Selecting Right Surface Roughness for CNC Machining

CNC machining is a highly accurate and precise manufacturing process capable of creating parts with tolerances as tight as 0.025 mm. As a subtractive manufacturing method, CNC machining leaves cut marks that introduce a certain level of roughness on the finished product's surface.

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What is Surface Roughness?

Surface roughness measures the average texture of a part's surface, particularly after CNC machining. Different parameters define surface roughness, with Ra (Roughness Average) being the most common. Ra is derived from the differences between heights and depths on a surface, measured in micrometres (x 10^-6 m). It is important to distinguish surface roughness from surface finish, which can be improved through various finishing methods like anodizing, bead blasting, and electroplating. Surface roughness, in this context, refers to the as-machined surface texture of a part.

How Are Different Surface Roughness Options Achieved?

A part's surface roughness after machining is not random. Specific roughness values are planned in advance, often adhering to industry standards specified in ISO 4287. These values, ranging from 25 µm to 0.025 µm, apply to all kinds of manufacturing and post-processing operations.

At Xometry Europe, we offer four surface roughness levels typically specified for CNC Machining applications:

• 3.2 µm Ra
• 1.6 µm Ra
• 0.8 µm Ra
• 0.4 µm Ra

Different applications require different roughness values. Lower surface roughness levels should be specified only when necessary because the lower the Ra value, the more machining effort and quality control is required, increasing costs and time. Specific surface roughness values typically do not include post-processing operations, as these can affect the dimensional tolerance of parts.

3.2 µm Ra

This is the standard commercial machine finish, suitable for most consumer parts. It is smooth but contains visible cut marks and is the default surface roughness unless otherwise specified. It is recommended for parts subject to stress, loads, and vibrations, as well as mating moving surfaces with light loads and slow motion. It is machined using high speeds, fine feeds, and light cuts.

1.6 µm Ra

This option has only slightly visible cut marks and is recommended for tight fits and stressed parts. It is sufficient for slow-moving and light load-bearing surfaces but not suitable for fast-rotating parts or those subject to intense vibration. Produced using high speeds, fine feeds, and light cuts under controlled conditions, this surface roughness adds approximately 2.5% to the production price for standard aluminum alloys, with potential increases for complex parts.

0.8 µm Ra

This high-grade surface finish requires very close control and higher costs. It is needed for parts exposed to stress concentration and can be used for bearings with occasional motion and light loads.

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This option typically adds around 5% to the production price for standard aluminum alloys, with potential increases for complexity.

0.4 µm Ra

This is the finest and highest quality surface roughness we offer, suitable for parts under high tension or stress, and rapidly rotating components like bearings and shafts. This surface roughness requires the most effort to manufacture and should be specified when smoothness is of primary importance.

This option generally adds approximately 11-15% to the production price for standard aluminum alloys, with possible increases for complex parts.

At Xometry, we can manufacture your CNC machined parts with any of these surface roughness options. Simply upload your model on our instant quoting platform and select your preferred surface roughness to get a quote in seconds.

Surface Roughness Tester - Stylus Question

NPL provides a comprehensive guide on The Measurement of Surface Texture Using Stylus Instruments, authored by R. K. Leach in Measurement Good Practice Guide No. 37. You can find it through a simple Google search for "NPL UK Guide no 37" or by visiting the NPL site to register and download.

Having a Laser Talysurf in the lab for several years, I learned to operate it, though I never fully understood its inner workings. For porous surfaces like oilite bushes, the roughness profile can be skewed, affecting instrument calculations. Sophisticated instruments have adjustments for this, but understanding the operation is essential. For simpler instruments, larger stylus radii can be a factory solution.

Contact us to discuss your requirements for a Surface Roughness Gauge. Our experienced sales team can help you identify the best options for your needs.