Rz

What is Rz (surface roughness)?

Rz is a measure of surface roughness that describes the average variation in height differences on a surface. It is most commonly defined as the average of the largest peak-to-valley differences over five consecutive measuring intervals (cut-offs). Unlike the Ra value, which measures the average roughness of a surface, Rz focuses on the surface's extreme height differences, thus providing a more detailed view of the surface's irregularities.

Rz in Surface Roughness Measurement

The Rz value is measured with a surface roughness tester, which moves along the surface and records the height differences. Since Rz is based on the largest peaks and the deepest valleys, it can be more sensitive to individual surface defects than Ra, making it especially useful when it is important to ensure there are no excessive individual deviations on the surface.

Rz Compared to Other Surface Roughness Parameters

Several different parameters are used for measuring surface roughness, and Rz is one of the most commonly used. It differs, for example, from the following:

• Ra (average roughness): Calculates the average deviation from the mean line, so it does not highlight individual deviations as much as Rz does.
• Rt (total roughness): Measures the entire height difference from the highest peak to the deepest valley over the entire measurement length, which can result in even greater variation than Rz.
• Rmax (maximum single height): Considers only the single largest height difference over a defined distance, which may be even more sensitive to individual deviations.

The Impact of Rz Surface Roughness in Machining

In machining and metalworking, controlling the Rz value is important because surface roughness affects the functionality, durability, and fit of a part. An excessively high Rz value can mean that the surface is too rough for sealing surfaces or bearings, for example, while an excessively low Rz can increase manufacturing costs without bringing real benefits.

Summary

Rz is a surface roughness parameter that measures the average of the highest peaks and deepest valleys, providing important information about the irregularity of a surface. It is especially useful when it is necessary to ensure that there are no large, individual deviations on the surface. In machining and the metalworking industry, controlling the Rz value is an essential part of quality assurance and optimizing the manufacturing process.