, cold working of

steels with a carbon content of over 0.4% will reduce tool life in some

cases (11).

Metals usually increase in strength and hardness when strained

or deforined. This property of "strain hardenability" is nornially

evaluated by the exponent n in Meyer's hardness relationship:

Load = ka

where:

k = Constant of the material corresponding to a fictitious,

underformed hardness, or a special relative hardness

value;

a = Diameter of the impression made by a Brinell ball at

varying values of load;

n = Slope of the resulting curve plotted on log-log graph

paper (7, 16).

17

A low value of n is reported to have beneficial effects on tool life and

surface quality, provided that comparisons are made on materials

of comparable hardness (7).

Relationship of Tool Life to Tool Variables

The two types of tool variables having an influence on tool life

are tool geometry and tool material. The standard single point tool

geometry is given in Figure 5. Of the seven factors describing tool

geometry, the three having the most pronounced effect on tool life

are the side rake angle, the side cutting edge angle, and the nose

radius (7, 11, 18).

The side rake angle is important because it directs chip flow

to the side of the tool holder and permits easier feeding of the tool

into the work. When the tool feeds more easily into the work, forces

on the tool tip decrease thereby increasing tool life.

The side cutting edge angle determines the true length of cut

which in turn determines the distribution of the cutting forces on

the tool. Increasing this angle also increases tool life, but only

up to a point. Beyond this critical point, chatter develops result-

ing in deterioration of surface quality and tool life.

The nose radius determines the quality of the machined sur-

face as v/ell as tool life. Increasing the nose radius generally

18

Nose ladius

/r= h End Cutting Edge Angle LJC

K J

Side Cutting Edge Angle

Side Rake Angle

Side Relief A

Back Rake Angle

^ .

^ / End Relief Angle

Fig . 5. Standard Tool Geometry Nomenclature,

19

improves surface finish, but if the increase is excessive tool chatter

may be induced which results in a tool life decrease.

As can be seen, compromises raust be made among these factors

depending on the application. Several sets of tabulated data are avail-

able which give satisfactory tool geometries for particular tool-v/ork-

piece combinations (7, 11, 13).

The remaining tool variable, tool material, has a more pro-

nounced effect on tool life than the geoiTietry (9). Obviously, the tool

material must be harder than the metal to be cut at the elevated

temperatures occuring during the cutting process. A very hard

material, however, is brittle and might chip or break easily. There-

fore, the basic properties of a good tool material should be:

1. Hardness greater than the workpiece,

2. High strength retention at elevated temperatures,

3. Microstructure which is wear resistant (9).

In summary, the general types of cutting tool materials in use

today, when they appeared, their maximum cutting speeds, maxi-

mum cutting temperature, and major constituents are listed in Table

1.

Relationship of Tool Life to^ Cutting Condition Variables

Based on the previous discussion, it should be obvious tb.ot when

metal cutting can be carried out under conditions of lower temperatures,

T A B L E 1

CUTTING TOOL MATERIALS AND CHARACTERISTICS (18).

20

M a t e r i a l

M a x i m u m MaximuiTi A p p r o x i m a t e Cutt ing Cutt ing

Y e a r of Speeds T e m p e r a t u r e s A p p e a r a n c e (fpra) (F)

Major Cons t i t uen t s

C a r b o n s t e e l 1800 25 400 0 . 7 - 1 . 2 % Car-bon

H i g h - s p e e d s t e e l

H i g h - s p e e d s t e e l

C a s t a l l oys

S u p e r - h i g h -s p e e d s t e e l

C a r b i d e s

C e r a m i c s

1850

1890

1915

1928

1930

1955

35

75

100

150

300

1600

500

1000

1500

1600

2000

2200

Tungs t en and Manganese

Tungs ten and C h r o m i u m

S te l l i t e s

Coba l t

Tungs ten , T a n t a l u m , and T i t a n i u m

Oxides

l o n g e r too l life can be expec t ed . Cutt ing t e m p e r a t u r e r educ t ion is

one of the funct ions of a cutt ing fluid. The o the r functions a r e to

r e d u c e f r i c t ion , p r o t e c t the mach ined su r f ace froi-n c o r r o s s i o n ,

and to c a r r y away chips f r o m the cut t ing zone (8, 18).

21

An incidental improvement which resu l t s from using a cutting

fluid is that the possibi l i ty of a bui l t -up-edge is reduced. Coincid-

ing with this reduction is a dec rease , or at least not an inc rease , in

tool -workpiece friction (8). When friction is reduced, tool t e m p e r a -

tu re will d e c r e a s e , thereby giving bet ter tool life.

Of the th ree types of cutting tool fai lures given in Figure 2 the

mos t commonly used c r i t e r ion to judge tool life endpoint is flank

wea r , or the wear land width. It is believed that going beyond a

c r i t i ca l wear land value causes an excessive te inpera ture inc rease

in the tool, thereby causing a decrease in the hardness of the cutting

edge (16). This softening of the tool m a t e r i a l quite natural ly resu l t s

in shor t e r tool life and a decline in the cutting p rocess efficiency.

Values of flank wear often used to determine tool life range from

0. 005 to 0. 060 inch, and a re governed a lmost exclusively by the

ma-chining environment (1, 16, 19), For exaraple, if surface quality

is the l imiting f ac to r , very little wear can be to lera ted . In addition,

wear land widths of only a few thousandths on a form tool give r i se

to la rge th rus t and longitudinal forces which may resu l t in large

tool deflections and a loss of dimensional stability (1). If meta l

r emova l is the only cr i te r ion , hov/ever, the tool might possibly be

run until complete breakdown of the cutting edge. The average wear,

land for carbide tool endpoint is usually chosen as 0.030 inch (10,

22

14, 18, 19). High speed s tee l tools a r e often run until a wear land

of 0.060 inch develops (14, 18).

C r a t e r wear is found predominate ly v/hen machining tough, s trong

s tee ls in conjunction with a continuous chip (5). Since this is a r a the r

l imited case , c r a t e r wear is l ess frequently chosen to de termine

tool fa i lure .

The two types of fa i lures just d iscussed a r e by no means all in-

c lus ive . A m o r e complete l ist ing is given by the Amer ican Society

of Tool and Manufacturing Engineers (ASTME) a s :

1. Coraplete failure - - Tool completely unable to cut;

2. Flank failure - - Occurrence of a cer ta in size of worn

a r e a on the tool flank (usually based on a cer ta in width

of wear land or a cer ta in volume of meta l worn av/ay);

3. Finish failure - - Occurrence of a sudden, pronounced

change in finish on the work surface in the direct ion

of e i ther improvement or deter iora t ion;

4. Size failure - - Occurrence of a change in dimension(s)

of the finished pa r t by a cer ta in amount;

5. Cutting force (or power) failure - - Increase of the

cutting force (tangential force), or the power con-

suiTLption, by a cer ta in amount;

6. Thrus t force failure - - Increase in the th rus t on the

tool by a cer ta in amount, indicative of end wear ;

23

7. Feeding force failure - - Increase in the force

needed to feed the tool by a certain amount, indicative

of flank wear (7, 11).

If flank wear is chosen as the tool life criterion and this value is

plotted against the length of cut (or time) for various speeds, the

family of curves shown in Figure 6 results. It can be seen that

there are three distinct phases to each curve. These phases are:

Phase I - Characterized by a small amount of flank wear

occurring in a very short period of time. This

phase is sometimes called the "break-in" pe-

riod and usually lasts from one to two minutes

depending upon the type of tool, workpiece,

etc.

Phase II - Characterized by a gradual increase in flank

wear. This phase, although considered by

many researchers to increase linearly, is

usually parabolic in shape and may be approxi-

mated by a straight line only in a small region

(5). The duration of this phase depends on the

machining conditions.

Phase III - Characterized by a rapid increase in flank v/ear

until tool failure. This transition zone has been

called the "tempcratare sensitive region" by

24

V1

25

Chao and Tr igger since it is at this point that

rapid i nc rea se s in tool t empe ra tu r e s a r e noted

which, as stated before, reduce the p rocess ef-

ficiency (1, 20).

Although c r a t e r wear is a more difficult measu remen t method

to ut i l ize and usually occurs in l imited cases , there is one impor-

tant point to cons ider . Both flank wear and c r a t e r wear may occur

s imultaneously under cer ta in cutting conditions, especial ly when

using carbide tools . The in ter re la t ionship between these two types

of vi/ear is very vague. One study v/as performed using radioactive

m e a s u r e m e n t techniques to evaluate the flank wear , c r a t e r wear ,

and total wear for var ious cutting speeds . The study concluded that:

1. Approximately sixty per cent of the total tool wear

took place at the c r a t e r .

2. Flank wear measurenaents gave a reasonably good

indication of the total tool wear (21).

Machining Economics

As noted previously, tool life has a m.ajor effect on the economics

of a cutting p r o c e s s . The total cost of a cutting operation, C , can

be broken down into many individual components, depending on the

si tuation. The major components a r e :

26

C- = Idle cos t / p i ece ,

C^ - Cutting cos t /p i ece ,

C^^ - Tool changing cos t /p iece ,

C^g = Tool regrinding cos t /p iece ,

C^(j = Tool deprecia t ion cos t /p iece ,

Cpf = P r e m a t u r e failure cos t /p iece (9).

The total cost equation would then be:

Cp = Ci + Cc + C^c + C^g + C^d + Cpf . [ l]

If it is a s sumed that disposable carbide inse r t s a re used, tool

regr inding costs can be eliminated and pre ina ture fai lures can be

assuraed to be relat ively infrequent and can also be eliminated froni

the cost equation. Therefore , a simplified total cost equation, a s -

suming a fixed depth of cut, can be given as :

Cp = xTc + y T c / T + xT^ (T^/T) + xT^ [2]

where :

X = Average operating cost per minute for labor and

overhead,

y = Cost of an individual cutting edge,

T = Tool life,

Tc = Cutting t ime per par t ,

T^ - Tool changing t ime,

Te = Non-cutting tiine (10).

27

This equation may be simplified and rewr i t ten a s :

Cp = x [ T e + T^ (1 + R / T ) ] [3]

where :

R = T j^ + y / x .

Using this re la t ionship, the minimum cost tool life, T=s may be

obtained a s :

T* - R (1 _ 1) [4]

where :

R = Defined previously,

n = Exponent in Taylor ' s tool life equation (10).

Assuming a fixed depth of cut, Taylor ' s tool life equation may be

wr i t ten to incorporate feed a s :

V T"" f"" = C , or V = C / T^ f"^ [5]

where :

V, C, T, n = Defined as before,

f = Feed,

rn = Dimensionless constant depending on work-

piece and tool var iab les (7, 11).

Substituting Equation [4] into this express ion resu l t s in:

V = C / (T*)" f'". [6]

The value for feed used in this equation will normal ly be adjusted

to some rnpximum value, f''', consistt^nt with the p rocess require inents

28

(excluding tool wea r ) . The resul t ing minimum cost cutting speed would

thus be:

Vmin = C/ (T=:=)" (*)"". [v]

It should also be real ized that some situations will be encount-

e red where it is n e c e s s a r y to produce at the maximum production

ra te consis tent v/ith the p roper levels of the machining va r i ab les .

In this situation, the var iable to rainimize v/ould be the production

t ime per p iece , Tp.

Given the same assumptions as before, the total production

t ime per piece can be given a s :

Tp = T^ + Td ( T^/T ) + T^ [8]

where :

T = Tool life,

Tc = Cutting t ime per par t ,

T 1 = Tool changing t ime,

Te = Idle or non-cutting t ime (14).

S imi lar ly , the tool life for maximum production ra te , T^^^^'

may be found to be:

Tmax = Td < ^ _ 1 t^l

Again, substituting this express ion into Equation [sj

r e su l t s in:

29

JLt should be noted that the minimum cost and maximum produc-

tion rate cutting speed expressions both contain the exponents "n"

and "C" of Taylor's tool life equation. If varying values of these

two constants are obtained when different cutting edges and methods

of tool life testing are used, then expressions [?] and [lO] will

reflect this difference. Therefore, the type of cutting conditions and

the production requirements actually encountered should determine

what values of these constants to use in arriving at the profjer cut-

ting speed. Investigation of these variances and their effects on

the economic models discussed here were the primary objectives of

this research.

Chapter II describes the equipment used in the experiment, the

experimental design, and the experimental procedure. Chapter III

presents the results of the experiment and the analysis of these re-

sults. Chapter IV gives the conclusions obtained from the experi-

rnent and recommendations for further research.

CHAPTER II

EXPERIMENTAL DESIGN, EQUIPMENT,

AND PROCEDURE

Genera l Considerat ions

This r e s e a r c h was conducted using a turning operat ion under

single point cutting condit ions. A single point turning operation

was selected for severa l r e a s o n s . F i r s t , it is a common industr ia l

operat ion uti l ized by manufacturing concerns which must generate

cyl indr ical shapes . Second, well defined economic models for

single point turning have been developed and could be used in the

ana lys i s . Third, an engine lathe and the a,ssociated exper imental

equipment were available for use . Finally, experiraental tooling

costs would have been excess ive if a drill ing or milling operation,

for example , was chosen.

A single type of workpiece ma te r i a l , SAE 1018 cold rolled steel ,

having the following chemical composition, was chosen for use in /

the exper iment :

1. Carbon 0. 15 to 0. 20%,

2. Manganese 0.60 to 0.90%, 30

31

3. Phosphorus ^ . 0 4 % maximum,

4 . Sulfur 0. 05% maximum,

5. Silicon 0. 10 to 0. 30% (7).

SAE 1018 s tee l was chosen p r i m a r i l y because of its wide indust r ia l

u se , re la t ive ly low cost, and availabil i ty from local s o u r c e s . This

type of v/orkpiece m a t e r i a l was a lso chosen because of its re la t ively

low machinabi l i ty rat ing of 65% (7). Metals in this raachinability

range usual ly produce a measurab le araount of flank wear in the cut-

ting tirae in terva ls used in the exper iment .

The tools selected were DoAll, type SPG-422, grade DO-16,

disposable carbide inse r t s having a chemical composition as follows:

1. Tungsten carbide 79%,

2. Cobalt 9%,

3. Ti tanium carbide 8%,

4. Tantalum carbide 4% (22),

The i n s e r t s were 1/2 by 1/2 by 1/8 inch with a 1/32 inch nose radius

and an 11 degree end relief angle. The cutting tools were mounted

in a 5/8 inch square shank tool holder having a 15 degree side cut-

ting edge angle . The DO-16 grade was found to be the type speci-

fied when inaking light roughing and finishing cuts , therefore , it was

ideally suited for the niachining conditions used in the exper iment .

Bes ides the fact that disposable i n se r t s were re la t ively incx]:>ensive

as compared to conventional high-speed s teel tools , they also reduced

32

the tool changing time in the experiment. Intermittent cuts utilized

in this research required removing and replacing the tool eight times

per insert under a particular cutting condition. Since the tool hold-

er was permanently mounted in the dynamometer, the inserts were

removed simply by loosening a screw clamp on the tool holder.

Thus, no change in the relative position of the tool and the work-

piece was necessary and consistency of the experimental procedure

throughout the research was assured.

A mechanical chip breaker was used to prevent the formation

of continuous chips which would foul the tool and workpiece. In ad-

dition, a safer working environment for the machine tool operator

was obtained.

It was decided not to use cutting fluids in the experiment as

dry cutting conditions facilitate faster tool wear and eliminate any

uncontrollable effect a cutting fluid may have on tool life.

Equipment

The equipment of prime importance in this research included

the engine lathe, tool dynamometer, Dynagraph recorder, and meas-

uring raicroscope. A complete listing of equipment is given in Table

2,

TABLE 2

LIST OF EQUIPMENT AND USE

33

Equipment Use

1. Engine lathe Perform turning operation

2. Tool dynamometer Measurement of tool forces

3, Beckman Dynagraph recorder

Record tool forces

4. Gaertner micrometer microscope and meas' uring fixture

Measurement of flank wear

5, Tool holder Hold carbide inserts

6. Chip breaker

7. Three-jaw universal chuck

Break up continuous chips

Hold work at headstock

8. Live center Support work at tailstock

9. Stop watch

10, Aluminum shims

Measure cutting time

Adjust height of dynamometer

11. High intensity desk larap Illuminate insert edge under microscope

34

Engine Lathe

The lathe available for use was a 1952 miodel, type DLNE,

Werkzeugmaschinen Fabr ik , 7.5 horsepower engine lathe equip--

ped with a t h ree - j aw un ive r sa l chuck and live center at the ta i l -

s tock. P rev ious r e s e a r c h e r s have deterinined spindle speed ac -

curacy to be approxiraately - 5 RPM (3, 5), therefore , no further

ver i f icat ion was felt to be needed for this r e s e a r c h . All mechanisms

were carefully checked for defects in order to prevent unnecessa ry

exper imenta l e r r o r from being introduced into the r e su l t s ; none

were found. After inspecting the machine and periodical ly through-

out the exper iment , it was lubricated to insure smooth operat ion.

Tool Dynamometer

The dynamometer used was a one-piece, three component tool

dynamometer consisting of two extended octagonal r ings , one above

the other . The bas ic operat ion of this type of force measur ing in-

s t rument was d iscussed in Chapter I. F igure 7 i l lus t ra tes the

dynamometer used in this r e s e a r c h and the three forces it is cap-

able of measu r ing .

Recorde r

A six channel Beckman Dynagraph r e c o r d e r was utilized to r e -

cord the output resul t ing fronn the dynamoineter s t ra in gages during

the cutting operat ion. The output, in t e r m s of m i l l i m e t e r s of pen

35

0) o ^1 o

^ 53 y o f1 L,

^ ;S o W) ^

Tl bJO.2 ;^ o rt U ^ K II II II

O 4-> ^1

P^ h h

iV

^1

0) 0) s

>> Q

(I)

II

(V5 O

1-1 CO

S P 0)

36

de f l ec t ion , w a s r e c o r d e d on s t r i p c h a r t s . P r o p e r c a l i b r a t i o n of pen

def lec t ion to too l f o r c e s f ac i l i t a t ed r ap id and a c c u r a t e fo rce d e t e r -

m i n a t i o n f r o m the s t r i p c h a r t r e a d i n g s . Two of the s ix ava i l ab l e

c hanne l s w e r e u s e d ; only the channe l for cut t ing f o r c e , F , and the

channe l for long i tud ina l f o r c e , F^, w e r e connected to the d y n a m o -

m e t e r . The t h i r d ava i l ab l e channe l for the r a d i a l f o r c e , F , was r

not u t i l i zed b e c a u s e of the ins ign i f ican t effect of th i s fo rce on tool

life (7).

Microscope

F l a n k w e a r m e a s u r e m e n t s w e r e t aken us ing a G a e r t n e r twenty

power m i c r o m e t e r m i c r o s c o p e equipped wi th t h r e e lens mounted

h a i r l i n e s ; one s t a t i o n a r y h a i r l i n e and two movab le p a r a l l e l h a i r -

l i n e s . The p a r a l l e l h a i r l i n e s and the i n s e r t edge w e r e a l igned

wi th the s t a t i o n a r y h a i r l i n e , which c o r r e s p o n d e d to a z e r o w e a r

land wid th . The p a r a l l e l h a i r l i n e s w e r e then moved unt i l they w e r e

c e n t e r e d o v e r the l ower edge of the w e a r land . The d i s t ance the

p a r a l l e l h a i r l i n e s w e r e moved to c e n t e r the ra ove r the lower edge

of the w e a r land, i . e . , the w e a r land width in un i t s of 0, 0001 inch,

w a s then d i r e c t l y r e a d f ro in the m i c r o m e t e r d r u m . A high in tens i ty

de sk l a m p w a s u s e d to i l l u m i n a t e the i n s e r t edge u n d e r the m i c r o -

scope to fac i l i t a te pos i t ion ing of the i n s e r t and to enable a c c u r a t e

m e a s u r e m e n t of the w e a r land wid th .

37

Des ign of the E x p e r i m e n t

Th i s e x p e r i m e n t was p e r f o r m e d us ing four independent v a r i -

a b l e s : t e s t i n g m e t h o d , cut t ing edge , cut t ing speed , and cut t ing t i m e ;

and t h r e e dependen t v a r i a b l e s : flank w e a r , cut t ing f o r c e , and long-

i tud ina l f o r c e . A s ind ica ted e a r l i e r in th i s c h a p t e r , w o r k p i e c e \ ^

m a t e r i a l , too l type , and too l ho lde r g e o m e t r y w e r e helii )fconstant V

t h roughou t the e x p e r i m e n t . A l s o fixed w e r e two mach in ing v a r i -

a b l e s : feed and depth of cut .

It w a s dec ided to hold the mach in ing v a r i a b l e s of feed and depth

of cut cons t an t for s e v e r a l r e a s o n s . F i r s t , feed and depth of cut

do not have a s g r e a t an influence on tool life a s does cut t ing speed

(9, 10, 11). Second, m o s t e conomic m o d e l s developed for s ingle

point t u r n i n g specify an op t imura cutt ing speed for m i n i m u m cos t

o r m a x i m u m p r o d u c t i o n r a t e a t a fixed depth of cut . Since a depth

of cut is g e n e r a l l y d e t e r m i n e d by the p a r t conf igura t ion and a su i t -

ab le feed is d e t e r m i n e d by c e r t a i n p r o c e s s r e q u i r e m e n t s , such a s

s u r f a c e f in ish , it s e e m e d log ica l to fix t h e s e v a r i a b l e s . F ina l ly ,

e c o n o m i c r e s t r i c t i o n s fo rced t h e s e v a r i a b l e s to be held cons tan t .

The e x p e r i m e n t would have g rown e n o r m o u s l y in s ize and expense

if m o r e than one l eve l of e a c h fac to r had been u s e d .

T e s t cu ts w e r e m a d e to d e t e r m i n e p r o p e r l eve l s of the fixed

m a c h i n i n g v a r i a b l e s , feed and depth of cut , which would yield

38

cha t te r free cutting and produce forces capable of being measu red

without damaging the dynamometer . The levels se lected we re :

1. Feed - 0.00315 ipr ,

2. Depth of cut - 0. 075 inches .

Independent Var iables

^' Test ing method. - - Two testing methods were uti l ized, an

in te rmi t ten t method and a continuous method. The bas ic exper i -

menta l p rocedures for each were discussed in Chapter I.

The in termi t ten t method utilized eight one-minute cutting tirae

in te rva ls per cutting edge. Three i n s e r t s , with two cutting edges

per inse r t , were uti l ized for each speed level tes ted. This combi-

nation resul ted in six cutting edges per speed level and a total of

240 minutes of cutting under the in termit tent method. In o rder to

e l iminate any effect on wear ra te due to heat build up in the inser t ,

a min imum of four minutes of cooling t ime was allowed between each

one minute cut.

The th ree t ime in tervals selected for use under the continuous

method were 4, 6, and 8 minutes . Continuous cuts were made at each

of these tirae in tervals and one flank wear measure inen t was taken at

the end of the per iod. Fo rce readings were taken continuously through-

out the exper iment . Again, th ree inse r t s and tv/o edges per inser t

were uti l ized at each of the three t ime in tervals for each speed level

39

so t es ted . This combination resul ted in a total of 540 minutes of

cutting under the continuous method.

2. Cutting edge. - - Opposite edges of the carbide inse r t s were

used in this r e s e a r c h in o rde r to extend the study of Leon (5) on

tool life differences between consecutive cutting edges of the same

carbide in se r t . In this way, the knowledge gained from Leon's

study was expanded and used to bet ter understand the wear of c a r -

bide i n se r t too ls .

3. Cutting speed. - - Cutting speeds normal ly encountered in

production situations for the tools and workpiece ma te r i a l specified

may range from 250 - 550 surface feet per minute depending on the

values of the machining var iab les used (7, 11, 16). Therefore ,

five equally spaced speed levels in this range were chosen for use

in the exper iment . Since the engine lathe did not have an infinitely

var iable spindle speed feature , equal speed spacing was accom-

plished by fixing the spindle speed and varying the workpiece dia-

m e t e r in equal i nc remen t s . To obtain the ranges of speeds des i red ,

the spindle speed was fixed at 710 revolutions per minute. The five

workpiece d iamete r s and the resul t ing cutting speeds a re given

in F igure 8.

4. Cutting t ime . - - Cutting t ime was measured with a Galco

dec imal stop watch. Measurement of cutting t ime with a stop watch

was determined to be of sufficient accuracy for the purposes of

40

this r e s e a r c h . Cutting t ime was measu red from, the instant of con-

tac t between the tool and workpiece to the end of the par t i cu la r tiine

in te rva l under invest igat ion.

Measu remen t s were taken at the end of each tirae period given

below:

1. In termi t tent method - 1, 2, 3, 4, 5, 6, 7, 8 minutes ,

2. Continuous method - 4, 6, 8 minutes .

WORKPIECE CUTTING DIAMETER SPEED

(inches) (fpm)

1.75 325.3

2 .00 371.8

2 .25 418.3

2.50 464.8

2 ,75 510.3

F ig . 8. Cutting Speeds Resulting F r o m the Specified Workpiece D iame te r s .

Dependent Var iables

1. Flank w e a r . - - Flank wear is one of the most common meth-

ods of determining tool life. Excess ive v/ear on the flank of the tool

r e su l t s in rapid i nc rea se s in tool t empera tu re with a result ing

41

decrease in cutting edge hardness (7). Coinciding with the soften-

ing of the tool raaterial is a rapid tool wear rate and a resulting

drop in cutting process efficiency. Therefore, flank wear was a

logical criterion for this research.

Measurements of wear land width were taken at the end of

each of the eight one-minute cuts under the intermittent method

and at the end of each specified time period under the continuous

method. The Gaertner microscope described previously was uti-

lized to make the flank wear raeasurements.

2. Cutting force and longitudinal force. - - The cutting and

longitudinal forces acting on the cutting tool were measured using the

three component dynaraometer coupled to a Beckman Dynagraph re-

corder. Previously developed calibration curves were then used to

convert in-process recorder readings of pen deflection to force

values.

At each speed level, three inserts were used for the inter-

raittent method and three inserts were used for each of the three

continuous time lengths, resulting in a total of 60 carbide inserts and

120 cutting edges being utilized. Total cutting time for the experi-

ment was 780 minutes. A total of 94. 5 feet of steel bar stock

was used.

42

Exper i raenta l P r o c e d u r e

The s tee l b a r s ut i l ized in this exper iment were previously pur-

chased for two separa te r e s e a r c h studies and consequently they had

been raachined to some extent. They w e r e , hov/ever, determined to

be usable for this r e s e a r c h and were machined to the proper d iameter

p r i o r to the beginning of the exper iment . The l a rges t d iameter 21

b a r s , each 24 inches long, were machined to the l a rges t d iameter

specified, 2 .75 inches . The reraaining 21 b a r s , each 30 inches

long, were machined to a 2. 0 inch d iameter . In this way, the f i rs t

t h r ee levels of cutting speed were run on the f i rs t set of 2 1 ba r s by

machining each ba r to the next lower d iameter after the experiraental

cuts were made . The las t two speed levels were run on the second

se t of 2 1 b a r s . This el iminated the need for machining the ba r s to

the next lower d iameter after the third cutting speed level, thereby,

conserving exper imenta l t ime . Since these ba r s were used previous-

ly, they contained the n e c e s s a r y countersunk hole for mounting in

the ta i ls tock l i ve -cen te r .

In o rde r to mount the dynamometer on the lathe, the tool post

was removed and the dynamometer was secure ly clamped in the

T-s lo t s on the lathe c a r r i a g e . The tool holder was mounted in the

dynamoraeter and an inse r t clamped in place . The proper tool tip

height was next adjusted by placing aluminum shims under the

43

dynamometer. This adjustment was necessary to insure that the

tool tip height coincided with the rotational axis of the workpiece.

One end of a workpiece was mounted in the three-jaw chuck

and the live center in the tailstock was inserted into the previously

drilled countersunk hole in the opposite end. The dynamometer was

connected to the recorder and the recorder pens zeroed before the

start of the cutting operation.

To insure that the proper cutting edge of the carbide insert

was being used, the opposite edges of each insert were numbered

" 1 " and "2" v/ith a felt tipped pen. The first three repetitions

under each testing method-cutting speed combination were run with

three inserts using the number " 1 " edge. The remaining three

repetitions for that combination were run using the sanae three

inserts but now using the number "2" edge. This method blocked

off'any variability existing betv/een cutting edges and faciliated the

analysis to determine if there were significant differences in tool

life values between opposite edges of the same insert.

When the tool was mounted in the tool holder and the dynamo-

meter and recorder were connected and checked, the desired depth

of cut was then obtained. This was accomplished by turning the

cross-feed handwheel mounted on the compound rest until the tool

tip engaged the workpiece surface, A micrometer slip rin^ mounted

on the handwheel was rotated to show zero depth of cut on the scale.

44

The tool was then moved longitudinally past the end of the workpiece

and the handwheel rotated to move the tool inward until the desired

depth of cut was obtained.

Next the p roper feed and spindle speed were selected by levers

on the headstock. The lathe power supply was turned on at this

point to facil i tate engaging of the gears in the headstock.

When these steps were accomplished, the cut was ready to

be made . The stop watch was checked for operation, spindle lever

engaged, and the feed mechanism engaged. At the instant of con-

tac t between the tool and workpiece, the stop watch was s tar ted .

When the p r e sc r ibed t ime period was over, the feed lever v/as d is -

engaged, the tool backed away from the work, and the spindle lever

disengaged ending the cycle. At this t ime the speed level, edge

number , repet i t ion number , and the t ime interval were noted on

the r e c o r d e r s t r ip char t adjacent to the pen deflection readings .

The tool was removed from the holder, mounted under the

mic roscope , and the flank wear measu red and recorded . The tool

was then r e in se r t ed in the tool holder after the 4 minute cooling

period or replaced with a new tool depending on the cutting method

being used. Each group of three tools , when through cutting,

were placed in envelopes with the speed and testing method noted

on the outs ide . Also placed in the envelopes was a sample of the

45

chips produced by the particular cut. The chip color, texture, and curl

characteristics aided in the analysis of the results of the experiment.

When the cuts on a workpiece were complete, a non-test car-

bide insert was placed in the tool holder and the workpiece was

machined to the next lower diameter. The bar was then set aside for

use at the next speed level and another workpiece was mounted in

the lathe. This cycle was repeated until all five speed levels had

been run.

Periodically during the experiment, the dynamometer calibra-

tion was checked to insure that no changes had taken place which

would affect the force readings obtained; no significant changes in

calibration were found.

After completion of the experiment, the recorder readings were

converted from inillimeters of pen deflection to force values by means

of the appropriate calibration curve equation given in Appendix A.

All data obtained in the experiment are given in Appendix B.

The following chapter presents an analysis of the flank wear

and force data.

CHAPTER III

ANALYSIS OF FLANK WEAR AND FORCE DATA

This chapter presents the analysis of the experimental flank wear

and force data obtained in the research. Inherent in the analysis were

two assumptions. First, it was assumed that flank wear values ob-

tained for a specific speed-tirae combination were normally distri-

buted about a mean value, W. This assumption was utilized by

Lambert (3), and was shown to hold true for high-speed steel tools

in a study at Purdue University (23). Second, the flank wear values

used to indicate tool life end point for each cutting speed were smal-

ler in this research than those discussed in Chapter I, since the

cutting time intervals v/ere necessarily shorter than normally used

in studies of this type. Therefore, it was assumed that the relation-

ship between flank wear and time was linear with tiine and that the

smaller flank wear values v/ould yield tPie same tool life-speed re-

lationship as larger values.

The analysis to be presented is divided into three sections as

follows:

1. An analysis of the flank wear data for opposite cut-

ting edges, and intermittent and continuous methods

. o

47

of tool life tes t ing;

2 . An analys is of the force data for opposite cutting

edges , and in termi t tent and continuous methods of

tool life tes t ing;

3, An analys is of the effect of differences between

flank wear values for both test ing methods on a

min imum cost and maximum production ra te eco-

nomic models of t h e cutting p r o c e s s .

Flank Wear Datai for Intermit tent and Continuous Methods

The f i rs t step in the data analysis involved determining if signifi-

cant differences between flank wear values obtained frora opposite

cutting edges at each speed- t ime corabination existed. To per form

the ana lys i s , a t - t e s t was utilized to determine if the differences

between the sample means , -x and y, of two normal ly distr ibuted

va r i ab l e s , X and Y, were significant. The t s ta t is t ic and its de-

g rees of freedom, d. f. , a re given by:

t = X - y

V 2 2 nxSx + nySy n n (n + n _ 2)

X y^ X y ;_

^x + ^y

and

d. f. = n^ + n - 2 x y

where

48

X = Sample mean from distr ibut ion X,

y = Sample mean from distr ibution Y,

S^ = Es t ima te of population var iance of distr ibution X, 2

S = Es t ima te of population var iance of distr ibution Y,

n^ = Sample size taken frora population X,

ny = Sample size taken from population Y (24, 25, 26).

The r e su l t s of the t - t e s t s a re presented in Tables 3 and 4.

F r o m these tab les , it can be seen that there a re no significant

differences between flank wear of opposite cutting edges for ei ther

tes t ing method. In s eve ra l c a s e s , however, the mean flank wear

values for opposite edges were widely separated, but the large

va r i ances assoc ia ted with the wear values kept the related t -values

smal l and insignificant.

These r e su l t s do indeed substantiate the work of Leon (5).

The r e su l t s of Leon 's study indicated that there was a "cumula-

tive effect" on flank wear when using the four cutting edges of an

in se r t consecutively. The p resen t study indicates that this effect

may be el iminated by using only the two opposite cutting edges.

Since there were no significant differences in flank wear be-

tween opposite edges at any speed- t ime combination for e i ther

tes t ing method, the th ree flank wear values for each edge were

grouped together , the resul t ing six values averaged, and this

r 49

T A B L E 3

I N T E R M I T T E N T METHOD t - T E S T S FOR FLANK WEAR B E T W E E N O P P O S I T E CUTTING EDGES

Speed T i m e t d.f. t Q25 4 Resu l t

510 (VI)

465 (V2)

418 (V3)

372 (V4)

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6

0 .229 0 .534 2 . 2 1 3 0. 356 0 .196 0. 171 0. 379 0 .845

0 .277 1.606 0. 169 0 .196 1. 512 0 . 5 3 3 0 .098 0, 383

0 .612 0 .478 0 .831 0 . 8 9 1 O.96I 0 .739 0 .892 1.452

0 .802 0 .919 0. 364 0. 571 1. 000 0 .400

4 4 II

II

II

II

II

II

11

II

II

II

II

II

II

II

II

II

II

II

II

It

II

II

II

II

II

II

II

2 . 7 8 2 . 7 8

II

II

II

II

II

II

II

II

II

11

II

II

11

II

II

II

II

II

II

II

II

II

II

II

II

It

II

Not S: Not S:

tl

II

II

II

tl

II

II

II

II

II

II

It

II

M

II

tl

II

II

II

II

II

II

II

II

11

II

II

Lgnificant Lgnificant

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

II

TABLE 3 - Continued

50

Speed Time d.f. ' . 025 , 4 Result

-

325 (V5)

7 8

1 2 3 4 5 6 7 8

0.242 0.267

1.706 1.260 1. 109 1. 183 1.400 1. 114 0.853 0.775

II

2.78 I I

Not Significant t l I I

I II

^ x = ^ y = 3

51

TABLE 4

CONTINUOUS METHOD t -TESTS FOR FLANK W^EAR BETWEEN OPPOSITE CUTTING EDGES

Speed Time ^^- *. 025 4 Result

510 (VI)

465 (V2)

418 (V3)

372 (V4)

325 (V5)

4 6 8

4 6 8

4 6 8

4 6 8

4 6 8

0.653 0. 195 0. 520

0.870 1. 332 0. 095

2. 516 0.252 1.911

0.280 0.204 0.516

0.843 2. 000 1.999

Not Significant

^x = ^ y - 3

52

value used as the mean flank wear value for each combination. The

resu l t ing flank w e a r - t i m e curves for both test ing methods a r e p r e -

sented in F igu res 8 and 9.

Some i m p o r t a n t observat ions may now be made from these two

cu rves . In F igure 8, the shapes of the curves general ly follow those

of other r e s e a r c h e r s as d iscussed in Chapter I. The slopes of the

curves in this figure a lso indicate that at the three highest speed

leve ls , the wear ra te increased more rapidly than for the two lower I i

speed leve ls . This observat ion shows the well established effect of

cutting speed on flank v/ear holds t rue in this r e s e a r c h . In Figure 9, I I

the "wear r a te for the three highest speeds is also higher than for , I

f the lowest two speeds , but only for the t ime interval from four to

six minutes of continuous cutting. F r o m six to eight minutes of

continuous cutting, the second and third highest speed levels actu-

ally have an equal or lower slope than the lowest two speed levels .

This indicates that the continuous method yields lower wear ra tes

than the in te rmi t ten t method as cutting t ime i nc r ea se s .

In addition, the re was a definite grouping of the data under

both tes t ing methods , with the mean flank wear values at the

highest th ree speed levels being separa ted from those of the low-

es t two speed leve ls . This seeras to indicate that a "c r i t i ca l " cut-

ting speed range for the given machining conditions occurred be-

tween 418 and 372 feet per minute, whereby flank wear values

53

600--

500

400

in I o

CO

o O

0

1!

300

200 -

100 --

510 fpm ^ 4 6 5 fpm

,. 418 fpm

^ 372 fpm

> 0 325 fpm

0

Time (Minutes)

" 1 -

6 8

Fig . 8. Mean Flank Wear -T ime Curves for Intermit tent Method.

54

600 - -

500 - -

400 - -

LO I

o

CQ (D O f1

d d)

0)

300

200

100

ot H y

/ .-J "^^^ fpna

5 10 fpm

418 fpm

5) 372 fpm

. . , - - 325 fpm

8

T i m e (Minutes)

F i g . 9. Mean F l ank W e a r - T i m e Curve for Cont inuous Method.

55

ahove t h i s r a n g e tend to be "higher and i n c r e a s e f a s t e r than w e a r

v a l u e s be low th i s r a n g e . Th i s c r i t i c a l r a n g e of speeds m a y be seen

m o r e c l e a r l y in F i g u r e s 10 and 11. Unde r the i n t e r m i t t e n t me thod ,

c h a n g e s in f lank w e a r r ang ing f r o m 218 x 10" inch a t e ight m i n u t e s

to 140 X 10" inch a t four m i n u t e s w e r e found b e t w e e n the t h i rd and

fo r th speed l e v e l s . Under the cont inuous me thod , the changes in

f lank w e a r b e t w e e n the s a m e two speed l eve l s r ange f ro in 190 x 10"

a t s ix m i n u t e s to 133 x 10"-' inch a t four m i n u t e s .

The second p o r t i o n of the a n a l y s i s involved d e t e r m i n i n g if dif-

f e r e n c e s e x i s t e d b e t w e e n flank w e a r va lue s of the two t e s t i ng m e t h o d s

F l a n k w e a r v a l u e s u n d e r the i n t e r i n i t t e n t method for four , s ix , and

e ight m i n u t e s of cut t ing v /e re c o m p a r e d v/ith those obta ined u n d e r the

con t inuous me thod at the s a m e t i m e i n t e r v a l s . The d i f fe rences w e r e

c o m p a r e d u s i n g the t - p r i m e s t a t i s t i c , t ' , d e s c r i b e d by Hoel (10).

Th i s s t a t i s t i c t e s t s the hypo thes i s tha t the s a m p l e m e a n s f r o m two

n o r m a l d i s t r i b u t i o n s a r e equa l when the v a r i a n c e s of each d i s t r i b u -

t ion a r e unknown and cannot be a s s u m e d to be equa l . When the

popu la t ion m e a n s a r e equa l iU^ ~MY^' *^^ s t a t i s t i c p o s s e s s e s an

a p p r o x i m a t e t d i s t r i b u t i o n . The t ' s t a t i s t i c is given by:

t ' = X - y

wi th a s s o c i a t e d d e g r e e s of f r e e d o m , d.f. , of:

56

600

500

400 - -

in I o r-l

w 300 0)

fI

200

100 -

8 rain.

/

^ 6 min.

0 T-^.v

4 min.

325 372 418 465 510

Cutting Speed (fpm)

Fig , 10, Mean Flank Wear v s . Cutting Speed for In termit tent Method.

57

in I o I(

CO

o

u 0)

ft

^ .

600 - -

500 '

400 - -

300

200 -

'^ 100

0 t.

i> 8 m i n .

6 min .

V-F

4 m i n .

325 372 418 465 510

Cutt ing Speed (fpm)

F i g . 11 . F l ank W e a r v s . Cutt ing Speed for Cont inuous Method.

58

2 2 S S ^ n n

d.f. = ^ 1 _ 2 2 2

S ^ S ^

,_ iL)2+(_y_)2

^x+1 ny+1

Where : All symbols a r e as descr ibed previously.

The r e su l t s of the t ' t e s t s for the flank wear values obtained

under each test ing method a r e given in Table 5. After four minutes

of cutting, the in termi t ten t flank wear was significantly g rea te r

than the continuous flank wear except at the 418 feet per minute

speed level . After six minutes , flank wear was significantly dif-

ferent between methods except at the 465 and 418 feet per minute

speed l eve l s . After eight minutes , flank wear at the 510 feet per

minute speed level was the only non-significant value, with the r e -

maining four speed levels having significantly different flank wear

between test ing methods . These resu l t s indicate that there was

slightly m o r e var iabi l i ty between test ing methods after four and

eight minutes of cutting than after six minutes of cutting.

These resu l t s were compared with those of Lamber t (3), at

four minutes of cutting and were found to par t ia l ly agree with his

r e s u l t s . Laraber t found no significant difference between testing

methods at his highest speed level of 465 feet per minute and lov/-

e s t feed level of 0. 00393 inches per revolution. In this r e s e a r c h .

59

T A B L E 5

t ' - T E S T S F O R FLANK WEAR B E T W E E N I N T E R M I T T E N T AND CONTINUOUS TESTING METHODS

Speed Time

510 4 (VI) 6

8

465 4 (V2) 6

8

418 4 (V3) 6

8

372 4 (V4) 6

8

325 4 (V5) 6

8

nx = ny = 6 I = Mean, inte

t '

5.608 2.452 1.606

3.937 0.618 4 .270

1.223 0.592 3.081

2.860 3.612 6.452

2. 181 3.090 6.496

rmi t ten t

d.f.

11 6

-6

12 7

11

9 11 10

11 11 12

12 14 12

method C = Mean, continuous method

^ ,025, d.f.

2 .201 2.447 2.447

2. 179 2. 365 2.201

2.262 2.201 2.228

2.201 2.201 2. 179

2. 179 2. 145 2. 179

Results

Significant, I > C Significant, I ^ C Not Significant

Significant, I ^ C Not Significant Significant, I ^ C

Not Significant Not Significant Significant, I ^ C

Significant, I ^ C Significant, I ^ C Significant, I ^ C

Significant, I ^ C Significant, I ^ C Significant, I ^ C

60

t he r e was a significant difference at the same speed level (level 2)

and cutting t ime . The feed, however, was slightly lov/er in this

study at a level of 0. 00315 inches per revolution. The remainder

of the data at four minutes of cutting appeared to agree very well

with Lamber t ; i . e . , as the speed decreased , flank wear values of

the ' two methods remained significant for the lowest two speeds .

The insignificance of the flank wear values after four rainutes of

cutting at the thi rd speed level seemed to further justify the previous

conclusion that this was a c r i t i ca l cutting speed region and that fur-

the r r e s e a r c h in the a r e a is needed.

The cutting t imes of six and eight minutes produced flank wear

compar i sons which were not ent i re ly unexpected. The insignificant

differences in flank wear between the two test ing methods after six

minutes of cutting at the second and third highest speed levels prob-

ably resu l ted because at these speeds a buipt-up edge was unlikely

to form. If a bui l t -up edge does not form, it has been shown the

tool forces tend to be higher than when a buil t -up edge is p resen t (16).

There fore , higher forces indicate the mean flank wear values during

the continuous cutting p r o c e s s would be higher and therefore c loser

to the flank wear during the in termi t tent cut. This fact raay be seen

m o r e c lea r ly upon re -examining F igures 8 and 9. At the second

speed level , the wear ra te can be seen to be ra the r rapid and a lmost

l inear in nature under the in termi t tent method. Under the continuous

61

method, the wear after six minutes at the second speed level was

slightly higher than for the first and highest speed level. There-

fore, insignificant flank wear differences between methods were

to be expected at this time interval.

At the longest cutting time of eight minutes, the only speed

at which the flank wear difference between raethods was insignifi-

cant was at the highest speed level. This result may be explained

by examining a plot of the difference in flank wear values between

testing methods as given in Figure 12. The highest cutting speed

is the only speed level where this difference decreases from sLx

to eight minutes of cutting time. In addition, it should be noted

that four of the five speed levels incidate a decrease in flank wear

difference between methods from four to six minutes of cutting.

Thus, Figure 12 indicates that the transition from six to eight

minutes of cutting time was the "critical" time period being sought

as a portion of this research.

It is important to note that in all research studies in this area

(3, 4), including the present study, flank wear values for the con-

tinuous testing method were lower than those for the intermittent

method. It is believed that this result stems from the fact that

a portion of the workpiece material remains on the cutting edge,

and during an intermittent cut this bit of material is reraoved each

time the tool contacts the work. Since this small portion of

in I o

en

0) V

14 0--62

4 6 5 fpm

1 2 0 - -

418 fpm

100

80

A 6 0 - -O

O

P 40 u

c It!

E 20

'I y ^

0 8

T i m e (Minutes)

F i g . 12. Di f fe rence Be tween I n t e r m i t t e n t and Cont inuous Method F l ank W e a r .

63

m a t e r i a l is p r e s s u r e welded to the cutting edge, when removed it

tends to chip away some of the edge. This leads to a more rapid

wea r ra te during the in termi t ten t tool life test ing method.

F o r c e Data for Inter rait tent and Continuous Methods

Tables 6 and 7 give the resu l t s of a comparison of cutting forces

and longitudinal forces between opposite cutting edges for the in ter-

mi t tent tes t ing method. It can be seen from Table 6 that the only

significant difference in cutting force between edges occurred at the

highest speed level during the sixth minute of cutting. Since the

second edge produced the lower cutting force, the significant dif-

ference can probably be at tr ibuted to variabi l i ty in the data r e -

sulting from the tools , workpiece s t ruc ture , and the machine tool

sys tem.

Tables 8 and 9 give the resu l t s when the forces for opposite

cutting edges under the continuous method were compared. The

information presented in the tables shows one t-value which is

significant. This value occurs in Table 9 when comparing longi-

tudinal force differences at the highest speed level after six min-

utes of continuous cutting. Although the t -value found in Table 8

for the cutting force at the same speed level after six minutes of

cutting is insignificant at the five per cent level, it is significant

at the ten per cent level . These resu l t s seem to verify the

64

T A B L E 6

I N T E R M I T T E N T METHOD t - T E S T S FOR CUTTING FORCES B E T W E E N O P P O S I T E CUTTING EDGES

Speed

510 (VI)

465 (V2)

.418 (V3)

372 (V4)

T i m e

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

t

0 .692 0 .460 0. 140 0. 525 0 .542 4 . 128 1.993 2 . 6 2 1

0. 166 0 ,449 0. 314 0. 007 0. 353 0 .221 0. 143 0. 000

0 .579 0. 325 0. 335 0 .405 0 .796 0 ,619 0 . 6 5 4 0 .617

2 . 160 1.674 0. 512 0 .835 0. 005 0. 178 0. 210 0 .712

d.f.

4 4

^ - 025, 4

2 . 7 8 2 . 7 8

II

II

II

tl

II

It

It

tl

tl

It

II

II

It

It

It

It

II

It

II

tl

II

II

It

II

It

It

It

II

II

II

Resu l t

Not S] Not S: It

It

II

Lgnificant Lgnificant

It

It

It

Signif icant Not S: It

It

II

It

II

It

11

It

It

It

II

It

It

It

II

II

tt

It

It

It

It

It

II

II

It

Lgnificant tl

It

tl

tl

It

It

It

It

It

It

It

II

II

It

tl

II

II

It

tl

It

It

II

It

II

tt

F

65

TABLE 6 - Continued

Speed Time d.f. -.025, 4 Result

325 (V5)

^x = ny =

1 2 3 4 5 6 7 8

3

1.193 1.738 1.273 1.570 1.293 0.909 1.221 1.415

2.78 Not Significant

66

TABLE 7

INTERMITTENT METHOD t -TESTS FOR LONGITUDINAL FORCES BETWEEN OPPOSITE CUTTING EDGES

Speed Time ^^- "^.025, 4 Result

510 ( V I )

465 (V2)

418 (V3)

372 (V4)

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

0 .577 0.226 0. 320 0.256 0. 004 0. I l l 0. 122 0. 002

1. 155 1. 155 0. 000 0.000 0.816 1. 155 0.817 0. 368

0.817 0 .551 1.092 1.092 1.804 2.065 1.731 0.710

0. 365 0. 311 0.518 0.412 0. 164 0 .004 0 .741 0.816

4 4

2 .78 2 .78

I I

I t

I t

t l

I t

I t

I I

I t

I t

I t

t l

I t

I t

I t

I t

I I

I I

I t

tt

I t

I t

I t

I t

t l

I I

I t

I I

I t

I t

I t

Not S Not S I t

I t

I t

t l

I t

t l

I t

I t

I t

I t

I I

I I

I t

t l

I t

I t

I t

I t

I t

I I

I t

I t

I I

t t

I I

I t

I t

I t

I I

I I

Lgnificant Lgnificant

I t

I t

I I

I I

I t

t l

I t

I t

I t

I t

I t

I t

I t

I I

I t

I t

I t

I t

I t

I t

I I

I t

I t

I I

I t

I I

I t

I I

t l

I t

. r

67

T A B L E 7 - Cont inued

Speed T i m e ^^- *. 025, 4 Re s al t

325 (V5)

. . . . -

1 2 3 4 5 6 7 8

0 . 8 1 4 1.442 0 . 8 6 4 1.585 1.586 0 .598 1. 135 1. 191

4 4

2 . 7 8 2 . 7 8

Not Signif icant Not Signif icant II It

It It

It It

It It

It tl

II 11

n X n.

68

TABLE 8

CONTINUOUS METHOD t-TESTS FOR CUTTING FORCES BETWEEN OPPOSITE CUTTING EDGES

Speed Time d.f. - .025, 4 Result

510 (VI)

465 (V2)

418 (V3)

372 (V4)

325 (V5)

nx = Hy =

4 6 8

4 6 8

4 6 8

4 6 8

4 6 8

: 3

1.008 2.242 0.834

0.497 0.488 0.714

1.026 1.039 0.732

0.691 1.000 0. 160

0.623 0.219 0.700

4 4 II

2.78 2 .78

Not Significant Not Significant

I t I t I t

69

T A B L E 9

CONTINUOUS METHOD t - T E S T S FOR LONGITUDINAL F O R C E S B E T W E E N O P P O S I T E CUTTING EDGES

Speed T i m e

510 (VI)

465 (V2)

418 (V3)

372 (V4)

325 (V5)

d.f. ' . 025 , 4 Resu l t

4 6 8

4 6 8

4 6 8

4 6 8 4 6 8

0.000 3..169 0. 309

0. 000 0.283 1.295

0.866 1,914 0. 327

0.614 1. 155 0.517 2. 160 1.411 0.489

4 4 It

2.78 2.78

t l

Not Significant Signif icant Not Significant

Not Signif icant

^x = ^y

70

conclusion that the six minute cutting speed range is an area where

more detailed research is needed. There were no significant dif-

ferences in either of the two forces measured after eight minutes

of cutting for both of the testing methods.

Force data for the intermittent method is plotted in Figures 13

and 14, and plotted for the continuous method in Figures 15 and 16.

The figures indicate that as speed increases, the cutting force for

both testing methods decreases; the primary exceptions being the

highest speed level in Figures 13 and 15 under the intermittent

method. This deviation from the general trend probably resulted

from a rapid increase in flank wear at the high speed level which

was observed in the discussion on flank wear in the previous

section.

It should also be noted that in all cases cutting forces exceeded

longitudinal forces. This result agrees with those reported by

Tourret (15).

To test if significant differences in forces existed between testing

methods, a t' test was perforraed on the data. The results of these

tests are presented in Tables 10 and 11.

The information in Table 10 shows that at four minutes, the

cutting force was significantly different between testing methods

at the first and second highest speed levels. After six minutes,

the cutting force was significantly different at the second, third.

71

en 'V Pi o

o o

o hi WD

!3 U

o

130

120 -

110

100 -J

90

80

0 t

^ -

510 fpra

,^ O 372 fpm

O 325 fpm

JO--O 418 fpiri j ^ ^ - ^ 465 fpm

-.0-"

vO^ 2r ^ . ^ '

o -

8

T i m e (Minutes)

F i g . 13. Mean Cutt ing F o r c e v s . Tirae for Inter-m i t t e n t Method.

72

130

120

110 - -

CO

O &. o o u o

^

Pi i H +J -P

u Pi o

100 - -

90 - -

80

0 t - f [-

j _ _ * fg) 9 325 fpm

^ 0 465 fpm

- ^ 372 fpm 5 10 fpm

418 fpm

"i"-I~H --! f-4 5 6 8

T i m e (Minutes)

F i g . 14. Mean Cutt ing F o r c e v s . T i m e for Cont inuous Method.

73

CO 13 Pi pi O

o o ;H o

r-l

PI 13 +-> H W) Pi o

Pi cv5 0)

9 0 - -

80

70

6 0 - -

50

. 0^

^-0 372 fpm 510 fpm

...-O 325 fpm

418 fpm

. . ^ 465 fpm

40

ot 7 8

Time (Minutes)

F ig . 15. Mean Longitudinal Force vs . Time for Intermit tent Method.

904-

74

8 0 - -

70-CO

'V PI O

g 60' ;^ o

P!

50 -

13

W) o

Pi

^ 40>-

ot

o^" *">0 325 fpm

J /^ f pm

^ 4 6 5 fpm

^

-

Time (Minutes) 8

Fig . 16. Mean Longitudinal Fo rce v s . Time for Continuous Method.

510 fpm

75

TABLE 10

t ' - T E S T S FOR CUTTING FORCES BETWEEN INTERMITTENT AND CONTINUOUS

TESTING METHODS

.Speed T i m e

510

464

418

371

325

^x = I =

4 6 8

4 6 8

4 6 8

4 6 8

4 6 8

ny = 6 Mean , inte;

t

5. 358 2. 179 2 . 6 4 7

2. 377 2 . 8 0 5 2. 100

1.733 2 . 6 7 0 1.208

1.036 3. 153 2 . 0 8 0

1.737 1.456 0 .867

r m i t t e n t

d.f.

12 9

11

8 6

10

12 10 12

11 10 11

9 6

12

m e t h o d

*. 025. , d.J

2 . 179 2 .262 2 . 2 0 1

2. 306 2 .447 2 . 2 2 8

2. 179 2 . 2 2 8 2. 179

2 . 2 0 1 2 .228 2 . 2 0 1

2 . 2 6 2 2 .447 2. 179

f R e s u l t

Signif icant , I ^ C Not Signif icant Signif icant , I ^ C

Signif icant , C ^ I Signif icant , C ^ I Not Signif icant

Not Signif icant Signif icant , C ^ I Not Signif icant

Not Signif icant Signif icant , I ^ C Not Signif icant

Not Signif icant Not Signif icant Not Signif icant

C = Mean, continuous raethod

F

76

TABLE 11

t ' - T E S T S FOR LONGITUDINAL FORCES BETWEEN INTERMITTENT AND CONTINUOUS

TESTING METHODS

Speed

510

464

418

371

325

^x = ^v

T i m e

4 6 8

4 6 8

4 6 8

4 6 8

4 6 8

. = 6

t '

2 . 5 5 1 3 .000 2 . 2 3 1

2 . 5 3 6 5 .005 4 . 8 1 1

0 .406 1. 140 1.239

0. 557 2. 388 1.206

2 . 0 1 2 2 . 2 5 9 0 . 7 2 4

d.f.

8 11 12

6 10

9

11 11 11

7 9

11

12 9

10

^ 0 2 5 , d.f.

2 . 306 2 . 2 0 1 2. 179

2 . 4 4 7 2 . 2 2 8 2 .262

2 . 2 0 1 2 . 2 0 1 2 . 2 0 1

2. 365 2 .262 2 . 2 0 1

2. 179 2 . 2 6 2 2 . 2 2 8

Resu l t

Signif icant , I ^ C Signif icant , I ^ C Signif icant , I ^ C

Signif icant , C}> I Signif icant , C ^ I Signif icant , C ^ I

Not Signif icant Not Signif icant Not Signif icant

Not Signif icant Signif icant , I ^ C Not Signif icant

Not Signif icant Not Signif icant Not Signif icant

C = Mean, continuous method

77

and fourth highest speed levels. Finally, at eight minutes, differences

between cutting forces v/ere significant only at the first and highest

speed level.

Table 11 shows similar results. After four rainutes of cutting,

longitudinal forces between methods were again significantly dif-

ferent at the first two highest speed levels. At six minutes, dif-

ferences were significant at the first, second, and fourth highest

speed levels. Eight minutes of cutting yielded differences which

were significant at the first and second highest speed levels.

In general, it can be concluded that as speed decreases, the

differences in force values between methods become insignificant.

This conclusion probably stems from the fact that a built-up-edge

forms more rapidly at the lower speed levels. The built-up-edge

changes the effective rake angle of the tool thus causing the cut-

ting and longitudinal forces to decrease. During an interraittent

cut, the built-up-edge is broken off at each contact with the work-

piece. This increases the flank wear and causes an increase in

the