By Sue Stuckey
The machine now pressing 2p coins at the Royal Mint in Llantrisant, Pontyclun in South Wales has a very important role: there is a national shortage of 2p coins.
The Rhomint high-speed coin press is the first British-made press since the 1960s to produce British coins. It is a remarkable achievement for metal-forming machinery maker Joseph Rhodes of Wakefield, which installed and commissioned the machine in October.
Julian Heyworth, Rhodes’ drawing office manager, a graduate in mechanical engineering design, was the link between the company and Huddersfield University where most of the research was done.
The most difficult part of the coining process is ‘gaining control’ of the blanks. The priority was to develop a feed system that could handle the high throughput. With millions of coin blanks thundering around a vibratory bowl the challenge is to get them into position at precisely the right time for the moment when, under the force of a 160kN press, they become legal tender at a rate of 750 coins a minute.
Heyworth says the patented feeder mechanism has a shovel mounted on a slider crank that bobs backwards and forwards like a woodpecker. The shovel pushes the lowest blank from stock in an overhead tube into position on an indexing table which operates in a similar but faster fashion than the dial of an early telephone.
Two feeders work simultaneously to deliver blanks between 10-34mm wide and from 0.7-3mm thick. Shapes can be tricky – Rhomint copes with the seven-sided UK 50p coin plus scalloped and other odd shapes used overseas, where the Royal Mint gets most of its business.
Hybrid hydrostatic bearings designed by Heyworth are maintenance-free. They prevent metal to metal contact between the machine frame and the crankshaft which ‘floats’ as it drives the twin dies through a series of linkages.
Machine frame harmonics are needed to match those created by 12.5Hz cycling frequency of the press, to avoid a sinusoidal build-up that Heyworth says can rip the frame apart. Finite element analysis by Leeds University allowed early design modifications which give the frame a natural frequency of 25-30Hz, so avoiding the build-up of harmonics induced by the impact of the two dies banging together.
Rhomint cost Rhodes around £1m to develop. Within 12 months it expects to capture about 8% of a potentially lucrative world market dominated by German press makers, in which 60 to 80 machines a year are sold at up to £500,000.