T

HP

ZD

a

ARAA

KHPBHSD

1

sititihicAtsA

Hbasnt

1h

Journal of Manufacturing Processes 15 (2013) 467–473

Contents lists available at ScienceDirect

Journal of Manufacturing Processes

j ourna l h o me page: www.elsev ier .com/ locate /manpro

echnical paper

igh speed ball nose end milling of hardened AISI A2 tool steel withCBN and coated carbide tools

hengwen Pu, Anshul Singh ∗

iamond Innovations, Inc., 6325 Huntley Road, Worthington, OH 43085, USA

r t i c l e i n f o

rticle history:eceived 18 May 2013ccepted 31 May 2013vailable online 11 July 2013

a b s t r a c t

High speed machining (HSM) of tool steels in their hardened state is emerging as an attractive approach forthe mold and die industry due to its potential for significant cost savings and productivity improvement.An experimental study was conducted to investigate the tool wear mechanism and surface integrityin high speed ball nose end milling of hardened AISI A2 tool steel using coated tungsten carbide andpolycrystalline cubic boron nitride (PCBN) tools. It is found that coated carbide tools can only be used

eywords:igh speed machining (HSM)CBNall nose end millingardened steelurface integrityie/mold

at low speed (120 m/min) while high content PCBN tools are suitable for HSM range (470 m/min). PCBNtools produce a damage free workpiece with better surface finish and less work hardening. Despite thehigher tool cost, HSM with PCBN tools lead to reduction in both total cost and production time per part.

© 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

. Introduction

High speed machining (HSM) of tool steels in their hardenedtate is emerging as an attractive approach for the mold and diendustry since machining and polishing take up more than 50% ofhe total production time [1]. Significant cost saving and productiv-ty improvement can be achieved through HSM compared with theraditional method where workpieces go through rough machiningn pre-hardened state, electrical discharge machining (EDM) afterardening and finally manual polishing to the required surface fin-

sh [2]. A 67% reduction in machining time and 58% reduction inost was achieved by using the HSM method for manufacturing anISI H13 die compared with the traditional method [2,3]. In addi-

ion, it was reported recently that the HSM approach led to betterurface integrity and improvement of fatigue life in machining ofISI H13 workpieces compared with the traditional method [4].

The definition of HSM varies with the workpiece materials. TheSM range for alloyed steels is about 500 m/min [1]. Coated car-ides are the most common cutting tools used currently for diend mold manufacturers due to its low cost. However, the cuttingpeed used is normally in the range of 100–300 m/min, which does

ot take full advantage of the HSM technique [5]. It is also reportedhat white layers were produced under all testing conditions when

∗ Corresponding author. Tel.: +1 614 438 2132; fax: +1 614 438 2829.E-mail address: anshul.singh@diamondinnovations.com (A. Singh).

526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Publishettp://dx.doi.org/10.1016/j.jmapro.2013.05.005

machining hardened P20 steel (41 HRC) at relatively high speed(301–754 m/min) with TiAlN coated carbide end mills [6].

Polycrystalline cubic boron nitride (PCBN) tools are claimed tobe more suitable for HSM of hardened steels by several researchers[2,4,5]. The cutting speed is in the range of 300–1200 m/mindepending on the workpiece materials. It was reported that 8 timeslonger tool life was achieved by PCBN tools compared with TiAlN-coated carbide when machining hardened D2 steels at 500 m/min[5]. A white layer free surface and compressive residual stresseswere reported in HSM of hardened H13 steels using PCBN tools atthe cutting speed 300 m/min and the fatigue life of the machinedcomponent was increased 30% compared with the traditionalmethod where EDM and manual polishing were employed [4].

Despite the known benefits of PCBN tools, there is still limitedknowledge on the influence of CBN content in the PCBN tools ontool life. Most researchers employed low content PCBN tools (about45 vol.% cubic boron nitride) for milling of hardened steels [4,7].Low content PCBN tools are normally recommended for hard tur-ning of hardened steels due to their good chemical resistance andmay not be suitable for hard milling applications where the cut-ting is more interrupted. Also, there is few studies on the surfaceintegrity generated by PCBN tools in the HSM range, especially forcold work tool steels with hardness >60 HRC. These two prob-lems are addressed in the current study. Ball nose end milling

of hardened A2 cold work steel at 64 HRC is conducted usingboth TiAlN coated carbide tools and PCBN tools with differentCBN content. The tool wear mechanism and surface integrity areinvestigated.

d by Elsevier Ltd. All rights reserved.

468 Z. Pu, A. Singh / Journal of Manufacturing Processes 15 (2013) 467–473

Table 1Chemical composition of AISI A2 cold work steel.

C Cr Mn Mo Si V

1.00% 5.30% 0.60% 1.10% 0.30% 0.20%

Fs

2

AToi

5iii1

Table 2Tool materials used for the milling tests.

No. Tool Material Coating CBN content Binder phase

1 Coated tungsten carbide TiAlN N/A Co2 PCBN A N/A 90% TiN3 PCBN B N/A 55% TiN

ig. 1. (a) Microstructure and (b) XRD phase analysis of the AISI A2 cold work toolteel.

. Experimental setup

The workpiece material used was hardened cold work tool steelISI A2 at 64 HRC. The chemical composition is shown in Table 1.he microstructure and the X-ray diffraction (XRD) phase analysisf the workpiece material are shown in Fig. 1. The major phasesnclude ferrite, untempered martensite and chromium carbides.

The milling tests were conducted on a Mori Seiki NMV5000-Axis high speed milling machine without any coolants. The max-

mum spindle speed is 20,000 RPM. The spindle drive motor power

s 18.5 kW. The workpiece was a plate (160 mm × 50 mm × 7 mm)nclined at 45◦ as shown in Fig. 2. The cutting tools employed were9 mm diameter ball nose end mills. The primary clearance angle

Fig. 2. (a) Experimental setup and (b) schematic of the milling process.

Fig. 3. Cutting edge radius measurement using Wyko NT1100 (a) coated carbidetools (15 �m) and (b) PCBN tools (5 �m).

for the coated carbide tool is 12◦ and 7◦ for the PCBN tools. The rakeangle is 0◦ for all the tools.

Three different tool materials were tested as shown in Table 2.One PVD-coated tungsten carbide grade and two PCBN gradeswith different CBN contents were tested. TiAlN coated carbide wasselected since it represents one of the best carbide tools used in thefield and its tool life was reported to be three times higher than TiCNcoated carbides [1]. The coated carbide tool has two teeth while thePCBN tools have one tooth.

The cutting edge radius of the cutting tools was found to havea critical influence on the tool life in milling applications for bothcoated carbide and PCBN tools. It was reported that honed coatedcarbide tools and upsharp PCBN tools led to the best tool perfor-mance [8,9]. The cutting tools were measured using Wyko NT1100optical profiler before the milling tests. Fig. 3 shows that the PVDcoated carbide tools are slightly honed and the average cutting edgeradius is 15 �m while the PCBN tools have sharper edge of 5 �m.

As shown in Fig. 2(b), due to the inclination of the workpiece andthe geometry of the cutting tool, the effective cutting speed is thelargest at Point A and is the smallest at Point B. For the milling test,the maximum cutting speed, Vmax, is the only variable and all otherconditions are the same as shown in Table 3. Both PCBN and carbidetools were tested at 470 m/min. Only carbide tools were tested at120 m/min due to their poor performance at the high speed.

The maximum flank wear, VBmax, was measured using an opti-cal microscope after the first pass (the tool finishes one total areaof the workpiece plate) and then every two passes. At the same

Table 3Milling conditions.

Axial depth of cut, ap 0.12 mmWidth of cut, ae 0.25 mmFeed per tooth, fz 0.18 mmMaximum cutting speed, Vmax 470 m/min (9500 RPM), 120 m/min

(2423 RPM, carbide only)

Z. Pu, A. Singh / Journal of Manufacturing Processes 15 (2013) 467–473 469

0

0.05

0.1

0.15

0.2

0.25

0.3

0 100 200 300 400

Fla

nk

we

ar

(mm

)PCBN A_470 m/min PCBN B_470 m/min

Coated carbide_470 m/min Coated carbide_120 m/min

F

tulomn

tToh21Nm

3

atc

Fig. 5. SEM pictures of coated carbide tool at 32 m cut length (a) 470 m/min, sec-

Length Cut (m)

ig. 4. Tool wears progression for different tool materials at different speeds.

ime, the surface roughness along the feed direction was measuredsing a portable roughness tester HOMMEL TESTER T500. The cut

ength per pass was 32 m. The tests were stopped after 9 passesf cut, which corresponds to 291 m cut length. Scanning electronicroscope (SEM) was used to observe the tool wear at high mag-

ifications.After the milling tests were completed, samples were cut from

he workpieces using a wire EDM for surface integrity analysis.he 3D surface topography was measured using Wyko NT1100ptical profiler. The surface finish was examined using SEM. Theardness was measured using an Instron Wilson-Wolpert Tukon100B Hardness Tester. Vickers’ indenter was used with a load of

kg. Then the sample was mounted, polished and etched using 2%ital (2% nitric acid + 98% ethanol by volume) solution to reveal theicrostructures near the machined surface.

. Tool wear analysis

Fig. 4 shows the tool wear progression for different tool materi-ls. It clearly shows that the coated carbide tool perform poorly inhe HSM range. The maximum flank wear reached 0.16 mm at 32 mut length at the cutting speed of 470 m/min. As shown in Fig. 5(a),

Fig. 6. SEM pictures of PCBN B at (a) 32 m, (b) 97 m and (c) 162 m c

ondary mode, (b) 470 m/min, backscattered mode, (c) 120 m/min, back scatteredmode and (d) at 291 m, 120 m/min, secondary mode (100× magnification).

a large crater formed on the cutting edge. The backscattered image,Fig. 5(b), shows that the coating was worn out and tungsten carbidesubstrate (bright white portion) was exposed. The flank wear wasreduced to 0.07 mm when the speed was 120 m/min. However, thecoating was already damaged at 32 m cut length due to the abra-sive nature of the workpiece. At the end of test, the wear on thecoated carbide tool was shown in Fig. 5(d). The wear mode is stillcrater wear at the low cutting speed of 120 m/min. It is noted thatthe each tooth on the coated carbide only cut half of the materialsthat PCBN cut due to the difference in number of teeth. Therefore,although the final tool wear after 291 m cut length is almost thesame, the performance of the PCBN tools are much better than thecoated carbide tool.

Both PCBN grades performed much better than the coated car-bide grade even at four times higher cutting speed at 32 m cutlength as shown in Fig. 4. However, the wear on the low CBN con-tent grade, PCBN B, grows significantly faster than the high content

ut length (470 m/min, secondary mode, 100× magnification).

470 Z. Pu, A. Singh / Journal of Manufacturing Processes 15 (2013) 467–473

F ation; (c) 226 m cut length, 100× and (d) 291 m cut length, 100× (470 m/min, secondarym

gatccst

fwiuacaaut

4

r[curcd

utrhatma

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

PCBN A_470 m/min PCBN B_470 m/min Coated carbide_470 m/min Coated carbide_120 m/min

Ra (μ

m)

Ra (32 m) Ra (291 m)

improvement when PCBN A tool was used compared with coatedcarbide tools. Sku is a measure of kurtosis, or the “spikiness” ofthe surface. A value greater than 3 indicates a spiky surface while

Table 43D surface roughness parameters.

ig. 7. SEM pictures of PCBN A at 97 m cut length, (a) 100× and (b) 500× magnificode).

rade, PCBN A. Fig. 6(a) shows that flaking and edge chipping occurst 32 m cut length on PCBN B, which suggest that the toughness ofhe tool is not high enough for the application. More flaking andhipping occurred at 97 m cut length. At 162 m cut length, largehannels formed on the flank of the tool, which suggested that abra-ive wear was dominant and could be caused by the hard phases inhe workpiece material.

At the same cut length of 97 m, the high content PCBN A per-ormed much better than PCBN B as shown in Fig. 7(a). The flankear was reduced to only 23% of PCBN A. At higher magnification,

t was shown that tool materials on the rake side were removedniformly near the cutting edge as shown in Fig. 7(b) which couldlso be caused by the abrasive carbides in the workpiece material. Ahipping on the right side of the flank occurred at 226 m cut lengthnd developed quickly at 291 m cut length. Flaking on the rake sidelso occurred. The failure mode on the PCBN A tool is related tonsatisfactory edge toughness as cutting forces increases due toool wear.

. Surface integrity

Surface finish is a critical factor for the die/mold industry. Theequired Ra for plastic molding is typically smaller than 0.1 �m10]. Manual polishing is required after finish machining, whichomprises 30% of the total time and 46% of the total cost for man-facturing a stamping die [11]. Therefore, reduction of surfaceoughness in the finish machining stage will lead to considerableost savings by reducing manual polishing time and improved pro-uctivity.

Fig. 8 shows the surface roughness on the workpieces machinedsing different tool materials at 32 m and 291 m cut length. It fur-her proves that coated carbide tools are not suitable in the HSMange since it produces the highest Ra value. The coated carbide toolas the same Ra value at 32 m length at 120 m/min cutting speed

s the PCBN A tool has at 470 m/min. However, at 291 m cut length,he surface roughness from the coated carbide tool increases much

ore significantly than PCBN A although their tool wears valuesre close as shown in Fig. 4. This could be caused by higher cutting

Fig. 8. Surface roughness on the workpieces machined using different tool materialsat 32 m and 291 m cut length.

forces generated by the coated carbide tool (larger cutting edgeradius) which led to more vibration. The low content PCBN toolgenerates higher surface roughness values and is as expected sincethe chipping and flaking occurred at 32 m cut length as shown inFig. 6(a).

Fig. 9 shows the 3D topography of the workpieces machinedusing PCBN A and coated carbide tools at 291 m cut length.The numbers in the figure indicate the scanned area size(0.9 mm × 1.2 mm). 3D surface roughness parameters were usedto quantitatively evaluate the surface produced by the two tools asshown in Table 4. The “Improvement” refers to the percentage of

PCBN A (470 m/min) Coated carbide (120 m/min) Improvement

Sa 0.39 �m 0.58 �m 49%Sz 3.24 �m 4.22 �m 30%Sku 2.99 3.45 15%

Z. Pu, A. Singh / Journal of Manufacturing Processes 15 (2013) 467–473 471

Fc

smlsm

rttibsccwu

Fm

bide tool at 120 m/min are shown in Fig. 13. It is noted that thethin white layer (<1 �m) near the edge of the workpiece is causedby the concentration of electrons since there is a gap between the

ig. 9. 3D topography of the workpieces machined using (a) PCBN A and (b) coatedarbide tools at 291 m cut length.

maller than 3 means a bumpy surface. A kurtosis value equal to 3eans a perfectly random surface [12]. The PCBN A tool not only

ead to about 50% reduction in Sa, it also produce a near randomurface which may improve the polishability of die/mold due to aore uniform surface finish.The hardness before and after milling using different tool mate-

ials at 291 m cut length is shown in Fig. 10. The initial hardness ofhe workpiece is 815 HV (64 HRC). Milling using coated carbide ledo a 12% increase to 913 HV (68 HRC) due to the work hardeningnduced during cutting. Much smaller increase (4%) was producedy PCBN tool and the hardness is 848 HV (65 HRC). The cuttingpeed of the PCBN tool was about four times higher than coatedarbide, which led to higher temperature of the workpiece duringutting and softens the workpiece. This is desirable as it causes less

ork hardening on the workpiece. The large hardness increase bysing the coated carbide tool will increase the difficulty for manual

500

550

600

650

700

750

800

850

900

950

Initial PCBN A_470 m/min

Coated carbide_120

m/min

HV

1.0

ig. 10. Hardness of the workpieces before and after milling using different toolaterials at 291 m cut length.

Fig. 11. SEM pictures of the surface machined using coated carbide tool at120 m/min after 291 m cut length (a) 100× and (b) 500×.

polishing and therefore leads to potential increase in the total costfor die/mold manufacturing.

Fig. 11 shows the SEM pictures of the surface machined usingcoated carbide tool at 120 m/min. It is clearly shown that the mate-rials on the workpiece were dragged on the surface and cracks weregenerated. Much less damage on the surface was produced on theworkpiece using the PCBN tool even at four times’ higher speed asshown in Fig. 12. The dragged materials will increase the difficultyof manual polishing while the more uniform surface generated byusing the PCBN tool is easier to polish which may lead to potentialcost savings.

The microstructures of the surface machined using coated car-

Fig. 12. SEM pictures of the surface machined using PCBN A tool at 470 m/min after291 m cut length (a) 100× and (b) 500×.

472 Z. Pu, A. Singh / Journal of Manufacturing Processes 15 (2013) 467–473

F1

mitsefaPpmh

F(

ig. 13. Microstructures of the surface machined using coated carbide tool at20 m/min (a) 1000× and (b) 5000×.

ounting material and the sample. It is not the “white layer” whichs commonly used in literature for untempered martensite forma-ion after machining of steels [13]. The microstructures are stillimilar to the initial microstructure as shown in Fig. 1(a). How-ver, grain elongation and distortion of the grain boundaries wereound near the surface machined using the coated carbide tools shown in Fig. 13(b). These changes did not occur when theCBN tool was used as shown in Fig. 14. These findings further

rove that milling using coated carbide tool at 120 m/min inducesore ploughing effects on the workpiece surface which leads to

igh surface roughness, large hardness increase, dragging of the

ig. 14. Microstructures of the surface machined using PCBN A tool at 470 m/mina) 1000× and (b) 5000×.

Fig. 15. Cost per part comparison between PCBN and coated carbide tools.

material, crack initiation and microstructural changes. All thesechanges have adverse effects on the performance of die/mold andalso increase the total cost by increasing the difficulty for manualpolishing.

5. Cost analysis

The higher cost of PCBN tools compared to coated carbide isa major economic consideration. This study compares total vari-able manufacturing cost models for finished workpieces producedusing both tool materials. It is assumed that the total cut lengthfor certain die/mold is 291 m and, conservatively, that a new toolis needed for each part; both PCBN and coated carbide tools havetwo teeth; the PCBN tool is used at the HSM range (480 m/min)and the coated carbide at the normal range (120 m/min), whichlead to four times higher material removal rate for the PCBN tool.The machine/labor/polishing costs are estimated from literature[11,14]. The polishing time for the part machined by PCBN tool isassumed to be 20% less due to the findings from this study thatthe surface roughness is significantly reduced and the hardnessincrease is smaller when the HSM technique is used. As shown inTable 5 and Fig. 15, despite the five times higher cost for PCBN tools,the total cost per finished part is reduced by 11%. The total time perpart is also reduced more than 40% from 16 h to only 9.5 h.

Two significant further savings may be obtained by using PCBNmilling tools. The PCBN tool’s higher productivity will decrease thesignificant investments for milling machines. Secondly, regrinding

the PCBN tools seems feasible and will offset a portion of the initialtool cost. The coated carbide tools cannot be reground due to thedamaged coatings.

Table 5Cost per part comparison between PCBN and coated carbide tools.

PCBN Coated Carbide

Tool cost per insert ($) 250 50Material removal rate (mm3/min) 102 25Machining time per part (h) 1.5 6Machining cost per hour ($) 40Total machining cost per part ($) 60 240Polishing time (h) 8 10Polishing cost per hour ($) 60Total polishing cost ($) 480 600Total cost ($) 790 890Total time (h) 9.5 16

acturi

6

whtrtatttf

tPicfian

tcPaats

R

[

[

[

[

[

products application development team focused on developing new applications ofpolycrystalline products. Before joining Diamond Innovations, he worked at Chro-malloy focused on development of high temperature coatings. Anshul has 11 years

Z. Pu, A. Singh / Journal of Manuf

. Conclusions

An experimental study was conducted to investigate the toolear mechanism and surface integrity in ball nose end milling ofardened AISI A2 tool steel using coated tungsten carbide and PCBNools. It is found that coated carbide tools cannot be used at the HSMange. Significant crater wear developed after short cut length andhe surface roughness is high. High content PCBN tools are suit-ble for HSM of hardened A2 steel. No chemical wear was found onhe PCBN tool. The failure mode is chipping/flaking due to lack ofoughness when the cutting edge is worn out. Low content PCBNool does not have the required toughness and abrasion resistanceor the application.

At the normal cutting speed of 120 m/min, the coated carbideool produces two times higher surface roughness compared withCBN tool at 470 m/min. It also led to remarkable work harden-ng on the workpiece, increased hardness, material drag, surfaceracks and microstructural changes. These changes increase the dif-culty for the following manual polishing process as well potentialdverse effects on the functional performance of die/mold compo-ents.

Although the cost for the PCBN tools are about 5 times higherhan coated carbide tools, the cost analysis shows that the totalost per part is reduced by 11% and the total time by 40% whenCBN tools are used due to the much higher material removal ratend reduction of polishing time. Therefore, high content PCBN toolsre suitable for HSM of hardened tool steels and have many advan-ages compared with coated carbide tools including significant costavings and productivity improvement.

eferences

[1] Fallbohmer P, Rodriguez CA, Ozel T, Altan T. High-speed machining of cast ironand alloy steels for die and mold manufacturing. Journal of Materials ProcessingTechnology 2000;98:104–15.

[2] Dewes RC, Aspinwall DK. A review of ultra high speed milling of hardenedsteels. Journal of Materials Processing Technology 1997;69:1–17.

[3] Aspinwal DK, Dewes RC. EPSRC case for support-ultra high speed machining ofhardened ferrous alloys. Birmingham, United Kingdom: School of MechanicalEngineering, University of Birmingham; 1994.

ng Processes 15 (2013) 467–473 473

[4] Axinte D, Dewes R. High-speed milling of AISI H13 hot-work tool steel usingpolycrystalline cubic boron nitride ball-nose mills: from experimental investi-gations and empirical modelling to functional testing of the machined surfaces.Proceedings of the Institution of Mechanical Engineers Part B: Journal of Engi-neering Manufacture 2010;224(1):15–24.

[5] Wojciechowski S, Twardowski P. Tool life and process dynamics in high speedball end milling of hardened steel. Procedia CIRP 2012;1:289–94.

[6] Pang JZ, Wang MJ, Duan CZ. White layer and surface roughness in high speedmilling of P20 steel. Advanced Materials Research 2007:45–54, 24–25.

[7] Koshy P, Dewes RC, Aspinwall DK. High speed end milling of hardenedAISI D2 tool steel (58 HRC). Journal of Materials Processing Technology2002;127:266–73.

[8] Bouzakis K-D, Michailidis N, Skordaris G, Kombogiannis S, Hadjiyiannis S, Efs-tathiou K, Erkens G, Rambadt S, Wirth I. Effect of the cutting edge radius and itsmanufacturing procedure, on the milling performance of PVD coated cementedcarbide inserts. CIRP Annals – Manufacturing Technology 2002;51:61–4.

[9] Elbestawi MA, Chen L, Becze CE, El-Wardany TI. High-speed milling of diesand molds in their hardened state. CIRP Annals – Manufacturing Technology1997;46(1):57–62.

10] Lee Hae Sung, Park Min Soo, Kim Min Tae, Chu Chong Nam. Systematic finish-ing of dies and moulds. International Journal of Machine Tools & Manufacture2006;46:1027–34.

11] Lo’pez de Lacalle Luis N, Lamikiz A, Sa’nchez JA, Arana JL. Improving the surfacefinish in high speed milling of stamping dies. Journal of Materials ProcessingTechnology 2002;123:292–302.

12] Veeco Company Product Manual, Surface Measurement Parameters for Wyko®Optical Profilers.

13] Umbrello D, Pu Z, Caruso S, Outeiro JC, Jayal AD, Dillon OW, Jawahir IS. Theeffects of cryogenic cooling on surface integrity in hard machining. ProcediaEngineering 2012;19:371–6.

14] Franci Pusavec, Janez Kopac. Sustainability assessment: cryogenic machiningof inconel 718. Journal of Mechanical Engineering 2011;57:637–47.

Zhengwen Pu is an Application Development Engineer at Diamond Innovationsfocused on developing machining solutions using PCBN and PCD products andproviding tool wear mechanism analysis to product development team as wellas customers. He holds a PhD in Mechanical Engineering from University of Ken-tucky. His research focus was on surface integrity in machining and its influence onfunctional performance of machined products.

Anshul Singh is a senior Product Development Engineer at Diamond Innovationsfocused on development of new PCD and PCBN products. He also leads the machining

of experience in materials research and holds a MS in Materials Science and Engi-neering and BS in Metallurgical Engineering.