Traditional grinding processes, when machining parts with hardness up to HRC 55, suffer from dispersed operations and extended lead times! However, the emergence of hard milling technology has quietly broken the HRC 62 barrier—enabling single-setup precision akin to surgical procedures, compressing delivery cycles, and reducing costs. From automotive dies and precision gears to aerospace components with ultra-hard parts, this metalworking revolution is making the "impossible" the new normal on the shop floor.

01 The Hardness Revolution: The Leap from HRC 30 to HRC 62

Hard milling refers to the milling of high-hardness steels. As technology advances, the definition of "hard" steel continues to evolve. In the early days of modern metal cutting, this threshold started around HRC 30, gradually rising to above HRC 40, then HRC 45, HRC 50, and finally surpassing HRC 55. Traditionally, when steel reached this hardness level, grinding was the prescribed process, not cutting. Today, however, milling steels with hardness up to HRC 62 and beyond has become commonplace.

02 The Efficiency Leap: Hard Milling Disrupts Traditional Processes

Why does hard milling spark such intense interest among manufacturers? What advantages does it offer machining workshops?

Introducing hard milling drastically reduces the number of setups, saving valuable time. More importantly, it brings manufacturers closer to a long-standing goal: completing a part in a single setup, without repositioning the workpiece between different operations. Clearly, with hard milling, the step of hardening the workpiece after machining becomes unnecessary. Therefore, hard milling provides manufacturers with a powerful tool to enhance efficiency, shorten delivery cycles—and even reduce production costs, especially for parts with complex geometries.

03 The Pain Point of Hard Milling: Overcoming Tool Wear at High Hardness

To better understand, let's revisit the concept of "hard" steel. From a cutting tool application perspective, these steels belong to the ISO H application group. According to the ISCAR material classification based on the VDI 3323 standard, this group includes steels with an average hardness of about HRC 55 and above. Additionally, hardened cast irons with hardness ≥ HB 400 (approx. HRC 43) and above, particularly difficult-to-machine, highly wear-resistant grades, are also included.

Thus, hard milling can be viewed as a method for machining steels and cast irons with hardness exceeding HRC 45 (as a guide value). The key factors for successful hard milling implementation are directly related to cutting tool selection.

Advancements in tool materials and cutting geometries have been the primary drivers enabling hard material machining. In the 1980s, the mold and die industry made significant efforts to drastically shorten cycles for new mold manufacturing and old mold repair. Hard milling emerged as an effective solution, and high-speed machining techniques also gained momentum. Over time, numerous hard milling strategies have incorporated high-speed machining concepts. Further developments in machine tool technology, powder metallurgy, and coating technology—giving rise to advanced machines, complex shape sintering capabilities, and innovative coatings—have accelerated the adoption of hard milling across the metalworking industry.

• Challenge: The inherent high hardness of the material drastically accelerates tool wear.
• Challenge: Machining hard materials requires greater cutting forces, significantly increasing mechanical load on the tool and machine, amplifying vibration, shortening tool life, and adversely affecting surface quality.
• Challenge: Increased cutting forces lead to higher cutting temperatures, potentially negatively impacting both tool and workpiece.

Therefore, compared to workpieces in their initial or normalized/annealed state, hard milling typically employs smaller machining allowances. This is precisely why hard milling is predominantly used for semi-finishing and finishing operations. However, by adopting multi-layer cutting or high-feed milling methods and controlling the allowance per cut, it is also applicable in roughing.

04 Tool Innovation: Three Major Breakthroughs in Hard Milling Materials

It's easy to understand that tool materials for hard milling must meet stringent requirements for toughness and hot hardness. Coated carbide remains the most commonly used tool material, with Cubic Boron Nitride (CBN) also applied (especially in indexable milling). Under certain conditions, particularly when machining hard cast irons, Polycrystalline Diamond (PCD) tools are worth considering.

The development of modern hard milling tools focuses on three key areas: advanced carbide grades (with a focus on innovative coating technologies); optimized macro and micro cutting geometries; and high-precision manufacturing, initially designed for high-speed machining end mills. In recent years, ISCAR, as a leading cutting tool manufacturer, has significantly upgraded its hard milling tool portfolio in line with these trends. These innovations cover both indexable and solid tooling domains.

05 Continuous Innovation: ISCAR's Diverse Hard Milling Applications

In indexable milling, ISCAR has expanded its range of high-feed mills for machining hard materials. The popular MILL-4-FEED family now includes the FFQ4 SOMW … T insert, suitable for workpieces up to HRC 60 hardness.

The LOGIQ-4-FEED series, employing bone-shaped inserts, provides a solution for machining materials up to HRC 49 hardness.

These two series are widely used for milling welded and post-weld-treated repair surfaces on molds and dies. The NEOBARREL series of single-insert end mills, featuring arc and lens profile designs, is specifically for semi-finishing and finishing complex 3D workpieces up to HRC 62 hardness.

ISCAR has also opened new horizons for indexable tool hard milling with inserts made from ceramic materials.

In solid carbide tools, new designs have enhanced the performance of the CHATTERFREE anti-vibration end mill series, which utilizes variable pitch concepts to suppress chatter. The new solid carbide end mills, with diameters ranging from 6-16 mm and a maximum cutting depth up to 2×D, are now available with grade IC608. This grade employs a hard submicron substrate and PVD coating, improving wear resistance and anti-oxidation wear capability. The introduction of IC608 enables effective machining of hardened steels and cast irons in the HRC 45-60 range under moderate to high-speed cutting conditions.

In the micro-tool series, new small-diameter end mills ranging from 0.3 to 4 mm have been upgraded. These end mills combine an improved geometric flute shape for enhanced rigidity, an ultra-fine grain IC602 PVD coated grade material suitable for hard milling to extend tool life, and tight dimensional tolerances (≤10 μm) ensuring high precision. They deliver optimized performance when machining steels up to HRC 65 hardness with these solid carbide end mills.

Hard milling is an extremely challenging machining task. However, growing industry demands make improving manufacturing process efficiency increasingly crucial. Therefore, tool manufacturers face new challenges and, like ISCAR, are committed to developing milling products that significantly simplify the machining of hard materials.