The grading of laser cut edges is described in detail in DIN 2310 Part 5 and ISO-9013. Assessment of laser cut quality is based on the following criteria:

• Cut kerf
• Roughness
• Right-angle accuracy
• Scoring
• Burr formation
• Cratering

The criteria are described below, along with possible causes of faults.

Cut kerf

For material thicknesses up to 3 mm the kerf is measured with a feeler gauge from the top.

For exact measurement of the kerf with material thicknesses over 3 mm a rectangle is cut and the edge length measured with the calliper gauge. The difference to the programmed edge length gives the kerf.

Causes of too large a kerf are:

• Incorrect focus position
• Dirty lens
• Lens coating damaged
• Thermal characteristics of the laser beam affected by contaminated resonator optics
• Beam path is not being purged properly; oil mists or aerosols produce the so-called gas lensing effect impairing the laser beam quality.

Surface roughness

With the laser cutting of sheets up to 2 mm thickness the average surface roughness Rz is measured in the middle of the cut edge, and with thicker sheets at 2/3 of the depth from the beam entry side. The surface roughness is the arithmetic mean of the individual roughness values of five adjacent separate measurement sections. The roughness of laser cut edges is very low in the thin sheet range, but it increases with sheet thickness.

When cutting small contours in carbon steel with reduced parameters (laser power, pulse frequency and cutting speed), a larger roughness occurs in the thin sheet range.

The roughness of cut surfaces on metallic materials can be caused by:

• incorrect gas pressure (too high or too low)
• incorrect cutting speed (too high or too low)
• incorrect laser power (too high or too low)

 

Right-angle accuracy

The right-angle tolerance is the separation of two parallel straight lines between which the cut surface profile must be at the theoretically correct angle, i.e. at 90° for vertical cuts. The right-angle tolerance includes both the straightness and flatness deviations.

A too large a deviation in the tolerance of the right-angle accuracy may be caused by:

• incorrect focus position (too high or too low)
• incorrect gas pressure (too high or too low)
• incorrect cutting speed (too high or too low)
• astigmatism in the laser beam
• incorrect polarization

Drag lines

Edges of metallic materials cut by laser exhibit a characteristic scoring pattern. When cut at the minimum cutting speed, scoring marks are produced on the cut edge almost parallel to the laser beam. When cut at the maximum speed, the marks run from the upper edge initially parallel to the laser beam, but at about 2/3 of the panel thickness the marks start trailing. The cutting marks bend here up to an angle of approx. 30°. With the high pressure cutting of stainless steel and aluminum alloys the scoring marks normally run parallel to the laser beam. The scoring distance is the greatest distance between two points of a mark measured in the cutting direction:

n = Scoring distance

1 = Reference line/beam axis

2 = Groove distance

3 = Cutting mark

4 = Cutting direction

Causes of heavy scoring and significant bending of the scoring marks for standard cutting with oxygen:

• cutting speed too high
• laser power too low
• focus position too low
• oxygen pressure too low

Causes of significant bending of the scoring marks for high pressure cutting with nitrogen (stainless steels and aluminium alloys)

• focus position too low
• cutting speed too high

Burr formation

Edges of metallic materials cut by laser should be largely or completely free from burrs.  In addition to material characteristics, a range of laser and process parameters may be responsible for the formation of burrs at the bottom of the kerf. Burr formation may involve a strongly adhering metallic burr or an adhering but easily removable slag.

Causes of burr formation for standard cutting with oxygen on carbon steel:

Problem: Burr as slag adhering to the side (easily removable)

• oxygen pressure too low
• cutting speed incorrect
• laser power incorrect
• focus position incorrect 

Problem: Metallic burr (only removable mechanically and with difficulty)

• oxygen pressure too high or too low
• cutting speed too high
• laser power too high
• focus position incorrect
• oxygen contaminated

Causes of burr formation during high pressure cutting with nitrogen, stainless steels:

Problem: Long, coarse burr formation; metallic burr, difficult to remove:

• cutting speed too slow
• focus position too high
• nitrogen pressure too low
• overheating of material, e.g. at very tight contours

Problem: Long burr formation (removable manually)

• cutting speed too slow

Problem: Formation of fine burrs:

• cutting speed too high
• focus position too low
• cutting gas pressure too high 

At the limits (max. sheet thickness, max. cutting speed, etc.) one of the cut edges may be free of burr, but the other side may exhibit burr formation.

Laser cutting which is almost burr-free can be carried out up to a max. sheet thickness of 4 mm. With thicker sheets the formation of fine burrs cannot always be avoided, but these burrs are usually easy to remove.

Cratering

Craters are eroded points of uneven width, depth and shape which interrupt an otherwise evenly cut surface.

Causes of cratering with standard cutting using oxygen on carbon steel:

• oxygen pressure too high
• cutting speed too slow
• focus position too high
• scales, paint, etc. on the sheet surface,   material defects (porosity, inclusions)

Cut Edge Not Completely Cut  

The cut edge must be completely cut according to the programmed contour without interruption.   Causes of incompletely cut edges when standard cutting with oxygen on carbon steel:

Problem: Cut edge not completely cut, parts can be knocked out

 

• oxygen pressure too low
• incorrect cutting speed
• incorrect laser power
• incorrect focus position
• starting hole not correctly pierced
• piercing time too short

 

Problem: Cut edge not completely cut and still connected by metal:

 

• oxygen pressure too low
• cutting speed too high
• laser power too low
• incorrect focus position
• oxygen contamination
• starting hole not correctly pierced
• piercing time too short

 

High pressure cutting with nitrogen on stainless steels and aluminum alloys:

 

Problem: Cut edge not completely cut

 

• cutting speed too high
• incorrect focus position
• nitrogen pressure too low
• laser power too low
• time for approach cut too short
• starting hole not correctly pierced
• piercing time too short
• cut interrupted over the work-piece support
• pointed contour transition, not rounded off with a radius
• machine axis acceleration set too high

 

Problem: No edge cut at all

 

• complete reflection of the laser beam due to a shiny, reflecting sheet surface

 

Carbon steel: Standard cutting with oxygen

 

Burning out with a wide kerf:

 

• oxygen pressure too high
• nozzle too large
• cutting speed too slow
• focus position too high
• laser power too high
• scales, paint, etc. on sheet surface
• beam path purge gas is not clean; check filter unit for contamination

 

Stainless steel: High pressure cutting with oxygen Problem:

 

Roughening at the bottom (often with fine burr formation)

 

• focus position too high 
• nitrogen pressure too high 
• cutting speed too high

 

Problem: Roughening over complete cut edge with fine burr formation

 

• focus position too low

 

Diversion mirrors

Contaminated diversion mirrors usually produce a constant worsening of the cutting quality independent of the cut length.

 

Effects:

• burr formation  
• increased roughness 
• tendency to cratering in carbon steel

ESAB’s Solution

ESAB’s Alpharex CNC Laser Cutting Machine automates virtually all process parameters, can be equipped with up to 6KW resonators, and handles large multiple large sheets.  It is truly the most automated laser cutting machine available.