Grounding the rail

Picture17
Rail held down deep in the groove by circumferential clamps

Small U-plates are screwed down to the beam on either side of the rail gripping a small round pin that is passed through a hole in the ring drilled flush with the surface of the beam.  Ten pairs of these were made from strips of galvanized sheet metal and arrayed at equal intervals around the circumference of the ring. 

Picture16
U-plates and pin dismantled for painting

These were perhaps not entirely necessary because the ring was well wedged into the wood and certainly the weight of the dome itself would make sure it would not budge. However they made it easier to handle the beam while the rail faced upwards and they also ensured no slippage of the rail around the trench.  

The low profile presented by these small anchoring stays did in no way effect the travel  of the rail around the castor wheels. Further anchoring of the rail to the beam by acrylic or other forms of cement was deemed unnecessary. 

What was felt necessary however, in view of the fact that the rail was not galvanized, was to paint the rail with two coats of zinc phosphate primer at this early stage because it was becoming fairly obvious that the project would carry on for quite some time.  Naturally the point of contact of the rail with the castors  would when the time comes be cleaned from the paint and covered with grease instead.

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