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Descriptions of The Forging
Processes |
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The original definition of forging is
the "mechanical deformation of metal when heated to
a plastic state". However, in recent years it has
come to include similar operations done in the cold or room temperature
state as well.
For our purposes here we'll treat three
basic types of forging:
- Closed Die Forging or Impression Die
Forging
- Open Die Forging
- Ring rolling
In each of the above areas there are
sub-categories and variations which we'll discuss as we go along. |
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In closed die forging, a negative image
of the part to be made is sunk into a diesteel block or pair
of blocks. The die set is keyed or otherwise clamped into
a press or hammer which supplies the energy for the deformation.
This method is used to make everything from cutlery and automotive
parts to parts for aircraft engines. Figure 1 shows a cut away
cross section of a pair of press dies used to make a gear blank
(This is over-simplified for discussion purposes since in
reality there would be a series of dies of increasing refinement
leading up to the finished forging). After forging in the
impression dies the flash or excess metal is trimmed off in a
press. Flash is metal that has been expelled from the die cavity
during forging- Its not necessarily waste but becomes trapped
in the flashline between the dies and aids in filling the die
cavity by creating a restriction that tends to keep the bulk
of the metal in the cavity. |
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Figure 1 Gear blank dies |
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Closed die forging is often done in
the cold state ("Cold Forging") or at somewhat elevated
temperatures but still below the "red hot" temperatures
required to put the material into a plastic state ("Warm
Forging")
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Open Die Forging is more akin to the ancient
art of blacksmithing
and is used to manufacture products like gear and bearing blanks
in smaller "non-production" quantities; shafting for
purposes like ship propulsion or power generating machinery. Other
applications are Oil tools, parts for agricultural and construction
equipment and special tools and dies for the automotive suppliers.
Some of the common shapes produced with this technique are rings,
discs, blocks, bars, and step-down shafts or spindles. |
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Figure 2
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Discs, Blocks, Bars & Shafts or Spindles |
Rings |
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Ring rolling is related to both open die and closed
die forging, depending on the nature of the parts being produced.
For example, a ring with a simple rectangular crossection can
be produced using open die forging techniques or by rolling.
The process used in either case is basically the same for
preparing the blank for subsequent diametral expansion. This
is done by upsetting or "pancaking" the starting billet
(reducing its height or length by working it perpendicular to
its axis), and piercing it partway through then punching out
the "slug". |
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Cut Round Billet |
Upset Blank |
Pierced from one side |
Punched Blank |
Figure 3 |
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Where the two processes part company is
in the subsequent diametral growth to attain the final diameters
and height, thickness or width. In the case of open die
forging this step is done by a process called saddling
by hanging the blank or "donut" on a mandrel and reducing
the wall section with a hammer or press with a series of hammer
blows or press strokes while rotating the workpiece either with
tongs , or, in the case of very large and heavy pieces, with
specially designed machinery . |
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Hammer or Press Saddling Set-up |
Figure 4 |
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In the case of ring rolling, the blank
is placed over a mandrel on a ring rolling mill and the diametral
growth is achieved by reducing the wall and width or height with
powered rolls as shown below. The example used here is
a horizontal mill but this process can also be done in the vertical
plane and with or without the Axial Rolls. |
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Starting The Rolling Process |
Finished Rolling |
Figure 5 |
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