OPTIMUM TOOL GEOMETRY OF CBN TOOL FOR CONTINUOUS TURNING OF CARBURIZED STEEL

FUJIMURA* Received 22 January 1988; in final form 14 September 1988 Abstract--The optimum geometry of a CBN tool for continuous turning of a carburized steel bar was determined for the

O P T I M U M T O O L G E O M E T R Y O F C B N T O O L F O R

C O N T I N U O U S T U R N I N G O F C A R B U R I Z E D S T E E L

K SmNTANI,* M UEKrt and Y FUJIMURA*

(Received 22 January 1988; in final form 14 September 1988)

Abstract The optimum geometry of a CBN tool for continuous turning of a carburized steel bar was determined for the angle and width of negative land, the nose radius and the honing radius Fine turning by

a tool with such an optimum geometry could be fully substituted for a grinding process on the basis of tool life and roughness of the finished surface Tool wear as a controlling factor of tool life was analyzed Particularly in the initial wear stage, frequent chipping of the cutting edge strongly influenced the life of tools with small negative land angles

NOMENCLATURE

f

L

LI

L2

N L

R~

Rac

rH

rn

l

VB

~c

w~

feed rate

cutting length (spiral)

cutting length to reach the wear life limit (VB=0.25 ram)

cutting length to reach the surface roughness life limit (R~=0.8 ~m)

tool-chip contact length

negative land angle

average roughaess height

calculated average surface roughness

honing radius

nose radius

depth of cut

flank wear width

cutting speed

negative land width

rake angle

INTRODUCTION

A CONSIDERABLE improvement has been achieved in the heat resistant and wear resistant performance of cutting tools in recent years However, low speed or inefficient machin- ing processes have not yet been improved for the finishing of extremely hard materials such as carburized steel because of the necessity of troublesome grinding If cutting could be substituted for the grinding process, many advantages would be obtained not only in reducing the cost of equipment but also in increasing the flexibility of machining facilities To achieve this substitution, cutting tools capable of machining the hardened steels are required For this purpose a sintered body of c-BN (cubic boron nitride) particles, which have been used as abrasive grains for grinding, can be used as the cutting tool The potential of CBN (sintered c-BN) as a cutting tool, however, has not yet been realized due mainly to lack of fundamental data on its cutting performance and lack of reliability as a tool, particularly in regard to its toughness

Experiments with turning of hardened steels using the CBN tool have been carried out by Hodgson and Trendler [1] and Chryssolouris [2, 3] In these studies, the optimum cutting conditions were determined based on the tool wear behaviour during cutting

The present study attempts to present the technological implications of effective use

o f t h e C B N t o o l f r o m t h e v i e w p o i n t o f o p t i m u m t o o l g e o m e t r y d e s i g n e d f o r i n c r e a s e d

t o o l life a n d i m p r o v e d c u t t i n g p e r f o r m a n c e T u r n i n g u s i n g t h e C B N t o o l e n a b l i n g t h e

c o n t i n u o u s f i n i s h i n g o f c a r b u r i z e d s t e e l a s a s u b s t i t u t e f o r t h e g r i n d i n g p r o c e s s w a s a l s o

i n v e s t i g a t e d

*Department of Mechanical Engineering, Kanazawa Institute of Technology, 7-10hgigaoka, Nonoichi Ishikawa 921, Japan

+R&D Laboratories I, Nippon Steel Corporation, 1618 Ida, Nakahara-ku Kawasaki 211 Japan

403

404 K SHINTANI et al

Ps

VIEW ON Po

VIEW ON Pf

rH

VIEW ON Ps

P~

FtG 1 G e o m e t r y of tool tip

EXPERIMENTAL PROCEDURES

1 Workpiece and tool materials

The workpiece material was carburized Cr-Mo steel (JIS-SCM420) of ~b98 x 450 mm The region from the surface to 1 mm in depth was used in the cutting experiments The hardness of the carburized layer was in the range of Hv 600-720 The tool material was cubic boron nitride (c-BN) sintered with binding phases such as TiN and AIN The size of the c-BN particle was about 3 p,m The volume fraction of c-BN was about 60% which is reported to be effective for cutting hardened steels [4, 5] The transverse rupture strength of the tool was about 800 MPa at room and elevated temperatures

[6]

2 Tool tip parameters

The tip geometry of the tool is shown in Fig 1 Various values of the tool parameters, such as negative land angle NL, negative land width WL, nose radius rn and honing

radius rH, were combined for each series of cutting experiments as denoted by (1)-(4)

in Table 1 The honing radius was finished by a #800 hand lapper to an accuracy of -0.01 ram Except for the experiments to determine the effect of WL on the tool life,

a considerably larger value of WL of 0.2 mm was used, which is larger than those of commercial tools produced in Japan (0.05-0.06 ram) The land acts as the rake face in the present case which is different to the general type of chamfering in [1] and the rake angle is N L plus - 5 °, the - 5 ° being the tool holder attaching angle

TABLE 1 EXPERIMENTAL SELECTION OF TOOL TIP PARAMETERS FOR CONTINUOUS CUTTING

1.2, 1.6

0.05, 0.085, 0.1

3 Cutting conditions and determination of tool wear

In order to substitute for grinding, fine cutting conditions were used with feed rate

f and depth of cut t chosen as 0.1 mm/rev and 0.1 mm respectively The cutting speed

vc was selected as 100 m/min (1.67 m/sec) based on preliminary experiments using tools with N L of 25 ° and 35 °

Tool life was determined by measuring both the flank wear width of the tool, VB, and the average roughness height of the workpiece, Ra, where the cut-off value was 0.8 mm The limiting values of these to determine the life of the tool were set as VB=0.25 mm and R~=0.8 Ixm VB was determined by measuring the mean distance between the initial cutting edge and the front of a worn portion of the tool excluding the wear notch Both measurements were carried out at each cutting length L of 50,

100, 500, 1000, 1500 and 2000 m For L longer than 2000 m, the measurements were made at each 1000 m interval The tool-chip contact length, ec was determined by measuring the scratched length on the painted rake face The profile of the wear crater

in the tool was also measured at the same time The wear and failure of the tool were observed using a scanning electron microscope (SEM), while the cutting force was measured and recorded using a tool dynamometer and a digital wave memory scope

RESULTS AND DISCUSSION

1 Effects of NL and WL on" the tool life

One of the measures of wear resistance, the cutting length to reach the life limit, was defined as L1 The dependent of L1 on N L is shown in Fig 2(a), indicating the

maximum wear resistance at an N L of 30-35 ° As seen in the figure, experiments were

performed 3-5 times under the same conditions and the results show a significant scatter

at NL less than 30 ° The relationship between Ra and the cutting length is shown in

Fig 3 Although a rapid increase in the roughness at the initial cutting stage occurred

in the tool with an N L of 15 °, R~ first decreased for the other tools to the minimum

due to the increase in the nose radius by wear, and then increased again by the progress

of groove wear in the end cutting edge The variations in the wear patterns can be discerned from the results presented in Fig 4, the variation of the surface roughness profile of the workpiece in the direction normal to the feed mark at different stages in the cutting process As an indication of wear resistance from the surface roughness point of view, the cutting length to reach the life limit was defined as the tool life L2 The relation between L2 and N L is shown in Fig 2(b) which exhibits a similar trend

to Fig 2(a), namely that L2 also has a maximum at around N L = 3 5 °, and that the experimental results are more scattered at lower N L values

406 K SHINTAN[ et al

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r , : O B m m ~ 25

r , : O.05mm o 35

• 45

~ ' d ' 12 ' 1~ ' 2o

C u t t i n g length L (km)

FIG 3 Variation of average roughness height, R~, with cutting length for various negative land angles

L ( k m )

0 0 4 1.80 4.50 5.00 5.60

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Width of negative land

FIG 5 Dependence of tool life (L,) on negative land width Wt_ for various feed rates

20.0

10.0

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251

Tool l i f e

o Li

• Lz

I

Nose radius r (mm) Flo 6 Dependence of tool lives L, and L on nose radius

Figure 5 shows the relationship between L~ and W L at a range of feed rates from 0.05 to 0.15 mm/rev L1 increased as WL initially increased and then reached a constant value for all feed rates• The WL at which L1 reached a constant value corresponds approximately to the value at which /~c becomes constant

2 Effects o f r,, and rH on the tool life

U n d e r condition (3) in Table 1, the effect of nose radius r on L1 and Lz was investigated and the results are shown in Fig 6 Initially L~ increased as r, was increased and it reached a certain constant value for rn larger than 0.8 mm T h e low values of L1 which occurred at small r, values were thought to be caused by thermal wear due

to the increased t e m p e r a t u r e rise in the tool tip because of the n a r r o w e d chip contact area and the increased chip thickness Such a t e m p e r a t u r e rise can also be confirmed

by the significant progress of groove wear in the side cutting edge due to oxidation as shown in the S E M micrograph of the tip in Fig 7

On the o t h e r hand, the m a x i m u m value of L2 was exhibited at r , = 0 8 mm A t r, larger than 0.8 mm L2 slightly decreased with increasing r, This may be caused by the severe groove wear in the end cutting edge with an increased rn as shown in Fig 8 Since the slice b e c o m e s thin with an increased rn resulting in the so-called size effect,

408 K SHINrANI et al

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,,.,11

FIG 7 SEM micrograph of cutting edge in tool with small nose radius at cutting length of L=5.2 km

(NL=35 °, rH=0.05 mm, r,=0.2 ram, vc=100 m/min, f=0.1 mm/rev, t=0.1 ram)

groove wear occurs markedly in the end cutting edge Although the calculated average surface roughness (Rac=f2/32r, [7] - 0.78 ~m) is approximately equivalent to the life limit of Ra=0.8 p,m at rn=0.4 mm, the surface roughness at this rn should be greater than the life limit because the experimental value is usually observed to be higher than the calculated one

The effect of the honing radius rH on L1 and L2 was investigated under condition (4) in Table 1 As shown in Fig 9, the maximum lives for Lt and L2 were exhibited

at rH=0.05 ram Similar to the variation of life with NL shown in Fig 2, the experimental data were scattered significantly at smaller values of rH For the tool with rH =0.10 mm, most of the cut was covered by the honing portion and therefore a shortened life similar

to that of the tool with an NL of 45 ° was observed It is apparent from the figure that the optimum honing radius can be considered as 0.05 mm

In order to investigate the reason why a significant scattering of experimental data and shorter lives were recorded in the tools with a honing radius smaller than 0.05 mm, the tip of an unused tool with ria=0.02 mm was observed by the SEM as shown in Fig

10 Some cracks were observed in the cutting edge These cracks were induced during grinding of the cutting edge from chips broken from the edge of a size of 0.01-0.03

mm For rH=0.02 mm, these cracks will remain without being eliminated completely during honing Such a chipped edge acts as the starting point for tool wear, resulting

in a shortening of the life

3 Optimum tool geometry

For maximum cost effectiveness in manufacturing the tools, a thinner tip is desirable for the CBN tool while a negative land is needed to make the rake angle negative for increased toughness in the tool However, the negative land width should be as small

as possible because successful treatment of the tool tip will become difficult for large widths According to the experimental results obtained, the optimum parameters of the CBN tool could be specified as in Table 2, including the nose radius and the honing radius

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410 K SHINTANI et al

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Tool life

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0.02 0.04 0.06 0.08 0.10 Honing radius rx (mm)

0.12

FIG 9 Dependence of tool lives Li and L 2 o n honing radius rH

FIG 10 Chipping observed at cutting edge in tool before use, where NL=35 °, rH=0.02 mm and r.=0.8 ram

T A B L E 2 O P T I M U M COMBINATION OF TOOL TIP PARAMETERS FOR CONTINUOUS CU'VI'ING OPERATION

Negative land angle Negative land width Nose radius Honing radius

ec: 0.08 0.13 mm

4 Continuous turning with the o p t i m u m tool

T h e progress of tool w e a r f r o m cutting is shown for tools with N L of 15-45 ° in Fig

11 as a variation of the cross-sectional profile of the rake face for cutting lengths f r o m

1 to 10 kin T w o different w e a r b e h a v i o u r patterns were o b s e r v e d in the tools with N L

of 15 ° In the tool d e n o t e d by subscript " a " , which exhibited the s h o r t e r life, the cutting edge was already extensively worn at a cutting length of 1.2 km H o w e v e r the tool

d e n o t e d by subscript " b " and o t h e r tools with larger values of N L showed a smaller

a m o u n t of w e a r of the cutting edge Especially at an N L of 35 °, which is the o p t i m u m

(a) ~ ~.~

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Fro 11 Variation of cutting edge profile with cutting length in tools with various negative land angles after

15°h, (c) 25 °, (d) 35 ° and (e) 45 °

value, a relatively sharp cutting edge was maintained on the tool even after a cutting length of 15 km (see Fig 8(b)) Two types of tool wear, defined by the decrease in the cutting edge, can be identified: one exhibits a significant backward decrease of the cutting edge as was observed in tool "a" with NL=15 °, while the other retains the

cutting edge for larger lengths, L, as was observed in the tool "b" with NL= 15 ° and the other tools with larger values of NL In the former type of tool wear the rake angle

becomes large with the progress of wear without any wear of the lower cutting edge, resulting in a steepening of the tool tip As seen in Fig 11, the rake angle in such tools

hardly changes during cutting with any value of NL

Although in general the cutting force increases with an increase in NL, the tool "a" with NL= 15 ° exhibited abnormal behaviour with a significant increase in the force at

the initial cutting stage for a small VB, namely that the force was larger than that for the tool with NL=45 ° at VB=0.05 mm The cutting force-time curve of the tool "a"

with an NL of 15 ° just after starting the cutting experiment was recorded as shown in

Fig 12 A step-wise increase in the force was observed According to the SEM observation of the tool tip in this case, many adhering particles from the workpiece and scratches (indicated by an arrow in Fig 13) in the rake face due to chipping were observed in a portion of the cutting edge and the flank wear surface Therefore, it can

be considered that the chipping must be the operative mechanism of tool wear in the initial stage of the cutting process The chipping is induced by the spalling out of particles from those adhering along the cutting edge just after starting the cutting experiment Then the step-wise increase in the cutting force as shown in Fig 12 must

be caused by the change in the geometry of the cutting edge due to the chipping The initial wear of the CBN tool can be summarized as follows In the tools with a

sharp cutting edge (small NL), micro-spalling of constituent particles in the tool material

occurs readily at the tip The workpiece material tends to adhere in the small cavities

412 K SHINTANI et al

r'

=

I

Cutting length Fro 12 Step-wise increase of cutting force in tool of N L = 1 5 °

Fro, 13 Chipped cutting edge after short time cutting at L=17 m, NL=15 °, rvt 0.05 mm and r, ffi0.8 mm

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= J M i c r o - s p a l l i n 8

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Adhesion of workpieee material

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in tool material

FIG 14 Schematic representation of chipping process of CBN tool