Ra and RMS | Surface Roughness

Ra and RMS are measurements of surface roughness. Both measurements use a profilometer, though calculations for Ra and RMS differ. Ra is the roughness average and RMS is the root mean square of a surface. Both measurements are based on the heights of peaks and valleys on the surface, however they each use the measurement in a different way. One peak on a surface will effect the RMS value more so than the Ra value.

The distance between peaks and valleys determines surface roughness. The larger the distance, the rougher the surface. The roughness of a surface determines how an object will interact with its surrounding environment. Rough surfaces tend to wear down more quickly in addition to having higher friction, though they tend to have better adhesion properties.

Ra, the arithmetic average roughness, is the most widely used parameter for roughness. Ra provides an overall description of the surfaces height variations and is less sensitive to large peaks and valleys. It is expressed in either micrometers or microinches.

The RMS average measures the average height deviations of the mean line. RMS is calculated by measuring the height of a surface’s microscopic peaks and valleys, squaring those measurements, determining the average of the squares, and finding the square root of that number. 

Ra and RMS Surface Roughness Test Requests

• Metallurgical materials laboratory needed to quantify surface roughness of thin metallic strips (interferometry, atomic-force microscopy AFM, etc.)
• Health company needs materials laboratory needed for non-destructive NDT optical testing including surface roughness analysis of non-flat surface.
• ISO metallurgical laboratory needed for surface roughness testing on Aluminum. Quantity: 10 – 15 pcs, size 120 mm x 95 mm. Material: Aluminum. Surface finish Rz and Ra.
• LONG-TERM TESTING Metallurgical laboratory needed for stainless steel tubing certification and testing for certain RA value, surface roughness
• LEGAL Materials laboratory needed for measuring surface of a flexible thin film material to characterize a few topographical features such as surface roughness Ra. Contact mode AFM testing in ambient; non-contact optical interferometric profilometer such as Wyko profilometer. Capable to scan 200×200 um area of sample, with resolution down to 0.5 nm roughness for smoothest materials.
• Mechanical Laboratory needed for motor testing of 24 motor assemblies measured for surface roughness (required spec. is Ra 0.4µm max). The surface we would like to measure is on a 8mm shaft within an axial location of .188, we would like both axial and circumferential measurements on these parts.
• Physical Laboratory needed for surface roughness testing on 40 parts. Tested on the cylindrical body and chamfeer on the part. I would prefer the testing to be done non-contact and results in the Ra and Rz mode.
• Physical laboratory needed for roughness testing.

See more Ra and RMS surface roughness requests.

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