1.3a) Cutting with oxygen: Flame cutting:
Principle- When cutting with oxygen (Gas purity 99.95 volume percent, 3.5) and a cutting gas pressure of maximum 6 bar, the material is melted and for the most part oxidized. The melting created is blown out together with the iron oxides out of the kerf.
The oxidized process supplies additional energy (exothermic reaction), which influences cutting process to the effect that higher cutting speeds are possible and greater material thicknesses can be machined then when cutting with nitrogen.
The oxidation layer, which builds up on the cutting surface, serves as corrosion protection for stainless steel surfaces. Cutting surfaces need to be refinished if parts are to be welded.
1.3b) Cutting with nitrogen: Laser fusion cutting:
Principle – Laser fusion cutting is done using nitrogen or argon as a cutting gas. This process also melts the material first and then with the help of a cutting gas – usually nitrogen – blows out the kerf. In practice, a gas pressure is used between 8 to 20 bar (so-called high-pressure cutting) with a nitrogen gas purity of 99.999 volume percent (5.0). With argon, there is a gas purity of 99.996 volume percent (4.6)
By using the high gas pressure, we can be assured that the cutting edges remain free of burr formation to a great extent and that no slag settles.
The use of inert gases provides oxid-free cutting edges, though it makes piercing at the beginning of the cutting process more difficult. For this reason, oxygen is used for piercing and cutting is then done with nitrogen.
1.4 Advantages and applications:
Advantages – In competition with the alternative slitting processes like plasma cutting, punching (blanking) and nibbling or wire erosion, laser cutting has the following advantages:
- Processing the work piece is possible without contact or force.
- As opposed to punching (blanking) and nibbling, almost every contour shape can be made, without requiring a single tool change.
- With the laser beam, both large cuts in any shape can be cut as well as small, filigree and complicated contours. Geometric shapes can be machined quickly with only a few piercing.
- Separation is precise. The extremely narrow kerf remains virtually constant. Maintaining tolerances as low as 0.05 mm is also possible in series production.
- The cutting speed is high, producing a significant acceleration of the production process in comparison to, for example, wire erosion.
- Due to high energy density, the heat-affected zone remains very small and confined. Hardness penetration depths from 0.1 to 0.2 mm are possible, an oxide film forms when cutting with oxygen. The small heat-affected zone, in turn, means that material distortion is minimal.
- The roughness of the cutting surfaces is kept to a minimum: smaller then 100 um. There is no need to refinish the work piece.
- The most commonly used steels can be cut without any burr formation, eliminating the need for subsequent burr removal.
- Due to rapid development of both laser as well as cutting technology, laser cutting has become a true commercial alternative to other techniques. If you also accept the argument of almost limitless flexibility in application, then it can be predicted, that laser cutting will gain more and more ground on the conventional techniques.
1.5 Marking and point marking with the laser:
Marking – In marking a minute amount of material on the surface is removed and coloured. This minute surface removal is e.g. no longer visible after the work piece has been enamelled.
Marking with the laser is possible with mild steel, stainless and aluminium alloys under the following conditions:
- Machining in all sheet thicknesses.
- The sheet surface must be completely free of oil.
- Low level laser output necessary.
- Position the focus position decidedly above the material surface.
- Oxygen as cutting gas is possible with mild and stainless steel, unsuitable with aluminium alloys, since the laser beam will reflect very strongly.
Centre marking – With the centre marking a spot indentation is produced in the material surface. This can be done in two ways:
- Circular centre marking – Two circles are described here with the laser beam. With mild steel, the circles have the same diameter of 0.6 mm, with stainless steel two concentric circles with a diameter of 0.3 mm and 0.6 mm.
Centre marking is possible preferably with mild and stainless steel. Oxygen is used as a cutting gas with mild steel and nitrogen is used with stainless steel.
Definition – Laser cutting produces a kerf which is usually narrower at the bottom of the cut than at the top.
The kerf – also referred to as the kerf breadth – is given in [mm].
Measuring the kerf – The kerf is measured in the slit of a rectangle