This is a bit of a new direction for us here at Shaggy Designs. We made this as a demonstration piece, and to test some new techniques. This represents the first time that we have worked to an external specification, rather than designing a piece ourselves.
So; a bit of background is probably in order. Venturi Bellmouth tubes are used to measure air velocity in a laminar (smooth) flowing air mass. They work in a similar manner to Venturi gauges that measure flow of liquids and gases in pipes. The critical difference though, is that they are used in an open flow situation, so have to keep the flow laminar. The pressure drop inside the parallel shaft of the tube is measured and used to calculate the mass flow rate. The interior surface of the Venturi bellmouth needs to be accurate and smooth for optimum performance.
So, after procuring a set of plans for a fairly small Venturi tube, we set to thinking about how to make one. We decided that using a wooden internal (male) mould would give a smooth, accurate finish on the critical surface. While wood is not an ideal material when high precision is needed due to dimensional changes with temperature and humidity, it would serve as a proof of concept for us. Aluminium tooling would be a better solution, but much more expensive.
To get the wood mould to an appropriate size and shape, we fashioned a baseplate and built up using pine blocks to an approximate shape. Then using a borrowed wood lathe (thanks Olaf!) we centered and shaped to an approximate outline. All of this work was done freehand with hand chisels.
Once we had this done, the final shaping was achieved using wooden forms, light chisel work and sanding to get a good finish. The wood was sealed with a couple of coats of Epoxy resin, and then back on the lathe to sand this layer smooth, then onto succesively finer grit size sandpaper and wax polish to give a release surface to be able to lay up onto.
The actual wet-layup was a comparatively easy part of the operation, building up 8 layers of 200gsm glass cloth and Epoxy resin. Once this had set and cured, we went back to the lathe again and more sanding to get the outside surface smooth, and the flange fitting to the right diameter. A final thin coat of epoxy on the outside surface finished the job nicely.