Ark Novin

Engineering Contrivance ,  Performance Sustainability

Surface Roughness

Definitions
The texture or topography of a surface is characterized by the surface roughness, waviness, and lay.
 
 
Surface Roughness
Surface roughness is a property of the material surface texture, which is recognized by an uneven topography as compared to an ideal flat surface. A rough surface is marked by finely spaced irregularities, protuberances, or ridges.
 
Surface Waviness
Waviness can be distinguished from roughness by the broader spacing between the surface irregularities.  Heat treatment, residual stress, and vibrations are some the common causes for waviness of surface texture.
 
Lay
Lay is defined as the direction of the predominant surface pattern.
 
Roughness Height
Roughness height is a measure of the height of the irregularities compared to a reference line.
 
Roughness Width
Roughness width is the distance between the two successive peaks or ridges of the predominant surface pattern, which is measured in a direction parallel to the surface.
 
Roughness Width Cut Off
Roughness width cut off is the largest spacing between the irregularities. This value should always be larger than the roughness width. The measure of waviness, on the other hand, stands outside the value of roughness width cut off.
 
Waviness Height
 Waviness height is a measure of the height of the irregularities compared to a reference line.
 
Waviness Width
Waviness width is the distance between the two successive peaks or ridges of the predominant surface pattern, which is measured in a direction parallel to the surface.
 
Surface Texture Roughness Height Visibility Some of the Possible Causes
Micrometer   Microinch 
Rough 45-160 1000-6400 Visible to the naked eye and felt upon touching   Rolling, casting in metallic molds, flame cutting,etc.
Smooth 10-40 400-1600 Visible to the naked eye but not felt upon touching Accurate Turning, milling, snagging, sawing, sand casting, hot rolling, planing, shaping, forging, etc.
Very Smooth 2.5-10 100- 640 Invisible to the naked eye Forging, drilling, chemical milling, milling, broaching, reaming, laser, boring, turning, etc.
Ultra Smooth 0.4-1 6-40 Invisible to the naked eye Electrolytic grinding, extruding, cold rolling, die casting, die forging, gravity die casting, filing, lapping, boring, turning, barrel finishing, reaming, honing, electro-polish, etc.
 

Surface Roughness Measurement Techniques:

Surface roughness or profile, Ra, is a quantitative calculation of the relative roughness measured across an area. Ra is determined via linear measurements of roughness through an area. Surface profile, Ra, is closely related to surface area roughness, Sa, in which several profiles are averaged and reported. In other words, Sa is an average of Ra though a surface.

Several methods can be utilised in order to measure the surface roughness, which can be classified into contact and non-contact techniques. In a contact-based method, which is the basic function of a profilometer, the surface roughness is measured via dragging a stylus across the surface. Electron microscopy is a common example of a device using non-contact method for surface profile measurement.

 Once the values of roughness are obtained, the relative value can be reported using several industry standard units. Surface profile, Ra, is generally reported as the arithmetic average of the peak heights and valleys from the mean line. Centre Line Average (CLA) is Ra reported in micro-inches. The value of Ra is calculated via the following formula:

 

The Root Mean Square (RMS) parameter is an alternative representation of Ra. The value of RMS is calculated using the following relation:

 

Based on the above relations, the RMS value of any given surface is approximately 11 % larger than the Ra equivalent.

Ten-point mean roughness, Rz, is an alternative parameter representing the surface roughness, in which the vertical distance from the five highest peaks to the five lowest valleys within the sampling lengths are measured and averaged based on the following formula:

 

The equivalent values of surface profile for surface roughness ISO Grade Numbers are provided in the following table:

 ISO Grade Number  Roughness Ra
 micro-meters (µm) 
Roughness Ra
 micro-inches (µin.) 
 RMS 

CLA
 micro-inches (µin.) 

N12 50 2000 2200 2000
N11 25 1000 1100 1000
N10 12.5 500 550 500
N9 6.3 250 275 250
N8 3.2 125 137.5 125
N7 1.6 63 64.3 63
N6 0.8 32 32.5 32
N5 0.4 16 17.6 16
N4 0.2 8 8.8 8
N3 0.1 4 4.4 4
N2 0.05 2 2.2 2
N1 0.025 1 1.1 1