Effect of diamond tool sharpness on minimum cutting thickness and cutting surface integrity in ultraprecision machining

J-d

Materials Processing Technology

.

Journal

of Materials

Processing

Technology

62 (1996)

327-330

Effect of diamond tool sharpness on minimum cutting thickness and cutting surface integrity in ultraprecision machining Z.J. Yuan , M. Zhou, S. Dong P.O.

Box

422, Dept.

ofMechanical

Engineering,

Harbin

Institute

of Technology,

Harbin

150001,

P.R.

of China

Abstract The diamond tool sharpness is a primary factor affecting the cutting deformation and the machined surface quality in diamond cutting process. In this paper, the relationship between the cutting edge radius and the minimum cutting thickness was analyzed. Cutting tests of aluminium alloys with two diamond tools of different sharpness were performed in order to investigate the effect of cutting edge sharpness on the machined surface integrity. Experimental results show that the surface roughness, microhardness, residual stress and the dislocation density of the machined surface layer vary with the cutting edge radius. Keywords:

Cutting

edge radius,

Minimum

cutting

thickness,

&&ace

1. Introduction Ultraprecision cutting aims at producing advanced components with not only a high dimensional accuracy but also a good surface integrity such as smaller surface roughness and residual stress[l][2]. Since the achievable machining accuracy is governed by the accuracy of the relative motion between the cutting edge and the workpiece, the performance of machine tools is of prime importance. Over the past decades, the machine tools used in ultraprecision cutting have reached an extremely high level in performance through scientific and engineering efforts[3]. However, few work was reported on the research of micro-cutting mechanism different from normal cutting. The cutting thickness in ultraprecision cutting is very small, only a few micrometer or less than one micrometer. In such case, a basic understanding of the effect of the cutting tool sharpness on the cutting process is necessary to produce a high quality surface. In this paper, the relationship between the cutting edge sharpness and the minimum cutting thickness was analyzed. The influence of the cutting edge radius on the machined surface integrity was studied from the viewpoints of the surface roughness, microhardness, residual stress and the dislocation density of the machined surface layer.

2. The relationship between the cutting the minimum cutting thickness 2.1. Theoretical

tool sharpness

and

analysis

In ultraprecision machining, the minimum cutting thickness available depends on the cutting edge sharpness and the physical and mechanical properties of the material being cut. Fig. 1 shows the relationship between the minimum cutting thickness and the 0924.0136/96/$15.00 0 PII 0924-0136(96)02429-6

1996 Elsevier

Science

S.A. All

rights

reserved

integn’ty,

Ultraprecision

machining

radius of the cutting edge p. It can be seen from Fig.l(a) that there is a critical point A in the cutting zone. One part of the workpiece material above point A is piled up and forms chips and the other part under point A undergoes the elastic and plastic deformation and forms the machined surface. There are two forces acting at point A, i.e., horizontal force Fx and vertical force Fy as shown in Fig. l(b). These two forces can be divided into normal force N and tangential force uN which can be calculated as: N=Fy cos8+Fx sinB (1) pN=Fxcos6’-Fysin8 where p is the friction coefficient between the cutting tool and the workpiece material. From equation (1) we can get: Fx-pFy (2) tg0= /lFx +Fy As point A is the critical point, the minimum cutting thickness b, is given by: a CINn=p(l-co@

=p(l-

&xi) Substituting equation (2) into equation (3), we have: Fy +pFx a cmin = p(l> (Fx2+Fy2)(1+$)

(3)

(4)

It can be seen from equation (4) that if the radius of the cutting edge p is a given value, the minimum cutting thickness acmln depends on the ratio of Fy/Fx and the friction coefficient p between the tool and the workpiece. In machining process, the ratio of Fy/Fx at point A depends on the strength, the elongation and the friction coefficient of the

328

Z.J. Yuan et al./Journal of Materials

Processing Technology 62 (1996) 327-330

-v Yo

Cutting tool

Cutting tool .; ca” Fx I I

(b)

(4

Fig. 1 The relationship between the minimum cutting thicknessand the cutting edge radius. workpiece and the position of the point A along the cutting edge. Generally, the empirical value of Fy/Fx at point A is 0.8-l .O.For the case of ultraprecision machining with diamond tools, the empirical value of Fy is 0.9Fx. We have measured the friction coefficient between diamonds and Al-alloys, The measured value is 0.06-O.l3(varying with different crystal planes and different friction directions). Noting that the difference in the friction condition between friction test and actual cutting process, the value of friction coefficient in diamond cutting processcan be assumed to be twice as much as that measured in friction test, i.e., n = 0.12-0.26. Therefore, we have: hrn = 0.322~ (when Fy = 0.9Fx, p = 0.12) hm,,,= 0.249p (when Fy = 0.9Fx, n = 0.26) 2.2. Analysis of some expen’mental

results

A diamond cutting of Al-alloys was performed in order to checkthe validity of the above analysis. The cutting edge radius of the tool used in the test was p = 0.3~~11.The cutting processis normal when the cutting thickness is 0.1 pm However, it is found to be unstable when the cutting thickness is lessthan 0. lprn. R.J.Chen[4] has investigated the attainable minimum cutting thickness in the cutting of steel with high-speed-steel or carbide tools. The experimental results of his test are as follows: Cutting of A3 steel with WlSCr4V tool: Cutting of 45” steel with WlKr4V tool: Cutting of A3 steel with YG8 tool: Cutting of 45* steel with YG8 tool:

h = 0.248~; aa,w,= 0.274~; &k= 0.3sop; am,= 0.377p;

The above experimental data gives a strong support to our theoretical analysis presented in Section 2.1. The minimum cutting thickness a-,, is about l/5-2/5 of the radius of the cutting edge p, The cutting edge radius of the diamond tools currently used in the practical production is about 0.2-0.6pm. The value of p can be as small as 0.05-O. 1pm for some carefully polished tools, The attainable minimum cutting thickness is 0.02-O.lpm. Ikawa[S] reported that an extremely small cutting thickness of

lmn was reached in the cutting of copper with a purpose-ground diamond tool. The tool was too sharp to be measured by the instrument available. The cutting edge radius of this tool is estimated to be 2.5-5nm according to our theory

3. Effect of tool sharpness 3.1. Experimental

on cutting

surface integrity

procedure

Cutting test was carried out on a ultraprecision 2-axis CNC diamond lathe ( Type MSG-32.5, made by Rank Pneumo Company of U.S.A.).The general specifications of MSG-325 are shown in Table 1. The workpiece material used in the experiment is aluminium alloys. Its chemical composition and mechanical properties are shown in Table 2. In order to remove the woke-deformed layer generated during the pre-machining process, the surface of the specimens was carefully electropolished prior to the cutting test. Table 1 Specifications of MSG-325 CNC lathe Item Description Radial nmout at spindle nose Spindle

Slide

Axial runout at spindle nose Radial stiffness Axial stiffness X slide horizontal straightness X slide vertical straightness Z slide horizontal straightness Z slide vertical straightness

Data o.ospm (v=lOOOrpm) O.OSpm (v=lOOOrpm) 88N/llm 14ONIpm 0.2Spm/3OOmm 1.2Sp.m/3OOmm 0.2Spm/2OOmm 1.2Scun/2OOmm

Table 2 Workpiece material property Chemical composition(%) Mechanical property Cu Mg Mn Hardness(Hv) Strength(MN/d) 3.8 0.6 105 350 1.6

Z.J. Yuan et al./Journal

of Materials

Processing

Technology

62 (1996)

329

327-330

Two diamond tools with 0’ rake angle and 4” clearance angle were used in the cutting test. All parameters of the two tools are the same, except the cutting edge sharpness The cutting edge sharpnessof these tools was measured by SEM. Fig.2 shows the SEM photography of the cutting edge radius. The cutting edge radius p of these two tools is 0.3~ and 0.6~ respectively.

(a) p = 0.3luti

a, = spnl R... = 0.o!nJlm

(a) p = 0.3l1ni

a, = 2#m R ,=O.O48fim

~‘O.Spm R-

= o.of$m

(b) p = 0.6pm Fig.3. Effect of the cutting edge sharpnesson the cutting surface roughness. tool with p = 0.3pm is H, = 167, and that machined by the tool is H, = 207. It can be seen from the above experimental data that the cutting edge sharpnessexerts a great influence on the machined surface hardness. The hardness of the surface machined with a sharp tool is smaller than that machined with a dull tool. In addition, it is also found that the measured value of the microhardness in the surface layer turned by the tool with p=O.3ltm is still higher than the original data of the bulk material. A much sharper tool is needed to get a better surface quality.

with p =0.6~

(b) p = 0.6~ Fig.2. The SEM photo of the cutting edge radius 3.2. Cutting surjace roughness

The cutting speed and feed rate used in the test are 3 14m/mm and 2.Qm/rev respectively. The surface roughness machined with these two tools is shown in Fig.3. It can be seen from Fig.3 that the cutting edge sharpnesshas a considerable intluence on the machined surface roughness. The measured value of surface roughness of the machined surface was found to vary with the cutting edge radius. The sharper the cutting edge, the smaller the surface roughness. In the case of machining at a depth of cut a,=OSpm, a surface roughness R,,=O.O35unr is obtained when cutting with a sharp tool while the surface roughness is R,,,,=O.O6pmwhen cutting with a dull tool. 3.3. Microhardness

of the machined su$ace

In order to investigate the effect of diamond tool sharpnesson the work hardening of the machined surface layer, the microhardnessof the machined surface was measured by a micro Vickers. The original microhardnessof the workpiece material is H,= 105. The microhardness of the surface machined by the

3.4. Residual stress in the machined sur$ace layer

Residual stress in the surface layer is an important criterion of surface integrity. In our experiment, the residual stress was measured by means of a MSF-2M X-ray diffractometer(made in Japan). The cutting speed and feed rate is 314m/min and S@rev respectively. The surface residual stress of AI-alloys machined with tools of different sharpnessis given in Table 3. It is found that the residual stressof the machined surface is largely affected by the tool sharpness. At the same depth of cut, the absolute value of residual stressvaries with the cutting edge radius. A large cutting edge radius corresponds to a big residual stress.It can also be found from Table 3 that in a certain range of depth of cut, the residual stress reduces with a decrease in the depth of cut. However, when the depth of cut reaches a critical value (e.g. ar,=0.5pm in this test), the magnitude of the residual stressgoes up as the cutting depth decreases.This is caused by so-called “size effect” of the removed chip. When the depth of

Z.J. Yuan et al/Journal

of Materials

Processing

Technology

62 (1996)

327-330

cut is at the same order of the cutting edge radius, the workpiece is actually removed by the tool with a large negative rake angle. In this case,the cutting processis accompanied by severerubbing or burnishing action. The specific machining energy will increase dramatically and the material near the vicinity of the tool tip will be subject to a large plastic deformation. As a result, the magnitude of residual stresswill increase greatly. Table 3 Surface residual stress machined with the tools of different sharpness Depth of cut (pm) Residual stress(MF’a) p = 0.3p.m p = 0.6pm 10 -67.0 -118.2 -62.3 -95.1 5 2 -28.4 -57.6 1 -20.3 48.0 -26.0 -60.8 0.5 3.5 Dislocation

(a) p = 0.3~.rm

in the machined suflace layer

The density of dislocation is a criterion of the cutting deformation of the machined surface layer, which affects the mechanical and physical properties of the metal. Fig.4 shows the TEM photography of the cutting surface layer machined with diamond tools of different sharpness. It can be obviously seen that the dislocation density in Fig.4(b), machined with diamond tool p = 0.6pm, is much higher than that in Fig.4(a), machined with tool p = 0.3pm. In other words, the plastic deformation in Fig.4(b) is larger than that in Figd(a).

4. Conclusions

1) The diamond tool sharpness exerts a great effect on the minimum cutting thickness. While the cutting edge radius of the diamond tools currently used is p = 0.2-0.6pm, the attainable minimum cutting thickness is 0.05-0.2pm. A more sharper tool is needed to reach a much smaller cutting thickness. 2) The diamond tool sharpnesshas a considerable inIIuence on the machined surface integrity. The surface roughness, microhardness,residual stress and the dislocation density of the machined surface layer have been found to vary with the cutting edge radius.

References

(b) p = 0.6pm Fig.4. Dislocation in the machined surface layer

[l]Z.J. Yuanetc.,Proc. offhe6fhMCC,(1993) 13. (21N. lkawa, Annals ofthe CZRP, 40 (199 1) 587. [3] K.Ueda, Annals of the CZRP, 40 (1991) 555. [4] R. J.Chen and Z.C. Lou, Annals of CSPE, (1988) 196 [S] N. Ikawa, S. Shimada and R.R. Donaldson, Preprints Spring Meeting

of JSPE, (1987) 465.

of