CNC milling of hardened steels is one of the most difficult tasks in machining. Materials with a hardness above 45 HRC offer exceptional resistance to tools, generate high temperatures, and accelerate blade wear. Every incorrect setting ends in a broken end mill or the rejection of an entire batch of parts.
The key to success is the precise selection of cutting parameters before the spindle is even started. The cutting speed Vc, feed per tooth fz, and depth of cut ap must work in harmony. The proper combination of these values determines the surface quality, tool life, and the stability of the entire machining process.
Cutting speed Vc for CNC milling of hardened steels
Cutting speed Vc is one of the most important parameters in the machining of hardened steels. A value that is too high leads to a sudden increase in temperature in the cutting zone and rapid wear of the tool’s protective coating. A value that is too low, on the other hand, causes chip adhesion and uneven loading of the blades.
In machining practice, the cutting speed for CNC milling is selected based on several factors at once. These include the hardness of the material, the type of tool coating, its diameter, and the machining strategy. Only a combined analysis of these variables allows for the determination of a safe and effective Vc range.
How steel hardness on the HRC scale affects the Vc value
The hardness of the material expressed on the HRC scale directly determines the upper limit of the permissible cutting speed. The higher the hardness, the lower the Vc that should be used. This is due to the increasing cutting resistance and heat generated in the contact zone between the blade and the material.
For steels with a hardness of 40–45 HRC, Vc values in the range of 60–100 m/min are typically used with carbide tools. At a hardness of 50–55 HRC, these values drop to 40–70 m/min. Above 58–62 HRC, characteristic of tool steels after full hardening, a Vc of 30–50 m/min is recommended using ball end mills with a very small depth of cut.
An increase in hardness of every 5 HRC units requires a reduction in Vc of approximately 20–30%. Adhering to this rule significantly extends blade life and protects against chipping of the cutting edge.
Formula for spindle speed and how to use it
The spindle speed n is calculated using the standard formula: n = (1000 × Vc) / (π × D), where D is the tool diameter in millimeters and Vc is the selected cutting speed in m/min.
Example: an end mill with a diameter of 10 mm, Vc = 60 m/min. Calculation: n = (1000 × 60) / (3.14 × 10) ≈ 1910 rpm. This is a starting value that can be adjusted up or down by 10–15% after the first pass. With ball end mills, it must be remembered that the effective cutting diameter is smaller than the nominal diameter, and this must be taken into account in the calculations.
Recommended Vc ranges for tool and high-alloy steels
Tool steels and high-alloy steels are characterized by different machinability, which is why they require separate Vc ranges.
Vc ranges for selected groups of hard steels:
- Cold-work tool steels (45–55 HRC): 50–80 m/min
- Hot-work tool steels (48–52 HRC): 40–70 m/min
- Hardened high-speed steels (58–65 HRC): 20–40 m/min
- High-alloy structural steels (40–48 HRC): 60–100 m/min
The values in the table above apply to carbide end mills with a PVD coating. For HSS (high-speed steel) end mills, cutting speeds are even several times lower and typically do not exceed 15–20 m/min in steels above 40 HRC. In practice, CNC operators often start at the lower end of the specified range and then gradually increase the values while observing the condition of the tool and the quality of the machined surface.
Influence of TiAlN and TiSiN tool coatings on permissible Vc
The tool coating is one of the key factors determining possible Vc values. The TiAlN (titanium aluminum nitride) coating is distinguished by its high oxidation resistance at temperatures up to approximately 800°C. It is effective for machining hardened steels up to 55 HRC, especially during dry milling or with minimal lubrication.
The TiSiN (titanium silicon nitride) coating features even higher hardness and resistance to temperatures reaching 1000–1100°C. End mills with this coating tolerate Vc values about 10–20% higher compared to tools coated with TiAlN. They are recommended for machining materials above 55 HRC and for high-speed machining (HSM) strategies. The choice of the proper coating should take into account not only the hardness of the material but also the machining strategy and cooling conditions.
Feed per tooth fz and depth of cut ap for hard steels
Feed per tooth fz and depth of cut ap are two parameters that directly affect cutting forces and the temperature in the machining zone. Their interrelation is particularly important for hard steels, where any exceeding of the recommended range risks chipping the cutting edge or causing machine vibrations. The correct combination of fz and ap allows for maintaining control over the process and achieving repeatable surface quality.
How to select feed per tooth fz based on the number of flutes
Feed per tooth fz is expressed in millimeters per one flute during one revolution of the end mill. The total feed rate per minute is calculated using the formula: Vf = fz × z × n, where z is the number of flutes and n is the spindle speed. For hard steels, fz values are significantly lower than for soft steels.
Typical fz values for solid carbide (VHM) end mills in hard steels:
- 2-flute end mill, steel 40–45 HRC: fz = 0.02–0.04 mm
- 3-flute end mill, steel 45–52 HRC: fz = 0.015–0.03 mm
- 4-flute end mill, steel 52–58 HRC: fz = 0.008–0.020 mm
- Ball nose end mill, finishing, steel 58–62 HRC: fz = 0.005–0.015 mm
End mills with a higher number of flutes generate lower cutting forces per flute, which protects the edge from cracking. At the same time, a higher number of flutes makes chip evacuation more difficult, which is why the depth of cut should be proportionally lower. Using an fz below the recommended minimum is just as harmful as exceeding the maximum, as it causes friction instead of cutting and accelerates tool wear.
Axial depth ap and radial depth ae when milling hard steels
The axial depth ap determines how deep the end mill penetrates the material along the tool axis. The radial depth ae describes the width of the material strip removed in a single pass. Both parameters must be balanced, especially with hard steels, where spindle power is heavily loaded.
When rough milling steel with a hardness of 45–50 HRC, it is recommended to use ap = 0.1–0.3 × D and ae = 0.05–0.15 × D, where D is the diameter of the end mill. For finishing operations, the values decrease to ap = 0.05–0.15 × D and ae = 0.01–0.05 × D. Such settings protect the tool from overloading and allow for achieving an Ra roughness below 0.8 µm.
The relationship between fz, ap, and the durability of VHM carbide inserts
The durability of VHM carbide inserts depends on the combined influence of fz, ap, and ae. Increasing any of these parameters above the recommended values causes an exponential increase in temperature in the cutting zone. This temperature is the primary factor destroying the coating and the carbide substrate of the tool.
Technological studies show that exceeding fz by 50% reduces tool life by up to three times. For this reason, it is better to increase efficiency through more frequent passes at small ap values rather than by increasing the feed rate in a single pass. The strategy of multiple light passes, popular in high-speed machining (HSM or HPC), allows for maintaining a stable blade temperature and extends its service life by 30–70% compared to classic conventional milling.
Which VHM end mills are suitable for milling steel with a hardness above 45 HRC
Selecting the right tool is the foundation of effective hard steel machining. VHM (solid carbide) end mills dominate this field because they combine high substrate hardness with the ability to apply advanced PVD coatings. However, not every carbide end mill is suitable for materials above 45 HRC. Geometry, number of flutes, carbide grain quality, and coating type all matter.
Features of a VHM end mill for hard steels:
- Substrate made of fine-grained cemented carbide (grains below 0.5 µm)
- PVD coating with a hardness above 3000 HV (TiAlN, TiSiN, AlCrN)
- Negative or zero rake angle on the cutting edge
- Reinforced cutting edge with preparation (honing, chamfer)
- Short tool overhang to minimize vibrations
The quality of the carbide substrate has a direct impact on resistance to brittle fracture during interrupted cutting. End mills with coarser carbide grains handle impacts better but lose sharpness faster. Therefore, for steels above 55 HRC, tools with ultra-fine grains and heat-resistant coatings are used.
End mill geometry, rake angle, and number of flutes for hardened steels
The geometry of an end mill when machining hard steels differs from the geometry used for soft steels. The rake angle should be zero or slightly negative (from 0° to -5°). A negative rake angle increases the strength of the cutting edge and reduces the risk of chipping under high cutting forces.
The number of flutes for steels above 45 HRC is typically 4–6. Four flutes are a compromise between chip evacuation and tool rigidity. Six flutes are used for finishing operations with very small fz and ap values, when surface quality is the priority. Ball nose end mills with appropriate flute geometry are indispensable for the contour machining of injection molds and dies.
When milling steel above 50 HRC, it is worth choosing an end mill with a shorter working length and clamping it as deeply as possible in the holder. This reduces vibration and allows for stable cutting parameters to be maintained throughout the entire machining cycle.
Solid carbide end mills versus indexable insert end mills for hard milling
Solid carbide VHM end mills and indexable insert end mills differ in their application, flexibility, and capabilities for machining hard steels.
| Feature | Solid carbide end mill | Indexable insert end mill |
|---|---|---|
| Minimum diameter | from 1 mm | from approx. 16–20 mm |
| Hardness of the machined material | up to 65 HRC | usually up to 58 HRC |
| Geometry flexibility | high (various coatings, angles) | limited to available inserts |
| Tool change time | requires replacing the end mill | quick replacement of the insert itself |
| Application | precision and profile machining | roughing and semi-finishing |
Solid carbide end mills are indispensable for the precision machining of small parts, pockets, and 3D contours in hardened steels. Indexable insert end mills perform better when removing large material allowances on parts with simpler geometry. The choice between these solutions depends on the material hardness, required accuracy, and available spindle power.
Tip: for machining hard steels above 55 HRC in finishing operations, it is always worth using solid carbide ball nose end mills with a TiSiN or AlCrN coating, mounted in hydraulic or shrink-fit holders. This reduces radial runout to a minimum and prevents premature edge wear.
Precision CNC metal machining at CNC Partner
Executing the machining of hard steels requires not only knowledge of cutting parameters but also access to modern machines and experienced staff. CNC Partner is a company with many years of experience in precision metal machining, serving industrial clients from the European Union. Orders are fulfilled via shipping, and delivery within the European Union is fast and on time.
The company produces both individual prototype parts and serial production runs numbering in the thousands. Every order undergoes rigorous quality control. This is confirmed by our ISO 9001 Quality Certificate, which guarantees repeatability and compliance of products with customer requirements.
Scope of machining services
The company provides a wide range of professional CNC metal machining, covering various cutting technologies. Each is adapted to the specifics of the material and the required accuracy of the part.
Available machining technologies:
- CNC Milling — precision contour and surface machining, including milling of hardened steels
- CNC Turning — production of turned parts with high dimensional accuracy
- CNC Grinding — surface finishing with roughness up to Ra 0.63
- Wire EDM — machining of materials with hardness up to 64 HRC
Each of the listed methods has its application for different types of steel and part geometries. Wire EDM is particularly effective where milling is impossible due to the contour shape or extreme material hardness. CNC grinding, on the other hand, ensures the highest surface quality while maintaining strict dimensional tolerances.
CNC Metalworking Services
Order fulfillment and contact
Quotes for each order are provided within 2 to 48 hours, and lead times range from 3 to 45 days, depending on the complexity of the project. Detailed information on the terms of cooperation is available on the service price list page. The company is highly rated by its clients, as confirmed by positive reviews of CNC Partner services on Google.
Orders are accepted from manufacturing companies, design offices, and enterprises outsourcing excess work. Those interested in cooperation or requiring technical consultation are invited to contact the CNC Partner team.
Cooling and lubrication during CNC milling of hardened steels
Managing temperature in the cutting zone is one of the most difficult aspects of machining hardened steels. The temperature at the contact point between the blade and the material can exceed 600–900°C, which, with improper cooling selection, leads to rapid tool degradation. Choosing between dry machining and various cooling and lubrication methods requires an understanding of blade wear mechanisms at different material hardness levels.
Oil mist cooling versus dry machining at hardness levels above 55 HRC
For steels with a hardness above 55 HRC, specialists most often recommend dry machining or machining with very limited lubrication. This is due to the risk of thermal shock to carbide blades. Sudden cooling of a heated tool with coolant causes micro-cracks in the carbide substrate and accelerates chipping.
Dry machining, with the appropriate tool geometry and correct Vc range, allows for a stable process temperature. Heat is then primarily dissipated through the chips, which should be gold or blue in color. Light, light-beige chips may indicate a cutting temperature that is too low and an insufficient process. With oil mist cooling used for 45–55 HRC steels, fine oil droplets reach the cutting zone without causing thermal shock. This method reduces friction and assists in chip evacuation while not disrupting the heat distribution at the blade.
Minimum Quantity Lubrication (MQL) and its impact on cutting zone temperature
Minimum Quantity Lubrication (MQL) involves delivering very small amounts of oil, from 10 to 50 ml per hour, in the form of an aerosol directly to the cutting zone. This method combines the advantages of dry machining with the benefits resulting from lubricating the cutting edge.
Technological studies confirm that MQL in the milling of hardened steels with a hardness of 45–58 HRC allows for a reduction in the temperature of the cutting zone by 15–25% compared to completely dry machining. At the same time, it extends tool life by 20–40% compared to milling without any lubrication. It is essential to match the pressure and angle of the aerosol delivery to the tool geometry, which has a direct impact on the effectiveness of blade lubrication.
For steels above 58 HRC and operations with ball-end mills, MQL with synthetic or ester oil is recommended. Ester oils have better metal wettability and penetrate the micro-gap between the blade and the material more effectively. Proper setup of the MQL system is just as important as the selection of the CNC milling cutting parameters itself.
Tip: when implementing MQL for machining hard steels, it is worth starting with the stream directed exactly at the cutting edge, not at the shank of the end mill. This increases lubrication efficiency and reduces oil consumption, maintaining tool protection throughout the entire machining cycle.
FAQ: Frequently asked questions
What cutting speed values (Vc) should be used when milling hardened steels above 50 HRC?
The cutting speed (Vc) when milling hardened steels above 50 HRC should be in the range of 40–70 m/min for VHM carbide end mills with a TiAlN or TiSiN coating. At a hardness of 55–62 HRC, it is recommended to lower the Vc to 30–50 m/min to protect the cutting edge from excessive temperature. Higher values risk chipping the protective coating and sudden tool breakage.
The spindle speed (n) is calculated using the formula: n = (1000 × Vc) / (π × D). For an end mill with a diameter of 8 mm and Vc = 50 m/min, the result is approximately 1990 rpm. It is recommended to always start at the lower end of the range and then adjust the values after observing the condition of the chips and the tool.
Should coolant be used when CNC milling hard steel above 55 HRC, or is dry machining a better method?
For steels with a hardness above 55 HRC, dry machining or Minimum Quantity Lubrication (MQL) is significantly safer than liquid cooling. Suddenly applying cold coolant to a heated carbide blade causes thermal shock, which leads to micro-cracks and accelerated chipping of the tool.
The MQL method, or minimum quantity lubrication, involves applying 10–50 ml of oil per hour in the form of an aerosol. It lowers the temperature of the cutting zone by 15–25% compared to completely dry machining, without causing thermal shock. For 45–55 HRC steel, oil mist cooling also works well. When choosing a method, the material hardness and the type of end mill coating are of key importance.
Which VHM end mill should be chosen for milling steel with a hardness of 45–60 HRC, and what should be considered when selecting it?
For milling steel with a hardness of 45–60 HRC, solid carbide VHM end mills made of fine-grained cemented carbide, coated with a PVD coating such as TiAlN or TiSiN, work best. The tool geometry should include a zero or slightly negative rake angle, ranging from 0° to -5°. Such a design strengthens the cutting edge and reduces the risk of chipping under high cutting forces.
The number of flutes should be 4–6. For finishing operations on steels above 55 HRC, six-flute ball end mills work well. Tool clamping is also important. A hydraulic or shrink-fit chuck minimizes radial runout and ensures process stability throughout the entire machining cycle.
What is the correct feed per tooth fz when milling hardened steel, and how is it calculated?
The feed per tooth fz for hardened steels is significantly lower than for soft steels. For a four-flute end mill working in steel with a hardness of 52–58 HRC, the recommended fz is 0.008–0.020 mm. For 40–45 HRC steels and a two-flute end mill, higher values can be used, reaching 0.02–0.04 mm.
The total feed rate is calculated using the formula: Vf = fz × z × n, where z is the number of flutes and n is the spindle speed. Using an fz below the recommended minimum values is just as harmful as exceeding it. Too low a feed causes friction instead of proper cutting and accelerates tool wear through abrasion rather than the normal machining process.
Summary
The proper selection of cutting parameters for CNC milling of hard steels requires taking into account the material hardness, tool geometry and coating, as well as the cooling and lubrication method. The cutting speed Vc, feed per tooth fz, and depths ap and ae form an inseparable system of dependencies. Changing one of them without adjusting the others almost always leads to problems with quality or tool durability.
Precise calculation of spindle speed, selection of a VHM carbide end mill with the appropriate coating, and implementation of the MQL method or dry machining are the pillars of effective hardened steel machining. This approach, based on proven data and technological precision, yields repeatable results, lower tooling costs, and high quality of machined surfaces.
Sources:
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- https://en.wikipedia.org/wiki/Milling_cutter
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- https://www.ijres.org/papers/Volume-11/Issue-6/1106279283.pdf
- https://www.academia.edu/122930054/Investigation_on_Effect_of_Material_Hardness_in_High_Speed_CNC_End_Milling_Process
- https://www.astrj.com/pdf-193805-115826?filename=The-Effect-of-Milling-Par.pdf
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