CNC milling is currently one of the most versatile heat treatment processes. It can be used not only for the production of parts that would take several days or weeks using traditional methods, but it can also be used to produce elements that would not be possible to create using other methods. Among several CNC milling techniques, micro milling is the latest achievement in the metal processing industry and is mainly used for steel hardening.
Compared to the conventional process, micro milling offers significant benefits when processing hardened steel, especially in terms of surface finish and dimensional accuracy. Nevertheless, this method of processing hardened steel is one of the most challenging machining operations. For a successful project execution, understanding several parameters that affect the entire machining process is essential. Any defect can lead to cutting errors, and even permanent machine failure. It is necessary to know how to handle in case of induction-hardened steel, what characterizes isothermally hardened ductile iron, and what are the main problems with low-carbon steel. Therefore, this article aims to explain a few key factors that should be considered when implementing CNC milling technology for the processing of hardened steel in your facility.
What should be considered when applying CNC heat treatment process?
Products and parts made of hardened steel are common in many industries, including automotive, military and defense, medicine, electronics, and semiconductors. This material is characterized by the highest strength and hardness and is perfectly suitable for use in various machine and device components. However, the process of micro-milling hardened tool steels is not simple. Before starting, several factors should be considered, including ceramic inserts, cutting edge, machining parameters, or alloy additives content. Other factors worth considering are listed below:
- During machining, choosing the right tool is extremely important. It's worth making a conscious choice between ball end mills, end mills, and sharp corner mills to match them to the machined object,
- During milling, especially when milling high-speed steel, maintaining a constant chip load on the cutting edge of the tool is crucial for the durability of the device and the quality of the parts,
- Machine beating is another important, but often overlooked aspect during the entire operation. Generally, the lifespan is halved when the beating exceeds 0.0004 inches. Minimizing rebounds becomes even more important when working with very small tools,
- It is not recommended to leave too much material for finishing milling. For mills with a diameter of about 1/8 inch and larger, it is recommended to leave about 1% of the diameter for final machining.
The technology discussed above is used for machining tools made of hardened steel. If this process is not performed correctly, it can result in poor quality of the finished product. There are many machining shops 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 functioning elements or cause defective operation of parts. Therefore, it is best to choose an affordable provider of such solutions with excellent customer service and innovative capabilities that meet your production needs. With many years of experience in this field, such a decision ensures that your parts are perfectly milled and machined. Moreover, it guarantees safe and efficient work, quick turnaround time, and greater precision.
What substrate should be used for grinding hardened steel?
When milling hardened steel HRc 50-70, it is not possible to use standard substrates made of carbide particles. The machined object is not only very hard, but dry work is often used to reduce heat fluctuations on the tool. For this purpose, devices containing the highest quality carbides, such as ultra-fine grain (e.g. YG-1 X-Power) or nano-grain (e.g. YG-1 X5070), are needed. These grades are denser, and therefore more resistant to wear and high temperature, making them ideal for steel hardened by crushing. In contrast, hard/ordinary sintered carbide or high-speed steel wears out quite quickly due to the extreme heat generated.
What type of coatings are required for hardened steel?
The coating of a tool can have a huge impact on its performance. Standard TiAlN or TiCN coatings withstand temperatures up to 800°C and quickly wear out during hardened steel machining. Specifically for this problem, YG-1 has developed a custom blue Y1200 coating, which can be found on X5070 tools. It has a very high temperature resistance up to 1200°C! The combination of this coating and nanograin 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 devices may be too hard, and tool durability may prove uneven when configured with older machines, machines with slower spindle speeds, or those exposed to wear or vibration. In such a case, a tool with ultra-fine particles, such as the YG-1 X-Power cutter, would be more appropriate. It is more tolerant and can work at lower cutting speeds due to the hardness of the carbide substrate.
Should you work wet or dry when milling hardened steel?
The durability of the tool is longer during milling or turning of hardened steel and is more even when dry (i.e., without coolant). For most engineers, this is counterintuitive, as they were told in college that the more lubrication, the better. However, hardened steel devices, such as the X5070, have coatings that can withstand much higher temperatures than those generated by dry cutting. Applying coolant to the tool can cause micro-cracks in the coating, which can in turn lead to its easier destruction and faster wear. This is similar to pouring cold water on a hot glass - it can also break, or more specifically, crack. With dry machining, durability of at least 40% can usually be achieved.
Can interchangeable milling cutters be used for machining hardened steel in CNC?
Most standard types of indexed tools can easily machine materials up to 45 HRc. When we start to process materials with hardness above 50 HRc, most dividing plates fail because they are not hard enough and have the wrong cutting edge geometry. For example, standard interchangeable 90-degree APMT/APKT mills are useless for milling hard steel - the edge is too weak to work with high feed and low cutting depth. The ideal indexing tool for hardened steel is a high-feed milling system, such as Korloy HRM Double or HFM High Feed Milling. They are characterized by a small cutting depth (0.3-1.6 mm), but they work with aggressive feed speeds, ideal for milling hardened steel (>1.2 mm per revolution). Excellent grades for hardened steel are PC2005, PC2010 or PC2015, with a flat plate (without a crusher), which can be machined up to HRc 65. This means that tools up to 200mm in diameter can be used for milling hardened steel!
What type of cutting edge geometry is required for hardened steel?
Tools for hardening materials require special cutting edge geometry to strengthen them. They typically have a geometry opposite to that of stainless steel tools. Hardened steel devices usually have negative grinding, shallow grooves, and thick cores. This type of geometry is very durable and designed for high speeds and small cutting depths.
Which tool mounting method is best for milling hardened steel?
After choosing the right tool, the next key is its proper mounting. If you have trouble investing in a high-performance milling cutter, using a universal holder can reduce the benefits that the device offers. Side lock and ER clamping sleeves are suitable for general milling, but they should be avoided when machining hardened steel.
Problems with ER collet chucks and side lock chucks
- Balanced - although they can be factory balanced to 15,000 rpm or more, this is not the case when mounted with clamping sleeves and fixtures,
- 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 vibrations of the tool and the workpiece. During the machining of hard materials with equally hard devices, they can lead to premature wear and low reliability.
Shrink fit holder in CNC
Heat shrink sleeves are ideal for crush-hardened steel, with excellent concentricity and precise 3-micron beating to help maintain balance at high cutting speeds. Long tools perform particularly well in heat shrink holders, and an additional advantage is the narrow neck and thin wall thickness, making them ideal for pocket machining where a narrow grip is required.
Hydraulic Grips in CNC
This is one of the favorite grips of many operators! Not only does it limit the beating to 3 microns and the balance to 25,000 rpm, but the hydraulic grip has the additional advantage of damping tool vibrations. In the case of long devices, it has an advantage over the retracted handle, thanks to a significant reduction in vibration. It also allows for quick tool change on the machine, and the clamping force increases with temperature! Also available with a YG thermal handle with a long neck (wall thickness 6 mm) or a narrow WTE HPH handle (wall thickness 3 mm).
Narrow GSK / HP3 Handles
The most universal and economical tool holders are Korloy GSK sleeves or ultra-thin Cutwel HP3. With a beat of 5 microns, they are characterized by a slim nut design and greater clamping force, significantly surpassing ER clamping sleeves. GSKs are designed with an 8-degree taper angle (compared to 16 degrees for ER sleeves). This eliminates the deflection and vibrations typical for ER clamping holders. It is especially recommended to use the Korloy GSK holder in combination with the YG-1 hydraulic holder for tools with a larger diameter.
What speeds and feeds should be used when milling hardened steel on a CNC machine?
Now that you have the perfect set of hardened steel, you need to think about how to use them – this will likely increase your chances of success. In milling, especially when machining high-speed hardened steel, the key to tool durability and product quality is maintaining a constant chip load on the cutting edge. Therefore, it is worth applying a higher cutting speed when turning hard materials and in the case of low-alloy steels. The chip load is equal to the feed divided by the spindle speed multiplied by the number of cutting edges. It is worth knowing that they are very diverse; if it is too small or too large, the tool wears out prematurely, bites or cracks. Many quick cuts and shallow feeds are the best way to ensure high product quality. However, remember that in the stainless steel cutting zone, separate milling recommendations may be worth implementing. Classic stainless steel will have different resistance than martensitic stainless steel. It is worth keeping this in mind to avoid chipping of the edges.