What is surface roughness?

Surface roughness is a measure of the average texture of a part’s surface, one of the most ubiquitous of these is Ra (Roughness average), which is derived from the differences between heights and depths on a surface. Ra surface roughness is measured microscopically and is usually in micrometres (x 10~⁶ m). Surface roughness here refers to the as-machined surface texture of a part.

Surface roughness values are planned in advance. In manufacturing, there are specific Ra values that are considered industry standards, as specified in ISO 4287. These are the values that may be specified during CNC machining. They range from 25 um to 0.025 um and apply to all kinds of manufacturing and post-processing operations.

USA Ra (um)	USA Ra (micro inch)	USA RMS (micro inch)	Machining Finish Method
50.0	2000	2200	The most coarse machining or good rough casting surfaces
25.0	1000	1100	Machining marks very obvious. Rough turning, boring, planning, drilling.
12.5	500	550	Machining marks obvious. Rough turning, planning, milling, drilling.
8.00	320	352	Machining marks visible. Normal turning, boring, planning, drilling, grinding.
6.30	250	275
5.00	200	220	Machining marks not obvious, but still visible. Normal turninq. borinq. planninq. drillinq, qrindinq.
4.00	160	176
3.20	125	137.5
2.50	100	110	Machining marks blur, but direction obvious. Number controlled turning, boring, planning, drilling, grinding.
2.00	80	88
1.60	63	69.3
1.25	50	55	Machining marks direction blur, but still visible. Number controlled turning, boring, planning, drilling, grinding.
1.00	40	44
0.80	32	35.2
0.63	25	27.5	Machining marks direction blur. Reaminq, qrindinq, borinq, rollinq.
0.50	20	22
0.40	16	17.6
0.20	12.5	13.75	Machining marks direction invisible. Grinding, super machining.
	10	11
	8	8.8
0.10	4	4.4	Surface dark gloss. Super machining.

There are four surface roughness levels that are also the values typically specified for CNC Machining applications:

3.2 μm Ra

1.6 μm Ra

0.8 μm Ra

0.4 μm Ra

The lower the Ra value, the more machining effort/operations and quality control will be required. Therefore, lower surface roughnesses should only be specified when necessary.

Otherwise, they can significantly drive up machining costs and time.

Post-processing operations aren’t usually applied when specific surface roughness values are required. This is because these operations cannot be controlled precisely and may affect the dimensional tolerance of parts.

3.2 μm Ra

3.2 μm Ra is the recommended maximum surface roughness for parts subject to stress, loads, and vibrations. It can also be used for mating moving surfaces when the load is light and motion slow. It is machined using high speeds, fine feeds, and light cuts.

1.6 μm Ra

Usually, there are only slightly visible cut marks with this option. This Ra rating is recommended for tight fits and stressed parts, and is sufficient for slow-moving and light load-bearing surfaces. However, it is not suitable for fast rotating parts and parts subject to intense vibration. This surface roughness is produced using high speeds, fine feeds, and light cuts under controlled conditions.

0.8 μm Ra

Considered high grade, this surface finish requires very close control to produce, costing more. It is required for parts that are exposed to stress concentration. When the motion is occasional and the loads are light, then it can be used for bearings.

0.4 μm Ra

his is the finest (“least rough” in technical terms) and highest quality surface roughness that are offered. It is suitable for parts that are under high tension or stress. It is also required for rapidly rotating components such as bearings and shafts. This surface roughness takes the most effort to manufacture and should only be specified when smoothness is of primary importance.