Figure 1 – Janvier’s (1902) patent drawing of his reduction lathe in profile. The hub (e) is on the left side, the larger galvano (a) is seen on the right side. As the reduction lathe rotates, a cutting tool on the hub mimics the design traced from the galvano.
by Russell Doughty
In 1837, the US Mint began using reduction lathes in the die making or production process. Three different lathes were used: the Contamin French portrait lathe (1837), the Hill reduction lathe (1867), and the Janvier reduction lathe (1907). Each lathe was of better quality than the one that preceded it. The lathes were very similar in nature and worked in the same manner as described below. The Janvier reduction lathe (Figures 1 & 2) is illustrated here as an example, but advanced research is encouraged.
The engraver would start by engraving his original design into wax. This original work was several times larger than the size of the coin for which the design is intended, and had a positive relief and orientation – exactly as seen on a finished coin.
Then, a metal cast would be made of the original work. The incuse metal cast would be filled with plaster. After drying, the plaster would be removed from the cast and final touches would be applied by the engraver. This plaster galvano is an exact, positive-relief reproduction of the engraver’s original work.
As seen in Figure 1, the galvano (a) is mounted aside the master hub (e). An arm (g) holds a cutting tool (i) over the hub, and a tracing tool (h) over the galvano. As the hub and galvano rotate in-sync, the cutting tool cuts the design into the hub. The cutting tool spins like a drill by a pulley-and-belt system (q, r, two unlabeled pulleys near the hub, and i), which is kept taught by a weight dangling from a fulcrum at pulley (q).
The hand-wheel (k4) turns a long threaded shaft, which moves the arm up and down. As the design is cut-away from the hub, the arm slowly drags the cutting and tracing tools across the faces of the hub and galvano. The process is very slow, taking up to two days. Both the galvano and the master hub are positive, or raised like a coin.
Master hubs produce incuse master dies by hubbing, or pressing the face of the master hub into an annealed, or softened, die. The process is repeated to produce working hubs, and again to produce working dies as illustrated in Figure 4. The die or hub that was receiving the impression had to be annealed between each hubbing.