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Machining processes and associated surface roughness

The correct surface finish significantly influences the functionality, service life and manufacturing costs of components. In this blog, you will learn everything you need to know – from the basics of surface roughness to measurement methods and international standards to hands-on examples of how roughness is called out in drawings and what machining methods deliver the desired quality. Immerse yourself in the world of precision and find out why even micrometers can make all the difference!

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What is surface roughness?

Surface roughness or the roughness of surfaces describes the microscopic unevenness and texture created by machining processes on the surface of a material. This unevenness is often too small to be perceived with the naked eye but significantly influences the mechanical, chemical and optical properties of a component.

Roughness is typically measured by the profile of the surface, wherein the deviations from an ideally smooth surface are recorded in the form of peaks and valleys. These deviations are measured in micrometers (μm) and are described using various characteristics. In industrial applications, surface roughness is a not insignificant aspect because it influences factors such as friction, adhesion, wear behavior, lubricity, and corrosion resistance. Optimum roughness can help extend the service life of components, increase machine efficiency, or improve the functionality of seals, bearings, and other components.

What types of surface roughness are there?

Surface roughness is described by various parameters that help to characterize the condition of a surface. Each of these parameters provides different information about the microstructure and the quality of a surface. Some of the most important parameters are explained below:

  • Roughness Ra (arithmetic average roughness value): Ra is the most commonly used parameter describing surface roughness. It measures the average distance of the roughness profiles from the center line of a surface. Ra provides a simple average of the height and depth of the surface deviations and provides a general roughness estimate.
  • Roughness Ry (maximum roughness): Ry refers to the highest individual peak and the deepest valley within a specific measurement section. It therefore describes the largest vertical distance on the surface.
  • Roughness Rz (ten-point height value): Rz describes the difference between the five highest peaks and the five lowest valleys in a measurement section. In contrast to Ra, which provides an average value, Rz focuses on extreme heights and depths and provides a more precise statement about the maximum roughness of a surface.
  • Roughness Rt (total height of the profile): Rt describes the distance between the highest and the lowest deviation of a surface over the entire length of the measured profile. It provides a total roughness value by taking into account the extreme peaks and valleys across the entire measurement field.

The exact calculation of these parameters can be quite complex and requires special measuring instruments and mathematical methods. If you are interested in a detailed calculation of the surface roughness, you will find more information in our blog post on the measurement and determination of surface finish and roughness graph.

Surface roughness measurement methods

Surface roughness is measured using different measuring instruments and measurement methods. The most common techniques include:

  • Sectional scanning method: In this case, a precision scanning tip scans the surface of the workpiece while recording the heights and depths of the surface profile. The sensor records the profile from which various roughness parameters such as Ra, Rz or Rt can then be derived. This method is suited for a wide range of surfaces, but only generates a two-dimensional view of roughness.
  • Optical roughness measurement: Optical measuring instruments generate a three-dimensional image of the surface. These methods are particularly useful for sensitive or soft materials that could be damaged by a mechanical tip.
  • Laser scanning: Laser-based methods use a focused light beam to measure the surface. This permits contactless measurement of surface structures with high speed and precision.

International standards for surface roughness

Surface roughness is measured and specified iaw. internationally recognized standards. The standard series ISO 25178, for example, refers to three-dimensional measurements of the surface texture and is becoming increasingly important because modern manufacturing processes often generate complex surface structures. International standards ensure that the measurement and specification of surface roughness remains consistent and comparable worldwide. These standards provide clear definitions and measurement guidelines used by the manufacturing industry to ensure components meet functional and qualitative requirements.

How is surface roughness called out in drawings?

Surface specifications in engineering drawings describe the specific quality of a surface, including its roughness, waviness, and machining methods. Engineers and manufacturing specialists use standardized symbols to accurately communicate surface finish requirements. The foundation for manufacturing components is created by specifying surface roughness, machining methods and grain directions in the drawings. These symbols and values follow international standards, such as ISO 1302, which ensure globally uniform specifications and measurement methods. Some basic roughness symbols are explained below.

The surface symbol is a standardized symbol used in engineering drawings to communicate surface finish requirements for a workpiece. It is used to present information on surface roughness, machining method, grain direction, waviness, and other relevant aspects of the surface.

a - Value of surface roughness Ra

b - Specifies machining method

c - Section spec, examined length

d - Specifies grain direction

e - Specifies machining tolerance

f - Parameters other than Ra

g - Specifies surface waviness

Machining methods for modifying the surface finish

Different methods, such as turning, milling, grinding or lapping, each generate different roughnesses that influence the functionality and quality of the manufactured components. Depending on the application, a coarse or particularly smooth surface may be required to minimize friction, wear, or susceptibility to corrosion. The following table lists roughness on surfaces and shows which methods can be used to achieve these roughnesses.

Machining method and achievable surface roughness - unit: μm
Surface roughness Ra (μm) Machining Method
0.025 - an almost mirror-smooth finish with only very minor, microscopic unevenness is achieved by methods such as micro-grinding, lapping, polishing or electropolishing - for sensitive high-precision components
0.05 - a precision finish with a uniform texture and barely visible roughness peaks and roughness valleys is generated by precise post-processing methods such as precision grinding, lapping, polishing or super finish methods - for applications with high precision requirements
0.1 - very smooth finish, but with a slightly more microscopic roughness - generated by precision grinding, grinding, lapping or polishing - ideal for instruments in precision mechanics and optics
0.2 - a precision, high-quality surface is achieved by grinding, precision grinding, lapping or honing - for fits, sealing surfaces and bearing surfaces
0.4 - a high-quality finish with noticeable but still small micro-unevenness is achieved by machining methods such as precision turning, milling, grinding and honing - for applications with moderate surface finish requirements
0.8 - a relatively smooth finish with more pronounced micro-unevenness is achieved by turning, milling, honing and grinding - for mechanical components, bearing surfaces and sliding surfaces that must permit smooth movements
1.6 - a good surface finish with tactile micro-unevenness is achieved by methods such as turning, milling, roughing or grinding - for plain bearings, shafts and components that work under moderate, controlled conditions
3.2 - a relatively rough finish with clearly perceivable peaks and valleys is generated by turning, milling or roughing - for components and joints that are machined or coated in subsequent operations
6.3 - a comparatively coarse finish with clearly visible and tactile unevenness is generated by turning, milling, casting, drilling, etc. - for components subject to high loads or intended for subsequent surface machining
12.5 - a very coarse surface with pronounced uneven surfaces is created by turning, milling, grinding or casting - for rough or non-critical components
25 - rough, inferior surface - created during sawing, rough turning, milling, etc. - for components and blanks before fine machining
50, 100 - extremely coarse surface - for applications tolerated or specified with high surface roughness

Importance of surface roughness in industrial applications

Surface roughness plays an important role in industrial applications, especially when it comes to reducing friction and wear. Depending on the roughness profile, surfaces can either cause higher friction or hold the lubricating film better and thus improve the efficiency of machine components. However, in order to gain a complete understanding of the relationships between surface finish and friction behavior, it is equally important to understand the concepts of friction and coefficient of friction. In this context, the surface roughness directly affects the coefficient of friction, which is decisively responsible for the contact between components. Want to learn more? Find more information on the basics of coefficient of friction, its measuring methods and its applications in engineering in our blog.