Casting has been used for shaping metal since the earliest days of civilization. A wide variety of sizes and shapes of simple and intricate nature can be produced in different metals. Casting is a process of producing metal or alloy components of desired shape, by pouring the molten metal/alloy into a prepared mould. After that it is allowed to cool and solidify. The solidified alloy or metal is known as casting.
Smithy is understood to handle relatively small jobs that can be heated in an open fire or hearth. The shop in which the work is carried out is known as the smithy or smith’s shop. The various forging operations are performed by means of hand hammers or small power hammers.
The different casting processes are as follows:
(a) Green Sand casting
(b) Permanent mould casting
(c) Pressure die casting
(d) Centrifugal casting
Green Sand Casting – It is the most widely used moulding process. The green sand used for moulding consists of silica sand, clay, water and other additives. One typical green sand mixture contains 10 to 15 percent clay binder, 4 to 6 percent water and remaining silica sand.
In this process the green sand mixture is prepared and the mould (cope and drag) is made by packing the same around the pattern. Cope and drag are assembled and the molten metal is poured while the mould cavity is still green, i.e. it is neither dried nor baked. Green sand moulding is preferred for making small and medium size castings. It is specially employed for producing non-ferrous casting.
Permanent Mould Casting – A permanent mould casting makes use of a mould which is permanent, i.e. the mould can be reused many times before it is discarded or rebuilt.
In this process, molten metal is poured into the mould under gravity only. No external pressure is applied to force the liquid metal into the mould cavity. However, the liquid metal solidifies under the pressure of extra metal in the risers. Permanent moulds are made of dense, fine grained, heat resistant cast iron, steel and bronze, anodized aluminium graphite and other suitable refractory materials. Thicker mould walls can remove greater amount of heat from the casting. This provides the desirable chilling effect.
Pressure Die Castings – In pressure die casting molten metal is forced into permanent mould (Die) cavity under pressure. The pressure is generally either pneumatic or hydraulic. The pressure varies from 70 – 5000 kg/cm²and is maintained while the casting solidifies. Externally applied high pressure helps the molten metal to be injected into the dies through the nozzle. This condition gives a unique capacity for the production of intricate components at low cost.
Centrifugal Casting – The principle of centrifugal casting was developed during nineteenth century. The process was introduced to manufacture cast iron pipes and a range of shapes of alloys. The unique feature of centrifugal casting is the introduction of liquid metal into a rotating mould. Centrifugal force plays a major role in shaping and feeding of the casting.
Forging is the process of shaping heated metal in a desired shape by the application of sudden blows. The application of heat lowers the yield point and makes permanent deformation easier. Forging may be done by hand hammer or by power hammer.
Forging by machine involves the use of dies and it is generally used for mass production. Hand forging is employed for small quantity production and for special work.
Forging Operations – The principal forging operations are:
(b) Drawing out or Fullering
(e) Punching and Drifting
(g) Setting down or Flatting
(h) Forge welding
Upsetting – Upsetting operation is carried out to increase the thickness (cross section area) of a bar and to reduce its length.
Drawing Out or Fullering – Drawing out or fullering is carried out to reduce the thickness of a bar and to increase its length.
Cutting – In cutting operation the metal is heated up to forging temperature and cutting is carried out with the help of hot set (chisel).
Twisting – In the twisting operation the metal (rod / square bar) is heated up to forging temperature. One end of the rod is clamped in the vice and other end is held with a tong or twisting dog. Twisting is carried out as per requirement.
Punching and Drifting – In punching operation the metal is heated to forging temperature. Then a punch is placed on the marked place and with the help of hammer, hole is made. Borax powder is sprinkled on the marked place as a flux, which will prevent the formation of scale. The punch is to be cooled periodically to avoid overheating. Punching is carried out from both the sides. Drifting is carried out to enlarge the hole size.
Bending – In bending operation the metal is heated up to forging temperature. One end of the job is supported on the anvil and hammering is carried out on the other end. Before bending upsetting is carried out to avoid the reduction in cross section.
Setting Down or Flatting – In setting down or flatting operation the metal is heated to forging temperature. With the help of flatter or set hammer setting down operation is carried out to remove the fullering marks/hammer marks or any waviness on the surface to produce a flat surface.
Forge Welding – In the forge welding the metals are heated to the forging temperature. Then upsetting is carried out at the both ends to be welded. Cleft shape is given to both the ends and inserted in each other. Flux (calcined borax) is sprinkled on the joint and hammering is carried out to weld the metals.