Surfaces in manufacturing applications must remain within desired roughness limits to ensure optimum quality of parts. Surface finishing has a crucial impact on the durability and performance of the product. Therefore, it is essential to learn about the surface roughness chart and its importance.

Rough surfaces often wear and tear more rapidly. The friction levels are higher than that in smooth surfaces, and irregularities in a surface’s smoothness tend to create nucleation sites. Breaks and corrosion occurring in these sites could then cause the material to wear easily.

Conversely, there is a degree of roughness that can give room for desired adhesion. Therefore, you must never leave the surface finish up for interpretation. Suppose you think surface finish does matter for your product, this guide is for you.

Ra – Average Surface Roughness

While most people refer to Ra as Center Line Average or Arithmetic Average, it is the average roughness between a roughness profile and the mean line. This is the most commonly used parameter for surface finish. The Ra surface finish chart is also one of the most used for absolute values.

Rz – Average Maximum Height of the Profile

Unlike Ra, Rz measures the average values of the five largest differences between peaks and valleys. The measurement is done using five sampling lengths, and it helps to eliminate error since Ra is quite insensitive to some extremes.

Micrometers
Rating
Microinches
Rating
ApplicationsSurface description
25100Rough, low-grade surfaces that result from saw cutting or rough forging. Therefore, such surfaces are suitable for certain unmachined clearance areas.Raw material
12.5500These are rough, low-grade surfaces resulting from coarse feeds and heavy cuts. While the cuts come from turning, milling, disc grinding, and more.Machined, coarse
6.32.5This type of surface finish results from surface grinds, disc grinds, milling, drilling, and more. Therefore, they are for clearance surfaces with stress requirements and design permitsMachined, medium finish
3.2125The roughest kind of surface is often recommended for parts. It is also used for parts subject to vibrations, loads, and high stress.Machined, smooth
1.663Good machine roughness/finish with its production under controlled conditions. It also involves fine feeds and relatively high speeds.Machined, very smooth
0.832A high-grade machine finish, which needs close control. It is relatively easy to produce with cylindrical, centerless, or surface grinders. It is also preferred for products that do not require continuous motion or large loads.Machined, extremely fine finish
0.416High-quality surface are often produced using emery buffing, lapping, or coarse honing. These finishes are therefore great options where smoothness is of high importance.Ground, EDM
0.28Fine, high-quality surface finish produced by lapping, buffing, or honing. Machinists use this where rings and packings have to slide across the surface grain.Lapped for seal joints
0.14A refined surface that is offered using lapping, buffing, or honing. Manufacturers use it only when there are mandatory design requirements. Therefore, it is the best finish in gauge and instrument works.Lapped, extremely fine finish, perfectly smooth
0.052Most refined surface finish produced with the finest buffing, honing, or superfinishing. Thus, they are best used for fine and sensitive precision gauge blocks.Superfinishing
Burnishing diamond paste
0.0251Most refined surface finish produced with the finest buffing, honing, or superfinishing. Thus, they are best used for fine and sensitive precision gauge blocks.Superfinishing
Burnishing diamond paste

Roughness Conversion Chart

Roughness Conversion Chart