Some years ago the SMEE stand at The Model Engineer Exhibition had a display of models from their collections, amongst them was an oscillating engine made by the late Charles Tidy. I thought it would make a nice project and, asking around, one of our club members came up with a copy of the original GA drawing by Mr Tidy and also a copy of a page from the original manufacturer’s catalogue showing the engine. Unfortunately the GA was a bit small and the engraving a bit vague as to details (the engraver who made the plate for the catalogue probably hadn’t a clue as to how a steam engine worked!) so I re-drew the whole thing to make sure I knew what went where and did what.

The original engines came in different cylinder sizes: 4”, 6” & 8” with larger sizes ‘on application’, in 1879 the 4” cylinder version would set you back £35.

The parts for the frame were cut from 1/8” mild steel to drawings scaled up from the GA, screwed together (left), then brazed up (right). After brazing the frame was sand-blasted to remove flux residue & scale. Finally car body filler was used to put a filet at the joins to give the appearance of a casting

Next to sort out was the cylinder, rather complicated in that the bearing trunnions & cylinder are one piece, I did think to make a pattern & have it cast but as I only intended to make one, this seemed a bit over the top so I decided on fabrication as with the frame. The various parts of the cylinder assembly including the trunnions, external steam passages and steam chest machined prior to assembly & silver soldering are shown on the left.

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