A Night Owl Emerges from the Dark - Part 4 Paul Perton, Marketer, Writer and Photographer Activities and production of parts for 4709 continue to make significant progress in several key work areas. Having finally achieved the casting of two 4709 cylinder blocks using polystyrene patterns, the castings have been stress relieved and delivered to a specialised engineering contractor in the Midlands for machining. Even in this modern high-tech world of locomotive restoration and construction, the casting of new cylinders is one of the less common activities. That may well change, as locos saved from Barry - most now in their eighties - are showing signs of longterm wear and tear, especially cylinders which not only suffer from wear, but also other degradation processes like rust and chemical attrition. The decision to cast new cylinders will largely be centred around the cost of wooden patterns and their cores. Pattern making is a highly skilled enterprise and given the complexities involved, high costs are a given. On 4709, we re-designed the cylinder block to accommodate the modern requirements of mainline running and conformance to the NR loading gauge. We also investigated ways by which the use of a wooden cylinder pattern could become economic and significantly reduce the risk of a casting failure - thereby lowering the unit cost. One line of enquiry was to review the pattern making process, to optimise the latest technologies. Another was to investigate the Swindon approach, whereby a standard cylinder pattern could be developed which had interchangeable sections to cover the maximum number of GWR classes. New technology won the day, with the CAD-based design for the revised cylinder being used to cut two identical patterns in blocks of polystyrene. On completion, 4709’s cylinders will be bolted together to make a siamesed pair. The positive benefits for other GWR locos needing new cylinders is now clear; much of the basic design work is complete, the technique of cutting the patterns in polystyrene is better understood with every new project and likewise, the casting process itself, more predictable and with outstanding results. After heat treatment, our cylinder blocks have each been sent to a specialised machine shop. Here, they have been set up on a horizontal boring machine for a thorough dimensional check to ensure that sufficient metal exists where it is needed - in the past, unaware machinists have taken too much metal from one end to find insufficient remaining at the other. Proven to be dimensionally correct and correctly set up, each cylinder then had one of its ends
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(Above and Below) Machining the cylinder blocks
Great Western Star Summer 2021
