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Introduction Cold forging in the karat gold jewelry industry has sometimes
been referred to as stamping, coining and die striking. This
process has been used throughout history in the making of coins
and medals as well as jewelry. Cold forging basics will always be
the same as they were during antiquity; however, the fine points
are constantly being refined. The process is simply described as
forming a finding by using extreme force to drive raw material
into a shaped cavity in a steel block. While it may sound simple,
there are many subtle conditions that will effect the outcome.
Findings such as shanks, tops, charms and trims are ideal products
for this method of manufacture.
Tooling Pressing a steel master or hub (also known as hob) into a soft
block of steel makes tools for cold forging. Up until the 1980’s,
hubs were cut by hand using hammer and chisels, gravers and
files. They were the height of toolmaking craftsmanship and
art; however, the skilled artisans needed to produce these
masterpieces became hard to find. Today, a hub can be
machined with conventional toolroom equipment or fabricated
with CAD/CAM technology. They still require a high level of
hand skills to finish and polish. The master is made from A2 tool
steel which is very tough and will withstand the pressure of the
die making.
After creation, the master is then heat-treated at 1750°F
(955°C) for one hour and then air-cooled. This process will yield
a Rockwell C hardness reading of 63. It is then tempered to 59
Rockwell C by placing in a furnace at a temperature of 450°F
(230°C) for one hour. The master is pressed into a steel block with
the aid of a hydraulic press and then the die is heat-treated at
1525°F (830°C) to a Rockwell C hardness of 60. The die is
tempered for two hours at 440°F (227°C) to 57 Rockwell C and
then the cavity is hand polished to aid extraction and impart a
superior finish to the finding. The steel used in the die itself is of
the L6 class of tool steel, having high impact and shock resistance.
Two hard and fast rules that apply to this type of tooling are
that there can be no undercuts and that a draft angle must be
applied to all cavities. Unlike castings, undercuts are impractical
with diestruck products. They will cause the metal to lock into
the cavity and become impossible to extract. Draft angles of
two to five degrees are necessary for easy removal of the part
from the die and often aid in reducing the time it takes to run
each job.
The stages in the toolmaking process are illustrated in
Figures 1-7.
Cold forging To illustrate the cold forging process, a typical shank job will be
used. This shank is of the type commonly used for solitaire rings.
The material must be fabricated into a shape that is close in size
to the final product. There are several ways of doing this. The
most common method is to chop thin strips from a sheet of
material or blank out shapes that resembles the outline of the die.
Blanking can cause problems if there are sharp notches in a blank.
This can cause a crack in the finding itself. A good example of this
is using a charms final trim tool as a blanker. The area where the
loop for the jump ring joins the body of the charm has a corner
that will be prone to cracking when it’s stamped. In this case,
using the trimmer for a blanker will result in the job being
scrapped. Rounding out notches on the blanker will reduce this or
prevent it from happening. This problem is magnified when
stamping nickel white gold. For the shank used in this article, 14-
karat gold square wire was used, Figure 8, because it is quick to
fabricate and it has a good shape for stamping.
The die is bolted into a pneumatic drop hammer, Figure 9, or
a hydraulic press, Figure 10. The drop hammer works on the
principal of a weight being raised by air pressure in a cylinder. The
air is evacuated from the cylinder and the weight falls forcing the
blank into the impression. However, the force created in a
hydraulic press is caused by oil being pressurized within a cylinder
that slowly drives the metal into the die. The methods of
generating the force are different and so are the results. The drop
hammer provides a quick, sharp hit while the hydraulic system
builds up pressure slowly. For the shank example, a drop hammer
was used.
The die is lubricated with a light coat of mineral oil, Figure 11.
Baby oil has been used since the early 1950’s and is ideal for the
job. To judge the proper amount, the brush is rubbed across the
fingernail and only dry oil sheen should be left. It shouldn't be
wet at all; however, there should be a light film left on the
fingernail. Too much oil will settle into the cavity and clog the
detail, preventing the part from fully forming. The first die used is
called a start die and the draft angle is exaggerated to allow the
metal to move into the cavity with less resistance. The blank is
positioned over the impression and the material is then struck
into the die. At this point, the finding will not be fully formed and
additional steps will be needed to bring up the definition of the
piece. The amount of pressure used with both systems can
destroy the die, if excessive.
The whole operation, if done incorrectly can be compared
with splitting wood. If the blank is hit too hard or is overworked,
it can act as a wedge and will split the die in half. This is an
extreme case of misuse. However, in lesser degrees, the
impression will crack at the bottom or the walls will start to
collapse. The development of the blank dimensions is also very
important. Too much material will result in extra strikes and trims
to bring the part to its final dimensions, while an undersize blank
will not fill in the impression. Using a force or male half of the die
can stamp hollow backed items with very thin sidewalls. Also
joining two die halves with impressions will permit both sides of a
finding to be detailed. This however can present challenges for
trimming tool geometry since the impression will need to be cut
into the face of the trimmer punch to prevent flattening.
Overflow or excess material is formed around the part, Figure
12. Thickness of the overflow will often range from 0.015” to
0.018” (0.38 - 0.46 mm). This must be reduced in order for the
material to flow freely again and a trimming tool is used to do
this. For a shank like the example, a rectangular shaped tool works
very well. This universal trimmer can be used for up to 80% of the
solitaire shanks offered by a company if designed properly,
Figures 13 and 14.
Each time the metal is pressed into the die, the material is
work hardened. For further forming, the material must be
annealed. Holding the workpiece at a constant temperature for a
set period of time and then uniformly cooling it does this. In many
factories, an atmosphere is maintained by using cracked
anhydrous ammonia or a nitrogen/hydrogen blend. These gases
provide an atmosphere that protects the metal from oxidation
while it’s being softened for additional operations. The sample
shank was annealed at 1400°F (760°C) for 20 minutes. Figures 15
and 16 illustrate the cold worked and annealed grain structures
respectively.
The shank is next stamped in a final die that has the finished
geometry in the impression. Some jobs call for a third strike to
bring up the final shape. In the example, only two dies were used to do this. The overflow will measure about 0.010” (0.25mm) at
this point since this thickness will yield an acceptable trim line.
After stamping, the overflow needs removal with a trimmer that
is specific to the style being forged, Figures 17 and 18. Proper
maintenance of the final trimmer is critical to the appearance of
the finding. If the plate is over sharpened, the opening will
increase, causing a step at the trimmed area. Also, a trimmer
plate that’s too tight will trim too high on the part and cause a
flat sidewall that detracts from the design.
Finishing operations Prior to rounding the shank, the ends need to be prepared for
mounting a setting. This involves shaping it for either four or six
prong settings, Figures 19 and 20. There are three methods of
doing this operation. Simple chopping with a punch and plate is
fast, however the quality of the cut is inferior to the other two
machining methods. Milling each end in a horizontal milling
machine is the best method. The shank is held in a clamp and a
milling cutter is run across the end making an accurate, clean cut,
Figures 21 and 22. The third method involves bending the ends
into a hook shape, clamping and milling. While it is similar to the
second method, it has the advantage of eliminating the problem
of the tip collapsing while bending. However, the accuracy of the
end bend is critical to repeated success. With both machining
methods, the angle that the shank is presented to the cutter is
important for a good fit. It needs to be set so that there are no
gaps at the top or bottom of the setting/shank joint.
Rounding is done with a series of bending steps and protecting the ends of the shank is very important. The operation is frequently performed with two end bending blocks, Figures 23 and 24. The first one sometimes has a plastic insert imbedded into the steel in the area where the block would mark the gold. The bend in the first block is little more than a kink in the piece made from a light hit. Where the block touches the shank is the area of most pressure and marks will develop if pressed too hard. The second blocks impression has the final bent shape and is made entirely from steel. The kink from the first block distributes the bending pressure along a larger surface area, preventing
gouging. After the ends are bent the rings are fully rounded,
Figure 25. Subsequent finishing to a final product is illustrated in
Figures 26 and 27.
Concluding remarks
The lack of porosity, as created in casting, and the superior
surface finish transferred by the tooling have made stamping gold
in dies a premium method of manufacture. The use of cold
forging can be applied to many types of findings in all colors and
karats. With proper annealing and care to avoid overworking,
even the most stubborn nickel white gold alloys can be
successfully cold forged.
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