I think I've been staring at this for too long and getting confused.
The etched coach side is approximately 24.5mm which doesn't sound far off 6'2".
I tried to redraw the roof profile from scratch using the drawings and numbers gathered so far.
Starting with a clear sketch I drawn an ellipse, major axis of 8'8 1/2",17.415mm and minor of 4'6 3/4", 9.125mm,
This, we are told is centred level with the bottom of the cant rail.
The cant rail is 2 1/4" high, .75mm so I draw a line .75mm up from the major axis and then down from where this meets the ellipse, squaring off the ends of the sides.
The overall width of the roof has been given as 8'8" or 34.66mm, my drawing gives that same dimension as 34.712mm which I think is close enough for P4.
Then we look at the height of the roof, The annotated drawing posted by Dave Bradwell appears to show a height from top of cant rail to top of roof of 1'11 7/8" (if my maths is right) which would be 7.875mm however my drawing now has this dimension as 8.375mm so the roof is profile appears taller than quoted.
We are told the height from the bottom of the body to the top of the roof is 8' 6 1/4", 34.08mm
We now have a coach side of 24.5mm, a roof height from bottom of cant rail to top of roof of 8.375 giving us an overall height of 33.625mm
roof-profile2.jpg
Measuring the Comet extrusion the height from bottom gutter / cant rail to the top of the roof comes in at 8.4mm, call it 2'3 1/2"
So total height if built as intended, reducing the height of the ends to match the sides, would be 8'5 1/2", again not far off the prototype 8'6 1/4 figure given by Dave Bradwell.
Rather than print a new roof I wondered about printing a tool which matches the profile and using it to sand the extrusion to a better shape.
David
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