Minting Processes

A View of the Birmingham Mint in the 19th Century

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Casting : Raw materials turned into cast strip.
Raw materials, are weighed into the assay proportions required. Additions of manganese and phosphorous are included as de-oxidants. The materials are charged to an electrical induction melting furnace and heated to the required temperature (1,000 to 1,500 Centigrade). A small sample is cast and the assay checked using an X-ray spectrometer. A water-cooled graphite die in the shape of the strip required is bolted to 3,000kg capacity holding furnace. When satisfactory the material is transferred to the holding furnace. Molten metal flows into the back of the die and solidifies. Approx. 18mm of strip is withdrawn allowing more molten metal to flow into the die, solidify and weld to the initial piece. By repetition strip is formed over a period of 1-5 days. The strip is cut into 1,000kg coils using a flying saw. On conclusion the holding furnace is emptied and the process repeated. Cast strip section is typically 212 x 14 mm.

Milling : Surface scale removed.
The cast strip surfaces are oxidised and copper rich. This layer is removed using a dedicated machine that un-coils the strip, feeds it between two cutters that remove the surface and re-coils the strip. The cutters have tungsten carbide inserts embedded in the surface and depth control is automatic.

Breaking Down : Strip rolled to ready gauge.
The material as cast is too thick for the production of coinage. The material is forced between rolls in a 4 high rolling mill. The gap between the rolls is set to approx. 20% less than the material thickness. As the material passes through the, water cooled and mineral oil lubricated, roll gap it's thickness is reduced. As a consequence the strip length increases pro-rata. Control of strip thickness is effected using flying micrometers, accurate to +/-1 micron, at the entrance to and exit from the roll gap. Compensation is effected via computer control to maintain the strip thickness within a tolerance of +/- 0.025mm even when rolling at 150 m per minute. The strip is typically processed to a ready gauge of 4 mm.

Salem Anneal : Material softened ready for further rolling.
The work involved reducing the strip thickness results in work hardening of the material. Attempts to reduce the strip thickness further would result in the strip cracking. Thus, the material is annealed prior to further work. The coils are loaded to an annealing base. They are covered with a nitrogen-filled stainless steel bell, to prevent oxidation, and an electrical heater. The coils are heated to around 800 Centigrade, soaked for 1 hour at temperature and allowed to cool slowly. This allows the material to soften ready for further processing.

Finish Rolling : Material rolled to finished thickness requirement.
The material is again rolled in between 3 and 9 passes to the required finished thickness. Control of the thickness of the strip is required to ensure that the blanks produced from the material conform to the stringent weight constraints required for coinage. A sample coil is produced and blanked at the commencement of each contract to ensure that the weight requirements are met.

Blanking : Blanks produced using multiple punch and die sets.
The strip is fed between multi-impression tooling. Each tool contains up to 20 impressions dependent upon the required blank diameter. Each impression consists of a punch and die. As the strip passes between the punch and die the blanking press closes. The punches force the strip into the die and punch out round or shaped discs. The presses operate at up to 650 strokes per minute. Typically blanks are in excess of 150 Hv30 after blanking.

Rimming : Edge of blank formed.
In order to pre-form the edge of the blank ready for the minting operation they are upset between a revolving ring and stationary segment, the ring and segment are cut to the profile required for the finished blank. This operation also reduces the possibility of blanks sticking at the annealing operation.

Plating : Plated layer added to surface of blank.
Steel blanks are loaded into revolving barrels. The barrels pass through a variety of cleaning operations prior to reaching a plating tank. Copper or Nickel is deposited from solution by a process of cathodic deposition. The thickness of the plating deposited is approximately 30 microns. For the Copper plated product subsequent annealing does not result in a metallurgical bond. The Nickel plated product does present a metallurgical bond after annealing and cleaning.

Blank Anneal : Blanks softened ready to receive impression.
The work involved in the finish rolling renders the blanks too hard to receive the impression. Thus, the blanks are annealed to soften them. This is achieved by passing them by either a belt or a drum containing an archimedes screw through an electrically heated chamber. The blanks are contained within a hydrogen/nitrogen atmosphere, to prevent oxidation, and are heated to around 950 Centigrade and the cooling rate controlled. This results in a hardness suitable for striking.

Cleaning: Blanks cleaned to required standard.
Following annealing the blank surface is not clean or bright enough to ensure a good finished coin. Thus, The blanks are de-scaled and polished in high energy cleaning machines. The blanks are mixed in a drum with stainless steel shot and revolved at high speed in order that the contact between the shot and the blanks results in burnishing of the blank surface. The process is helped by the introduction of sulphuric acid at the de-scaling stage and soaps at the burnishing stage. The blanks are clean and bright following this operation.

Sorting/View : Final blank sorting prior to feed to striking presses.
Overlooking of blanks to remove any foreign blanks or surface defects that result from the annealing and cleaning operations

Striking : Impression embossed onto blank surfaces.
The forming of the impression is achieved by striking the pieces between two dies carrying the required impression. During the application of the load the piece is retained within a collar to control diameter. The pieces are fed automatically via an indexing plate to the striking position at a rate of up to 700 pieces per minute dependent upon the shape of the coin and the press used.

Count : Finished coin and blanks counted into the required quantity prior to packing.
The pieces are counted through specialist telling machines and check-weighed to ensure complete accuracy. Packaging specifications can range from 1 tonne of blanks loose within a cardboard box to a few pieces counted into a plastic sachet or bag or cotton bag.

Pack : Counted parts packed to the required specification.
Bags of coin containing 1,000-4,000 pieces are traditionally packed into small wooden boxes that are then stacked and strapped on pallets.