Grinding parameters and finishes

Grinding is an abrasion process that uses a rotating wheel and achieves flatness tolerances below ±0.0025 mm on surfaces of 127 x 127 mm (5 x 5 in). Cutting action occurs through the interaction of abrasive grains with the material, generating microscopic chips and intense heat that requires abundant coolant. It is mainly applied to improve surface finish, correct deformations after hardening, and machine parts of high hardness that cannot be worked by conventional material removal.

Operating parameters

The parameters that govern grinding include depths of pass between 0.005 and 0.1 mm and wheel speeds between 15 and 35 m/s, adjusted according to the material. The following table lists typical values for the most common materials.

Material	Process	Wheel speed	Depth of pass	Feed
Carbon steel (annealed)	Surface grinding	28 – 33 m/s / 5512 – 6496 ft/min	0.01 – 0.05 mm / 0.0004 – 0.002 in	15 – 25 m/min / 49 – 82 ft/min (longitudinal)
Alloy steel hardened (50‑60 HRC)	External cylindrical grinding	30 – 35 m/s / 5906 – 6890 ft/min	0.005 – 0.02 mm / 0.0002 – 0.0008 in	0.02 – 0.1 mm/rev / 0.0008 – 0.004 in/rev
Gray cast iron	Surface grinding	20 – 25 m/s / 3937 – 4921 ft/min	0.01 – 0.04 mm / 0.0004 – 0.0016 in	10 – 20 m/min / 33 – 66 ft/min (longitudinal)
Stainless steel (304)	Surface grinding	25 – 30 m/s / 4921 – 5906 ft/min	0.01 – 0.03 mm / 0.0004 – 0.0012 in	10 – 18 m/min / 33 – 59 ft/min
Aluminum	Grinding with open‑pore wheel	15 – 20 m/s / 2953 – 3937 ft/min	0.02 – 0.1 mm / 0.0008 – 0.004 in	20 – 35 m/min / 66 – 115 ft/min

Surface finishes

Grinding can achieve surface roughness Ra between 0.08 and 1.6 µm (3.1 to 63 µin) depending on the abrasive grain size. The following table relates the U.S. mesh grain size to the average roughness (Ra) obtainable under normal operating conditions.

Grain size (U.S. mesh)	Approximate mesh opening	Achievable surface roughness (Ra)
46	354 µm / 0.0139 in	0.7 – 1.1 µm / 27.6 – 43.3 µin
60	250 µm / 0.0098 in	0.35 – 0.7 µm / 13.8 – 27.6 µin
80	177 µm / 0.0070 in	0.2 – 0.4 µm / 7.9 – 15.7 µin
100	149 µm / 0.0059 in	0.17 – 0.25 µm / 6.7 – 9.8 µin
120	125 µm / 0.0049 in	0.14 – 0.2 µm / 5.5 – 7.9 µin
150	105 µm / 0.0041 in	0.12 – 0.17 µm / 4.7 – 6.7 µin
180	88 µm / 0.0035 in	0.1 – 0.14 µm / 3.9 – 5.5 µin
220	74 µm / 0.0029 in	0.08 – 0.12 µm / 3.1 – 4.7 µin

Typical tolerances

The dimensional precision of grinding covers IT4 to IT6 grades, and can maintain flatness tolerances below ±0.0025 mm on a steel surface of 127 x 127 mm (5 x 5 in) properly supported. On cylindrical parts, roundness of the order of 0.001 mm and very tight concentricities are achieved, meeting the requirements of bearings and high-demand components.

Compatible materials

Grinding is effective on materials whose hardness exceeds 45 HRC, extending the application range to virtually any metallic material and many non-metallic ones. The list of processable materials includes:

• Carbon, alloy and tool steels (any hardness).
• Stainless steels and heat-resistant alloys (nickel, cobalt).
• Gray and ductile cast irons.
• Sintered carbides (with diamond wheels).
• Advanced ceramics (alumina, zirconia, silicon nitride).
• Glass and some metal-matrix composites.

Advantages

Grinding reduces surface roughness from typical rough turning values (Ra ≈ 6.3 µm) to values below 0.1 µm, an improvement of more than 60 times. It also offers extremely tight dimensional tolerances (IT4-IT6), the ability to machine hard materials without significant plastic deformation, and excellent control of final geometry on flat and rotational parts.

Limitations

The temperature at the cutting interface can exceed 1000 °C (1832 °F), requiring abundant and constant cooling to avoid thermal damage to the part and wheel. The main limitations include:

• Low material removal rate, lower than in milling or turning.
• High cost of precision wheels and grinding machines.
• Possibility of introducing residual surface stresses that require subsequent relief.
• Risk of vibration and surface burn if parameters are not rigorously controlled.

Selection criteria

When dimensional tolerances below 0.01 mm (0.0004 in) or a finish better than Ra 0.8 µm (31.5 µin) are required, grinding is usually the ideal choice. This process is preferred over others if:

• Material hardness exceeds 50 HRC.
• Distortion after hardening must be corrected.
• Surfaces must withstand cyclic loads and a fine roughness is needed to avoid stress concentrators.
• The geometry to be machined cannot be achieved with single-point tools to the required precision.

Frequently Asked Questions (FAQ)

What surface roughness (Ra) can be achieved with precision grinding?

Grinding delivers finishes with Ra between 0.1 µm and 1.6 µm (3.9 to 63 µin), depending on grain size and pass strategy.

What flatness tolerance does grinding guarantee on a 127 x 127 mm surface?

With proper clamping, flatness less than ±0.0025 mm (±0.0001 in) is achieved on a steel plate of 127 x 127 mm (5 x 5 in).

What abrasive grain size (mesh) is needed to obtain a Ra 0.4 µm finish?

A 80 mesh grain (opening of 177 µm / 0.007 in) allows reaching an Ra in the range of 0.2 to 0.4 µm (7.9 to 15.7 µin) under normal conditions.

How much material is removed per pass in rough and finish grinding?

In roughing, depths of 0.02 to 0.05 mm (0.0008 to 0.002 in) are used, while in finishing it is reduced to 0.005 – 0.02 mm (0.0002 – 0.0008 in).

What is the recommended peripheral wheel speed for grinding hardened steel?

The optimal speed is between 30 and 35 m/s (5906 – 6890 ft/min) for hardened steels, ensuring effective cutting without burn.

What depth of pass is recommended to avoid surface burn in surface grinding?

To avoid thermal damage, it is advised not to exceed 0.025 mm (0.001 in) per pass on carbon steel, combined with abundant cooling.

References

• engineeringtoolbox.com: https://www.engineeringtoolbox.com/surface-roughness-abrasive-grain-size-d_1648.html
• efunda.com: https://www.efunda.com/processes/machining/grind.cfm
• manufacturingguide.com: https://www.manufacturingguide.com/en/ordlista/surface-roughness