Surface Roughness: Tudo o que precisa de saber

When components are manufactured along with dimensions and tolerances, surface finish is also a crucial aspect. The right surface roughness is maintained to achieve durability, performance, and quality.

If you are not aware of anything about surface roughness and want to know what is surface roughness and what its units are, read the guide below.

What is Surface Roughness in Machining and Manufacturing?

The surface nature is called surface finishing or surface roughness. It is the deviation that the surface may have compared to a perfectly flat plane. The term surface finish refers to the level of finish and polishing that is applied to a component. The use of components is dependent on their surface roughness.

Units of Measurement for Surface Roughness (Ra)

What are Ra Units?

Ra refers to roughness average, which is a unit that measures surface roughness in manufacturing. It provides a way to quantify the smoothness and texture of a surface and gives an insight into its appearance and functionality. It is measured in micro-inches or micrometers.

Ra Value in Surface Roughness

A value in surface roughness represents the deviation on average that the surface has from the mean line from a given sampling length.

Terms of Surface Roughness

The different terms used in determining the surface roughness are given below.

Ra

Ra is the most common measurement unit for assessing surface finish. It represents the arithmetic mean deviation, which includes the roughness of a surface. It quantifies the variation of roughness from the mean line.

Rz

Rz is a measure of the maximum average height of the surface of a component. It represents the average of five deviations that lie between the lowest and highest peaks. Sometimes, when Ra is inconsiderate due to imprecise measurements, the Rz unit helps to prevent errors.

Rp

Rp represents the maximum peak height from the mean line. It is the height of the tallest peak, and it is a very crucial parameter as it determines the maximum surface irregularity. It is measured in micrometers or microinches.

Rv

Rv is the maximum depth of the valley; it measures the deepest value in the sampling length from the mean line. It indicates the depth of indentation on a given surface, and factors like stress concentration may impact it.

Rmax

When assessing the surface roughness, scratches and burrs are very significant, and max helps in identifying these irregularities.

RMS

RMS stands for Roughness Magnitude Surface; it measures the surface roughness by taking the height values and squaring them. It gives weight to the extreme values and gives a detailed insight into the texture of the surface.

What is the Standard Surface Finish in Machining?

3.2μm Ra is the standard surface finishing and is opted in general application. It balances manufacturability as well as functionality and does not need extensive post-processing. This range of Ra value represents a moderately smooth surface, which has some visible tool marks but there are no significant irregularities.

Types of Machining Finishes

The different types of machining finishes are as follows:

Acabamento como maquinado

It is the finish that appears right after machining and has tool marks. This surface has a Ra value ranging between 3.2 to 6.3 µm. It is opted for parts where aesthetics are not important like functional prototypes.

Smooth Finish

A smooth finish is a fine finish that is achieved by using a shape tool and when machining is done at a slow speed. It has a value of Ra between 0.8 to 1.6 µm. It is used in applications like medicine, aeroespacial, and automobiles, where smooth surfaces help reduce friction.

Textured Finish

A textured finish is a surface that has been roughened deliberately with the help of processes and tools like bead blasting and serrilhado. It has a value of Ra above 6.3 µm. It is used for enhancing grip, reducing glare and for decorative purposes.

Mirror Finish

A mirror finish is a surface that is polished and has seamless smoothness. The value of Ra in this finish is 0.2 µm. Such a finish is required in optical products.

Anodized Finish

An anodized finish is achieved by treating the surface chemically and an anodized layer is formed, which improves aesthetics as well as corrosion resistance. The Ra value of anodized finish usually starts at 3.2 µm and is used for aerospace and electronic components.

How are Different Levels of Surface Roughness Achieved?

There are different surface roughness levels, and they are achieved using different methods, as discussed below.

As-machined

As -machined surfaces are achieved directly after machining and they do have some tool marks. The surface roughness of these surfaces is around 3.2 to 6.3 µm Ra. It is a cost-effective method and is suitable for applications that do not need aesthetically appealing parts.

Bead Blasted

Bead blasting is a surface finishing process that uses beads like fine glass beads which are blasted on the surface at a high pressure. It creates a uniform and matte finish, and the range of Ra in such a finish is between 1.6 to 6.3 µm Ra. It helps to smoothen the sharp edges and enhances the aesthetics of components.

Anodização

Anodização is the process of treating the surface chemically, which creates an anodized layer on the surface of the components. The two different types of anodizing processes are as follows:

Anodized Type II

Type II anodizing creates a layer of oxide on the surface which is thin and has a surface roughness of 5-25µm. It offers resistance against corrosion and helps to achieve different colors. It is lightweight and hence is mostly opted for electronic enclosures.

Anodized Type III

Type III anodizing is a hard layer of oxide that has a surface roughness of 25 to 100µm. This type of anodizing gives the surface high resistance against corrosion as well as wear. It is mostly opted for military applications.

Surface Roughness Options Achieved in CNC Machining

The different types of surface roughness options achieved using CNC machining are as follows:

3.2 μm Ra

3.2 μm Ra is the finish which is for the chemical machines; it is mostly acceptable for the consumer components and has smooth surfaces. However, you can see some visible tool marks on these components.

1.6 μm Ra

1.6 μm Ra has cut marks, and it is mostly opted for the parts that pass through high stress or need tight fittings.

0.8 μm Ra

0.8 μm Ra is a surface finish that offers high-end surfaces and is also costly. However, it is important for such parts which are subjected to high amounts of stress.

0.4 μm Ra

0.4μm Ra uses a surface roughness that is opted for components that need a seamless finish as a top priority. It is suitable for such components which pass through excessive tension and stress.

Main Effects of Surface Roughness on Parts

The impact of surface roughness on the components is as follows:

Resistência ao desgaste

When the surface is rough, the connection between the mating components creates an increased friction. This leads to quick wear and tear.

Stability of Fit

Rough surfaces are less wear-resistant, and hence, the clearance increases during the use. The micro-peaks are flattened during the assembly process of the interference of fits and it reduces the effective interference and weakens the strength of connection.

Resistência à fadiga

Rough parts have larger valleys that work like stress concentrators. These impact the fatigue strength of the components.

Resistência à corrosão

Due to roughness, the surfaces may let air and moisture penetrate the metal components, which leads to making the parts less resistant to corrosion.

Sealability

Rough surfaces cannot create enough sealing capability, and this may lead to leaks between the components that are assembled together.

Contact Stiffness

Contact stiffness is the ability of the components that mate together to resist deformation when subjects do external forces. The stiffness of a machine depends on the contact stiffness that is present between the parts.

Measurement Accuracy

The roughness of the surface, if a component is measured using the measuring tool, impacts the accuracy of measurement.

Surface Roughness Conversion Table

The surface roughness conversion table is as follows.

How to Measure Surface Roughness?

There are different methods to measure surface roughness, which are discussed below:

Profiling Method: This method involves measuring surfaces using a probe with high resolution.

Area Technique: This method involves measuring the finite area of a given surface. The measurements give a statistical average of troughs and peaks. This technique uses capacitance probes, ultrasonic scattering, or optical scattering.

Microscopy Technique: It is a qualitative method that measures the contacts. It gives information about the valleys and peaks of the surfaces.

Conclusão

Surface roughness is a crucial aspect of manufacturing components because their performance and durability are dependent on it. If you are looking to manufacture components with the finest surface finish for your custom applications, we at DEK can help you. We have experienced engineers who consider the significance of the surface roughness in components very carefully, ensuring exceptional perfection.

FAQs

How surface roughness is generated?

Surface roughness is generated through the manufacturing process or surface finishing techniques.

What to pay attention to when marking surface roughness?

Ra is the parameter used for describing the surface roughness and while marking it, the average profile height’s deviation from the mean line is measured.

What are the characteristics of the surface shape when the surface roughness Ra is 3.2μm?

3.2 µm Ra shows visible marks of tooling and has a moderately rough surface.