In CNC machining, dimensional accuracy is only one of the factors that determine part quality. Even a component manufactured according to the technical drawing may fail to perform its intended function if the machined surface does not meet the project requirements.
For this reason, technical documentation often specifies surface roughness requirements using parameters such as Ra and Rz. Surface roughness affects not only the appearance of a component but also its durability, sealing performance, and interaction with other parts.
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Why Surface Roughness Matters in CNC Machining
Every machined surface contains microscopic irregularities created by the cutting action of the tool. Their size and characteristics depend on many factors related to the manufacturing process.
Surface quality requirements should always be adapted to the function of the component. Not every surface requires the same level of finish, but in the case of mating surfaces, guide surfaces, and sealing surfaces, roughness can directly affect the performance of the finished product.
Excessive surface roughness can accelerate component wear, increase friction, or make it difficult to achieve the required sealing performance. On the other hand, unnecessarily strict surface quality requirements often lead to higher production costs.
Surface quality is particularly important wherever even minor deviations can affect component functionality. A good example is the medical industry, which we discuss in more detail in the article: “CNC Machining in the Medical Industry – Surface Quality Requirements”
Ra and Rz – What Do They Tell Us About a CNC-Machined Surface?
Ra and Rz are among the most commonly specified surface roughness parameters in technical drawings. Both describe surface condition, but each provides information about a different aspect of surface quality.
In CNC machining, roughness parameters affect component interaction, sealing performance, and durability during operation. Understanding the differences between Ra and Rz is therefore important when defining quality requirements.
Why the Ra Parameter Does Not Show the Whole Picture
Ra represents the arithmetic average of surface profile deviations over the measured length. It is the most commonly used roughness parameter and appears in a large proportion of technical documentation.
However, Ra provides only an average value. Two surfaces may have the same Ra value while differing significantly in the shape and depth of their irregularities. From the perspective of sealing performance, friction, or component interaction, these differences may be important.
For this reason, when planning a CNC machining process, evaluating surface quality based on a single parameter does not always provide enough information to determine the actual requirements of the component.
When Rz Becomes More Important
Rz describes the height of surface irregularities, taking into account the differences between the highest peaks and deepest valleys of the profile. This makes it possible to evaluate not only the average surface condition but also larger irregularities that may affect component performance.
This parameter is particularly useful when evaluating mating and sealing surfaces, as well as components exposed to operational wear. It can help identify local irregularities that may affect sealing performance or component durability.
For this reason, in many industrial applications, Rz analysis provides valuable additional information alongside Ra measurements.
What Determines Surface Roughness After CNC Machining?
Achieving the required surface quality in CNC machining is not the result of a single technological decision.
Even when working from identical technical documentation, the final result may vary depending on machining parameters, material properties, and tool condition. Surface roughness requirements should therefore be considered in the context of the entire production process rather than a single machining operation.
Cutting Parameters and Tool Condition
Machining parameters are among the most important factors affecting surface quality. Feed rate, cutting speed, and depth of cut directly influence the characteristics of the marks left by the cutting tool. In our work, we often encounter situations where a seemingly minor adjustment to machining parameters has a greater impact on the final surface quality than the choice of material itself.
Tool condition is equally important. A worn cutting edge can reduce surface quality, increase surface irregularities, and produce unwanted machining marks.
Workpiece Material and Process Rigidity
Not every material behaves in the same way during machining. Aluminium, structural steel, stainless steel, and brass have different properties that affect the cutting process.
For this reason, machining parameters should not be selected solely on the basis of component geometry. Material properties and the required surface quality after CNC machining are equally important. Parameters that work well for one material may not produce the same result when machining another component.
The rigidity of the entire machining system also plays an important role. Vibrations generated during machining can leave marks on the surface that become visible during quality inspection.
Where Are Surface Roughness Requirements Particularly Important?
Not every surface of a component requires the same level of finish. Surface roughness requirements should therefore result from the function that a given feature performs in the finished product. We often work with components where surface quality requirements are just as important as the dimensional tolerances specified for CNC machining.
Particular attention is paid to:
- Surfaces interacting with seals,
- guideways and sliding components,
- mounting surfaces,
- bearing seats,
- surfaces intended for protective coatings,
- components used in industries with increased quality requirements.
Hydraulic components are a good example, as surface quality directly affects sealing performance and system durability.
Surface Roughness Inspection After CNC Machining
Achieving the required surface quality depends not only on the manufacturing process but also on effective inspection.
Surface roughness is measured using specialised measuring instruments that make it possible to determine the required parameter values. This allows compliance with technical documentation to be confirmed before the component moves to subsequent production stages.
We discuss our approach to quality control in more detail in the article: “Quality control in CNC machining – how do we ensure precision at SIM Gdynia?”
Does Lower Surface Roughness Always Mean a Better Part?
One of the most common mistakes is assuming that the lowest possible Ra value will always be the best solution. In our work, we often encounter situations where surface roughness requirements are stricter than the actual function of the component requires.
Overly restrictive surface roughness requirements can lead to:
- Longer CNC machining times,
- the need for additional finishing operations,
- increased tool wear,
- higher production costs,
- longer project lead times.
For this reason, surface quality requirements should be defined according to the actual function of the component as early as the design stage. This helps avoid unnecessary manufacturing operations and allows production costs to be optimised more effectively.
Summary
Surface roughness is one of the parameters that directly affect the functionality of a finished component. In CNC machining, it influences not only the appearance of the part but also component interaction, sealing performance, connection durability, and the ability to carry out subsequent manufacturing processes.
The best surface is not always the one with the lowest Ra value, but the one that meets the actual requirements of the application.
Are you planning to manufacture components that require precision CNC machining and surface quality inspection? Contact us to discuss your project requirements.