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CNC milling is currently one of the most versatile heat treatment processes. Not only can it be used to produce parts that take days or weeks using traditional methods, but it can also be used to produce parts that would be impossible to create using other methods. Among several CNC milling techniques, micro milling is the latest development in the metalworking industry and is mainly used to harden steel. Compared to the conventional process, micro milling offers great advantages when developing hardened steel, especially in terms of surface finish and dimensional accuracy.

Nevertheless, this method of processing hardened steel is one of the most difficult machining operations. For a successful project, it is necessary to understand several parameters that affect the entire machining process. Any flaw can lead to cutting errors and even permanent machine failure. It is necessary to know how to handle induction-hardened steels, what characterizes isothermally hardened ductile iron, and what the main problems are with low-carbon steels. No cutting tool engineering magazine will give you as much knowledge as practice and this article. Therefore, this article aims to explain some of the key factors to consider when implementing CNC milling technology for hardened steel machining in your facility.

What should you consider when using a CNC heat treatment run?

Products and parts made from hardened steel are common in many industries, including automotive, military and defense, medical, electronics, and semiconductors. The material is characterized by superior strength and hardness and is ideal for use in a variety of machinery and equipment components. However, the process of micromilling hardened tool steels is not simple. Before starting it, several factors must be taken into account, including ceramic inserts, cutting tools, machining parameters, and alloy additive content. Here are examples of hard milling solutions:

  • When machining, it is extremely important to choose the right machine tool. It is worth making an informed choice between spherical milling cutters, cylindrical milling cutters, and milling cutters with sharp corners to match the workpiece,
  • When milling, especially when milling high-speed steel, maintaining a consistent chip load on the cutting edge of the tool is critical to the life of the machine and the quality of the part,
  • Machine run-out is another important but often overlooked aspect throughout the operation. Generally, life is halved when runout exceeds 0.0004 inches. Minimizing runout becomes even more important when working with very small tools,
  • Leaving too much material for finishing milling is not recommended. For cutters about 1/8 inch in diameter and larger, it is recommended to leave about 1% of the diameter for finishing.
  • The technology discussed above is for machining hardened steel tools. If this process is not done properly, it can result in a poor-quality finished product. There are many machining companies that offer CNC milling services, but finding the right one can be a challenge. An unreliable company not only takes up valuable time but can also damage working parts or cause parts to malfunction. That’s why it’s best to choose an affordable provider of this type of solution with excellent customer service and innovative capabilities to meet your manufacturing needs. With years of experience in the field, such a decision ensures that your parts are perfectly milled and machined. In addition, it guarantees safe and efficient work, fast turnaround times, and greater precision. 

What substrate should be used for grinding hardened steel?

When milling HRc 50-70 hardened steel, it is not possible to use standard substrates made of carbide particles. Not only is the workpiece very hard, but dry working is often used to reduce heat fluctuations on the tool. For this purpose, you need devices containing the highest quality carbides, such as ultra-fine-grained (e.g., YG-1 X-Power) or nano-grained (e.g., YG-1 X5070). These grades are denser and therefore more resistant to wear and high temperatures, making them ideal for crush-hardened steels. In contrast, hard/ordinary carbide or high-speed steel wears out rather quickly due to the extreme heat generated.

What kind of coatings are required for hardened steel?

A tool’s coating can have a huge impact on its performance. Standard TiAlN or TiCN coatings can withstand temperatures up to 800°C and wear out quickly when machining hardened steel. Specifically for this problem, YG-1 has developed a customized Y1200 blue coating, which can be found in X5070 tools. It has a very high-temperature resistance of up to 1200°C! The combination of this coating and nano-grained carbide means that our X5070 tools can machine high hardness steels up to 70 HRc at extremely high speeds and feeds with incredible durability! X5070 machines can be too hard, and tool life can be uneven when configured with older machines, machines with slower spindle speeds, or those subject to wear or vibration. In this case, a tool with ultra-fine particles, such as the YG-1 X-Power cutter, would be more suitable. It is more forgiving and can operate at lower cutting tool speeds due to the hardness of the carbide substrate.

Should I work wet or dry when milling CNC hardened steel?

Tool life is longer when milling or turning hardened steel and is more consistent dry (i.e., without coolant). For most engineers, this defies logic, because in college they were told that the more lubricant, the better. However, hardened steel devices such as the X5070, for example, have coatings that can withstand much higher temperatures than those produced by dry cutting tool. Applying coolant to the tool can cause micro-cracking of the coating, which can in turn lead to easier deterioration and faster wear. This is similar to pouring cold water on a hot glass – it too can break, or more specifically, crack. With dry machining, you can usually achieve durability of at least 40%.

Can indexable cutters be used to machine CNC high-hardened materials?

Most standard grades of indexable tools can easily machine materials up to 45 HRc. When you start developing plastics with hardnesses above 50 HRc, most indexable inserts fail because they are not hard enough and have the wrong cutting-edge geometry. E.g., standard interchangeable 90-degree APMT/APKT cutters are useless for milling hard steel – the edge is too weak for high feed and shallow depth of cut. The ideal indexing tool for hardened steels is a high feed milling system such as Korloy HRM Double or HFM High Feed Milling. They feature a shallow depth of cut (0.3-1.6 mm), but operate at aggressive feed rates, ideal for hardened steel milling (>1.2 mm per revolution). Excellent grades for hardened steel are PC2005, PC2010, or PC2015, with a flat plate (no breaker) that can be machined to HRC 65.

This means that tools up to 200mm in diameter can be used to mill hardened material!

What type of cutting-edge geometry is required for hardened steel in CNC?

Tools for hardening materials require special cutting-edge geometries to strengthen them. They usually have the opposite geometry to stainless steel tools. Hardened steel devices usually have negative grinding, shallow grooves, and thick cores. This type of geometry is very strong and designed for high speeds and shallow depths of cut.

Which tool clamping method is best for hardened steel milling?

Once you have chosen the right tool, the next key is to mount it correctly. If you’re having trouble investing in a high-performance cutter, using a universal chuck can reduce the benefits the tool offers. Side lock and ER collets are suitable for general milling but should be avoided when machining hardened steel.

Problems with ER handles and side handles

Balanced – although they may be factory balanced to 15,000 rpm or more, this is not the case when installed with collets and tooling, Runout – runout up to 10 microns at a tool length of 3xD. However, if the length is 8xD, it will increase to 50 microns. For a 5 mm tool, this is a huge number, causing vibration of the tool and workpiece. When machining hard materials with equally hard equipment, these can lead to premature wear and poor reliability.

Heat shrink chuck

Heat shrink sleeves are ideal for crush-hardened steel, with excellent concentricity and precise 3-micron runout to help maintain balance at high cutting speeds. Long-length tools perform particularly well in heat shrink chucks, with the added advantage of a narrow neck and small wall thickness, making them ideal for pocket machining where a narrow chuck is required. 

Hydraulic chucks

This is one of the favorite chucks of many operators! Not only does it limit runout to 3 microns and balance to 25,000 rpm, but the hydraulic chuck has the added benefit of dampening tool vibration. On long-length equipment, it gives an advantage over a reversed handle by significantly reducing vibration. It also allows quick tool changes on the machine, and the clamping force increases as the temperature increases! Also available with the YG long-neck thermal handle (6 mm wall thickness) or the WTE HPH narrow handle (3 mm wall thickness).

Narrow handle GSK / HP3

The most versatile and economical tool holders are Korloy GSK or Cutwel HP3 ultra-thin bushings. With a runout of 5 microns, they feature a slim nut design and greater clamping force, far superior to ER collets. GSKs are designed with a taper angle of 8 degrees (compared to 16 degrees for ER collets). This eliminates the deflection and vibration typical of ER collets. It is highly recommended to use the Korloy GSK chuck in combination with the YG-1 hydraulic chuck for larger diameter tools.

What speeds CNC and feeds should I use when milling hardened steel?

Now that you have the perfect set of hardened steels, you need to think about how to use them – this will cultivate your success. In milling, especially in high-speed machining of hardened steels, the key to tool life and product quality is maintaining a constant chip load on the cutting edge. It is therefore worth using a higher cutting speed when turning hard materials and for low-alloy steels. The chip load is equal to the feed divided by the spindle speed multiplied by the number of cutting edges. It’s worth knowing that they vary widely; if it’s too low or too high, the tool will wear prematurely, bite or break. Many fast cuts and shallow feeds are the best way to ensure a high-quality product. Keep in mind, however, that in the cutting zone of stainless steel, there may be times when separate milling recommendations are worth implementing. Classical stainless steel will have different resistance and martensitic stainless steel will have different resistance. This is worth bearing in mind in order to avoid edge chipping. We hope that the machining hardened steels or milling hardened steels will no longer be a major problem for you.