By way of background to the work process ,it all starts off with a rough sketch on paper to get a feel for the item and how it might be drawn giving particular note to lines of symmetry.
Sourcing some photos of the original is paramount along with an idea of the actual size. For the roof vents Google came up with several images but was not so useful when it came to finding dimensions. These were quickly tracked down from Keith Parkin's excellent book entitled British Railways Mark 1 Coaches.
Drawing in Onshape
The important thing, whichever drawing package is used is that it should have the ability to export the drawing file in .stl format (stereo lithography) - more later. Armed with dimensions, it's a case of into the drawing program - in this instance - Onshape, a cloud based service, which is ideally suited to drawing regular geometric shapes.
A cross section was drawn first.
Then the 'stem' was revolved. This was made long enough 10mm, and wide enough 2mm, to make for easy fixing in the model.
Next came the inner dome created by revolving an arc around the central axis
and finally the overall 'strap' was put in place. Even though this was done in O Gauge, the detail of any of the internals or even the fixing bolts was considered to be too small to be modelled. A check of some spare white metal vents which happened to be to hand confirmed this.
Finally, the vent was multiplled and fixed to an horizontal bar for the purposes of printing and exported as an .stl file which is basically a file standard describing the drawing in a format which the Slicer software (see below), can understand.
The downloaded file is loaded into in a slicer program. These are often supplied by the printer manufacturers. The final article is either built up by depositing plastic filament, one layer on top of another or, as in this case, by exposng photosensitive resin to a UV light one layer at a time and it is the slicer programme that works out what is printed on each layer.
The slicer program provided by Elegoo is called Chitubox and here is a screen grab from the process;
The slicer then exports the file in a format that can be handled by the printer.
This was a very small print and was completed in under 20 minutes.