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Hot machining processes
Posted Date: 28-Nov-2009
hot machining processes
Hot machining is the process which is used for easy machining and to eliminate the problems of low cutting speeds, feeds and heavy loads on the machine bearings. This problem arises when machining process is being done on the new and tough materials. The basic principle behind this process is the surface of the work piece which is to be machined is to be pre heated to a temperature below the re-crystallization temperature. By this heating the shear forces gets reduces and machining process becomes easy.
Some of the
of hot machining process are
1) Easy formation of chip
2) Lessened shocks to the tools
3) Good surface finish of the work piece
On the other hand the main disadvantage of this process is the work piece’s micro structure may get disturbed due to heating.
to be taken are
1) Heat generated should be localized
2) Heating is to be done just before the machining
The heat requirements for this process should satisfy the following conditions
1) Heat input rate should be very high such that the work piece gets heated up in very short time
2) The heat generated should heat only the shear zone. if heat generated penetrates to higher depths then there may be a problem of thermal damage.
3) Constant temperatures over a wide range should be generated
4) The installation cost and operating cost should be less
5) It should be easy to set up and control.
Methods of heat generation
Heat generation is done in various methods
1) Flame hardening
2) D.C. arc welding
3) Plasma arc welding
4) Furnace heating
5) Resistance heating
6) Radio frequency resistance heating
7) Induction Heating
It is one of the easiest methods of heating. In this process oxyacetylene (or) propylene (or) town gas is employed. Multi-jet head is employed when high concentration of heat is required. The equipment required for this method is relatively inexpensive and is very effective in milling narrow jobs. But when it is employed for wider jobs problem of localization of heat arises. Another problem which may arise due to this process is oxidation. If this happens post machining is required
D.C. Arc heating
This method is employed for machining non magnetic (or) refractory materials. A carbon (or) tungsten electrode is used for making an electric arc. In this process the negative terminal of the arc is connected to work piece and positive terminal is connected to an electrode which keep moving the electric arc forward. Magnetic field may also be used for removing the problems of wandering. The main disadvantage in this process is maintenance of constant temperature is very difficult.
Plasma arc welding
Though the initial cost of the equipment is very high the method is very effective. In this method the plasma arc contains a restricted arc through which the gas selected is allowed to flow. In this process temperatures of very high range can be generated (20,000 K). low permeable materials are machined by this method as heat flow can be easily controlled. High speed and depth machining can be done if adequate power and gas supply is given proper safety measures are to be taken to avoid the radiations affecting us.
In this process a furnace is employed for heating the work piece. The work piece material is placed in the furnace and is heated till the surface temperature gets equal to furnace temperature. Then the work piece is taken out and is machined. This process is economical when the required furnace is already there. The machine on which machining operation will be done should be placed near to the furnace such that the temperature loss of the work piece is very less. There are problems in this method
1) Heat cannot be regulated
2) The whole body gets heated
3) Damage of work piece
In this method 50 cycle A.C current is passed through the work piece body to heat it. Much difficulty is required in maintaining a constant temperature. A proper cooling is required otherwise damage occurs. In this process arcing should be strictly restricted. This is an inexpensive process and is more suitable in milling process.
Radio frequency resistance heating
Radio frequency currents are employed in this process. These currents follow low impedance paths. So by this process the heating can be done to only the pre determined area as current doesn’t flow to other directions. The low impedance can be quite easily obtained by placing a current carrying conductor near to the work piece. This current carrying conductor generates reverse currents. The main advantage of this method is heat can be localized. This method is gaining importance now-a-days. Similar to resistance welding arcing should be restricted.
This method uses an alternating current of 3000C/s to 1.2 MC/s for the generation of heat. Transformer action is used for inducing the current. Localization of heat is not so difficult. Temperature control, depth of heat zone can be repeated without any undue problems. The equipment can also be used for various other engineering works but it is very costly.
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