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Milling is a common subtractive machining process that uses a rotary cutting tool to cut and shape the material into the desired form. Hard milling is a specialized variant of this process, applied to materials that have been hardened to increase wear resistance and durability.
This article uncovers the essentials of hard milling, the types of materials that require this method, and the tools & equipment needed to machine those hard materials.
Understand the Concept of Hard Milling
If you know a bit about industrial materials, you probably know that some undergo a hardening process to boost their hardness and wear resistance. These materials are tough to machine with standard tools and methods. Wire EDM is an option for cutting such hard materials, but it is costly and not every machine shop can handle the cost.
Today, advanced tools and machines can tackle these hard materials. Hard milling refers to the machining of materials whose hardness rating falls between 45 and 65 on the Rockwell scale. At times, this term also defines machining other tough materials like titanium and some nickel alloys, which might not be as hard on the hardness scale but are still a challenge to machines.
Hard milling is a controlled approach to machining materials. It effectively deals with efficient and precise material removal via advanced cutting tools and supporting equipment. The primary difference between standard milling and hard milling is in the materials and tools used. Hard machining is performed on harder materials and requires sharper, and more geometrically optimized cutting tools that remove the material in a controlled manner.
Advantages of Hard Milling
Hard milling is becoming popular in the industrial landscape due to its ability to handle hard materials more efficiently, saving both time and costs.
Complex Geometries
Hard milling makes use of sharp and hard tools on multi-axis CNC machines, that allow the creation of complex and intricate geometries through a single setup. You don’t have to juggle between machines to get the job done.
Cost Savings
Hard milling saves costs in two ways. First, it eliminates the finishing processes like roughing and grinding that add up to extra cost. Second, the setup and labor cost of hard milling is significantly less than the other alternative for hard machining, Wire EDM.
Faster Lead Time
In hard milling, high material rates are possible. And unlike other traditional processes, you don’t need any secondary finishing processes; the specimen is ready for shipping, right after hard milling.
Surface Finish
Hard milling has advanced to a point where it can rival the surface finishes achieved by grinding. A surface roughness study on alloy steel (molds and dies) found that the surface finish of hard milling was comparable to that achieved by grinding.
Materials That Undergo Hard Milling
For hard milling, you are dealing with two types of materials: those with a high hardness value and those that are tough to machine due to their unique properties.
Here’s a closer look at popular materials machined via hard milling.
Hardened Tool Steel
Hardened tool steels can withstand wear and maintain hardness, even under extreme conditions. Their hardness falls between 50 to 65 HRC and hard milling seems a reasonable solution for machining them. Common grades include D2 and A2 – both used in die and mold applications.
Alloy Steel
These steels are heat-treated to achieve hardness values suitable for hard milling, typically between 45 – 55 HRC. One example is alloy grade 4140, commonly machined for mold bases and crankshafts. Another one is 4340, used for structural components like shafts and gears due to its superior strength.
Super Alloys
Super Alloys like Inconel 718 are hard to machine using typical methods. Their hardness may be less than that of hardened tool steels, yet they pose a machining challenge, which hard milling solves.
Specialized Tools and End Mills for Hard Milling
Hard machining requires unique practices and specialized tools that can handle wear and tear and withstand high cutting forces.
Carbide Cutting Tools
Carbide cutting tools are the staple for hard milling operations due to exceptional hardness that surpasses that of materials being cut. They can easily cut material for hardnesses up to 68 HRC. Carbide geometries usually feature negative rake angles to reduce chipping and enhance edge stability.
Thermal Barrier Coatings
Due to higher hardness and high frictional forces, heat is a common occurrence in machining. This heat can damage both the workpiece and the tool itself. To manage that large amount of heat, milling cutters often bear a thermal barrier coating of AlTiN (Aluminum Titanium nitride).
Ball Nose Endmills
Ball Nose End mills have a rounded tip for 3D contouring and machining complex surfaces with smooth finishes. They are particularly useful in mold and die-making. 4 flute ball nose end mills, made up of nanograin carbide, can handle materials with hardness up to 70 HRC.
CBN Tools
Cubic Boron Nitride (CBN) is one the hardest materials available for cutting tools – it’s just second to diamond. CBN has remarkable wear resistance. Machining with CBN tools is done in a dry mode. Also, carbide may lose a chunk of hardness over 800℃ but CBN remains unchanged at least, up to 1200℃.
Considerations When Milling Hard Materials
When milling hard materials, it’s advisable to weigh in all aspects of the cutting tool and material under consideration. Here are a few things that should be considered when going forward with hard machining.
Tool Selection and Geometry
Hard milling requires tools with exceptional wear resistance. Coated carbide end mills and cubic boron nitride (CBN) tools are the most common choices. Carbide tools work well for general applications, while CBN tools are better for high-hardness materials (60+ HRC) and finishing operations.
The geometry of the tool also matters. Tools with negative rake angles and sharp cutting edges are ideal for maintaining precision and minimizing heat generation. For roughing, indexable cutters with strong chip breakers are a good option, particularly when dealing with less critical surface finishes. Always select the largest tool diameter and shortest overhang possible to reduce deflection and vibrations.
Material’s Hardness and Machinababily
Material mechanical and chemical do affect the machining process and tools needed. Hardened steels (45-65 HRC) and alloys like D2, Inconel, and titanium require slower cutting speeds and lighter feed rates to avoid tool wear and overheating.
Machinability doesn’t solely depend on hardness – it’s also influenced by material properties. For example, D2 tool steel machines are more like 62-65 HRC despite having a hardness closer to 60 HRC due to their high chromium content. Similarly, titanium alloys pose challenges due to their poor heat dissipation, even though they are not as hard on the Rockwell scale.
Cutting Parameters
Machining parameters have to be adjusted based on the material hardness. Here’s detail on how speed, feed rate, and depth of cut need to be adjusted:
- Speed: For materials up to 45 HRC, start with cutting speeds between 600 to 1,000 sfm. For harder materials (45–58 HRC), the range drops to 400 to 600 sfm. Extremely hard materials above 60 HRC require slower speeds of 200 to 400 sfm. These values depend on the tool diameter, so calculate spindle speed using the effective diameter for ball nose tools or smaller cutters.
- Feed Rate: Feed rates need to vary by material hardness and tool size. For materials up to 45 HRC, the feed per tooth can be 3–4% of the tool diameter. For 45–58 HRC, reduce it to 2–3% of the tool diameter, and for 60+ HRC, go lighter with 1–2% of the tool diameter. Extended tool lengths or deep pockets may require even lower feed rates.
- Cutting Depth: Both radial (step-over) and axial (step-down) depths depend on hardness. For up to 45 HRC, radial depth can go up to 50% of the tool diameter, while axial depth is limited to 10% of the tool diameter.
For 45–58 HRC, these values drop to 45% radial and 7% axial, and for 60+ HRC, keep them below 45% radial and 5% axial. Lighter depths help achieve better surface finishes and prolong tool life.
Cooling Strategies
Traditional flood coolant is useful in softer materials, but it can create thermal shock in hard milling. This happens because the tool’s cutting edge rapidly heats up during cutting and then cools down suddenly when flooded with coolant, causing micro fractures.
Compressed air cooling is a better choice for hard milling. Air cooling reduces heat buildup without causing thermal shock and aids in chip evacuation.Oil-based mist coolants are another option. They add lubrication to the cutting edge, reduce friction, and further improve surface finishes. Also, these coolants do not absorb heat like water, so they avoid the rapid cooling effect that damages tools.
ToolPath Methods
Machining efficiency also depends upon the toolpath. Some effective toolpath strategies for hard milling include:
- Trochoidal Milling: Uses a circular toolpath with light cuts to reduce heat and stress, ideal for deep pockets and hard-to-reach areas.
- Contour Milling: As the name suggests, the mill follows the shape of the part, making it great for finishing curved or complex surfaces.
- Helical Interpolation: Creates circular features like holes or slots by moving the tool in a spiral path.
- High-Feed Milling: Uses tools with low lead angles for faster feed rates, which makes it efficient for roughing large areas or deep pockets.
Richconn Hard Milling Services
If you’re looking for reliable hard milling services, you can count on Richconn CNC Milling Services. We handle over 60 materials with precision, from hardened tool steels to superalloys like Inconel 718. Our machines and technicians are equipped to work with even the toughest materials.
We offer 3-axis, 4-axis, and 5-axis CNC milling, capable of machining any complex or intricate geometry. Our tolerances go as low as ±0.002mm.
Have questions? Need assistance with your project? Contact our experts today for a free consultation.
