Blowfly Construction Notes


I started construction of a modified Blowfly in April 2005 based on the construction series published in "Australian Model Engineering" magazine in 1989. I am building in 7¼" gauge rather than 5" to provide a more stable locomotive, all other dimensions are unchanged. The locomotive will be the same size as the original design, only the frames and gauge will be wider.

The 7¼" gauge has wider treads and hence thicker wheels than 5" gauge, this required a change in the patterns for the wheel and cylinder castings to accommodate the widerwheel tread width. I borrowed patterns and had the wheel and cylinder patterns modied to suit. Grigsby's Foundry in Ballarat did the castings which were excellent. I specified gunmetal for the cylinders to avoid rust problems and cast iron for the wheels, axleboxes and eccentric straps.

The frames are made from 10mm x 100mm black steel plate, this was cleaned up in a bath of hydrochloric acid followed by oiling to prevent rusting, this removed the scale and gave the appearance of BMS(bright mild steel). The clean steel was easier to mark out , looks nicer and paint will adhere to it better than the original scaley black steel. I bolted the frames together using 4mm bolts with angle at the corners instead of welding as recommended in the construction notes. This has avoided welding distortion and resulted in square parallel frames which makes the alignment of cylinders, cross head slides, connecting and side rods easier and more accurate. Another advantage of this is the ability to dissassemble the frames to carry out further work on them such as the drilling & counter boring for the cylinder holding bolts.

I found that because the eccentric strap patterns were only 10mm thick this resulted in hard almost un-machineable cast iron castings due to chilling of the cast metal in the small pattern space. Grigsbys were more than happy to anneal the castings resulting in a soft readily machinable material.

I have used needle roller bearings instead of plain bushes in the axleboxes. I am using "O" rings on the pistons and in the stuffing boxes. I have modified the valve gear and have includied two small stainless steel springs between the valve and valve nut to lightly hold the valve onto the vertical valve face, this avoids a design fault which allows the valve to fall away from the valve face when the steam chest is not under pressure. I have extended the valve rod, the end of which runs in a "tail"extending from the steam chest to counter side pressure on the valve rod from the springs.

I am currently at the assembly stage having painted the frames and major components. I have just successfully run on compressed air (June 10). I intend to construct my own copper boiler.

Please note that the wheels in the photographs are not finished, they will be profiled in the lathe between centres once they are loctited to the axles to ensure they run true.



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