Determination of the Real Cutting Edge Wear Contact Area on the Tool-Workpiece Interface in the Light of Cutting Forces Variations

Determination of the Real Cutting Edge Wear Contact Area on the Tool-Workpiece Interface in the Light of Cutting Forces Variations

SAMY E. ORABY

Department of Mechanical Production Technology, College of Technological Studies, PAAET, P. O. Box. 42325 Shuwaikh 70654, KUWAIT

Email: [email protected]

Keywords: Cutting edge wear and deformation; Wear contact area, Tool-workpiece interface; cutting forces.

Abstract. The determination of actual stresses over the tool-workpiece interface has long been a

matter of debate among researchers. Evaluation of the nature and the geometry of the wear contact

area were always associated with many, sometime impractical, assumptions. The indeterministic

fashion of edge wear and deformation requires a more realistic way to predict the actual wear contact

area. In the current study, many wear area patterns are proposed considering the different wear modes

of the cutting edge. The selection of the most correlated pattern to a specific edge deformation is

justified using the relevant variations in the radial and the axial force components. For a regular wear

over the entire cutting edge, a wear pattern that considers nose and/or flank is justified. When the

cutting edge plastically fails, a pattern that considers only nose wear is preferred. As the cutting edge

is subjected to many types of irregular disturbances of edge fracture and chipping, a wear pattern

considering both flank and nose wear is selected.

Introduction

In machining, the tool edge exhibits many wear forms such as flank wear, crater wear, nose wear,

chipping, etc, Fig. 1. The flank wear has long been used as a criterion measure to determine the tool

useful time. Usually the flank wear is assessed by its direct measurement as specified by

ANSI/ASME B94.55M-1985 standard, Fig. 2. However, such a direct measurement usually involves

the frequent interruption of the machining process and this is no longer accepted when the advanced

fully automated processes are considered. This called for a more efficient indirect assessment of edge

performance through the inherent variations in one or more of the measures cutting output; cutting

force, system dynamic characteristics, cutting temperature, consumed power, etc. [1,2].

The edge deformation on the tool-workpiece interface is a result of thermal-mechanical-chemical

interaction with a severe friction and plastic-elastic contact. In such situations, the contact bearing

area is usually a matter of interest since it determines the extent of the induced stresses. According to

Waldorf [3], the contact is so complex that the ratio of the normal and contact forces does not follow

those obtained in standard mechanical tests [4]. One unrealistic approach was the assumption of a

constant wear width along the flank contact area [3]. Moreover, a dispute still on [4-6] regarding the

nature of the contact and the induced stresses on the flank interface. One of the reasons behind such a

disagreement is the lack of a realistic approach to determine the actual pattern of the stresses bearing

contact area. Unfortunately, the edge wear and deformation has rarely been with an even distributed

area. In most cases several wear and deformation modes exist over different locations on the

tool-workpiece contact interface. Online or, in-process monitoring of the state of the cutting edge for

possible replacement decision requires a better understanding of the mutual interrelationship between

the wear topography on the flank interface and the corresponding variations in the measured cutting

force components. In more simple words, it is thought that the use of the wear area rather than wear

land width may produce an accurate measure to judge the state of the cutting edge and its

performance. Changes in the contact area as wear propagates can be monitored inprocess through the

corresponding variations in the measured cutting force. Verification of such an approach is the main

objective of the current study.

Applied Mechanics and Materials Vols. 325-326 (2013) pp 1406-1411Online available since 2013/Jun/13 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.325-326.1406

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 80.184.58.151-08/11/13,08:57:35)

a) Edge wear and deformation b) ANSI/ASME B94.55M-1985 standard

Figure 1 Cutting edge wear topography and standard

Geometrical Considerations of the Wear Contact Area on the Flank Face

Various Patterns of Wear Contact Area. Many possible patterns for wear contact area are

proposed as schematically described in Fig. 3. First wear contact area pattern Aw1 is proposed

considering the nomenclatures assigned by ANSI/ASME B94.55M-1985 standard, Fig. 1.b, where

the edge is of zero radius. As shown by Fig. 3.a, the wear contact area is:

( )( )

++==

r

NBCavc1wsin

ap.VBmaxVBVB.

3

1VB.bA

κ (1)

where bc is the cutting length considering zero nose radius, ap is the depth of cut, κr is the main edge

(approach) angle and VBav is the arithmetic average wear of the three wear measures on flank face.

With a rounded nose, Fig. 2, the contact length usually comprises of two length parts; the straight

bs and the rounded br. The second patterns Aw2 is proposed, Fig. 3.b considering the nose wear

domination with a linear decrement toward the contact end points at the notch and the round nose:

( ) ( )( )

−+

−=+=+= rn

r

nCrsCrs2w

2.r

sin

rap.VB.

2

1bb.VB.

2

1aaA κ

πκ

(2)

in which rn is the nose radius while bs and br are the straight and the rounded part of the contact length.

The third pattern Aw3, Fig. 3.c, considers the nose wear along with the notch and the end_nose:

( ) ( )( )

−+

−=+=+= rn

r

nCrsCrs3w

2.r

sin

rap.VB.

4

3bb.VB.

4

3aaA κ

πκ

(3)

Finally, the pattern Aw4, Fig. 3.d, is assumed to include both the max flank wear and the nose:

( )( )

( )

−++

−=++= rnCBC

r

nr2_s41_4s4w

2.r.VB.

2

1maxVB.2VB.

sin

rap.

4

1aaaA κ

πκ

(4)

Applied Mechanics and Materials Vols. 325-326 1407

Figure 2 Real wear land on the conventional tool-workpiece interface

Figure 3 Various assumptions of wear contact area on tool-workpiece interface

Experimental Procedures and Evaluation

A set of turning experiments were carried out using multicoated carbide inserts to cut 709M40 alloy

steel. Inserts configuration was 6°, 5°, 0°, 60°, 30° normal rake, clearance, inclination, approach and

side approach angles respectively. For each experiment, wear land measured sequentially at three

regions: nose VBC, flank VBBmax, and notch VBN, Fig. 1.b. Three cutting force components: main Ft,

feed (axial) Fa and radial Fr, Fig. 1.b, were measured using a three-component dynamometer.

Regular Evenly Distributed Wear Land. Figure 4 shows the wear and the force experimental

results using 100 m/min, 0.6 mm/rev and 2.25 mm speed, feed and depth of cut respectively. Both

SEM micrograph, Fig. 4.a and wear-time graph, Fig. 4.b, show an almost regular wear on both the

edge flank and nose. The radial force component Fr, Fig. 4.c, is observed to be more sensitive for

wear land than the axial force Fa one [7]. In this case, wear pattern resembles that proposed by Aw2,

Fig. 3.b. As shown in Fig. 4.d, the wear pattern Aw1 proposed by ANSI/ASME B94.55M-1985

standard overestimated the real contact area and, consequently produced a misleading values of the

normal stresses on the flank face. Also, it is observed that the pattern Aw3 did not comply with the

case where there no notch wear developed. Generally, the regular evenly distributed wear may by

represented either by pattern Aw2 or Aw4.

1408 Manufacturing Engineering and Process II

a) SEM micrograph of the edge wear b) Wear-time plot

c) Force-time plot d) Wear area according to different patterns

Figure 4 Testing results and data evaluation for evenly deformed cutting edge

Edge Failure due to Softening. The use of a high cutting speed of 206 m/min led to the failure of

the cutting edge due to material softening and plastic deformation, Fig. 5.a. Failure eventually

occurred at the nose and extended to the flank zone. Again, the edge deformation was detected well

by the radial force component. While the proposed wear patterns Aw2 and Aw4 conformed well to the

experimental results, the other proposed patterns especially Aw1 overestimated the wear contact area.

Combined Nose and Flank Wear together with Edge Fracture. For a cutting speed, feed and

depth of cut were 145 m/min, 0.12 mm/rev and 2.0 mm respectively, wear-time plot, Fig. 6.b,

indicated that the cutting edge practiced a sudden fracture at the flank zone at an early stage followed

by a nose fracture after about 16 min. These were detected by the radial and the axial force

components, Fig. 6.c. Wear-force-time plots, Fig. 6.b&c, show a good correlation between the edge

deformation and corresponding force variation using Aw1 and Aw4 patterns. This is basically because

the patterns Aw2 and Aw3 do not involve the flank wear. Besides, the notch wear VBN is found

insignificant to invoke itself in the pattern Aw3. Although the pattern Aw1 seems to qualitatively

represents the case, the SEM micrograph of the cutting edge, Fig. 6.a, strongly suggests that its values

are overestimated. Generally, whenever similar radial and axial magnitudes with fluctuated nature is

observed, it is probably better to suggest a wear pattern of the type Aw4.

a) SEM micrograph of the plastic deformed cutting edge

VBN

VBBmax

VBc

0,1

0,2

0,3

0,4

2 5,17 8,84

Wear

(mm

)

Time (min)

Fr

Fa

Fra

0

500

1000

1500

2 5,17 8,84

Fo

rce (

N)

Time (min)

Aw2

Aw4

Aw3

Aw1

0

50

100

2 5,17 8,84

We

ar

are

a (

x1

00

mm

2)

Time (min)

Applied Mechanics and Materials Vols. 325-326 1409

b) Force-time graph b) Wear area for different patterns

Figure 5 Testing results and data evaluation for plastically deformed cutting edge

a) SEM micrograph shows nose and flank wear b) Wear-time plot

c) Force-time graph d) Wear area for different patterns

Figure 6 Testing results and data evaluation for combined edge wear and fracture

Edge Fracture at Nose, Flank and Notch Zones. Low cutting speed of 72 m/min in combination

with large values of feed and depth of cut of 0.3 mm/rev and 2.5 mm led to a sequential edge fracture

on notch, flank and nose zones, fig. 7.a. The radial force component responded very well to edge

fracture especially at the end of the experiment when nose fracture dominated. Considering the

presence of the notch deformation, it is shown; Fig. 7, that force and wear may be expressed by either

Aw1 or Aw3 patterns. Before the nose failure, both the axial and the radial force components were of

almost equal values with much greater magnitude of Aw1. The sudden increase of Aw3 at the final

failure stage suggests that the earlier values of Aw1 overestimated the contact area. Generally,

whenever multiple random disturbances are observed with minor effect on force signals, the case may

be attributed to either Aw2 or Aw3 patterns. Escalated rate of increase of the radial force component

suggests the wear extraversion over the nose zone.

a) SEM micrograph of multi fractured edge b) Force-time plot

FrFaFra

0

2000

4000

6000

8000

2 4 6

Fo

rce (

N)

Time (min)

Aw2Aw4

Aw3Aw1

0

200

400

2 4 6

We

ar

are

a (

x1

00

mm

2)

Time (min)

0,00

0,20

0,40

0,60

2 11 22 33 45 57 67W

ea

r (m

m)

Time (min)

VBN VBBmax VBN

FaFrFar

0

500

1000

1500

2 11 22 33 45 57 67

Fo

rce (

N)

Time (min) Aw2

Aw4

Aw3

Aw1

0

50

100

2 11 22 33 45 57 67

We

ar

are

a (

x1

00

mm

2)

Time (min)

0

1000

2000

3000

2 13 23 33

Fo

rce (

N)

Time (min)

Fa Fr Far

1410 Manufacturing Engineering and Process II

c) Wear contact areas according to different proposed patterns

Figure 7 Testing results and data evaluation for edge fracture at nose, flank and nose

Conclusions

In metal cutting, there are always much disagreement between the analytical and the experimental

results regarding the nature of contact over the tool-workpiece interface. Among factors behind such

a conflict is the incorrect estimation of the real wear contact area. In this study, the determination of

the most appropriate wear area pattern is verified in the light of the variations encountered in the

cutting forces. A pattern depending on the proposed tool life standard with zero nose radius is found

to produce overestimate values of the wear contact area. For a regular wear mode over the entire

cutting edge, a wear pattern that considers nose and/or flank is justified. When the cutting edge

plastically fails, a pattern that considers only nose wear is preferred. As the cutting edge is subjected

to many types of irregular disturbances of edge fracture and chipping, a wear pattern considering both

flank and nose wear is selected. However, in practice, the notch wear is found to have a diminishing

influence on either the cutting force or on the wear contact area.

Acknowledgements

The author would like to thank the Public Authority for Applied Education and Training PAAET,

KUWAIT for supporting this study under the research support agreement: TS-11-11. Also, author

thanks the Kuwait Foundation for the Advancement in Sciences (KFAS) for their support.

References

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Materials (2013).

0

50

100

150

2 13 23 33W

ea

r a

rea

(x

10

0

mm

2)

Aw4 Aw2 Aw3 Aw1

Applied Mechanics and Materials Vols. 325-326 1411

Manufacturing Engineering and Process II 10.4028/www.scientific.net/AMM.325-326 Determination of the Real Cutting Edge Wear Contact Area on the Tool-Workpiece Interface in the

Light of Cutting Forces Variations 10.4028/www.scientific.net/AMM.325-326.1406