Fundamental Study on Chip Formation Mechanism in New PCD Ball-End Mill

(4th Report) Analysis of Chip and Cut Surface Formation Mechanisms

新しい PCDエンドミルにおける切りくず生成に関する基礎的研究

（第 4報）切りくず生成および切削面形成機構の解析

日本工業大学 神雅彦, 〇カスリヤピラポン 日進工具 渡辺 健志，後藤 隆司

1. Introduction Surface finish is one of the essential control parameters to ensure that

the functional surface of manufactured parts confirms to the specified

standards. Also, it can significantly affects friction, wear, fatigue, and

corrosions. Generally, in the mold manufacturing process, the mirror-like

surface finishing process is needed very long and high-cost by second

operation, such as manual grinding and polishing process. Especially the

mold has a small and complex shape that the polishing tool cannot be

access. In order to solve them, the mirror finishing process by using the

PCD ball-end mill on the CNC machining center is studied. The

distinctive feature of the PCD ball-end mill is simple spherical shape and

smooth surface with a high concentration of diamond particles. So, it is

achieved in nanometer scale of surface roughness. In this study, we

experimentally investigated the chip formation mechanism under

demonstration of the cutting a hardened stainless steel. Furthermore, the

crystal structure of the cut surface was investigated by using the scanning

ion microscope SIM. As a result, the work-affected layer by the PCD ball-

end mill was strongly distorted it is clarified.

2. Feature of PCD ball end mill In the shaping process of PCD tool is generally applied to WDEM

process. However, the typical surface processed by WEDM is covered

with a transformed layer caused by EDM, which including carbide and

oxide from diamond. The WDEM processed surface is rough with deep

crater generation and it not suitable for a high-precision tool with ductile-

mode machining. Hence, we have developed the surface finishing by

diamond grinding wheel, which is completely removed EDM machined

layer from the surface of ball-end mill as shown in Fig 1 (a) and (b). A

distinctive feature of the PCD ball-end mill has a simple spherical shape

without the conventional sharp-cutting edge. And it is a smooth surface

but includes several concave pits that appeared owing to the lacking

cobalt binder at the boundary between diamond particles as shown in Fig

1 (c) and (d). We inferred the consecutive of diamond particles are

importance controllable the surface within the plastic deformation zone

in a crack-free condition.

3. Experimental setup and cutting conditions The experiments was performed on hardened stainless steel for small

and medium size of molding part with AISI 420 grade modified and

commercially called Stavax ESR with hardness of 54 HRC, The

experiment was carried out an ultra-precision machining center (Toshiba

UVM-450D). Positioning accuracies is less than 10 nm with the high-

rotation speed of air spindle up to 60,000 rpm. The workpiece size is

20(x) x 20(Y) x 6(z) mm. It is mounted with the inclined fixture of 45°.

The experiments was carried out by the consecutive process as shown in

Fig. 2. There are: roughing by using Tungsten carbide (WC) ball-end

mill, semi-finishing by using cubic boron nitride (cBN) ball-end mill and

the mirror finishing by using the PCD ball-end mill under the

experimental conditions as shown in Table. 1

4. Experimental result and discussion The results can see a clear mirror surface from reflective of a print

image on the finished surface using the spherical PCD ball-end mill, as

shown in Fig. 3. Therefore, a mirror surface has been completed in

finishing operation on the CNC machining center without the polishing

process. With the surface roughness is measured. Ra and Rz values of the

surface roughness are reduced in a consecutive process: in roughing using

WC ball-end mill with Ra: 0.109µm/Rz: 0.569µm to semi-finishing using

cBN ball-end mill with Ra: 0.076µm/Rz: 0.369µm and a mirror finished

surface using PCD ball-end mill with Ra: 0.010µm/Rz: 0.048µm

respectively.

Fig. 1 SEM images of a spherical PCD tool with ground surface

Fig. 2 The consecutive of cutting process

Table 1. Experimental conditions.

Cutting tool Tool

radius

Spindle

speed

Feed rate

(f)

Depth of

cutting

Cutting

types

mm min-1 mm/min apxae (µm)

WC ball-end mill 0.5 40,000 1000 50x50 Down-cut

cBN ball-end mill 0.5 40,000 1000 20x20 Down-cut

PCD tool 0.5 40,000 500 5x5 Up-cut

Coolant : oil (flood)

In the case of the segmented chip-shape is generated by the PCD ball-

end mill as shown in Fig 4 (a) and (b) that thin plat-like the segmented

chip has a width of approximately 5-10 µm different from common flow

type chip and resembles strongly compressed was clarified.

It can clarify the chip formation of the PCD ball-end mill in the

consecutive process. The compression force removed the chips with the

plowing phenomenon occurs on the cutting surface. This phenomenon is

the plastic deformation, and it will flow sideward the cutting paths are

overlapped like segmented chip shape by the next cutting track, as shown

in Fig. 4 (c).

Magnified view Whole view

200µm 20µm

(a) (b)

10µm

(c) Diamond particles

Cobalt

2µm

(d)

Magnified view of surface finish Magnified view of diamond particles

2020 年度精密工学会春季大会学術講演会講演論文集

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Figure. 6 shows the scanning ion microscope (SIM) image of the

metal structures on the cross-sections photograph under the cutting

surface in each cutting operation. The observation zone will be classified

in A and B zones, as shown in Fig. 5. In this case, in the cutting with

tungsten carbide ball-end mill, the metal-structure has not changed due to

it has been removed into chips by sharp cutting-edge, as shown in Fig.

6(a3, b3). In contrast, in the cutting using cBN with the negative cutting

edge. The figure indicates the material flow layer is occurred due to

plastic deformation existed near the region of the cutting surface and

sheared leftward, as shown in Fig. 6 (a2, b2). Finally, the cutting of the

PCD ball-end mill, the figure demonstrates the regions where grains

strongly distorted by compressive of PCD ball-end mill. The grains were

crushed, and the plastic flowing leftward corresponds to the up cutting

direction, as shown in Fig. 6(a1 and b1), it can be clarified that is the

work-hardening by using a new PCD ball end mill.

Fig. 3 The cut surfaces by consecutive process on CNC machining

Fig. 4 schematic of the chip formation of a new PCD ball end mill

(a)removed chips (b)magnified of removed chip,

and (c)chip formation mechanism

Fig. 5 observation zone of the SIM analysis

Fig. 6 The cross-section view of crystal structure, along feed direction

(A) and cross-section with feed direction (B)

5. Conclusion We have developed a new spherical polycrystalline diamond (PCD) ball

end mill for complete the mirror-finished surface on the small mold and

die on the CNC machining center. The following results were obtained.

• In the case of cutting part of the PCD ball-end mill that relatively

uniform diamond particles and hardness of itself, also continuous series

of diamond particles are importance to control plastic deformation zone

and chip formation.

• In the consecutive process by automating tool change on the CNC

machine tool, the mirror surface finish on stainless mold steel has been

achieved with 10 nm Ra and 48nm Rz without the polishing process.

•In the cutting of the spherical PCD tool, as the result demonstrates the

regions where grains strongly distorted by compressive of the PCD ball-

end mill. It can be clarified that is the work-affected layer.

•The chip formation of the PCD ball-end mill, the compression stress and

the plowing phenomenon is the main of the chip formation mechanism.

As a result of the removed chip shape is generated in the consecutive

process that is a thin plate-like the segmented chip has a width of

approximately 5-10 µm different from common flow type chip and

resembles strongly compressed was clarified.

6. References 1) L. Grandguillaumea, S. Lavernhea, Y. Quinsata, C. Tournier, Mold

manufacturing optimization: a global approach of milling and polishing

processes, Procidia CIRP 31, 2015, p 13-18

2) F.J. Shiiou and J. T. Chiu, surface finishing of plastic injection mold steel with

ball burnishing and spherical polishing processes on machining center,

Materials Science Forum Vols 505-507, 2006, p.799-804.

3) Z. Zhang, H. Peng, and J. Yan, “Micro-cutting characteristics of EDM

fabricated high-precision polycrystalline diamond tools,” Int. J. Mach. Tools

Manuf., vol. 65, 2013, pp. 99–106.

4) F. Klocke, Manufacturing process 2 , grinding, horonning, lapping, Springer,

e-ISBN 978-3-540-92259-9, 2009, pp8-9.

Ra:0.109μm/Rz:0.569μm

Ra:0.076μm/Rz:0.369μm

Ra:0.010μm/Rz:0.048μm

WC

cBN

PCD

FIB milling

(A3) (B3)

45µm

(A2) (A1)

(B2) (B1)

2µm 2µm 2µm

Feed direction

(A1) WC (A2) cBN (A3) PCD

Cutting surface

2µm 2µm

(B1) WC (B2) cBN (B3) PCD

Plastic deformation

Removed chip (RC) 1

(RC) 2 (RC) 1

Vt

(a)

(b)

2µm

(c)

2020 年度精密工学会春季大会学術講演会講演論文集

Copyright Ⓒ 2020 JSPE

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