Support Instructions Measuring Surface roughness measurement

Surface roughness measurement

Surface roughness is a critical attribute that significantly affects the functionality of materials and components across various industries. It influences mechanical properties such as friction, wear resistance, and corrosion resistance; thus, its precise measurement and control are necessary. This guide details the principles of surface roughness measurement, the methods used, significant surface roughness parameters, and their importance.

Basic Profiles of Surface Measurement

The profiles used in surface measurement can be divided into three basic profile types:

P-profile (Primary Profile):

  • This profile is obtained when the actual surface is cut perpendicular at a selected level.
  • It includes all surface deviations and has not been subjected to any electronic filtering.
  • The P-profile forms the basis for other profiles and provides the most comprehensive view of the surface structure.

R-profile (Roughness Profile):

  • This profile is obtained by filtering out long-wavelength components from the P-profile using an appropriate measurement interval (cut-off).
  • The R-profile focuses on surface roughness and is deliberately formed from the P-profile.
  • The most common surface roughness measures (Ra, Rz, Rt) are calculated from this profile.

W-profile (Wave Profile):

  • This profile is further obtained by filtering out long-wavelength components from the P-profile.
  • The W-profile describes the wavy components of the surface and is particularly important when assessing the macrogeometry of the surface.

Measurement Basics

When measuring surface roughness, a specific, defined measurement interval is used. The choice of measurement interval is critical, and if the interval is not specified in the workpiece drawings, the measurer must appropriately define it.

Measurement Length and Interval:

  • lt = total length (includes starting point, measurement length, and endpoint).
  • ln = measurement length (generally comprises five measurement intervals).
  • lr = measurement interval (cyclic length).

Selection of Measurement Method:

  • Chosen based on the needs and specifications of the object to be measured, including surface sensitivity and geometric complexity.

Surface Roughness Measurement Methods

There are several methods available for measuring surface roughness:

Contact Methods:

  • Commonly used and involves a measurement tip being slid across the surface being measured.
  • The movements of the measurement tip are recorded into the device’s memory and converted into surface roughness data.
  • A relatively easy and quick method suitable for most surfaces.

Optical Methods:

  • Does not require physical contact between the measuring device and the workpiece.

  • Uses, for instance, lasers or light beams for surface inspection.

  • Suitable for measuring sensitive or complex-shaped surfaces.

Hybrid and Advanced Methods:

  • Combine the best aspects of contact methods and optical methods.
  • Improve measurement accuracy and allow for the analysis of more complex surface shapes.

Common Surface Roughness Measures Based on R-profile

Ra (Arithmetic Mean Deviation):

  • The arithmetic mean of profile deviations over the measurement length, regardless of the vertical direction.
  • A simple numerical value that describes how "rough" or "smooth" a surface is on average.
  • The Ra value is the most common standard and is widely used in manufacturing and quality control.

Rz (Average Maximum Profile Depth):

  • The average of the individual profile depth values calculated from the highest peaks and deepest valleys within a measurement interval.
  • Provides more information about individual peaks and valleys than the Ra value.

Rt (Total Profile Depth):

  • Total profile depth between the highest profile peak and the lowest profile valley over the measurement length.
  • Rt review can detect the largest deviations on the surface.

Rp (Maximum Profile Peak Height, Average):

  • Describes the distance of the highest peaks from the centerline within a measurement interval.
  • Together with the Rz value, provides a more detailed picture of surface characteristics.

Rmr (Material Ratio, Abbott-Firestone Curve):

  • Expressed as a percentage, describing how much of the profile is covered by material starting from a certain cut level (c).
  • Useful for assessing profile wear reserve and surface defects.

Measurement Conditions and Quality Control

Measurement Conditions (EN ISO 4288 and DIN EN ISO 3274):

  • Include detailed instructions for the surface roughness measurement process.

  • Important when selecting measurement points, setting measurement devices, and interpreting results.

Drawing Symbols (EN ISO 1302):

  • Use standardized symbols to indicate surface roughness requirements on technical drawings.

Other Measurement Conditions:

  • Maximum Rule: All surface roughness parameters with the "max" suffix represent the maximum of averages from five sample lengths.
  • 16% Rule: Parameters without the "max" suffix represent the average of five sample lengths, with no more than 16% allowed to exceed the limits.

Tips

  • Selection of Measurement Tip: The correct radius and measurement frequency of the measurement tip is essential for ensuring measurement accuracy.
  • Surface Shape and Sensitivity: Choosing optical methods is beneficial for complex and sensitive surfaces.
  • Selection of Measurement Length: Ensuring a sufficient number of measurement lengths and sample paths from all directions is important to ensure accuracy.

Summary

Understanding and measuring surface roughness play a central role in improving production processes, ensuring component quality, and extending their service life. The use and interpretation of different profiles (P-profile, W-profile, and R-profile) and their parameters (Ra, Rz, Rt, Rp, and Rmr) provide a comprehensive view of surface characteristics.

Advanced measurement methods and strictly standardized measurement conditions ensure that surface roughness measurement is accurate and reliable. This helps manufacturers and designers optimize their processes and ensure high-quality final products.