Tools that Make Hard Milling Easier

Hard milling relates to the milling of high-hardness steel.
Over time, the threshold defining what qualifies as “hard” steel has evolved alongside advancements in technology.
In the early days of modern metal cutting, this threshold began at around HRC 30, later increased past HRC 40, then to HRC 45, HRC 50, and eventually exceeded HRC 55.
Traditionally, this threshold indicated when steel should be machined by abrasive grinding rather than by cutting. However, today it is no longer uncommon to mill steel hardened to HRC 62 or even higher.

Why, then, is hard milling attracting such interest from manufacturers? What benefits does it offer to a machining shop?

First of all, hard milling fundamentally changes the entire technological process by allowing for the elimination (or at least significant reduction) of grinding operations.
Traditionally, the process is divided into several main steps: machining the workpiece in its soft or pre-hardened state, heat treating it, and then grinding the hardened material.
This approach requires leaving machining allowances in the first stage to compensate for potential deviations and defects resulting from heat treatment.

By introducing hard milling into the process, the number of setups can be greatly reduced, saving valuable time.
Furthermore, this moves manufacturers closer to achieving a longstanding goal: complete machining of a part in a single setup, with no need to reposition the workpiece between operations.
Clearly, with hard milling, post-machining hardening of the workpiece is no longer necessary.
As a result, hard milling provides manufacturers with a powerful tool to boost efficiency and shorten delivery time - especially when machining parts with geometrically complex shapes - while also reducing production costs

Let us now return to the concept of “hard” steel for a clearer understanding.
From the perspective of cutting tool applications, this type of steel falls under the ISO H application group.
According to the ISCAR Material Classification, which is based on the VDI 3323 standard, this group includes steels with an average hardness of around HRC 55 and above.
Additionally, hardened cast iron, particularly the difficult-to-cut, highly wear-resistant grades with hardness HB 400 (~HRC 43) and more, are also included in this group.

Therefore, hard milling can be considered a machining method for steel and cast iron with a hardness exceeding, as a guideline, HRC 45.
The key factor for success in hard milling is directly related to the choice of the cutting tool.

Advancements in tool materials and cutting geometries have been the main factors enabling the machining of harder materials.
In the 1980s, the die and mold industry made significant efforts to drastically reduce production times for both manufacturing new molds and restoring worn ones.
Hard milling emerged as one effective solution.
Another approach was the adoption of high-speed machining (HSM), and, over time, many hard milling strategies incorporated HSM principles.
Further progress in machine tool engineering, powder metallurgy, and coating technologies - resulting in leading-edge machines, the ability to sinter complex shapes, and the development of innovative coatings - accelerated the integration of hard milling into the metalworking industry.

At the same time, despite its significant advantages, hard milling presents challenges that make this machining method particularly demanding.
The natural high hardness of the material greatly intensifies tool wear.
Additionally, cutting hard materials requires much greater cutting forces, which substantially raises the mechanical load on both the cutting tool and the machine.
This amplifies vibration, shortens tool life, and negatively impacts surface finish.
Moreover, increased cutting forces lead to intensive heat generation, which can adversely affect both the tool and the workpiece

As a result, hard milling typically uses smaller machining allowances compared to machining workpieces in a soft or pre-hardened state.
This is why hard milling is usually applied to semi-finishing and finishing operations. However, it can also be suitable for rough machining by using multi-pass cutting or high feed milling (HFM) methods, while maintaining low machining stock per pass.

Understandably, cutting materials for hard milling must meet strict requirements for toughness and hot hardness.
Coated carbides remain the most common cutting material, while cubic boron nitride (CBN) is also used, particularly in indexable milling.
In some applications, especially when machining hard cast iron, polycrystalline diamond (PCD) offers an option that is worthy of consideration.

The modern development of tools for hard milling has focused on advanced carbide grades, with emphasis on progressive coatings; optimized macro- and micro-cutting geometries; and high precision, initially for HSM endmills.
In recent years, following these trends, ISCAR, as a leading cutting tool manufacturer, has significantly updated its hard milling tool program.
These innovations have impacted both indexable and solid tool designs.

In indexable milling, ISCAR has expanded its line of HFM tools for machining hard materials.
The popular MILL-4-FEED family now includes FFQ4 SOMW … T inserts, intended for milling workpieces with hardness up to HRC 60 (Fig. 1).
Meanwhile, the LOGIQ-4-FEED family, featuring bone-shaped inserts, offers solutions for machining materials with hardness of up to HRC 49 (Fig. 2).
Both lines are widely used for milling repaired surfaces of dies and molds after welding and post-weld treatment.
The NEOBARREL family of single insert endmills with barrel and lens cutting profiles is designed for semi-finishing and finishing complex 3D workpieces with hardness up to HRC 62 (Fig. 3).

According to ISCAR, promising prospects for indexable hard milling are opened by applying inserts made from ceramic materials.

ISCAR’s solid tool line has been expanded with new designs that enhance the company’s CHATTERFREE endmill family, which utilizes a variable-pitch concept for chatter dampening.
The new solid carbide endmills (SCEM), available in diameters from 6 to 16 mm and capable of achieving a maximum depth of cut up to two tool diameters, are now offered in the IC608 carbide grade.
This grade features a hard submicron substrate and PVD coating, improving resistance to abrasive and oxidation wear.
The introduction of IC608 enables effective machining of hardened steel and cast iron with hardness of HRC 45–60 at moderate to high cutting speeds.

In the miniature tool family, new small diameter endmills ranging from 0.3 to 4 mm have replaced the previous generation of cutters.
Incorporating improved geometry for greater rigidity, an ultra-fine IC602 PVD-coated carbide grade for extended tool life in hard milling, and tight dimensional tolerances (within 10 μm) for higher accuracy, the endmills deliver optimized performance when SCEM are used for machining steels with hardness of up to HRC 65 (Fig. 4).

Hard milling is a highly demanding machining application.
However, growing industry needs have made it increasingly necessary to enhance the effectiveness of manufacturing processes.
As a result, tool manufacturers are facing new challenges and, like ISCAR, are working hard to develop milling cutters that make machining hard materials significantly easier. ■