The laser cut surface reveals a specific form of unevenness. As either semicircular grooves or proper grooving are the consequence of the focused laser beam shape, the cutting velocity and formation process, as well as of the removal and hardening of the molten material at the cut place.

Observation of the cut surface can reveal two zones: the upper one in the area of the laser beam entrance side and the lower one, in the area of the laser beam exit side. The former is a finely worked surface with proper grooves whose mutual distance is 0.1…0.2 mm while the latter has a rougher surface characterised by the deposits of both molten metal and slag.

That is why it is determined to measure roughness of the cut surface at the distance of one third of sheet thickness from the upper cut edge. There is a difference between the cut surface roughnesses in the direction of the laser beam fand that in the direction perpendicular to the laser beam axis that is in the cutting direction. The former is of no crucial importance in considering the problem of the cut surface roughness due to the fact that the laser is applied to thin sheet cutting. The latter is a more obvious phenomenon that can be observed and analysed.

In laser cutting, the edges of the workpiece have a characteristic grooved pattern. At low cutting speeds, the grooves run almost parallel to the laser beam. As the cutting speed increases, the grooves bend away from the direction of cutting. Groove lag refers to the greatest distance between two drag lines in the direction of the cut. The groove lag is evaluated visually. The evaluation is carried out on a picture of cut with the aid of a magnifying glass or a microscope.

Parameters that are most often used for accessing the surface roughness are the standard roughness (ten point height of irregularities) Rz. and the mean arithmetic profile deviation Ra. The standard roughness Rz is the arithmetic mean calculated from the roughness (scallop height) of five consecutive, representative, individual measured sections.

The standard roughness Rz is measured e.g. with a brush analyzer corresponding to ISO 3274. The measuring itself is carried out at continuous distances in the cutting direction, in accordance to ISO 4288. The point at which the roughness is measured is dependent on the sheet thickness and the material type.