ISO 21920 series
ISO 21920 surface texture grades with Ra and Rz values, the process that reaches them and what the number actually controls.
ISO 21920 series · 2021 · unit: µm
Ra and Rz describe different profile parameters, so the ranges are typical values for the grade and not a guarantee for every process.
| Roughness grade | Ra nominal (µm) | Ra range (µm) | Rz nominal (µm) | Rz range (µm) | Typical use | Typical process |
|---|---|---|---|---|---|---|
| N1 | 0.012 | 0.008 – 0.020 | 0.025 | 0.020 – 0.040 | Precision optics, gauge parts | Superfinishing, lapping |
| N2 | 0.025 | 0.020 – 0.040 | 0.05 | 0.040 – 0.080 | Precision optics, gauge parts | Superfinishing, lapping |
| N3 | 0.05 | 0.040 – 0.080 | 0.1 | 0.080 – 0.160 | Optical and hydraulic surfaces | Grinding, honing |
| N4 | 0.1 | 0.080 – 0.160 | 0.2 | 0.160 – 0.320 | High-precision turned and ground parts | Fine turning, grinding |
| N5 | 0.2 | 0.160 – 0.32 | 0.4 | 0.320 – 0.630 | Seal and bearing surfaces | Fine turning, boring |
| N6 | 0.4 | 0.32 – 0.64 | 0.8 | 0.630 – 1.25 | Hydraulic and pneumatic parts | Fine turning, milling |
| N7 | 0.8 | 0.64 – 1.25 | 1.6 | 1.25 – 2.50 | General machined surfaces | Turning, milling |
| N8 | 1.6 | 1.25 – 2.50 | 3.2 | 2.50 – 5.00 | General machined surfaces | Turning, milling |
| N9 | 3.2 | 2.50 – 5.00 | 6.3 | 5.00 – 10.0 | General engineering parts | Turning, milling |
| N10 | 6.3 | 5.00 – 10.0 | 12.5 | 10.0 – 20.0 | Semi-finished and covered surfaces | Rough turning, milling |
| N11 | 12.5 | 10.0 – 20.0 | 25 | 20.0 – 40.0 | As-forged and as-cast surfaces | Forging, casting |
| N12 | 25 | 20.0 – 40.0 | 50 | 40.0 – 80.0 | As-forged and as-cast surfaces | Forging, casting |
| N13 | 50 | 40.0 – 80.0 | 100 | 80.0 – 160 | As-forged and as-cast surfaces | Forging, casting |
Ra is the arithmetic average of the profile and Rz is the peak-to-valley height, so they are not interchangeable one to one. Always state the parameter, the value and, for Rz, the cutoff or sampling length. The table links each ISO 21920 grade to a typical use and to the process that normally reaches it.
Reference source
ISO 21920 series
Ra is the arithmetic average of the absolute deviation from the mean line. Rz is the maximum height of the texture profile, measured from the deepest valley to the highest peak. Ra is the normal default because it is stable and easy to measure, while Rz reacts strongly to isolated defects and to the sampling length chosen.
Match the value to what the surface actually does. A sealing face usually needs Ra 0.8 to 1.6 µm, a sliding bearing surface Ra 0.4 to 0.8 µm, an optical or gauge surface Ra 0.05 to 0.2 µm, and a general machined surface Ra 3.2 to 6.3 µm is normally acceptable.
Yes, but always state the cutoff or sampling length next to it, for example Rz 6.3 or Rzmax 6.3, because Rz depends heavily on how the trace length is chosen. If a surface has to be both inspected by the customer and produced by the supplier, Ra is the safer choice.
A good three-axis milling operation with sharp carbide tooling reaches Ra 0.8 to 1.6 µm on aluminium and 1.6 to 3.2 µm on steel. Turning with a wiper insert reaches Ra 0.4 µm, and grinding or lapping is needed below Ra 0.4 µm. Any tighter value should be quoted as a finishing operation with its own cost.
Yes, strongly. Turning and milling leave feed marks whose pitch and depth follow the feed per tooth, so a finer feed gives a finer finish. A boring operation can push the finish below the theoretical value from the tool because of the change in chip thickness. The values in the table are typical, not guaranteed.
The ISO 21920 series replaced the old ISO 1302 roughness grade numbers with a continuous scale. An old grade N7 corresponds roughly to Ra 0.8 µm, N8 to Ra 1.6 µm and N9 to Ra 3.2 µm. When you see an old drawing, convert the N number to the nearest Ra value and then state Ra.
Yes. Paint, powder coating, anodising and plating all add thickness and change the surface. Specify the roughness on the substrate where you need a reliable bond, and specify the coating thickness separately with its own tolerance.
Send the drawing and the requirements to Evefab’s engineering team. We reply with manufacturability feedback, not just a price.
