High Precision Cutting of Hardened Steel with Newly Developed PCBN Tools Tomohiro Fukaya, Yasuyuki Kanada, Takayoshi Wakabayashi, Junichi Shiraishi, Kunihiro Tomita, Tetsuo Nakai SUMITOMO ELECTRIC INDUSTRIES, LTD. 1-1-1, Koya-Kita, Itami, Hyogo,664-0016, Japan Tel: + 81-727-71-0689 Fax: +81-727-72-1545 Email: [email protected] ABSTRACT: Consequently, PCBN (Polycrystalline Cubic Boron Nitride) compacts has been used as tools for cutting hardened steel parts in various industries. The demand for PCBN tools is growing rapidly due to the trend of replacing grinding processes with cutting to reduce machining cost, energy consumption and machining waste. Although the trend of processing is to use more cutting, there still remains grinding in many processing fields. High precision machining has been such field. In this study it has been verified that a novel PCBN tools can be applied to high precision machining with the surface roughness Rz1.6 µ m . Thus cutting has become applicable to those machining and the trend of replacement of grinding with cutting will be enhanced. In this report, these newly developed PCBN tools will be introduced.

KEYWORDS: Cubic boron nitride, Hard turning, High precision

1.Introduction Just a few decades ago it was believed that grinding was the only method to industrially process hardened steels and it was impossible to process them commercially by cutting. In 1977 the first PCBN tools was developed in Japan which successfully replaced grinding processes with cutting processes for the finish machining of hardened steel products such as automotive parts and machine parts [TAB 78]. Since then, PCBN cutting tools have been widely applied for machining hardened steels and manufacturers in various fields such as automobiles, aircraft, and so on have been benefited by improving productivity. This benefit was due to the excellent properties of PCBN tools such as high hardness, high thermal conductivity and chemical inertness to ferrous materials, resulting in longer tool life, increased work piece precision, and faster processing time. Up to now several kinds

IT6•@•@•@ IT7

Tolerance (class)

of PCBN materials have been developed to machine work pieces which were thought to be difficult to be cut before [SHI 97][KUK 99]. The demand for PCBN cutting tools is steadily growing because cutting has great advantage over grinding in efficiency and flexibility of machining. Moreover the recent concern over global environment makes cutting more advantageous because it was reported by Yoshimura et al. that cutting consumes less Cutting energy per amount of with newly developed PCBN Tools machining and creates less industrial waste Cutting [YOS 98]. with conventional The trend of PCBN Tools manufacturing process is towards cutting from grinding, but still there remain some Grinding processes where cutting cannot be applied. High 0.8• @@ •@ •@ •@ •@ •1.6• @@ •@ •@ •@ •@ •3.2• @@ •@ •@ •@ •@ •6.3 precision machining Surface Roughness Rz (µm) have been such applications. The Fig. 1 Applicable area of newly developed PCBN tools problem of cutting for those applications exists in the unreliability of tool life, which can be found as the deterioration of work surface against precision machining. To solve this problem, a novel PCBN were invented and are introduced in this paper. 2. High precision hard turning Until recently it was generally thought that hard turning with PCBN tools could be applied when the surface roughness was allowed over Rz 6.3µ m. When it comes to the surface roughness of Rz 3.2µ m and smoother, it was believed that only grinding could achieve this level. As mentioned before; however, the trend of machining is toward cutting from grinding because of the necessity for improving productivity as well as the concern over environmental factors. Thus high precision machining has been a challenging task for turning. To respond to such demand, the new PCBN tools were developed. Fig. 1 shows the applicable area with new PCBN tools. This PCBN tools can be applied practically in the cutting of hardened steel parts such as gears and shafts which requires Rz 1.6-3.2 m surface roughness.

3. Improving the surface roughness with newly developed PCBN tools 3.1 Difficulties in High precision Turning There are two major factors which make a rough work piece surface in hard turning as shown in Figure 2; 1) the peak to valley feed mark which is the track of the tool edge, and 2) an waviness due to the rigidity and accuracy of the machine, tool, and chuck. When performing high precision turning of a hardened steel with PCBN tools, the primary factor of the surface roughening is the enlargement of the peak to valley feed mark due to Table 1 PCBN tool properties the notch wear development. Hardness HV TRS GPa Thus in order to obtain a good 1.0 - 1.1 PCBN-A 3,200 ~ 3,400 surface finish, the notch wear development must be controlled 0.8 - 0.9 PCBN-B 2,800 ~ 3,200 within a certain level. To control 1.0 - 1.1 PCBN-C 3,200 ~ 3,400 this notch wear, new PCBN tool was developed. 3.2 Experimental Procedure Experimental cutting was performed to verify the performance of newly developed PCBN tools above mentioned. Table 1 is the list of tool properties. “PCBN-A” is the newly developed PCBN tools. To compare the performance, Fig. 3 The newly developed PCBN tools conventional PCBN tools “PCBNB” and “PCBN-C” were also tested. Fig. 3 shows the newly developed Rmax PCBN tools, which has W o r k : 3 4 C r M o 4 H RC 58-62 a coated layer on the Insert :2NU-CNMA120408 Waviness PCBN substrate. Peak toCValley o n d i tFeed i o n smark : V=160m/min. d=0.1m m f = 0 . 0 8 m m / r e v . W E T Rigidity, Accuracy Conventional PCBN Shape of Cutting edge of machine and chuck tools are non-coated inserts. Fig. 4 shows Notch wear: Small Notch wear: Large Insert the test conditions. PCBN Tool face Work material is a Work carburized steel with Feed Direction Machined HRC58-62 hardness. Surface The test was carried out Chuck Fig. 2 Factors causing surface roughness in continuous cutting, Fig. 4 Test conditions of newly developed PCBN tools and the surface roughness and flank

wear width were measured. 3.3 Results and discussion Fig. 5 shows the test results, surface roughness and flank wear width. Although the wear resistance of PCBN-B and PCBN-C are better than that of PCBN-A, PCBN-A shows the most stable surface roughness smoother than Rz 2 µ m. Fig. 6 shows the cutting edge SEM images at the cutting length 3km. Compared to the 0.20 PCBN-B 3

PCBN-C

2 PCBN-A 1

0

0

2 4 Cutting Length (km)

Flank Wear Width VB (mm)

Surface Roughness Rz (µm)

4

6

PCBN-A 0.15

PCBN-C

0.10

PCBN-B

0.05

0

0

2 4 Cutting Length (km)

Fig. 5 Cutting test Results (surface roughness and flank wear width)

PCBN-A Fig. 6 Cutting edge SEM images

PCBN-C

6

cutting edge of PCBN-C, which the notch wear developed, the cutting edge of PCBN-A worn smoothly. So PCBN-A shows good surface roughness. The cutting edge of PCBN-A consists of the coated layer, which is homogeneous, while those of PCBN-B and PCBN-C consist of PCBN sintered compacts made from CBN and binder materials particles. This is considered to be the reason why PCBN-A wears smoothly and shows good surface roughness. 4.”Altering feed rate” cutting with newly developed PCBN tool 4.1 “Altering feed rate” cutting To control the notch wear and get a better surface roughness, “Altering feed rate” cutting method was reported [YOS 99]. Fig, 7 show the effects of this cutting method. With this cutting method, the cutting edge smoothly wear, so the surface roughness is improved. 4.2 Experimental procedure To examine the effects of this cutting method to newly developed PCBN tool, cutting test was carried out. New “PCBN-A” and conventional “PCBN-B” in table 1 were tested. Fig. 8 shows the test conditions. Work material is also the carburized steel with HRC58-62. The Constant Feed Rate Altering Feed Rate test was carried out in the Last continuous cutting, and the Cutting Edge f f surface roughness was measured. 4.3 Results and discussion Stationary Notch Location

Shifting Notch Location

Fig. 9 shows “altering Notch wear reduction feed rate” cutting test Notch wear developing (balanced) results. The surface roughness of PCBN-A, new Surface Roughness Improved Bad Surface Roughness coated PCBN insert, is Fig.7 “Altering feed rate” cutting method better than that of PCBN-B, conventional non-coated PCBN insert. The surface roughness of PCBN-A is smoother than Rz1.6µ m.

“Altering feed rate” cutting method has an effect not only conventional non-coated PCBN insert but also new coated PCBN insert. PCBN-A is considered to be applicable to the cutting of hardened steel parts which requires Rz1.6µ m surface roughness with this cutting method.

work : 34CrMo4 HRC58-62 Insert : PCBNA , PCBNB 2NU-CNMA120408 V : 180 m/min. f : 0.04,0.05,0.06 mm/rev. Changing f with every 4pcs.

d : 0.1 mm Coolant : WET 30

5. Application Examples φ70

Surface Roughness Rz (µm)

φ40 Fig. 10 and Fig.11 show the application examples of these newly developed PCBN tools. In these applications, the work is automobile parts. Fig. 8 Test conditions of “altering feed rate” cut Required accuracy is Rz3.2µ m surface roughness and IT6 class 16µ m tolerance. PCBN-A makes the practical 3 use of such high precision cutting of hardened steel possible with the high rigidity and high PCBN-B accuracy lathe 2 In the first example, Fig. 10, the work material is induction hardened high carbon steel. The 1 criteria is surface roughness. The PCBN-A tool life of new developed PCBN tools, PCBN-A, is more than 0 0 5 10 20km in cutting length, while that Cutting Length (km) of conventional PCBN tools, PCBN-C, is about 5km in cutting Fig. 9 Test results of “altering feed rate” cut length. With PCBN-A, the tolerance is still good when the cutting length is 20 km even in dry cutting condition.

In the second example, Fig. 11, the work material is carburized steel. The criteria is also surface roughness. The tool life of new developed PCBN tools, PCBN-A, is more than 10km in cutting length, while that of conventional PCBN tools, PCBN-C, is about 4km in cutting length. In this application, to get a better tolerance, wet condition is adopted.

Surface Roughness Rz (µm)

5.Conclusion

4 PCBN-C

3 2 PCBN-A

1 0

0

10 20 Cutting Length (km)

Diameter: 50mm Surface roughness: Rz 3.2µm Tolerance: 16µm (IT6 class) Work : C50 HRC58-62 Insert : PCBN-A, PCBN-C 2NU-CNMA120408 V : 190 m/min. f : 0.08 mm/rev. d : 0.1 mm Coolant : DRY

Surface Roughness Rz (µm)

Fig. 10 Application example of newly developed PCBN tools 1

4 Diameter: 40mm Surface roughness: Rz 3.2µm Tolerance: 16µm (IT6 class)

PCBN-C

3 2 PCBN-A

1 0

0

5 Cutting Length (km)

10

Work : 34CrMo4 HRC58-62 Insert : PCBN-A,PCBN-C 2NU-CNMA120408 V : 180 m/min. f : 0.1 mm/rev. d : 0.1 mm Coolant : WET

Fig. 11 Application example of newly developed PCBN tools 2

In this article, newly developed PCBN tools have been introduced and been explained using actual cutting data. The newly developed PCBN tools makes high precision hard turning commercially possible. High precision machining of hardened steel used to be done with grinding and thus cutting was thought to be an inappropriate method. The newly developed PCBN tools explained here have changed this situation. They can be applied to such processing formerly inappropriate. The application area of cutting, which has the advantage over grinding, becomes wider and newly developed PCBN tools will enhance the trend of replacing grinding with cutting. In conclusion 1. Newly developed PCBN tools with coated layer shows good surface roughness and it is applicable for high precision hard turning which requires Rz 3.2µ m surface roughness. 2. Newly developed PCBN tools can achieve Rz 1.6µ m surface roughness with “altering feed rate” cutting method. 6.References [TAB 78] TABUCHI, N., HARA, A., YAZU, S., KONO, Y., ASAI, K., TSUJI, K., NAKATANI, S., UCHIDA, T., MORI, Y., “Performance of SUMIBORON BN200”, Sumitomo Electric, No.18, pp.57-65, 1978. [SHI 97] SHIRAISHI, J., FUKAYA, T., OGATA, Y., NAKAI, T., “Hard turning with SUMIBORON BNX10”, 1st French and German Conference on High Speed Machining, 1997. [KUK 99] KUKINO, S., KANADA, Y., FUKAYA, T., SHIRAISHI, J., TOMITA, K., NAKAI, T., “Development of a new grade of SUMIBORON BNX25 and new NS-style oneuse inserts for high-speed interrupted cutting of hardened steel”, SEI TECHNICAL REVIEW, No. 48, pp.49-52, 1999. [YOS 98] YOSHIMURA, H., ASAKAWA, N., SATO, M., “The Current Status and the Problems of Diamond and CBN Tools in Automobile Industries”, Proceedings of Industrial Diamond association 50th Commemorative Seminar, pp.6-1-6-8, 1998. [YOS 99] YOSHIDA, K., KUKINO, S., HARADA, T., FUKAYA, T., SHIRAISHI, J., NAKAI, T., Manufacturing science and engineering-1999 ASME 1999, MED-vol.10, pp.825830, 1999.