OSG’s latest carbide ball end mill series for high-hardness steel applications

Oji Kawaguchi, OSG Corporation Applications Engineer (End Mill Development Division)

In recent years, the die and mold industry has been demanding for shorter production time than ever before. Distortion caused by heat treatment and time reduction of heat treatment are being frown upon, prompting a greater call for capabilities to directly mill high-hardness steels. In addition, requirement for higher precision is also increasing. In order to fulfill the current industry standard, OSG has recently developed three new products to accommodate these needs.

To complete a mold with high precision, not only is the accuracy of the cutting tool critical, but also is the longevity of the tool. Even if a cutting tool is made with high precision in a new state, if it lacks durability, wear will progress early, the contour of the tool will collapse, dimensions may change, and the accuracy of the mold will decrease. To resolve these challenges, OSG is pleased to introduce three new ball end mills, the AE-BM-H, AE-BD-H, and AE-LNBD-H, with capabilities to contribute to faster mold production and higher precision.

The AE-BM-H, AE-BD-H, and AE-LNBD-H are coated with OSG’s innovative new coating “DUROREY” engineered for hardened steels, which was released in April 2019. As depicted in figure 1, the DUROREY coating is composed of a super-fine nano-periodic laminated structure that has an excellent wear and heat resistance layer containing silicon carbide (SiC) on the surface layer and exhibits excellent toughness underneath. As a result, even with work materials exceeding 60 HRC, chipping resistance is much greater than the conventional hardened steel coating, and tool life can be significantly prolonged.

Figure 1. Properties of DUROREY coating

AE-BM-H: 4-flute High-efficiency Carbide Ball End Mill

The AE-BM-H is a 4-flute carbide ball end mill designed based on the concept of high-efficiency and stable processing of high-hardness steels. It is most suitable for forging dies, press dies and die-cast dies. The AE-BM-H features a sharp spiral curve for reducing cutting resistance compared to conventional specifications. In addition, the AE-BM-H’s unequal flute spacing geometry enables greater control of harmonic vibration commonly generated during milling with multiple flutes in hardened steels, thereby allowing higher efficiency milling. Figure 2 illustrates a comparison of cutting force. Amount of edge contact with the workpiece is at its highest at the corner region. With the conventional product, cutting resistance spikes at the corner, whereas that of the AE-BM-H increases only slightly.

Figure 2. Effects of sharp spiral curve and unequal flute spacing enable stable milling with low resistance

Next, figure 3 demonstrates tool durability comparison in high-hardness steel milling. As depicted below, the AE-BM-H is able to achieve stable, high-efficiency and long tool life even in materials with a high degree of cutting resistance such as SKH51 (65 HRC).

Figure 3. Tool durability comparison in high-hardness steel milling

Processing time can be significantly reduced with the AE-BM-H’s capability to enable high-efficiency machining of high-hardness steels, which is difficult to achieve with conventional products. Manufacturers who are currently performing roughing and semi-finishing with a 2-flute ball end mill can particularly benefit from this 4-flute end mill to experience much improvement in machining efficiency and stability. The AE-BM-H is available from R1 to R6, with a total of eight items.

AE-BD-H: 2-flute High-precision Finishing Carbide Ball End Mill

The AE-BD-H is not just a change of coating from the conventional product. The shape of the cutting edge and the carbide base material have been completely reviewed and revamped with special attention to the machining surface accuracy. By adopting a variable negative spiral gash and a thick center core geometry, the cutting edge of the ball tip is less likely to be deformed during machining. By optimally utilizing the outer peripheral cutting edge, the tool is able to make better contact with the workpiece to improve processing quality. 

Moreover, the helix angle is also different from the conventional product. The conventional product adopts a 30-degree helix angle while the AE-BD-H adopts a 25-degree angle. By reducing the helix angle, the strength of the cutting edge is improved and the reliability of the cutting edge in machining hardened materials is increased. The R accuracy is +/- 5 μm (1 / 1,000,000 in micro size) for all sizes from R0.5 to R6. The AE-BD-H’s superior ball R precision ensures a stable radius accuracy across 180-degree as illustrated in figure 4.

Figure 4. Superior ball R precision

Furthermore, the AE-BD-H is engineered with a superior shank accuracy that supports h4 tolerance, with specifications that surpass those of conventional products. Its short shank type lineup help minimize tool protrusion, enabling stable machining. Last but not least, the AE-BD-H and AE-LNBD-H are processed with a smooth surface treatment to remove hard foreign substances called droplets (excluding AE-LNBD-H that are R0.25 or under). The surface quality has been improved significantly as depicted in figure 5.

Figure 5. Smooth surface treatment

Figure 6 shows the excellent durability of the AE-BD-H versus two other competitors in high-hardness steel SKD11 (60 HRC). 

Figure 6. Long tool life in SKD11 (60 HRC)

As described above, the AE-BD-H is a high-precision finishing ball end mill that is ideal for machining environments where the overhang length can be suppressed, such as in 5-axis machining, and for machining with a relatively short overhang length. The AE-BD-H is available from R0.5 to R6, with a total of 17 items.

AE-LNBD-H: 2-flute High-precision Finishing Long Neck Carbide Ball End Mill

Although the AE-LNBD-H is positioned as a long neck type version of the AE-BD-H, it does not only offer extended neck length, but is also capable of achieving good surface finish with excellent accuracy even in unstable long overhang length machining environment. It is a product that has been optimized specifically for long overhang length with exceptional precision. The R accuracy is +/-3 μm for R0.25 or less, and +/-5 μm for all items exceeding R0.25. For R0.25 or less, higher accuracy is achieved compared to conventional products. In addition, as with the AE-BD-H, high precision machining is made possible with the tool’s superior ball R precision across 180-degree and the superior shank accuracy construction that supports h4 tolerance.

Bending of the tool is prong to occur with the long neck configuration, which is the biggest challenge in developing a long neck tool. If the tool is bent, it will not be finished to the intended dimensions and will directly affect accuracy of the mold. The AE-LNBD-H uses a cemented carbide base material different from the AE-BD-H, which requires a short protrusion length. In order to minimize deflection, the AE-LNBD-H features a thick center core to help prevent deformation of the ball tip to improve chipping control. In addition, the adoption of a teardrop-shaped outer periphery strong back taper geometry enables milling by point, which prevents chattering and chipping, resulting in improvement of surface accuracy (see figure 7; excludes R2 or above).

Figure 7. Teardrop-shaped outer periphery

The AE-BD-H employs a 25-degree helix angle while the AE-LNBD-H adopts a 30-degree angle. This is because the AE-BD-H has a short overhang length and emphasizes on the reliability of the cutting edge. The AE-LNBD-H, on the other hand, is designed for high overhang length machining, which tends to be more unstable, thus rather than emphasizing on a sharp cutting edge, it emphasizes on superior biting properties.

Figure 8 demonstrates AE-LNBD-H’s superior durability versus two other competitors in SKD11 (60 HRC).

Figure 8. AE-LNBD-H exhibits superior durability in SKD11 (60 HRC)

Figure 9 shows the machined surface between the AE-LNBD-H and two other competitors in STAVAX (53 HRC). As with the AE-BD-H, a smooth surface treatment is applied to the AE-LNBD-H to help attain high accuracy and high-quality mirror-finished surface.  

Figure 9. AE-LNBD-H exhibits excellent durability and surface finish in STAVAX (53 HRC)

As depicted in figure 8 and figure 9, the AE-LNBD-H offers the best specifications for long neck applications, with exceptional performance in comparison to other competitor tools. However, specifications alone would be incomplete without a full lineup since there are many scenarios where long neck end mills are required. To satisfy manufacturers’ various needs, the AE-LNBD-H is offered from R0.05 to R3, with a total of 261 items to accommodate a wide range of applications.

In order to manufacture high-precision dies, it is also important to select high-precision machine tools, high-precision cutting tools, and high-precision tool holders. OSG offers high-precision holders made by HAIMER of Germany, and all of the test data introduced this article are processed using the company’s high-precision shrink holders. By employing these end mills in combination with OSG’s high-precision holders, maximum performance can be guaranteed in high-hardness steels. To manufacturers who are working in high-hardness steel applications, look to the AE-BM-H, AE-BD-H and AE-LNBD-H carbide ball end mill series to experience greater performance.

For more information on the AE-BM-H, AE-BD-H and AE-LNBD-H carbide ball end mill series for high-hardness steel applications