Surface roughness measurements
Roughness measurements according to the European standard NEN-EN 1090 (the standard for load-bearing steel and aluminium structures) are intended to verify the surface structure and quality of thermally cut edges.
The standard prescribes how surface roughness must be quantified and checked (particularly in Part 2 of the standard, section 6.4.3 on validation of cutting work).
Why roughness measurements according to EN 1090?
During the thermal cutting of steel (such as laser cutting, plasma cutting, or oxy-fuel cutting), small irregularities (grooves and melt residues) occur on the cutting edge.
EXC classes: Depending on the Execution Class (from EXC 1 to EXC 4), stricter requirements apply to structures. For higher classes (EXC 2 and higher), the tolerances for perpendicularity and roughness of cutting edges are stricter. Weld quality: An excessively rough surface can lead to adhesion problems with coatings, poor penetration during welding, or stresses in the material.
The measurement parameters (Ra, Rz, and Rmax)
The standard prescribes that surface roughness be measured and assessed based on standardized values:
(Ra) (Average roughness): The arithmetic mean of all absolute deviations (peaks and valleys) relative to the zero line.
(Rz) (Maximum average roughness): The average distance between the five highest peaks and the five deepest valleys within a standardized measurement range.
When assessing cutting work according to EN 1090, the focus is often on the (Rz) value, because it is more sensitive to extreme deviations in the cutting surface.
How are the measurements performed?
This is usually done using a roughness tester. This is an instrument with an extremely sensitive probe (needle) that is drawn across the cutting edge of the material at a standardized speed. Cutting Procedure Qualification (SPK): Construction companies must validate their cutting machines. Before a machine is allowed to cut in production for, for example, EXC 3 or EXC 4, test pieces are cut. These test pieces are subjected to a roughness measurement and perpendicularity check. The measurement results are recorded in an official measurement report to qualify the machine.
