CNC grinding is one of the most accurate machining methods used in modern industry. It allows for dimensional tolerances on the order of a few micrometers, making it essential wherever precision matters. From gears to hydraulic pump components, every detail requires strict control of dimensional deviations and surface quality.
The precision achieved through CNC grinding results from a combination of advanced control algorithms, rigid machine structures, and appropriately selected grinding wheels. Numerical control systems enable the achievement of tolerances below 0.02 mm even in serial production, while maintaining full repeatability. It is precisely this feature that gives CNC grinding a special place among surface finishing methods.
Requirements regarding dimensional accuracy are constantly growing along with the development of the automotive, aerospace, and tool industries. Parts must meet rigorous geometric and roughness standards to ensure proper fit and long service life. Therefore, knowledge of the real capabilities of precision grinding is crucial when designing technological processes.
What dimensional tolerances are achievable with CNC grinding?
CNC grinding belongs to machining processes with the highest class of dimensional accuracy. The ISO standard classifies tolerance fields in classes from IT01 to IT18, where a lower number indicates higher precision. Precision grinding falls within the range of classes IT4 to IT6, which corresponds to deviations from 2 to 16 micrometers, depending on the nominal diameter and the type of process.
Tolerance ranges IT5 and IT6 obtained during precision grinding
Tolerance classes IT5 and IT6 are the basic range achieved during finishing and precision grinding. For a nominal dimension from 18 to 30 mm, class IT5 is 9 µm, and class IT6 is 13 µm. For larger dimensions, in the range from 50 to 80 mm, IT5 corresponds to 13 µm, and IT6 to 19 µm.
Dimensional tolerances in class IT4 are achievable with superfinish grinding and plunge grinding using fine-grain grinding wheels. They correspond to deviations on the order of 4 to 8 µm for dimensions up to 50 mm. Such results are realistically available on modern CNC grinders with active vibration compensation systems.
Below is a summary of tolerance values for selected classes and dimensional ranges:
| Nominal dimension (mm) | IT4 (µm) | IT5 (µm) | IT6 (µm) |
|---|---|---|---|
| up to 3 | 3 | 4 | 6 |
| 3 – 6 | 4 | 5 | 8 |
| 6 – 18 | 5 | 8 | 11 |
| 18 – 50 | 7 | 11 | 16 |
| 50 – 120 | 10 | 15 | 22 |
| 120 – 250 | 14 | 20 | 29 |
Even IT6 class allows for transition and interference fits used in shafts, bearings, and guide components. IT5 class is the standard for machine tool spindles and precision journals. IT4 class is the domain of superfinish grinding used in measuring equipment components.
Form and position deviations after surface and cylindrical grinding
In addition to dimensional tolerances, form and position deviations, i.e., geometric errors of the part, are equally important. When grinding cylindrical shafts, the achieved roundness is within the range of 0.001 to 0.003 mm. Surface flatness after surface grinding is between 0.003 and 0.01 mm per 100 mm of length.
Centerless grinding, used for precision shafts and piston rods, allows for limiting the cylindricity deviation to below 0.002 mm. Surface grinding with a wheel or the periphery of the grinding wheel reduces straightness errors to a level below 0.005 mm. These are values unattainable with milling or finish turning.
Geometric errors accumulate with dimensional deviations and, as a result, determine the final pairing of components. Therefore, when designing CNC grinding tolerances, both the IT class and the requirements regarding geometric shape are always taken into account.
Limits of CNC grinding accuracy for various steel grades
The achievable dimensional accuracy depends not only on the machine but also on the properties of the material being machined. Soft and medium-carbon steels are easier to grind and allow for routinely achieving IT6 class, and sometimes IT5. Hardened steels have higher hardness and reduce grinding wheel wear, but they generate more heat in the cutting zone.
Tool steels with a hardness above 60 HRC require grinding wheels made of regular corundum or high-grade corundum. When grinding them, dimensional deviations can range from 3 to 8 µm at IT5 class. High-speed steels, due to their high tungsten carbide content, allow for achieving IT6 class tolerances without the risk of surface burning.
How does surface roughness Ra relate to CNC grinding tolerance?
Surface roughness and dimensional tolerances are closely related. A higher class of dimensional accuracy generally requires lower Ra values, because surface irregularities directly affect the measurement of the actual dimension. Therefore, when designing a grinding process, both parameters are considered together.
Ra values of 0.1 to 0.8 µm achieved during finish grinding
Finish grinding achieves a roughness Ra in the range of 0.2 to 1.6 µm under standard production conditions. Using fine-grained grinding wheels and low feed rates, Ra values drop to 0.1 µm. Such surface finishing corresponds to classes N3–N5 according to the ISO 1302 standard.
Rough grinding generates Ra in the range of 1.6 to 6.3 µm and is mainly used to remove machining allowance. Semi-finish grinding achieves Ra from 0.8 to 1.6 µm. Only finish grinding ensures Ra below 0.8 µm, which is a requirement for most precision applications.
Types of grinding according to roughness:
- Rough grinding – Ra 3.2 to 6.3 µm, stock removal
- Semi-finish grinding – Ra 0.8 to 1.6 µm, geometry shaping
- Finish grinding – Ra 0.2 to 0.8 µm, final smoothness
- Superfinish grinding – Ra 0.05 to 0.2 µm, sealing and bearing surfaces
The differences between successive grinding stages translate not only into smoothness but also into the fatigue life of the part. Lower roughness reduces stress concentration on the surface, which improves crack resistance. Therefore, for dynamically loaded components, an Ra below 0.4 µm is often a design requirement.
ISO 286 standards and their application in evaluating grinding results
The ISO 286 standard defines the system of tolerances and fits for dimensions up to 3150 mm. It specifies both IT tolerance classes and fundamental deviations denoted by letters for holes and shafts. When evaluating CNC grinding results, it is used as a basis for verifying the dimensional compliance of manufactured components.
The tolerance zones defined by the ISO 286 standard contain values from IT1 to IT18 for 20 nominal dimension ranges. Precision grinding falls within the IT4–IT6 class range, which the standard assigns to connections requiring accurate fits. Rolling bearing connections, spindles, and guideways are designed precisely based on these classes.
Influence of grinding wheel grit on final surface roughness
The grit of the grinding wheel has a direct impact on the resulting surface roughness Ra. Coarse-grained wheels (grit 46–60) remove material quickly but leave a surface with an Ra of 1.6 to 3.2 µm. Fine-grained wheels (grit 120–220) allow for achieving an Ra of 0.2 to 0.8 µm.
Grinding wheel grits and their applications:
- Grit 46–60 – rough grinding, large stock removal
- Grit 80–100 – shaping grinding
- Grit 120–180 – finish grinding
- Grit 220 and finer – superfinish grinding and lapping
The choice of grit is not the only factor affecting Ra. The hardness of the grinding wheel bond, the peripheral speed of the wheel, and the use of coolant are also significant. A grinding wheel that is too hard does not regenerate properly, which leads to an increase in roughness despite the fine grain. Regular dressing of the grinding wheel maintains the consistency of cutting parameters.
Measurement of roughness and tolerances on the production line
Quality control in CNC grinding requires appropriate measurement methods. Roughness is measured using contact or optical profilometers, which provide Ra, Rz, and Rq values. Dimensional tolerances are checked with pneumatic micrometers, dial indicators, or coordinate measuring machines.
In modern facilities, in-process measurement is used, meaning directly on the grinder during machining. Active compensation systems correct the dimension in real time based on the signal from the sensor. This eliminates errors resulting from grinding wheel deflection and thermal deformations of the machine.
What determines dimensional accuracy in CNC grinding?
Many factors simultaneously influence dimensional accuracy in CNC grinding. The machine, material, machining parameters, and environmental conditions create a complex system of dependencies. Understanding these relationships allows for conscious process planning and helps avoid the most common errors.
Grinding machine rigidity and dimensional repeatability of parts
The static and dynamic rigidity of a CNC grinding machine directly affects the dimensional repeatability of machined parts. Any deflection in the machine-fixture-part system causes dimensional deviations from the target value. Precision grinders are built from gray cast iron or granite, which effectively dampen vibrations and limit deformations.
Contact rigidity in the joints of guideways, the spindle, and the table is just as important as the rigidity of the spindle itself. Backlash in the drive system translates into positioning repeatability errors. CNC grinders equipped with ball screws and linear roller guideways achieve positioning repeatability on the order of 0.001 mm.
Machining temperature and its impact on dimensional deviations
Heat generated during grinding is one of the main factors reducing dimensional accuracy. Studies show that after an hour of continuous operation, a change in coolant temperature from 17°C to 45°C causes an increase in dimensional deviations of up to 7 µm. The thermal expansion of the spindle and the part changes the effective cutting dimension without changing machine settings.
The ambient temperature of the production hall also matters. Fluctuations of 1°C can change the length of the spindle by 0.5 to 2 µm. Therefore, precision grinding of IT4 and IT5 classes is performed in air-conditioned rooms maintaining a temperature of 20°C ±1°C.
Using an efficient cooling system with coolant temperature control helps limit the impact of heat on the process. The coolant cools both the cutting zone and the machine body. Chilling units connected to the coolant tank maintain a constant temperature and stabilize the dimensions of the machined parts.
Before grinding elements in class IT5 or higher, it is worth performing a machine warm-up lasting at least 20 minutes so that the thermal stabilization of the spindle system reduces the risk of dimensional deviations after starting production.
Selection of grinding parameters for maintaining tight tolerances
The peripheral speed of the grinding wheel, depth of cut, and table feed create a triangle of parameters that determines grinding quality. An excessive depth of cut increases cutting forces and system deflection, which translates into dimensional errors. Too low a feed rate extends the contact time of the grinding wheel with the material and generates excessive heat.
Principles for selecting parameters:
- Depth of cut for finish grinding – from 0.002 to 0.010 mm per pass
- Peripheral speed of the grinding wheel – from 25 to 35 m/s for conventional grinding wheels
- Longitudinal table feed – from 1/3 to 2/3 of the grinding wheel width per revolution
- Coolant flow rate – minimum 10 l/min for surface grinding
Adjusting parameters between rough and finish grinding is crucial for achieving tight tolerances. Rough grinding removes 80–90% of the allowance with increased feed, while finish grinding performs a precise dimensional correction. This division allows for combining efficiency with accuracy.
Tip: When finish grinding, it is worth turning off the infeed for the last few passes and performing spark-out grinding without additional grinding wheel penetration. Such “sparking out” eliminates system deflection and stabilizes the final dimension.
Precision CNC metal machining at CNC Partner
CNC Partner is a specialized company with many years of experience in precision metal machining. The facility handles both individual and serial orders covering thousands of pieces. We serve clients from manufacturing companies, design offices, and enterprises outsourcing production overflows. Orders are fulfilled via shipping with fast delivery throughout the European Union.
Every order undergoes rigorous quality control before shipment. Quotes are provided within 2 to 48 hours, and lead times range from 3 to 45 days, depending on the complexity of the project. This approach allows for the efficient execution of both simple series and complex precision parts.
Scope of machining services
The company provides professional CNC metal machining in four main technological areas. Each method is tailored to the specific material and geometric requirements of the part.
Available machining methods:
- CNC Milling – precise shaping of flat and spatial components, including injection molds
- CNC Turning – machining of rotating parts from metals and plastics, with driven tools
- CNC Grinding – surface finishing to a roughness of Ra 0.63 µm and high tolerance classes
- Wire EDM – precise cutting of materials with a hardness of up to 64 HRC, unattainable by other methods
Each of the listed services is performed on modern machines that are regularly upgraded, which guarantees consistent dimensional repeatability. The machine park includes grinders with a working area of up to 2000 x 1000 mm and milling machines with areas up to 1700 x 900 x 800 mm.
CNC Metalworking Services
Quality, Execution, and Contact
The company has gained recognition from clients in many European Union countries, including France, Germany, Denmark, Switzerland, and Belgium. Positive feedback confirms the high quality and punctuality of completed orders, which can be verified in the customer reviews of CNC Partner. Detailed information regarding the scope of services and terms of cooperation is available on the CNC machining price list page.
Orders are fulfilled via shipping with delivery within 48 hours within the European Union for standard orders. For larger contracts, the company organizes its own transport directly to the client’s headquarters. Any questions regarding orders, technical specifications, and production capabilities can be directed through the form on the contact CNC Partner page.
How do CNC grinding tolerances compare when machining different materials?
Different materials react differently to CNC grinding, which directly affects the achievable dimensional tolerances and surface quality. The hardness, thermal conductivity, and microscopic structure of the material determine the selection of the grinding wheel, parameters, and machining method.
Achievable accuracy in grinding hardened and tool steels
Hardened steels with a hardness of 58–65 HRC are among the most frequently ground materials in the tool industry. CNC grinding allows for achieving dimensional tolerances of IT5 and IT6 classes with Ra from 0.2 to 0.8 µm on these materials. The high hardness of the material helps avoid surface plasticization, which promotes geometric stability.
A risk when grinding hardened steel is tempering burn, which is a local reduction in hardness caused by excessive heat. It occurs with overly aggressive parameters and a lack of coolant. It is detected by nital etching tests or magnetic methods. Burnt surfaces require re-heat treatment.
High-speed tool steels (HSS) containing tungsten, molybdenum, and vanadium are ground with regular corundum or ceramic aluminum oxide grinding wheels. The achieved dimensional accuracy is IT5 class with tolerances from 4 to 13 µm for typical working dimensions. CNC Partner uses these methods in the machining of precision tools and injection molds.
When grinding hardened steel, a water-based coolant with corrosion inhibitors should be used, and its concentration should be checked regularly. Coolant that is too diluted does not dissipate heat effectively and increases the risk of burns that affect dimensional deviations.
Grinding tolerances for technical ceramics and cemented carbide
Technical ceramics, such as aluminum oxide or silicon nitride, are characterized by hardness comparable to tool steels, but they are brittle. Grinding ceramics requires diamond grinding wheels and slow feed rates. The achieved dimensional tolerances range from ±0.002 to ±0.005 mm, which corresponds to the boundary between IT5 and IT6 classes.
Cemented carbides based on tungsten carbide are materials with a hardness of 85–92 HRA. Diamond grinding of these materials allows for achieving dimensional accuracy at the level of ±0.001 mm with a properly selected grinding wheel and a rigid grinding machine. Ra roughness reaches values from 0.1 to 0.4 µm.
Ceramics and carbides are materials that are not resistant to tensile stresses, so cutting forces must be minimized. The grinding wheel speed is selected so that cutting occurs through micro-chipping rather than plasticization. Incorrect parameters lead to subsurface micro-cracks that reduce the strength of the component.
Tip: When grinding cemented carbide, it is worth regularly checking the condition of the diamond grinding wheel with a magnifying glass. A dulled diamond grain does not cut but rubs against the material, which generates heat, cracks, and dimensional deviations exceeding the assumed tolerance class.
FAQ: Frequently asked questions
What is the minimum dimensional tolerance achievable with CNC grinding?
CNC grinding allows for achieving dimensional tolerances in the range of IT4 to IT6 classes according to the ISO 286 standard. For typical working dimensions from 18 to 50 mm, the IT5 class corresponds to a deviation of 11 µm, and the IT4 class is 7 µm. By using superfinish grinding and fine-grained grinding wheels, it is possible to go below 5 µm for small nominal dimensions.
Such tight tolerances require meeting several conditions simultaneously. The machine must have high static and dynamic rigidity, and machining should take place under stable thermal conditions. The production hall temperature, maintained at 20°C ±1°C, eliminates errors resulting from the thermal expansion of the part and the grinder spindle.
Why does Ra roughness matter when evaluating CNC grinding tolerances?
Surface roughness Ra directly affects the measurement result of the actual part dimension. Microscopic surface irregularities fall within the dimensional tolerance field and can distort the reading of the measuring instrument. Therefore, for tolerance classes IT5 and higher, an Ra roughness below 0.8 µm is required.
Finish grinding typically achieves an Ra of 0.2 to 0.8 µm, while superfinishing allows for an Ra below 0.2 µm. Lower roughness reduces the risk of measurement errors and improves the fatigue life of the component. This translates into a longer service life for precision moving joints, bearings, and seals.
What factors most often cause dimensional tolerances to be exceeded in CNC grinding?
The most common cause of exceeding tolerances is the heat generated in the cutting zone. An increase in coolant temperature of just over a dozen degrees Celsius can increase dimensional deviations by several micrometers. Overly aggressive grinding parameters, such as high depth of cut and high feed rate, exacerbate this effect.
Another factor is the condition of the grinding wheel. A dull grain does not cut but rubs against the material, which causes vibrations and process instability. Regular dressing of the grinding wheel and performing spark-out grinding without infeed on the final passes effectively eliminates errors accumulated by the deflection of the grinder, fixture, and part system.
Machine vibrations and backlash in the drive system are the third group of causes. Linear guides with play or worn spindle bearings generate positioning errors that exceed the intended tolerance class. Regular calibration and maintenance of the CNC grinder are prerequisites for maintaining repeatable accuracy.
Is CNC grinding suitable for brittle materials, such as technical ceramics?
CNC grinding is the appropriate method for machining technical ceramics, but it requires diamond grinding wheels and precisely selected parameters. Ceramics, unlike metals, do not deform plastically but crack. Excessive cutting forces lead to the formation of subsurface microcracks, which reduce the strength of the component.
With properly performed grinding of aluminum oxide and silicon nitride ceramics, the dimensional tolerances achieved range from ±0.002 to ±0.005 mm. The Ra roughness is in the range of 0.1 to 0.4 µm when using fine-grained grinding wheels. Tungsten carbide-based cemented carbides allow for tolerances of even ±0.001 mm, which corresponds to the limit of the IT4 class.
Summary
CNC grinding allows for achieving dimensional tolerances in classes IT4 to IT6, which means deviations from 2 to 22 µm depending on the nominal dimension and the type of process. The Ra roughness is in the range of 0.1 to 1.6 µm, and with superfinishing, it drops below 0.2 µm. This accuracy is possible thanks to a combination of rigid machines, properly selected grinding wheels, and stable thermal conditions.
The results achieved depend on the material being machined, the class of the CNC grinder, and the technological discipline of the entire process. Hardened steel, technical ceramics, and cemented carbide require different approaches, but each of these materials allows for industrial precision that meets ISO 286 standards. Conscious management of grinding parameters, cooling, and quality control is the foundation of repeatable production of precision parts.
Sources:
- https://en.wikipedia.org/wiki/IT_Grade
- https://en.wikipedia.org/wiki/Engineering_tolerance
- https://en.wikipedia.org/wiki/Engineering_fit
- https://pl.wikipedia.org/wiki/Tolerancja_wymiaru
- https://www.iso.org/obp/ui/
- https://www.ijert.org/optimization-of-surface-grinding-process-parameters-by-taguchi-method-and-response-surface-methodology
- https://ijrtsm.com/wp-content/uploads/2022/09/Shivam-07-11.pdf
- https://www.scirp.org/journal/paperinformation?paperid=132614
- https://www.academia.edu/110639728/Thermal_Stiffness_a_Key_Accuracy_Indicator_of_Machine_Tools
- https://www.scitepress.org/Papers/2018/75347/75347.pdf
- https://kesugroup.com/machining-precision-and-tolerance-grades-explained/