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 operationThe oxy acetylene flame cutting process makes use of a cuttingtorch. The mixes the acetylene and the oxygen in correctproportions to produce a preheating flame and also the torchsupplies 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 ignitiontemperature (900°C), i.e., reddish yellow colour by the preheatflames, keeping the flame cones 1.5 to 3 mm above the surface ofmaterial, to be cut, a thin jet of high purity oxygen is then directedor shot at this heated spot.The jet quickly penetrates through the steel. The iron and oxygencombine to form iron oxide. The oxygen jet blows the reactionproducts from the joint and thus the torch moves progressivelyforward over the metal surface, cutting a narrow slot or kerf alongthe desired line of severance.In actual practice, the top surface of the material is frequentlycovered with mill scale or rust. They must be melted away by thepreheating flame to expose a clean metal surface to the oxygen jet.The cutting action is self progressing provided the (cutting) oxygenjet is sustained and the pre heating flame is maintained on the topsurface of the metal object being cut.
5. Summary(i) Oxy acetylene flame preheats the metal to the ignitionpoint at the place to be cut. It also provides a protectiveshield between the cutting oxygen stream and theatmosphere.(ii) Cutting oxygen combines with iron to form iron oxide.(iii) Cutting oxygen jet blows away molten iron and ironoxide thereby cutting a narrow slit or kerf in the metalobject.
6. CUTTING TORCHA cutting torch is made up of a yellow brass body. It:(a) Mixes acetylene and oxygen and carries the gas mixture tothe orifices where it is ignited to produce preheating flames.(b) Carries cutting oxygen to the central orifice from where as itemerges, it oxidizes the metal and blows the same away toform an open slot (kerf).There are two types of cutting torches:(a) The injector type, in which the acetylene is delivered to thetorch at pressures below 1 psig. The acetylene is drawn into thetorch with the help of an injector. Acetylene and oxygen for thepreheating flames are mixed in the tip of the cutting torch.(b) The equal pressure type, in which oxygen and acetylene aredelivered at pressures above 1 psig. The mixing of the gasestakes place within the torch (and not in the tip).
8. GAS PRESSURE REGULATORSGas pressure regulators, etc., are similar in design to thoseused for oxy acetylene welding.
9. USES AND APPLICATIONSUses/Applications of Oxy-Fuel Gas Cutting:(i) To prepare edges of plates for bevel and groove weld jointdesigns.(ii) To cut small sized work pieces from bigger plates forfurther processing.(iii) To cut rivets gates and risers from castings.(iv) To cut many layers of thin sheets at the same time (stackcutting) 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 DISADVANTGESAdvantages(i) Shapes and sizes difficult to be machined by mechanical methodscan 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 amachine tool, i.e. mechanical cutting machine.(iv) Flame cutting equipment being portable also, can be used for thefield 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 tolerancesare poor.(iii) The place of cutting needs adequate ventilation and proper fumecontrol.(iv) The expelled red hot slag and other particles present fire andburn hazards to plant and workers.
11. METALS CAN BE CUT USING FLAME CUTTING1. Plain Carbon Steel.2. Low Alloy Steel.3. Manganese Steel.4. Low-content Chromium Steel.
12. METALS CANNOT BE CUT BY FLAME CUTTING1. Stainless Steel.2. Carbon Steel.3. Aluminium.4. Non-ferrous Metals.
13. PLASMA ARC CUTTINGIt is an arc cutting process wherein the severing of the metal isobtained by melting a localized area with a constricted arc andremoving the molten material with a high velocity jet of hot,ionized gas issuing from the orifice.
14. PRINCIPLE OF OPERATIONPlasma arc cutting makes use of DCSP (electrode negative) witha constricted transferred arc* struck between a tungstenelectrode situated within (and not protruding) the torch andthe workpiece to be cut.The cutting arc between the electrode and the workpiece isinitiated by a pilot arc established between electrode andnozzle. The nozzle is connected to ground (+) through acurrent limiting resistor and a pilot arc relay contact.The pilot arc is initiated by a high frequency generatorconnected to the electrode and nozzle. Ionized orifice gasfrom the pilot arc is blown through the constricting nozzleorifice. This forms a low resistance path to ignite the main arcbetween the electrode and the workpiece. Once the main arc isignited the pilot arc goes off.
15. A high-frequency electric arc thus established is passedthrough a stream of inert gas (usually nitrogen) the latter isionized. Both the ionized gas column and the arc are forcedthrough a small orifice in the torch nozzle. The nozzle, havinga relatively small orifice, constricts the arc and thus increasescurrent density and arc temperature. This high temperaturearc is localized and concentrated upon a small area of the platewhere its intense heat melts the metal to the cut.The gas which is heated by the arc cannot expand due to theconstriction of the nozzle orifice and it emerges in the form ofa supersonic jet. The base metal continuously melted by theintense heat of the arc is removed by the jet-like gas stream(issuing from the torch nozzle) to form a narrow kerf andsmooth surface. The combined heat and force of the arcstream produce a high quality, saw-like cut.
16. GASES USED IN PLASMA ARCThe gases that are used in plasma-arc cutting:1. Nitrogen2. Nitrogen + hydrogen3. Nitrogen + argon4. Compressed air
17. ADVANTAGES AND DISADVANTAGES OF PLASMA ARC CUTTINGAdvantages(i) It cuts carbon steel up to 10 times faster than oxy-fuelcutting, with equal quality more economically.(ii) It leaves a narrower kerf.(iii) Plasma cutting being primarily a melting process can cutany metal.(iv)Arc plasma torches give the highest temperature availablefrom many practicable sources. The energy seems to beunlimited in this method.DisadvantagesHigh initial cost of the equipment
18. APPLICATION OF PLASMA ARC1. 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.2. Plasma cutting can be used for stack cutting, plate bevelling, and shape cutting and piercing.3. With some modifications, plasma arc cutting can be used under water.4. Plasma arc cutting finds applications in many industries such as shipyard, chemical, nuclear and pressure vessel.5. It is used for removing gates and risers in foundry.6. It cuts hot extrusions to desired length.7. It is used to cut any desired pipe contour.8. It is also employed for gouging applications.9. It finds use in the manufacture of automotive and railroad components.