The choice of steel machining method determines whether the finished component will meet rigorous dimensional standards. CNC grinding and milling are two different processes, each of which is suitable for specific conditions. The question of when CNC grinding is better than milling arises every time a designer or technologist faces the choice of a finishing method for precision parts.
Milling removes a large amount of material quickly and efficiently. It allows for the creation of complex shapes, grooves, threads, and channels. However, it is not a tool for achieving surfaces with very low roughness or for machining steel hardened above 60 HRC. That is where CNC grinding comes in, which operates on completely different principles.
The boundary between the two processes is defined not only by the hardness of the material, but primarily by the required geometric accuracy and surface finish class. When a tolerance within the range of ±0.005 mm proves insufficient, and the project requires deviations on the order of a few micrometers, grinding becomes the only rational answer.
When does CNC grinding outperform milling in steel machining?
The performance limit of CNC milling is well known to production engineers. When finishing high-hardness steel or components requiring a very smooth surface, milling ceases to be sufficient. CNC grinding then takes on the role of the final process, which brings the part into compliance with the technical documentation.
Dimensional tolerances and surface roughness after CNC grinding
CNC grinding enables the achievement of dimensional tolerances of class IT4 to IT6, which means deviations on the order of 2 to 10 micrometers. Milling, even with the most careful setup, yields class IT8 to IT11, which means tolerances from over a dozen to over a hundred micrometers. The difference is therefore not just qualitative, but quantitative, and directly determines whether the component will work correctly with other parts of the mechanism.
Surface roughness measured by the Ra parameter after grinding is between 0.1 and 1.6 μm. Milling achieves Ra in the range of 0.8 to 6.3 μm, and during roughing operations, values reach 5 to 20 μm. For components requiring tight fits or working under seals, such differences have a direct impact on durability and reliability.
Geometric precision obtained through CNC grinding includes more than just linear dimensions. The concentricity of cylindrical shafts can be maintained below 0.005 mm, and the flatness of flat surfaces can be reduced to values of less than 0.01 mm. Milling is unable to provide such parameters in serial production.
Machining hardened steel, where milling loses efficiency
Steel hardened above 45 HRC poses a serious challenge for milling. Cutting tools wear out faster, cutting forces increase, and the risk of vibration and thermal deformation rises significantly. At a hardness above 60 HRC, milling is simply unprofitable due to tool wear and machining time.
Abrasive grinding removes material through micro-cutting with abrasive grains. Each cutting grain acts as a separate, very fine blade. The heat generated during grinding is dissipated with coolant, and the allowance is removed layer by layer. This allows the structure and hardness of hardened steel to be maintained without the risk of tempering the surface layer.
Regular aluminum oxide grinding wheels are used for grinding structural steels, while CBN (cubic boron nitride) wheels are effective for hardened steels with a hardness above 55 HRC. Diamond grinding wheels, in turn, are intended for the hardest materials, including tool steel after full hardening. The selection of the grinding wheel for the material directly affects the efficiency of the process and the quality of the surface.
Industry requirements that mandate grinding instead of milling
Industrial standards in several manufacturing sectors explicitly indicate grinding as the required finishing method. The aerospace, medical, and precision mechanics industries operate within roughness ranges of Ra from 0.1 to 0.8 μm, which milling is unable to consistently achieve.
Industries requiring CNC grinding:
- Aerospace: engine components, pins, and turbine shafts with tolerances of ±10 to ±50 μm and Ra 0.1–0.8 μm
- Medical industry: implants and surgical instruments with Ra below 0.05 μm and a blade angle below 15°
- Automotive: cylinder bores, camshafts, and seals with Ra 0.1–1.6 μm
- Energy: turbine and generator shafts with concentricity below 0.02 mm
ISO standards regarding bearing fits require IT5 to IT7 class tolerances on shafts and in bearing housings. Achieving these classes after milling is only possible with very slow feed rates and multiple passes, which makes CNC grinding after hardening an economically justified process. The precision of manufacturing cutting tools, such as drills and end mills, requires sharpening after hardening specifically by the grinding method.
How do the technical parameters of grinding and milling steel differ?
Comparing both methods through the prism of technical parameters allows one to understand why there is no single “better” method for every situation. Grinding and milling complement each other in the technological process, and the choice between them results directly from the documentation requirements and the production stage.
Geometric accuracy and tolerance range of both methods
The tolerances achieved by milling and grinding differ by an order of magnitude. CNC milling typically achieves tolerances of ±0.1 mm for rough machining and ±0.005 mm for finishing operations. CNC grinding goes down to values of ±0.001 mm, and specialized grinding centers achieve accuracies close to ±0.00254 mm.
The table below compares the key technical parameters of both methods when machining steel:
| Parameter | CNC Milling | CNC Grinding |
|---|---|---|
| Typical tolerance | ±0.005 to ±0.1 mm | ±0.001 to ±0.005 mm |
| Roughness Ra | 0.8–6.3 μm | 0.1–1.6 μm |
| IT tolerance grade | IT8–IT11 | IT4–IT6 |
| Optimal materials | Mild steel, aluminum, brass | Hardened steel, cemented carbide |
| Material removal per pass | Large (up to several mm) | Very small (2–20 μm) |
| Main application | Shaping, rough machining | Finishing, dimensional precision |
IT tolerance classes define the degree of accuracy: IT01 is the tightest tolerance, and IT16 is the loosest. CNC grinding is at the end of the scale requiring high precision, while milling is in the middle classes, corresponding to general mechanical requirements.
Cutting speed and material removal rate
Milling is a more efficient process in terms of the mass of material removed. Face mills can remove several millimeters of material in a single pass, making milling a suitable tool for shaping a part. Grinding works differently, removing layers from a few to a dozen micrometers per pass.
The cutting speed for creep-feed grinding of hardened steel ranges from 5 to 10 mm³/mm/s. These values are significantly lower than in milling; however, each pass provides a predictable and accurate dimensional result. Grinding time is longer, but it eliminates the need for subsequent corrective operations.
Types of grinding wheels and milling cutters used for steel machining
The choice of cutting tool determines the final result of the machining. Milling cutters for hardened steel have a special geometry with a negative rake angle, shallow flutes, and a thick core. This geometry increases the tool’s resistance to high cutting forces when machining materials with a hardness above 45 HRC.
Most commonly used grinding wheels in steel grinding:
- Aluminum oxide (Al₂O₃): intended for structural and soft tool steels
- CBN grinding wheels: intended for hardened steel above 55 HRC, they maintain their shape for a long time
- Diamond grinding wheels: intended for fully hardened tool steel and cemented carbide
A CBN grinding wheel is characterized by durability many times higher than that of a conventional grinding wheel. It can be profiled (dressed) thousands of times while maintaining a constant profile shape. In the serial production of hardened steel components, this provides dimensional repeatability unattainable for standard cutting tools.
Influence of cutting temperature on steel surface quality
The temperature in the cutting zone is one of the main factors affecting the surface quality of steel. When milling hardened steel, high temperatures can locally change the material structure, causing micro-cracks or a change in the hardness of the surface layer. This effect is particularly visible at small depths of cut and high speeds.
Grinding using coolant effectively dissipates heat from the contact zone between the grinding wheel and the material. The cooling liquid lowers the temperature of the abrasive grains and the workpiece surface. Proper use of coolant prevents the phenomenon of so-called grinding burn, which destroys the hardness of the steel’s surface layer.
Ra roughness parameters from 0.06 to 0.08 μm are obtained at very small grinding depths (0.004 mm) and low table feed speeds. Exceeding a feed speed of 1.8 m/min during the grinding of steel parts leads to a clear increase in surface roughness.
In which applications is CNC grinding the only right choice?
Some machine components must meet such rigorous requirements that there is simply no alternative to CNC grinding. This mainly applies to parts operating under high dynamic loads, where every micrometer of deviation affects the service life and safety of the assembly.
Precision parts for bearings, shafts, and guides
Rolling bearings require shaft fits with IT5 to IT6 tolerance classes. For high-precision electric motors, the recommended shaft tolerance class is IT5, which corresponds to a deviation of 4 to 8 micrometers for diameters of 30 to 80 mm. Milling does not provide such repeatability in mass production.
Precision shafts for bearings are cylindrically ground. Specialized shaft grinders with support systems prevent long shafts from deflecting during machining. CNC grinding centers, such as those used by CNC Partner, achieve concentricity of less than 0.005 mm and surface roughness down to Ra 0.63 μm. These values directly correspond to industry standards for precision bearings.
Types of precision parts requiring grinding:
- Electric motor and turbine shafts: diameters from 30 to 2000 mm, Ra 0.4–1.6 μm
- Linear guides: flatness below 0.01 mm, Ra 0.2–0.8 μm
- Bearing seats and journals: IT5–IT7 classes, concentricity below 0.005 mm
- Hydraulic cylinder piston rods: Ra below 0.4 μm, hardness above 55 HRC
Linear guides for CNC machines must have a surface with a flatness of less than 0.01 mm per meter of length. Surface grinding meets this requirement in a repeatable manner. The accuracy of tool movement depends directly on the quality of the guides, which is why their grinding process affects the entire precision of the machine.
Cutting tools requiring sharpening after hardening
Cutting tools, such as drills, end mills, taps, and reamers, are manufactured from high-speed steel or cemented carbide. After hardening, their hardness reaches 64 to 68 HRC. Cutting edges can only be shaped by grinding, as no milling cutter can cut material of such hardness.
Sharpening cutting tools requires achieving a blade angle of less than 15° with a surface roughness of Ra below 0.1 μm. CNC grinding on specialized 5-axis machine tools allows for the precise shaping of chip flutes, cutting edges, and rake faces. The accuracy of the geometry directly affects the tool life and the quality of the machining it performs.
Tip: When sharpening cutting tools after hardening, use fine-grain CBN grinding wheels (150 to 320) with a low feed rate to avoid burning the surface layer and losing edge hardness.
Precision CNC metal machining and services from CNC Partner
For demanding industrial projects, it is not just the quality of the machines that counts, but above all the experience and the range of available technologies. CNC Partner is a company with many years of experience in precision metal machining, executing both individual and serial orders involving thousands of pieces. Fast order fulfillment and delivery throughout the European Union mean that the company serves clients from France, Germany, Denmark, Switzerland, and Belgium.
Every order undergoes rigorous quality control. A price quote for an order is available within 2 to 48 hours, and the lead time ranges from 3 to 45 business days, depending on the complexity of the project.
Scope of CNC machining services
Professional CNC metal machining at CNC Partner covers four main technological areas, adapted to various industry requirements:
- Metal milling: CNC milling of complex components with the highest geometric accuracy, for both individual and serial production
- Precision turning: CNC turning of parts with varying degrees of complexity with high surface quality
- Finishing grinding: CNC grinding with dimensional accuracy down to Ra 0.63 μm, intended for hardened steel and precision components
- Wire EDM: wire EDM machining enabling precise shaping of materials with a hardness of up to 64 HRC
The CNC Partner machine park includes milling centers with a working area of up to 1700 x 900 x 800 mm, lathes with driven tools and angle heads, grinders with a working area of up to 2000 x 1000 mm, and two wire EDM machines.
CNC Metalworking Services
Clients and order fulfillment
CNC Partner services are provided to manufacturing companies, design offices ordering prototypes, and enterprises outsourcing specialized operations. Every order is shipped, and for larger contracts, the company delivers components using its own transport directly to the recipient. Fast delivery within the European Union is a standard for every order.
Before deciding to commission production, detailed information on the terms of cooperation is available on the metal machining price list page. Opinions from existing clients confirm the high quality of completed orders, and a full collection of CNC Partner client reviews is available on Google Maps. Detailed quote requests and technical consultations are accepted through the contact CNC Partner page.
How to select a steel machining method based on technological requirements?
Selecting a machining method begins with an analysis of the part’s technical documentation. Material hardness, required tolerance class, expected surface roughness (Ra), and production batch size are the four parameters that determine the appropriate technological path.
Steel hardness and choosing between grinding and CNC milling
Steel hardness measured on the Rockwell scale (HRC) is the primary selection criterion. Steel with a hardness of up to 38 HRC can be milled without major difficulties using standard carbide end mills. Above 45 HRC, milling is possible but requires end mills with special geometry and shallow depths of cut.
At hardness levels above 55 HRC, CNC grinding becomes the dominant method. Cutting forces during the milling of such hard materials are many times higher than during grinding, which leads to rapid tool wear and the risk of part deformation. Precise grinding removes material without significant radial forces, which protects the geometry of thin-walled components.
Production batch and machining time as decision factors
For one-off part production, grinding may prove justified even when milling accuracy would be technically sufficient. Machining a single shaft on a grinder takes significantly longer than milling, but this time is predictable and safe for product quality. Large production batches justify the investment in CNC grinders with automatic part changing.
Factors determining the choice of method:
- Required dimensional tolerance and IT grade of the documentation
- Material hardness after heat treatment (HRC)
- Permissible roughness Ra from the technical drawing
- Batch size and machining cycle time
- Production stage: rough machining or finishing operation
In serial production, both processes are often combined. Milling performs the roughing work and shapes the part, while CNC grinding removes the final 0.01 to 0.05 mm, bringing the dimensions to IT5 or IT6 grade. This sequence of processes minimizes grinding wheel wear and reduces grinding time to the necessary minimum.
Surface finish and Ra standards used in the industry
The Ra standard describes the arithmetic mean deviation of the roughness profile. When designing mating surfaces, technical documentation always specifies the permissible Ra. Finishing milling reaches Ra 0.8 μm as its limit, while CNC grinding goes down to Ra 0.1 μm in a standard process.
For hydraulic seal surfaces, an Ra below 0.4 μm is required. For sliding surfaces of plain bearings, the standard indicates Ra 0.2 to 0.4 μm. Both values lie below the limit achievable by milling, which makes CNC grinding a mandatory method for such applications.
Surgical instruments and medical implants require Ra even below 0.05 μm. These values can be obtained only through precision grinding with fine-grained grinding wheels and strict control of process parameters. Materials such as 440C stainless steel used for surgical tools require special grinding wheels adapted to the properties of corrosion-resistant steels.
Tip: When selecting a machining method, always start with the Ra value from the technical drawing. If the Ra is lower than 0.8 μm, CNC grinding is the only method that will ensure repeatability in serial production.
FAQ: Frequently asked questions
Can CNC grinding be used instead of milling for every steel machining process?
CNC grinding does not replace milling in every case. Milling works well for shaping, drilling holes, and rough machining of soft steel up to 38 HRC. CNC grinding enters the process as a finishing stage or the only possible method for steels hardened above 55 HRC.
Both processes complement each other in the production cycle. Milling performs the shaping work, and grinding brings the part to the required tolerance grade and surface roughness. Combining both methods reduces grinding time and extends the service life of grinding wheels.
What surface roughness Ra is achieved with CNC grinding of steel?
CNC grinding of steel allows for achieving an Ra roughness from 0.1 to 1.6 μm under standard production conditions. With very fine-grained grinding wheels and small depths of cut, Ra can drop below 0.05 μm, which is required for medical instruments and hydraulic seal components.
Finishing milling reaches Ra 0.8 μm as its limit. Any application requiring an Ra below 0.8 μm requires a grinding process. The technical drawing always specifies the permissible Ra, which is why the selection of the appropriate machining method starts with this value.
How does steel hardness influence the choice between grinding and CNC milling?
Hardness measured on the Rockwell scale (HRC) defines the limit of cost-effectiveness for milling. Steel up to 38 HRC can be milled with standard carbide end mills without major difficulties. Above 45 HRC, cutting forces increase rapidly, tools wear out faster, and the risk of part deformation rises.
At hardness levels above 55 HRC, CNC grinding becomes the only justifiable process. CBN (cubic boron nitride) grinding wheels maintain their profile shape for a long time, even with steels hardened above 60 HRC. Using the correct grinding wheel for the appropriate steel hardness class determines the quality and efficiency of the machining process.
In which industrial sectors is CNC grinding a mandatory process?
The aerospace industry requires tolerances of ±10 to ±50 μm and surface roughness of Ra 0.1 to 0.8 μm for engine components and turbine shafts. Milling does not consistently achieve such parameters in mass production. CNC grinding is a mandatory process when manufacturing precision components for the aerospace industry.
The medical industry sets even higher requirements, with Ra below 0.05 μm for implants and surgical instruments. The automotive industry requires grinding of cylinder bores, camshafts, and sealing surfaces with Ra from 0.1 to 1.6 μm. Manufacturers of precision bearings use CNC grinding for every component requiring tolerance classes IT5 to IT6.
Summary
CNC grinding prevails over milling wherever geometric accuracy and surface quality become the deciding factors for a part’s functionality. Tolerance classes IT4 to IT6, surface roughness Ra from 0.1 to 1.6 μm, and the ability to machine steel hardened above 55 HRC are three areas where grinding is an irreplaceable process. Milling remains a roughing and shaping tool, while CNC grinding completes the production cycle with full dimensional precision.
The decision on which steel machining method to choose should be based on an analysis of four parameters: material hardness, required tolerance, permissible Ra, and production batch size. At hardness levels above 45 HRC and tolerances below ±0.005 mm, CNC grinding ceases to be an option and becomes a technological necessity. Knowing the boundary between both methods allows for avoiding costly errors and shortening production implementation time.
Sources:
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