In 1820, Sir William Congreve patented a process known as compound-plate printing, to overcome the problem of forgery of bank notes. It entailed the construction of elaborately engraved printing plates, which would print complicated designs in two colours.
Bryan Donkin built a unique geometric lathe, or ‘rose engine’ to engrave the security printing plates, and designed and constructed an unusual printing press to accommodate the dual-inking process. Compound-plate printing was used for official and commercial printing until the mid 20th century. In the 19th century, paper duty labels were printed by the method to prevent forgery, and in 1839, postage stamps printed on a compound-plate printing press from plates engraved on the rose engine were highly commended in the Treasury Competition for the first penny postage stamps, but were rejected in favour of the penny black. No printing press has survived, but the rose engine has been restored together with the pantograph milling machine which played a part in shaping the dies.
The Rose Engine is now in th custody of the Science Museum. Some excellent photographs can be seen on their web site.
PANTOGRAPH ENGRAVING MACHINE
This particular machine was originally made for cutting strainer plates for paper machinery, the pantograph being added to it at a later date for engraving purposes and for cutting of dies. The work to be engraved is mounted on a small clamp on .the left hand arm of the pantograph. On the table at the near end of the pantograph, the copy of the design or pattern which is to be engraved is placed. This table, together with the pantograph arm supporting it, can be freely moved by hand in a horizontal direction. The pencil or style is held in a supporting bracket with its point immediately above the pattern, and the latter is then moved by hand so that the various lines contained thereon are followed by the point of the style. A tool holder is mounted on a sliding saddle which works across the bridge of the machine. This tool holder is rotated by means of a belt and can be fitted with various types of milling tools in the chuck at its lower end. The tool is set in motion and lowered to give the correct depth of cut, and the design is then moved beneath the style as described above, when the rotated tool cuts an exactly similar design upon the work. The position at which the clamp holding the work is fixed upon the pantograph arm and also the adjustment of the pantograph fulcrum pin decide the scale to which the work will be reduced relative to the original pattern. The speed of rotation varies from 2,000 to 7,000 r.p.m., when engraving in gun-metal the speed is 5,000 to 7,000 r.p.m., depending upon the size of the tool. In addition to engraving, the machine can be used for making compound dies for printing paper money and can cut the parts to such a degree of accuracy that they will fit and interlock together without any further handwork, the line of demarcation being practically invisible. It should be noted that the fulcrum pin; the point of the tool, and the point of the style must be in one straight line, otherwise distortion of the pattern to be cut will occur.