Trial pieces are struck as prototypes in preparation for the minting of a new coin. They are distributed, not as legal tender, but as samples to the automatic vending industry for the purpose of re-calibrating machines prior to the circulation of the new coin.
The first stage in the manufacture of a Two Pounds coin is the melting and alloying of its constituent metals in electric induction furnaces at temperatures of between 1200 and 1400 degrees Celsius.
The bi-colour nature of the new coin means that two different alloys must be processed separately. In either case, once examination of a sample by x-ray fluorescence spectrometry has confirmed that the alloy is correct, the molten metal is continuously cast, from a holding furnace, through water-cooled graphite dies. The emerging strip, 212mm wide by 16.5mm thick, is cut into 100m or 2.5 ton lengths and its upper and lower surfaces are scalped by up to 0.50mm to remove oxide discolouration.
Two passes through a 3.2mm to 4.0mm tandem rolling mill reduce its thickness to 3mm and, in so doing, double its length. Because the nickel-brass will now have become work-hardened, the strip must be softened by an annealing process of seven hours duration in a furnace at 600 degrees Celsius.
The strip is reduced to the desired thickness for coining by being accurately rolled on a reversing finishing mill. At this point the now extended strip is accurately reduced to coin thickness. Cupro-nickel and, at the stage, whole nickel-brass coin blanks are punched out of the strip at a rate of up to 5000 a minute with a force of 100 tonnes. The metal residue from the blanking process, known as scissel, is returned to the furnace.
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The blanks are softened in annealing furnaces operating at temperatures of up to 950 degrees Celsius. After cooling they are fed into automatic blank finishing machines, where stains are removed by a solution of sulphuric acid. They are then burnished and finally rinsed in water and dried off.
The outer blanks of nickel-brass are given raised edges by being rolled under pressure between the fixed segment and rotating wheel of a rimming machine. It is also at this stage that the coin's edge lettering is applied. Central holes, which will take the smaller cupro-nickel blanks, are punched out on a piercing press. The rimmed inner blanks meanwhile receive an edge groove as a key to bonding.
A coining press is served by two separate hopper and feeding systems for the different blanks. The first system feeds the pierced outer blank into a dial plate segment, which then moves on to the second operation. There the inner blank is dropped into position, loosely located inside its outer. Held by the segment, the coin parts pass on to be brought to rest on top of the lower dies whose upwards movement pushes the blanks into a restraining collar. Continued upward movement of the lower die squeezes the blanks against the upper one with a 100 ton force, so that both impressions are received in a single operation. By the same action the metal, being forced outwards, takes up the pattern of the milled collar, and the union of the two coin parts is made.
The completed bi-colour Two Pounds coin is ejected from its collar by a downward movement of the upper die.
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