Introduction
1.
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.
2.
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.
Casting Processes
3.
The different casting processes are as follows:
(a) Green Sand casting
(b) Permanent mould casting
(c) Pressure die casting
(d) Centrifugal casting
4.
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.
Fig
1 Casting Operation
5.
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. (Refer
Fig 1 & 2)
Fig
2 Finished Product of Casting
6.
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.
7.
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. (Refer Fig 3)
Fig
3 Permanent Mould Casting
8.
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/cm² and 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. (Refer Fig 4)
Fig
4 Pressure Die Casting
9.
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 roll in shaping and
feeding of the casting. (Refer Fig 5)
Fig
5 Centrifugal Casting
Forging
10. 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. (Refer Fig 6)
Fig
6 Forging
11. 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.
12. Forging
Operations. The principal
forging operations are:
(a) Upsetting
(b) Drawing out or Fullering
(c) Cutting
(d) Twisting
(e) Punching and Drifting
(f) Bending
(g) Setting down or Flatting
(h) Forge
welding
13. Upsetting.
Upsetting operation is carried out to increase the thickness (cross section
area) of a bar and to reduce its length. (Refer Fig 7)
Fig
7 Upsetting
14. Drawing
Out or Fullering. Drawing out or fullering is carried out to reduce the
thickness of a bar and to increase its length. (Refer Fig 8)
Fig 8 Drawing Out
15. Cutting.
In cutting operation the metal is heated up to forging temperature and
cutting is carried out with the help of hot set (chisel). (Refer Fig 9)
Fig
9 Cutting
16. 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.
(Refer Fig 10)
Fig
10 Twisting
17. 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. (Refer Fig 11)
Fig
11 Punching and Drifting
18. 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.
(Refer Fig 12)
Fig
12 Bending
19. 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.
(Refer Fig 13)
Fig
13 Flatting
20. 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. (Refer Fig 14)
Fig
14 Forge Welding
