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.
This measurement is most commonly shown as “Ra” for “Roughness Average” and that value is used to determine compliance of equipment with various industry standards.
Commonly specified values are: - Ra < 3.2 µm = very rough finish possible - Ra < 1.2 µm = a relatively easy to achieve finish - Ra < 0.8 µm = a fairly fine finish; I think this is the most common limit in practice to indicate a maximum Ra value.
A surface finish of 32 means that the surface has an average roughness of 32 microinches. A roughness of 32 microns is the equivalent of 1/32 inch. The closest surface finish to a 32 finish would be a 6 surface finish and smoother.
Ra measures within a certain sampling length the average of the peaks and valleys of the metal surface, including the deviation from the mean line. That can be done with a profilometer or laser scanner. The greater the deviations, the rougher the surface and if the Ra is small, the surface is smooth.
Ra (average roughness) measures the deviation of a surface from a mean height. The horizontal line through the profile represents the arithmetic mean height. The blue areas represent the deviations from that line. Ra, then, is the total blue area divided by the length of the profile.
A smooth, shiny surface — a mirror finish — would produce an Ra measurement of around 6 to 12, while stainless-steel with a standard, brushed finish would likely produce a measurement of 20 to 30 Ra.
The surface is completely free of grit lines, achieving the perfect mirror finish. This particular finish is most often applied to stainless steel for use in architecture and design projects. The #8 Mirror finish is considered the industry standard for a highly reflective, mirror appearance.
Grinding is a machining process specially indicated for finishing operations in hard materials, in order to obtain low surface roughness (Ra 0.1 m to 2 m) and tight tolerances. The cutting tool is the grinding wheel which is formed by abrasive particles attached in a bond.
The mirror surface is generally between Ra0. 8-0.08um when removed, while the non-cutting method (with mirror tool) is generally between Ra0. 4-0.05um. There is no limit to the hardness of the material being processed.
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.
Ra values most commonly measure different surface roughness levels. A surface roughness chart can show different types of surface finish with Ra roughness values ranging from 12.5 μm Ra (very rough) to 0.4 μm Ra (very smooth).
One of the popular finishes is the Ra 30 surface finish or the #4 finish. It is also known as satin finish. It is an electronic polishing method and produces very low roughness. Hence, it is a very fine finish used in a variety of industries. It exhibits a 180-grit grain, which renders a dull finish.
In the world of seals, surface roughness is typically used to describe the finish condition of a sealing surface. The Roughness Average (Ra) value is most commonly used parameter, expressed in micro inches or micrometers. The lower the value, the smoother the surface.
RMA(RA) Stand for Root Mean Square(Roughness Average), it is a term used to measure the roughness of a surface,RMS 63 is a smooth surface finish, RMS 125 is an average manufacturing surface, RMS 25 is a very clean smooth surface finish.
For precise and consistent results, the surface finish should be specified in a range or maximum level of roughness average (Ra). This is normally expressed in micrometers (Microns) or microinches.
For comparison, a good surface finish from sand casting is 250 μin (6.4 μm Ra).
Ra is defined as roughness average and is the most accurate measure of surface finish roughness. We measure the Ra using a surface roughness tester or profilometer. Often times the desired level of polish is called out in terms of grit.
Ra, the roughness average, is a common measurement to report surface roughness. The Ra is an average calculated from the peaks and valleys measured along the surface, in units of micrometres (μm) or microinches (μin).
Average roughness is calculated by finding the average deviation of the peaks and valleys, indicated in the shaded areas. The valleys are then inverted to obtain a positive value. The horizontal mid-line represents the average roughness, Ra, of this surface.
Term RMS refers to the mathematical Root Mean Square which is an average of peaks and valleys of a materials surface profile. Ra stands for roughness average. Ra is a average of only peaks; therefore, to get an Ra, multiple your 3.2 to 6.3 by 2 to get 6.4 to 12.4 Ra.
Ra is the average roughness of a surface. Rz is the difference between the tallest “peak” and the deepest “valley” in the surface.
Sa is the extension of Ra (arithmetical mean height of a line) to a surface. It expresses, as an absolute value, the difference in height of each point compared to the arithmetical mean of the surface.
Typical absolute roughness values are ε = 0.0006 in for new and ε = 0.009 in for used well tubing. There are several formulae describing friction factors for different flow conditions.