We offer solutions and technologies to carry out surface finishing processes to control the roughness of a surface. These processes are carried out to obtain a stable and predictable roughness that facilitates the adhesion of a coating, or the realization of other subsequent surface finishes. The controlled roughness produces an artificial surface with a succession of engravings and protuberances (valleys and peaks).
Controlled roughness is a concept used to refer to the manipulation and control of the rough characteristics of a surface or material, more specifically the ability to design and adjust the texture and irregularities of a surface in a precise and predictable manner.
Roughness can be intensified or smoothed, depending on the application. For example, in some cases, a rough surface is sought to improve the adhesion of paints or coatings, or to increase the holding capacity of lubricating oils on a surface. In other cases, roughness is minimized to achieve a smoother, slipperier surface.
In general, coatings with lower thicknesses require “softer” profiles, while in thicker coatings the etches generated by sandblasting are also usually more pronounced and deeper to increase adhesion.
Surface roughness control can be achieved through surface processing and finishing techniques, which allow roughness parameters, such as height and distribution of irregularities, to be adjusted to meet the specific requirements of an application. These technologies include blasting, shot blasting, sanding, polishing, chemical etching and the use of micro-manufacturing tools.
Controlled roughness has numerous applications in different fields, such as:
Controlled roughness in metal surface finishing processes involves manipulating and adjusting the roughness characteristics of a surface in a precise and predictable manner in order to achieve functional properties, improve adhesion, obtain desirable aesthetic finishes and increase fatigue resistance.
To adjust the roughness characteristics of a metal surface accurately and predictably, parameters such as surface roughness, shape and distribution of irregularities, and other factors affecting the surface texture must be controlled.
Properly handling surface roughness is important because it can have a significant impact on material properties and performance.
Some of the reasons for seeking to control roughness in these processes are as follows:
In metal surface finishing processes, various techniques are used to achieve controlled roughness, such as sanding, polishing, shot peening/shot peening, burnishing and applying chemical treatments. These techniques allow to eliminate or control the irregularities of the metal surface to achieve the desired roughness.
Each of these techniques has its own variations and specific settings depending on the application and desired roughness requirements. It is important to consider the material and geometry of the part, the target roughness, as well as any other relevant factors when selecting the appropriate technique and applying it correctly.
There are occasions when roughness control may require the combination of several techniques or processes in sequence to obtain the desired result.
To ensure that these technologies obtain the desired results, measuring instruments, such as roughness meters, are used to evaluate and verify the roughness obtained and adjust the process if necessary.
There are other more specialized and advanced techniques to achieve controlled roughness, such as microfabrication and the use of lasers. These techniques can offer greater accuracy and control in the generation of roughness on metal surfaces, but their application is usually limited to more specialized manufacturing environments.
Shot peening is a process used to improve the strength and durability of metal surfaces while controlling roughness. It consists of bombarding the surface with small particles (generally spheres or shot) at high speed and at controlled angles. These particles impact the surface, creating small plastic deformations in it and generating a residual compression layer.
Shot peening is divided into the following steps:
During shot peening, roughness is controlled in two main ways. First, the impact of the particles tends to remove the roughness and irregularities of the surface, improving the surface roughness. Second, the formation of the residual compression layer causes a contraction of the surface, which also contributes to roughness control.
It is important to note that shot peening cannot correct structural problems or deep defects in the material; its main function is to improve the resistance and durability of the surface.
Polishing is a finishing process used to obtain very smooth and shiny metal surfaces. Although the primary objective of polishing is not to generate roughness, it is possible to control roughness by proper selection of abrasives and polishing compounds.
Vibratory polishing on a vibrating drum is a widely used technique to achieve a controlled roughness on different materials, including metals. In this method, the parts to be polished are placed in a vibrating drum along with abrasive media and polishing compounds, and are subjected to vibratory movements to generate the polishing action.
It is an efficient and cost-effective option on softer metal surfaces, when a uniform finish is required, or when complex shaped parts must be processed. It is important to consider the specific application specifications and requirements before selecting this polishing technique.
The proper selection of abrasive media, polishing compounds and polishing time will allow to control the roughness obtained. It is important to adjust the polishing parameters, such as the speed and amplitude of vibration, the load on the parts and the materials used, to obtain the desired results.
In addition, vibratory polishing offers the following advantages:
It is important to note that the efficiency and effectiveness of vibratory polishing can vary depending on the nature of the parts and the characteristics of the surface to be treated.
Specialized machinery can be used, such as automatic polishers, hand polishers or wheel polishers.
Before starting polishing, it is important to prepare the metal surface properly. This may include deburring sharp edges, removing impurities and cleaning the surface to ensure a more effective polishing process.
Polishing is performed by rotary or linear movements of the metal surface in contact with the abrasive and the polishing compound. During the polishing process, controlled pressure and speed are applied to remove minor surface irregularities and achieve a smooth finish.
Often, the polishing process with these alternative techniques is divided into several stages using abrasives of different particle size and polishing compounds with different properties. This allows the larger irregularities to be removed in the initial stages and the surface to be gradually refined to obtain a controlled roughness. Each stage is performed with a specific abrasive and polishing compound, and time and pressure guidelines must be followed to achieve the desired finish.
Abrasives and polishing compounds used in polishing are essential to achieve controlled roughness. Fine abrasive materials such as aluminum oxide, diamond or silicon carbide are used and applied as a paste or suspension in water or oil. The choice of particle size and composition of the polishing compound is based on the desired roughness.
Shot blasting is a process that uses small particles, called shot, to generate a controlled plastic deformation on the metal surface. Shot blasting is widely used to improve the fatigue strength and durability of metal components.
The roughness generated by shot blasting can be controlled by adjusting key parameters such as particle size and shape, blast velocity, impact density and exposure time. These parameters are selected according to roughness requirements and material properties.
These are the key aspects to obtain a controlled roughness by shot blasting:
It is important to mention that, in both polishing and shot blasting, controlled roughness can be evaluated and measured using instruments that provide information on roughness parameters, such as average roughness (Ra), roughness height (Rz) and the distribution of irregularities on the surface.
These are the fundamental parameters used to measure roughness.
Arithmetic mean roughness (Ra)
Ra is the mean value of the absolute values of the profile heights, with respect to the midline, over the evaluation length, based on the R curve (filtered).
Ra is the most commonly used parameter to control roughness, as it provides relevant information on the following concepts:
Average roughness height (Rz)
This measurement gives the average of the heights of the peaks and valleys found on the surface of the part. It represents the mean of the absolute values of the 5 highest profile ridges and the 5 deepest valleys within the evaluation length.
Root mean square root, RMS
The root mean square takes the average of the square of the distance between peak height and mean length, and then calculates the square root of that value. This value represents the surface roughness in the form of a sine wave, each of which represents the distance from the midline.
The root mean square gives an approximate value of the roughness of the surface, but does not represent an accurate view of the final surface finish. It is related to the average roughness Ra, by a factor of 1.1.
Maximum ridge height (Rp)
It is the distance from the highest point of the profile to the midline based on the P-curve (unfiltered). It is a valid parameter to evaluate the quality of pressed fits and surface wear of parts.
Maximum profile height (Ry)
Distance between the deepest valley and the highest crest of the profile in the evaluation length, based on the P curve (unfiltered).
This parameter is very sensitive to the presence of local defects such as blows, tool breakage at that point, and other defects that may falsify the value obtained.
Maximum peak-to-valley height (Rt or Rmax)
Provides the distance between the highest peak and the deepest valley in the evaluation length based on the R-curve (filtered).
The maximum roughness depth does not give an accurate value of the surface roughness, as any scratch can distort the value obtained.
Direct method
This method uses a stylus, held perpendicular to the surface, to measure its roughness. The pencil directly generates the roughness profile and, from this profile, the values of various surface units can be calculated.
Non-contact technique
This technique uses light or sound to generate the roughness profile. Instead of a stylus, this process uses optical and vocal sensors. A light or sound wave is bombarded onto the surface, which then reflects it. It is this reflected wave that allows the surface roughness profile to be measured.
Comparison technique
This method uses the comparison of a sample of known surface roughness with samples of unknown surface roughness. The technician can then compare the surface roughness of both samples using different visual techniques and analyze them to assign value to the material. This technique is generally not accurate due to its high dependence on human subjectivity.
Technique in process
The use of electromagnetic induction to measure surface roughness is called the in-process technique. Magnetic induction is used to measure the distance between the peaks and depths of the roughness profile. However, this method is useful only for magnetic materials.
