CNC Machining

In CNC machining, the roughness markings on engineering drawings often cause confusion. Ra, Rz, and Rq - these three seemingly similar parameters have distinct differences. How should they be chosen in actual production? This article will help you clarify this.

1. What do they represent?

Ra (Arithmetic Mean Roughness): The most commonly used indicator. It measures the absolute deviation of each point on the contour from the centerline. Simply put, it is "how much the surface fluctuates on average". Ra has the advantage of involving all points within the sampling length in the calculation, resulting in stable and repeatable results.

Rz (Ten-point Height): Measures the average distance between the five highest peaks and the five lowest valleys within the sampling length. It is more sensitive to extreme values - if a surface has several deep scratches, Rz will increase significantly, while Ra may not change much.

Rq (Root Mean Square Roughness): Calculates the average of the square of the contour deviations and then takes the square root. Compared to Ra, Rq reacts more strongly to abnormal values (particularly high peaks or deep valleys) because it amplifies the deviation degree.

2. When to choose which one?

Choose Ra: For most regular scenarios

If you have no special requirements, the Ra marking on the drawing is sufficient. Ra is an internationally recognized "standard language", and measurement instruments are widely available, resulting in stable and repeatable results. It is suitable for most general surfaces of mechanical parts, such as ordinary structural components, non-mating surfaces, and appearance surfaces. Example: Ra 3.2, Ra 1.6.

Choose Rz: For scenarios sensitive to extreme peaks/valleys

When the part is sensitive to "individual deep grooves" or "local high points", Ra may mask the problem, while Rz can capture it. Applicable to: sealing surfaces (deep grooves can cause leakage), thin-coated surfaces (high points may pierce the coating), substrate surfaces before electroplating or spraying. Example: Rz 6.3.

Choose Rq: For scenarios with extremely high surface functional requirements

The amplification effect of Rq on abnormal values is more meaningful in certain high-precision scenarios. Applicable to: optical element surfaces, precision bearing raceways, and mating surfaces requiring the calculation of actual contact area. However, the use frequency of Rq is much lower than Ra and Rz. Unless there is a clear functional requirement, it is not necessary to deliberately use it.

3. A simple judgment logic

1. Does this surface have sealing requirements?

Yes → Prioritize Rz (deep grooves affect sealing)

No → Continue to the next question

2. Will the surface be coated or electroplated?

Yes → Prioritize Rz (high points and deep valleys affect coating quality)

No → Continue to the next question

3. Is this a regular mechanical machining surface?

Yes → Only Ra is needed

No (such as optics, precision mating) → Consider Rq

4. Suggestions for processing factories

When communicating with customers, if the drawing only indicates a roughness value without specifying the parameter type, it is assumed to be Ra. If you encounter a drawing with Rz or Rq markings, it is recommended to first confirm the customer's intention, as not all inspectors are clear about the differences between these three parameters. If necessary, you can explain to the customer: Rz is more sensitive to scratches, and whether extreme values really need to be controlled?

Understanding the differences between these three parameters is not only a basic skill for reading drawings but also a practical skill to avoid quality disputes and reduce rework rates.