Scholarly article on topic 'Tool Life and Process Dynamics in High Speed Ball End Milling of Hardened Steel'

Tool Life and Process Dynamics in High Speed Ball End Milling of Hardened Steel Academic research paper on "Materials engineering"

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{"tool life" / dynamics / "ball end milling" / "hardened steel"}

Abstract of research paper on Materials engineering, author of scientific article — Szymon Wojciechowski, Paweł Twardowski

Abstract In this paper, a comparison of tool life of sintered carbide (with TiAlN coating) and cubic boron nitride (CBN) ball end mills is presented. Experiments were carried out on hardened steel X155CrVMo12-1 plate during milling process with a constant surface inclination angle (α) and variable cutting speeds (vc) and feeds (fz). Cutting forces (Fi) were measured and presented in function of tool wear (VBB). Specific cutting pressures were also estimated in function of tool wear (VBB). Their values are applied to cutting force model formulation, which includes tool wear. The research revealed that in certain range of cutting speed tool life acquired for sintered carbide can be higher than that obtained for cubic boron nitride tool. It was observed, that for cubic boron nitride cutter abrasive wear was dominant independently of cutting speed. In case of sintered carbide, catastrophic wear of cutting edge was found for cutting speed vc = 500 m/min.

Academic research paper on topic "Tool Life and Process Dynamics in High Speed Ball End Milling of Hardened Steel"

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Procedia CIRP 1 (2012) 289 - 294

5th CIRP Conference on High Performance Cutting 2012

Tool life and process dynamics in high speed ball end milling of

hardened steel

Szymon Wojciechowski *, Pawet Twardowski

"Poznan University of Technology,Piotrowo 3, Poznan 60-965, Poland" * Corresponding author. Tel.: +48-061 6652608; fax: +48-061 6652200.E-mail address: sjwojciechowski@o2.pl._

Abstract

In this paper, a comparison of tool life of sintered carbide (with TiAlN coating) and cubic boron nitride (CBN) ball end mills is presented. Experiments were carried out on hardened steel X155CrVMo12-1 plate during milling process with a constant surface inclination angle (a) and variable cutting speeds (vc) and feeds f). Cutting forces (F) were measured and presented in function of tool wear (VBB). Specific cutting pressures were also estimated in function of tool wear (VBB). Their values are applied to cutting force model formulation, which includes tool wear. The research revealed that in certain range of cutting speed tool life reached for sintered carbide can be higher than that obtained for cubic boron nitride tool. It was observed, that for cubic boron nitride cutter abrasive wear was dominant independently of cutting speed. In case of sintered carbide, catastrophic wear of cutting edge was found for cutting speed vc = 500 m/min.

© 20122 The Authors. PEblished by EElsevier B.V. Selection and/or peer-review under respons ibility ofProfessor Wonrad Wegener Keywords: tool life, dynamics, ball end milling, hardened steel,

1. Introduction

The great popularity of free form surfaces high speed machining of hardened steels using ball end mills results from the many advantages of this technology. These advantages are: the increase of material removal rate, improvement of surface roughness [1], as well as the reduction of technological process through the elimination of some machining operations conducted in a tempered state [2]. Nevertheless, HSM technology of hardened steels has also some significant drawbacks. The growth of cutting speed induces the increase of cutting temperature, which as result causes excessive tool wear [3]. Furthermore, the occurrence of dynamical cutter's radial run-out induces the inequality of cutting teeth load and hence excessive tool wear of a certain tooth [4]. Another problem occurring in hardened steel machining is the growth of the thrust force during cutting. Investigations [5, 6] revealed that, the growth of

tool wear on the flank face VBc caused intense growth of friction force and as a result also of the thrust force.

Specific technological and kinematical conditions of HSM processes, influencing tool wear mechanisms, impose appropriate selection of tool material. The most popular cutting tool's materials are sintered tungsten carbides (WC) and cubic boron nitrides (CBN) [7, 8]. According to [1], the application of CBN cutters is most effective in the range of high cutting speeds, because of the presence of hot machining mechanism. The growth of cutting speed increases cutting temperature and thus the ductility of work material. In the range of low cutting speeds, work material remains hard and brittle, which in turn can cause intense abrasive tool wear or cutting edge chipping of CBN cutter. Therefore, the authors of [1, 7] indicate that CBN cutters can be effectively applied to the machining of hardened steels in the range of cutting speeds vc = 300 ^ 1200 m/min. Sintered carbides has significantly lower hardness and maximum cutting temperature than CBN materials. However their great popularity is related to the low price, as well as higher

2212-8271 © 2012 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Professor Konrad Wegener http://dx.doi.org/10.10167j.procir.2012.04.052

ductility and fracture toughness than cubic boron nitrides. According to [7], the primary wear mechanism of CBN and WC materials during high speed milling of hardened steel is chipping induced by milling process dynamics. Sintered carbides with mean grain size lower than 1p,m have higher hardness and abrasive wear resistance. Above mentioned properties improve tool's life in HSM conditions. From the point of view of tool life, the anti-wear coatings have also significant importance. According to [9], the tool life is critically affected by the certain properties of its coating such as, hot hardness, oxidation resistance and thermal stability. The most popular coatings on sintered carbides are TiAlN and TiCN, because of their anti-wear properties [1]. According to [10], modern TiAlN coatings obtained using PVD technology can increase tool life 3-fold in comparison to TiCN coatings. In spite of many advantages of sintered carbides, their hardness in high temperatures is significantly lower than CBN materials. Thus, in [11] it is recommended that sintered carbides can be applied to hardened steel machining in the range of cutting speeds vc = 150 - 350 m/min. According to [12] tool life in curvilinear milling is also affected by the machining strategy as well as surface inclination angle a. During ball end milling of curvilinear surface, one should avoid surface inclination angle «=0, because the cutting speed near tool's tip is equal to zero and consequently, large mechanical loads on cutting edge can induce cutter's chipping [8, 11]. Investigations [8, 11, 12, 13] reveal, that from the point of view of tool wear, surface inclination angle a should be selected in the range of 15-30°.

In this study, a comparison of tool wear and tool life of sintered carbide (with TiAlN coating) and cubic boron nitride (CBN) during inclined surface milling is presented. The secondary purpose of this paper is the analysis of edge specific coefficients Kie in function of tool wear. Obtained Kie values can be applied to formulation of the cutting force model including tool wear effect.

tool had joining and shank part made of tungsten carbide and the following geometry of the cutter: yo = 0°, ao = 6°, local helix angle Xs = 0° and radius of tool arc cutting edge rn = 5 ^m.

2.2. Research range and method

The main purpose of this paper was a comparison of tool wear and tool life of sintered carbide (with TiAlN coating) and cubic boron nitride (CBN) during inclined surface milling. The cutting forces were measured in function of progressing tool wear. Their value was applied to the estimation of the edge specific coefficients Kie. Cutting parameters applied in the research are presented in the Tab. 1.

Table 1. Cutting parameters applied in the research

f [mm] « [°] n [rev/min] vc [m/min] ap [mm]

0.02-0.1 45 4502- 100-500 0.1

interval 22253 interval 200

Experiments were conducted on 5-axes CNC milling workstation (DECKEL MAHO Co., model DMU 60monoBLOCK), in upward ramping conditions with surface inclination angle a = 45° (Fig. 3). In all investigated cases tool's effective diameter was lower than the value of pick feed - De<br.

2. Experimental details

2.1. Work and tool materials

Investigations have been carried out on hardened cold-work tool steel X155CrVMo12-1 plate with hardness approx. 60 HRC and length Lf = 320 mm. Monolithic ball end mills made of sintered tungsten carbide (WC) and cubic boron nitride (CBN) with diameter d=10 mm and number of teeth z=2 were selected as milling cutters. Milling tools made of finegrained tungsten carbide had anti-wear TiAlN coating and the following geometry: orthogonal rake angle yo = -15°, local helix angle Xs = 30°, radius of tool arc cutting edge rn = 5 ^m and orthogonal flank angle ao = 6°. CBN

Fig. 1. The upward ramping process with surface inclination angle a = 45°

The measured quantities in the carried out research were tool wear on the flank face VBB and cutting forces fixed in machine tool coordinate system (Fx, Fy, Fz). Tool wear was investigated for the constant value of feed per tooth fz = 0.1 mm/tooth and variable cutting speed (see Tab.1). Tool wear on the flank face VBB was measured -in the preliminary wear progress phase with the intervals of 5 passes, and in the stable wear progress phase with the intervals equal to 50 passes (length of one pass -sample length Lf = 320 mm). The stereoscopic Carl Zeiss microscope was applied to measure tool wear VBB. The method of tool wear measurement is depicted in figure 2.

Fig. 2. Tool wear measurement method

The analysis of research results concerns the mean arithmetic value of tool wear VBB calculated for the two cutting edges of milling tool. Dullness criterion for both investigated tools was selected as: VBB Cr = 0.2 mm. Cutting time was calculated from the equation:

Lf fz ■ z ■ n

where: i - number of passes.

The hook up into bed of a machine piezoelectric force dynamometer was used to measure total cutting forces components. Its natural frequency is equal to 1672 Hz. In order to avoid disturbances induced by proximity of forcing frequency to gauge natural frequency, the band -elimination filter was applied. Cutting force components were measured (in machine tool coordinates - Fig. 3), in following directions: dir. X - feed normal force Fx [N], dir. Y - feed force Fy [N], dir. Z - thrust force Fz [N].

Fig. 3. Cutting force components in machine tool coordinates

Cutting forces were measured in the same intervals as tool wear. However, in the particular phases of progressing tool wear, cutting forces were measured additionally for the remaining investigated feed per tooth values.

2.3. The edge specific coefficients determination

Cutting forces in ball end milling process can be estimated on the basis of Lee and Altintas [14] mechanistic model. According to this model, the elemental tangential dFt, radial dFr, and axial dFa cutting forces acting on the cutter, are expressed by:

dFt = Ktedl + Ktchz( <p,q>r)db, dFr = Kredl + Krchz( <p,ç>r )db,

dFa = Kaedl + Kachz (V,Vr)db>. (2)

where: Kte, Kre, Kae - edge specific coefficients [N/mm], Ktc, Krc, Kac - shear specific coefficients [N/mm2], dl - infinitesimal length of cutting edge [mm]. h((p, - undeformed chip thickness per tooth, depending on angles ^ and yr,

db - infinitesimal undeformed chip width [mm]. The shear coefficients (Ktc, Krc, Kac) in equation (2) are related to shear mechanism due to the chip generating process on the rake face. Their value is depending mainly on the cutting conditions. The edge coefficients (Kte, Kre, Kae) include the effects of ploughing and rubbing mechanisms on the flank face. It is very often assumed, that their value is constant for the particular work and tool material. However, progressing tool wear on the flank face VBB can have significant influence on the edge coefficient values. Therefore, in this step, the estimation of edge specific coefficients will be discussed. In this study direct calibration method of specific cutting coefficients was applied. These coefficients were obtained from the root mean square (RMS) values of measured force signals (Fx, Fy, Fz). Subsequently RMS values of measured forces were substituted into the edge specific coefficient equations:

Fze sin <pr + Fxe cos ç ■ cos (pr + Fye sin ç ■ cos (pr l

_ Fye cosff~ Fxe sin^

Fxe cos ç ■ sin <pr + Fye sin ç ■ sin <pr - Fze cos <pr

The value of cutting edge length l and cutting edge element position angles in equation (3) can be

calculated on the basis of [14]. The root mean square values of qr and l calculated in the range of one tool revolution were substituted into equation (3). The Fxe, Fye, Fze denote edge forces obtained by extrapolating the measured forces to zero chip thickness [14]. Subsequently, these edge specific coefficients were expressed in function of the mean arithmetic value of the tool wear VBB.

3. Experimental results

Figures 4 and 5 depict the time courses of tool wear VBB = f(ts) for the investigated tool materials in the range of three different cutting speeds. It was found that, for the sintered carbide tooling material, tool wear intensity (expressed as the growth of tool wear per the time unit) grows together with the cutting speed growth (Fig. 4), which is the typical dependency occurring in metal cutting processes. In case of cubic boron nitride tool material, the influence of cutting speed on tool wear intensity is characterized by non-monotonous dependency. The lowest wear intensity has occurred for the cutting speed vc = 300 m/min. It confirms the fact, that in case of CBN materials one should avoid the low values of cutting speed, because in this range work material remains hard and brittle, which in turn can cause intense abrasive tool wear or cutting edge chipping of CBN cutter. Investigations revealed, that for the cutting speeds vc = 100 m/min and vc = 300 m/min, independently of tool material, abrasive wear mechanism was dominant.

vc2 - 300 m/min VC1 - 100 m/min T2 = 181min| -347 min

0 50 100 150 200 250 300 350 400 ts [min]

Fig. 4. The time course of tool wear VBB = f(ts) for the sintered carbide

0 10 20 30 40 50 60 70 80 90 100 110 120 ts [min]

Fig. 5. The time course of tool wear VBB = f(ts) for the cubic boron nitride

In case of highest investigated cutting speed (vc =500 m/min), abrasive wear on the flank face occurred in CBN material, whereas in sintered carbide the catastrophic chipping of tool was observed (Fig.6). In both materials tool wear was concentrated near the area of maximum chip thickness formation.

Fig. 6. The view of flank wear VBB after milling with the cutting speed vc =500 m/min for the: a) sintered carbide, b) cubic boron nitride

Figure 7 depicts the comparison of tool life T in function of cutting speed vc for the investigated tooling materials.

0 100 200 300 400 500 600

vc [m/min]

Fig. 7. Tool life T in function of cutting speed vc

From the figure 7 it can be seen, that for the sintered carbide tool material, tool life decreases monotonously with the cutting speed growth. This observation stays in agreement with the Taylor's dependency. In case of lower investigated cutting speeds (vc =100 m/min, vc =300 m/min), sintered carbide cutter has significantly higher tool life than that, obtained for the cubic boron nitride. It confirms previous investigations [1, 7], stating that, CBN cutters should be applied in the range of cutting speeds higher than vc =300 m/min, because of hot machining mechanism appearance, which can be defined as the work material softening, due to temperature growth. Furthermore, CBN materials should work in ultra-finishing conditions i.e. ap = 0.01 mm and fz = 0.01 mm/tooth and the presented results are outside the recommended range.

From the point of view of cutting efficiency, tool life can be expressed as the summed cutting length in the tool's life period LT (Fig. 8). It was found, that cutting speed growth from the vc =100 m/min to the vc =300 m/min caused cutting length LT to increase. In [15]

related to milling of hardened AISI D2 steel (58 HRC) qualitatively similar results are presented. On the other hand, for the CBN cutters the previous observations were confirmed, i.e. for the lower cutting speeds sintered carbides have higher tool life, however in the range of vc > 500 m/min, CBN cutters are significant superior during milling of hardened steel.

500 400 ■j^ 300 .J 200 100

0 100 200 300 400 500 600

vc [m/min]

Fig. 8. Cutting length in tool's life period LT in function of cutting speed vc

Figure 9 and 10 depict the influence of flank wear VBB on the RMS values of cutting forces for the investigated tools. It can be seen, that a clear relation between cutting forces and progressing tool wear is found. This relation is described by the correlation coefficient R2>0.88. The occurrence of this phenomenon is related to the progressing attrition of the tool's flank face during cutting, which in turn induces friction force growth. From the figures 9 and 10 it can be also seen, that flank wear's VBB growth to approx. 0.2 mm, incurs 3 to 6 fold growth of cutting force components. This observation has significant importance, since in industrial applications cutting tools are applied, whose tool wear very often exceeds 0.2 mm. Consequently, it has significant influence on the cutting forces, and thus on some technological effects of cutting process (e.g. surface texture). Therefore, it is relevant to include the tool wear effect in the cutting force model.

100 t-1-

WC, Vc = 300 m/min 80 " fz = 0.1 mm/tooth ap = 0.1 mm

Fp = 472.99 VBB R2 = 0.942

160 140 120 100 E 80

60 40 20 0

CBN, vc = 300 m/min fz = 0.1 mm/tooth ap = 0.1 mm

FfN = 467.6 VBb + 10.1

„2_„„„ "*!"

R = 0.88 ♦

0,05 0,1 0,15 0,2

VB b [mm]

Fig. 10. RMS values of cutting forces as function of tool wear VBB for cubic boron nitride

Figures 11 and 12 show the edge specific coefficients Kie as function of flank wear VBB for the investigated tools. Their values were determined on the basis of equation (3). It was found, that progressing tool wear has significant influence on the edge specific coefficients, independently of applied tool material. As it was mentioned before, these coefficients include the effects of ploughing and rubbing mechanisms on the flank face. Above mentioned mechanisms are also affected by the progressing flank wear, and thus they have direct influence on the edge specific coefficients values.

0,16 0,2 VBB [mm]

Fig. 11. Edge specific coefficients as function of tool wear VBB for sintered carbide

From the figures 11 and 12 it is resulting, that flank wear's growth induce the increase of edge specific coefficients absolute values. The highest intensity of flank wear's effect can be found for the axial Kie and radial Kie specific edge coefficients, for both tooling materials. The negative values of axial edge coefficient Kie results from the applied directions of cutting forces in machine tool coordinates (see Fig. 3).

Fig. 9. RMS values of cutting forces as function of tool wear VBB for sintered carbide

0) -90

Ke - 49,481 VBB + 32,656

Ve -♦

CBN, vc = 300 m/min fz = 0.1 mm/tooth ap = 0.1 mm

0,000 0,050 0,100 0,150 VBB [mm]

Fig. 12. Edge specific coefficients as function of tool wear VBB for cubic boron nitride

4. Conclusions

The investigation revealed, that in case of lower cutting speeds (vc =100 m/min, vc =300 m/min), sintered carbide cutter has significantly higher tool life than that, obtained for the cubic boron nitride. It confirms the fact, that in case of CBN materials one should avoid low cutting speeds, because in this range work material remains hard and brittle, which in turn can cause intense abrasive tool wear or cutting edge chipping of CBN cutter. It was also found, that for the cutting speeds vc = 100 m/min and vc = 300 m/min, independently of tool material, abrasive wear mechanism was dominant. In case of highest investigated cutting speed (vc =500 m/min), abrasive wear occurred in CBN material, whereas in sintered carbide the catastrophic tool chipping was observed.

Tool wear has also significant influence on the cutting forces, and thus on the specific edge coefficients. The progressing attrition of the tool's flank face during cutting induces the growth of friction force, which is related directly to the cutting and edge forces. Therefore, it is relevant to include the tool's wear effect in the cutting force model.

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