Cutting forces and surface finish when machining medium hardness steel using CBN tools

Radial thrust cutting force was the largest among the three cutting force components and was most sensitive to the changes of cutting edge geometry and tool wear.. The surface finish pro

Cutting forces and surface finish when machining medium

hardness steel using CBN tools

Wuyi Chen

Beijing University of Aeronautics and Astronautics, Beijing, PR China

Received 11 November 1997

Abstract

Cutting forces generated using CBN tools have been evaluated when cutting steel being hardened to 45–55 HRC Radial thrust cutting force was the largest among the three cutting force components and was most sensitive to the changes of cutting edge geometry and tool wear The surface finish produced by CBN tools was compatible with the results of grinding and was affected by cutting speed, tool wear and plastic behaviour of the workpiece material  1999 Published by Elsevier Science Ltd All rights reserved.

Nomenclature

1 Introduction

Machining of hardened steel using advanced tool materials, such as CBN, has certain advan-tages over the traditional cutting–hardening–grinding practice in terms of improved fatigue

0890-6955/00/$ - see front matter  1999 Published by Elsevier Science Ltd All rights reserved.

PII: S 0 8 9 0 - 6 9 5 5 ( 9 9 ) 0 0 0 1 1 - 5

a Because the depth of cut was far smaller than the nose radius, the major cutting edge angle was not practically func-tional.

b 1 mm nose radius CBN 2 insert was used only in cutting force tests.

strength of the machined parts, increased productivity and reduced energy consumption [1–3] Although CBN tools offer excellent performance on fully hardened steels, the results on steels

of medium hardness have been challenged by other members of the tooling family, e.g ceramic tools or even some new carbides In this paper the performance of CBN tools is investigated when machining steel hardened to 45–55 HRC Since CBN tools are normally used in finishing operation and the cutting regime employed is likely to generate large radial thrust force which may cause chatter and deteriorate machining quality, understanding the changing patterns of cut-ting forces and surface finish is therefore important

2 Experimental work

2.1 Materials

The cutting tools used were high concentration CBN compacts, referred to in this paper as CBN 1 and CBN 2 The materials and geometric parameters of the tool inserts are detailed in Table 1 The workpiece material used in the tests was hardened GB699-88 55 steel hardened to 45–55 HRC The compositions of the workplace material are shown in Table 2

Table 2

Compositions of 55 (GB699-88) steel

2.2 Experimental procedure

Multivariate tests were performed to measure cutting forces and machined surface roughness

0.1 mm In addition, the tools with chamfered/unchamfered cutting edge and with different tool nose geometry were used in certain tests All tests were conducted dry under continuous turn-ing conditions

3 Results and discussion

3.1 Cutting forces

3.1.1 Cutting force components

Cutting forces can be divided into three components: feed force (F x ), radial thrust force (F y)

and tangential cutting force (F z) Usually the tangential cutting force is the largest of the three components, though in finishing the radial thrust force is often larger (see Figs 1–3), while the feed force is minimal This arrangement in finishing can be explained by studying the particular cutting regime and tool geometry used in the tests From the tool geometry and the cutting con-ditions outlined in Section 2.2, it is clear that the depths of cut (0.025–0.10 mm) are far smaller than the nose radii of the tools (0.3–1.2 mm) Under such conditions the tool nose, i.e the curved part of the cutting edge, performs the whole cutting job, thus the acting cutting edge angle varies along the tool–work contact arc of the tool nose The largest value of the angle appears at the position where the cutting edge meets the original work part surface as in Fig 4 The maximum cutting edge angle can be obtained from:

Kr⫽ arccos r⑀ ⫺ ap

where Kr is the cutting edge angle, r⑀ is the tool nose radius and ap is the depth of cut

If r⑀ ⫽ 1 mm, ap ⫽ 0.025 mm, then Kr ⫽ 12°8⬘ Such a small cutting edge angle is seldom

used in metal cutting, moreover, if considering the average value along the tool–work contact arc the angle is even smaller As the cutting edge angle decreases the horizontal component of the

cutting force F xy will alter direction clockwise; see Fig 5 As a result, F y will increase whereas

F x will decrease F z will also increase but to much less extent [4] Eventually F y will surpass F z,

and F x will reduce to a negligible quantity

The increase in F y can lead to instability through vibration From this point of view the tool nose radius should be kept as small as possible This is not ideal however in respect of good surface finish In addition, this may also cause temperature concentration at the tool nose and increase the likelihood of spalling, resulting in a short tool life

3.1.2 Cutting regime vs cutting forces

With an increase in cutting speed, both the radial thrust force and the tangential cutting force showed a decrease (Fig 1) This is a standard effect when cutting most metals with carbide tools

Fig 1. Cutting forces vs cutting regime and edge chamfer using CBN 1 (a) f ⫽ 0.15 mm/rev, ap⫽ 0.05 mm; (b) v

⫽ 95 m/min, a ⫽ 0.05 mm; (c) v ⫽ 95 m/min, f ⫽ 0.15 mm/rev.

Fig 2. Cutting forces vs feed rate and tool nose radius using CBN 2 v ⫽ 85 m/min, ap ⫽ 0.1 mm.

Fig 3. Cutting forces vs tool wear and edge chamfer with CBN 1 v ⫽ 95 m/min, f ⫽ 0.15 mm/rev, ap ⫽ 0.05 mm.

Trent [5] attributed this phenomenon in part to the softening of the workpiece material at high temperature and in part to a decrease in tool–chip contact area owing to a thinner chip

When the feed rate was increased the forces also increased, but the radial thrust forces generated

by CBN 1 tools (Fig 1) appeared to be not as sensitive to the change as those produced by CBN

2 tools (Fig 2)

Depth of cut seemed to influence cutting forces more significantly than cutting speed and feed

rate In fact, the feed force (F x) showed visible changes only when increasing DOC On

substitut-ing the tool nose radius used, r⑀ ⫽ 1.2 mm, in the cutting force tests into Eq (1), it can be seen

that when the depth of cut increases from 0.025 to 0.1 mm, as in Fig 1, the maximum cutting edge

Fig 4 The maximum cutting edge angle with a large tool nose radius and small depth of cut.

Fig 5. The influence of cutting edge angle on the direction of F xy.

angle increases from 11°3⬘ to 23°3⬘ This is a major reason for the increase in F x, as illustrated in Fig 5

3.1.3 Edge geometry and cutting forces

Chamfered and unchamfered CBN inserts were used in the cutting force tests It can be seen from Figs 1–3 that all three force components generated by the chamfered tools were greater than those recorded when using the unchamfered ones The radial thrust force was affected the

most On the chamfered tools, F y was doubled or even tripled, yet the increase in F z was only about 10–50%

There are other observations that may also be related to geometric parameters In the cutting force tests, CBN 2 inserts were ground to different nose radii yet were tested under otherwise identical cutting conditions It can be seen from Fig 2 that as the nose radius increases from 0.3

to 1 mm, F y increases by about 30%, whereas the changes in F x and F z are negligible This phenomena can be explained using Eq (1) When the nose radius changes from 0.3 to 1 mm, for

a depth of cut of 0.1 mm, the maximum cutting edge angle decreases from 48 to 26° Such a

change may turn the horizontal component of the cutting force (F xy in Fig 5) clockwise, then the radial component of the cutting force increases

3.1.4 Influences of tool wear on the cutting forces

It can be seen in Fig 3 that tool wear had a negligible influence on feed force and tangential cutting force, however, the radial thrust force showed a 90–150% increase when the wear land

VBB had an increment of about 0.18 mm

Because the tool nose radius is much larger than the depth of cut, the flank wear land may almost be parallel to the feed direction, thus the force normal to the flank wear land will be

approximately in the direction of the y-axis Meanwhile the friction force on the flank wear land

is always in the direction of the z-axis The fact that F z changes only slightly while F y increases dramatically seems to indicate that either the increase in the normal force on the flank wear land does not lead to the increase in friction force, or the friction force on the flank face is too small

to have significant influence on the total force in the z-direction.

3.2 Surface roughness

3.2.1 Hardness vs roughness

The majority of Ra data collected during the tests were summarised by using histograms; see Fig 6 The horizontal axis of the graphs represents the observed roughness readings and the vertical axis gives the frequency of the readings The graphs are able to show the variations of surface roughness with the changing workpiece hardness

From Fig 6, it is evident that the harder the workpiece material, the lower is the surface roughness obtained for a given set of operating parameters This phenomenon may be explained

by a finding presented by Usui [6]

In orthogonal cutting, the material flow is mainly two-dimensional, on a plane normal to the cutting edge The deformation in the third direction, i.e the direction parallel to the cutting edge,

is usually disregarded However, this deformation does exist and causes slight lateral plastic flow

of the workpiece material in the region adjacent to the two free surfaces, e.g the internal and external surfaces if a thin wall tube is used as the workpiece when conducting orthogonal cutting

on a lathe When there is only one free surface, as in turning a solid bar, the lateral flow on the constrained side may increase the peak-to-valley height of the machined surface profile as in Fig

7 By increasing the workpiece hardness, the plasticity of the workpiece material is reduced and

so is the level of the lateral plastic flow As a result the surface roughness becomes lower

3.2.2 Influence of cutting regime

Surface finish was shown to be improved by increasing cutting speed (Fig 8), though the improvement was very limited Producing a better surface finish at higher cutting speed is not something unusual in metal cutting, but the conventional explanations are usually related to BUE [4] That is, the formation of a built-up-edge is favoured in a certain range of cutting speed By increasing cutting speed beyond this region, BUE will be eliminated and as a result the surface finish will improve When hardened steel was machined under present cutting conditions, the cutting speeds adopted were higher than those favouring BUE formation [5] Indeed BUE was not apparently observed even at the lowest speed of 56.5 m/min Therefore, the phenomenon needs further explanation Two possible reasons are given below

According to Liu [7], the properties of metals are influenced by the deformation velocity The higher the velocity, the less significant the plastic behaviour will be Based on the reasoning in

Fig 6 Surface roughness vs workpiece hardness (a) By CBN 1 tools; (b) by CBN 2 tools.

Fig 7 Additional increase in surface roughness caused by lateral plastic flow.

Fig 8. Surface roughness vs cutting speed and tool geometry with CBN 1 f ⫽ 0.1 mm/rev, ap ⫽ 0.1 mm.

Section 3.2.1, the lateral plastic flow of the workpiece material along the cutting edge direction may increase the peak-to-valley height of the surface irregularity If the material presents less plasticity by increasing cutting speed and hence deformation velocity, the surface finish can be improved as a result of less significant lateral plastic flow and thus less additional increase in the peak-to-valley height of the machined surface roughness

The second possible reason is based on SEM observations At low cutting speed, grooves developed on the flank wear land (Fig 9) When such a cutting edge is engaged with a workpiece, the defects will in part be copied on to the newly generated surface In any event it is likely that the surface will be rough With an increase in cutting speed the grooves will gradually be reduced,

Fig 9 Wear scar of CBN 1 tool: (a)⫻ 60; (b) ⫻ 1250 v ⫽ 82.5 m/min, f ⫽ 0.2 mm/rev, a ⫽ 0.025 mm.

Fig 10 Wear scar of CBN 1 tool: (a)⫻ 60; (b) ⫻ 1250 v ⫽ 145 m/min, f ⫽ 0.2 mm/rev, ap ⫽ 0.025 mm.

thus the cutting edge and wear land will become smoother (see Fig 10), as will the workpiece surface The influence of wear land grooves on surface roughness was also observed by Solaja [8], and Ansell and Taylor [9] They demonstrated that with the development of the grooves the surface finish deteriorated

The roughness increases with increases in feed rate (see Fig 11), but the trend is less significant for the tools with large nose radius A recommendation is therefore made to the tool users that

if the inserts of 1 mm nose radii are used, feed rates as large as 0.3 mm/rev may be used in order

to promote productivity when finishing without significant deterioration in surface roughness However, low DOC should be used in order to reduce the tendency to chatter

The DOC has little direct influence on the surface roughness, however, with increases in DOC, chatter may result causing degradation of the workpiece surface Therefore, if the tool–work

Fig 11. Surface roughness vs feed rate and tool nose radius v ⫽ 85 m/min, a ⫽ 0.1 mm.

system is not very rigid, such as in cutting slender parts, very fine DOC should be employed to avoid chatter In this way very good surface finishes can be obtained For example, when a DOC

achieved using CBN 1 inserts, which is compatible with grinding

3.2.3 Influences of tool wear

As mentioned in the last section, large surface roughness values produced at low cutting speed probably resulted in part from the grooves on the wear scars of the tools It can be seen from Fig 12 that the roughness is also associated with the width of the flank wear land The relationship may be explained as follows

When a new insert starts to work, the machined surface is determined, for a given feed rate,

by the geometry of the fresh tool edge If the DOC is far smaller than the nose radius, then the principal geometric parameter is only the nose radius As the tool wears, however, the round corner becomes flatter, in other words the nose radius increases substantially As a result the machined surface finish improves With the development of excessive flank wear, however, increased cutting force and temperature may destabilise the machining process and the surface quality is degraded

4 Conclusions

1 When finish cutting of hardened steel, the radial thrust force (F y) became the largest among the three cutting force components and was the most sensitive to the changes of cutting edge chamfer, tool nose radius and flank wear Although an unchamfered tool with small nose radius

generated low F yand hence reduced the tendency to chatter, such geometry decreased tool life

Fig 12. Surface roughness vs tool wear (1) v ⫽ 82.5 m/min, f ⫽ 0.2 mm/rev, ap⫽ 0.025 mm; (2) v ⫽ 121 m/min,

f ⫽ 0.1 mm/rev, a ⫽ 0.1 mm.