Thermal & mechanical cutting

1. Thermal & Mechanical Cutting Group Discussion

2. CUTTING OF METALS Cutting of metals implies severing or removal of metal. Cutting is the process of separating metals, i.e., a metal piece is separated or split into two parts. Cutting of metals is an everyday practice in industry. It is employed for the following purposes: ( i ) Cutting desired lengths and shapes of (rolled) metal pieces for assembly and other processing operations to be carried out on different machine tools and presses. Many times it is required to cut a gear blank from a plate or a blank is cut from a plate for subsequent forming operations. (ii) For preparing the edges of plates for welding them together. (iii) For cutting gates and risers from the castings. ( iv) For salvage work.

3. FLAME CUTTING Also known as Oxy-Fuel Gas Cutting It is an oxygen cutting process wherein the severing of metals is effected by means of the chemical reaction of oxygen with the base metal at elevated temperatures, the necessary temperature being maintained by means of a gas flame obtained from the combustion of a fuel gas (such as acetylene, hydrogen, propane, etc.) and oxygen.    

4. Principle of operation The oxy acetylene flame cutting process makes use of a cutting torch. The mixes the acetylene and the oxygen in correct proportions to produce a preheating flame and also the torch supplies a uniformly concentrated stream of high purity oxygen (known as cutting oxygen) to the reaction zone. After a spot area along the line of cut is heated to ignition temperature (900°C), i.e., reddish yellow colour by the preheat flames, keeping the flame cones 1.5 to 3 mm above the surface of material, to be cut, a thin jet of high purity oxygen is then directed or shot at this heated spot. The jet quickly penetrates through the steel. The iron and oxygen combine to form iron oxide. The oxygen jet blows the reaction products from the joint and thus the torch moves progressively forward over the metal surface, cutting a narrow slot or kerf along the desired line of severance. In actual practice, the top surface of the material is frequently covered with mill scale or rust. They must be melted away by the preheating flame to expose a clean metal surface to the oxygen jet. The cutting action is self progressing provided the (cutting) oxygen jet is sustained and the pre heating flame is maintained on the top surface of the metal object being cut.

5. Summary ( i ) Oxy acetylene flame preheats the metal to the ignition point at the place to be cut. It also provides a protective shield between the cutting oxygen stream and the atmosphere. (ii) Cutting oxygen combines with iron to form iron oxide. (iii) Cutting oxygen jet blows away molten iron and iron oxide thereby cutting a narrow slit or kerf in the metal object.

6. CUTTING TORCH A cutting torch is made up of a yellow brass body. It: (a) Mixes acetylene and oxygen and carries the gas mixture to the orifices where it is ignited to produce preheating flames. (b) Carries cutting oxygen to the central orifice from where as it emerges, it oxidizes the metal and blows the same away to form an open slot (kerf).   There are two types of cutting torches: (a) The injector type, in which the acetylene is delivered to the torch at pressures below 1 psig. The acetylene is drawn into the torch with the help of an injector. Acetylene and oxygen for the preheating flames are mixed in the tip of the cutting torch. (b) The equal pressure type, in which oxygen and acetylene are delivered at pressures above 1 psig. The mixing of the gases takes place within the torch (and not in the tip).

7. 1. Cutting oxygen tube 2. Pre-heat oxygen tube 3. Cutting oxygen valve lever 4. Oxygen valve 5. Acetylene valve 6. Acetylene tube 7. Cutting oxygen orifice 8. Preheat orifices

8. GAS PRESSURE REGULATORS Gas pressure regulators , etc., are similar in design to those used for oxy acetylene welding.

9. USES AND APPLICATIONS Uses/Applications of Oxy-Fuel Gas Cutting: ( i ) To prepare edges of plates for bevel and groove weld joint designs. (ii) To cut small sized work pieces from bigger plates for further processing. (iii) To cut rivets gates and risers from castings. (iv) To cut many layers of thin sheets at the same time (stack cutting) to reduce both time and cost for production work. (v) To pierce holes and slots in steel plates. (vi) For salvage work

10. ADVANTAGES AND DISADVANTGES Advantages ( i ) Shapes and sizes difficult to be machined by mechanical methods can be easily cut by flame cutting. (ii) The process is faster than mechanical cutting methods. (iii) The cost of flame cutting is low as compared to that on a machine tool, i.e. mechanical cutting machine. (iv) Flame cutting equipment being portable also, can be used for the field work. (v) Multitorch machines can cut a number of pieces simultaneously. Disadvantages ( i ) Flame cutting is limited to the cutting of steels and cast iron. (ii) As compared to mechanical cutting, the dimensional tolerances are poor. (iii) The place of cutting needs adequate ventilation and proper fume control. (iv) The expelled red hot slag and other particles present fire and burn hazards to plant and workers.

11. METALS CAN BE CUT USING FLAME CUTTING Plain Carbon Steel. Low Alloy Steel. Manganese Steel. Low-content Chromium Steel.

12. METALS CANNOT BE CUT BY FLAME CUTTING Stainless Steel. Carbon Steel. Aluminium. Non-ferrous Metals.

13. PLASMA ARC CUTTING It is an arc cutting process wherein the severing of the metal is obtained by melting a localized area with a constricted arc and removing the molten material with a high velocity jet of hot, ionized gas issuing from the orifice.

14. PRINCIPLE OF OPERATION Plasma arc cutting makes use of DCSP (electrode negative) with a constricted transferred arc* struck between a tungsten electrode situated within (and not protruding) the torch and the workpiece to be cut. The cutting arc between the electrode and the workpiece is initiated by a pilot arc established between electrode and nozzle. The nozzle is connected to ground (+) through a current limiting resistor and a pilot arc relay contact. The pilot arc is initiated by a high frequency generator connected to the electrode and nozzle. Ionized orifice gas from the pilot arc is blown through the constricting nozzle orifice. This forms a low resistance path to ignite the main arc between the electrode and the workpiece . Once the main arc is ignited the pilot arc goes off.

15. A high-frequency electric arc thus established is passed through a stream of inert gas (usually nitrogen) the latter is ionized. Both the ionized gas column and the arc are forced through a small orifice in the torch nozzle. The nozzle, having a relatively small orifice, constricts the arc and thus increases current density and arc temperature. This high temperature arc is localized and concentrated upon a small area of the plate where its intense heat melts the metal to the cut. The gas which is heated by the arc cannot expand due to the constriction of the nozzle orifice and it emerges in the form of a supersonic jet. The base metal continuously melted by the intense heat of the arc is removed by the jet-like gas stream (issuing from the torch nozzle) to form a narrow kerf and smooth surface. The combined heat and force of the arc stream produce a high quality, saw-like cut.  

16. GASES USED IN PLASMA ARC The gases that are used in plasma-arc cutting : Nitrogen Nitrogen + hydrogen Nitrogen + argon Compressed air

17. ADVANTAGES AND DISADVANTAGES OF PLASMA ARC CUTTING Advantages ( i ) It cuts carbon steel up to 10 times faster than oxy-fuel cutting, with equal quality more economically. (ii) It leaves a narrower kerf. (iii) Plasma cutting being primarily a melting process can cut any metal. (iv)Arc plasma torches give the highest temperature available from many practicable sources. The energy seems to be unlimited in this method. Disadvantages High initial cost of the equipment

18. APPLICATION OF PLASMA ARC Plasma cutting is used to cut particularly those nonferrous and stainless metals that cannot be cut by the usual rapid oxidation induced by ordinary flame torches. Plasma cutting can be used for stack cutting, plate bevelling, and shape cutting and piercing. With some modifications, plasma arc cutting can be used under water. Plasma arc cutting finds applications in many industries such as shipyard, chemical, nuclear and pressure vessel. It is used for removing gates and risers in foundry. It cuts hot extrusions to desired length. It is used to cut any desired pipe contour. It is also employed for gouging applications. It finds use in the manufacture of automotive and railroad components.