More Busker Organ Thoughts and Pitfalls
by Tony Goldsworthy
|Bellows and Bellows Fabric etc.
I used medium density fibre (MDF) boards of various thicknesses for the bellows boards and the wind chest. It's flat and stable (and I had it in the waste box anyway). As long as you regard it as fairly stiff cardboard and amend your woodworking techniques to accommodate this fact its ok. In Australia (and no doubt in the rest of the world too) there are now available fast helix narrow shanked 'man made timber' wood screws. They work well into MDF as long as you don't overtighten them and strip the hole. If it's a critical or load bearing screw I try to back the MDF with a bit of proper timber and screw clean through into that. I seal the surfaces and cut edges with shellac or sanding sealer. Both of these sealing methods present problems if you have to glue onto the sealed surface later and you will have to abrade the surface with glass paper to destroy the sealed surface and get to the fibres below.
The traditionalist in me thought leather bellow fabric would be 'pukker'. Unfortunately the combined thicknesses of the folded leather prevented the bellows from closing to the 1/4" required and hence the throw of the crankshaft (7/8" crank) became too great and the crank would not go over 'top-dead-centre'. The solutions were to remake the bellows with rubberised fabric (too many hours - and I had almost got an airtight seal on the system) or remake the crank shaft. I chose the latter (two and a half hours work) and opted for a 17mm crank ie 34mm (1 5/16") from fully closed to fully open. I hope it will generate enough air volume from this reduced throw. Time alone will tell. Another minor problem I have created is that I backed up my leather flap valves with thin MDF to keep them flat. They work like a treat but the air system now has a distinct 'clap, clap, clap' as the various valves seal. The motto there is 'Do it like the book says'.
Bellows and Air Sealing
If I ever have to make paired (or multiple) bellows again I would consider making them individually and them sticking them onto a thin intermediate board. They would then be flat and probably air tight. The 6mm MDF wind chest was pinned (12mm pins) and PVA glued together. When the glue was dry all joints had a fillet of CSS squidged into them with a finger tip. I attempted to make an air seal around the take up drum drive shaft by forcing the shaft through a patch of 1mm shim rubber which was stuck over the bearing block and bearing hole. The Jury is out on this one as the wind chest is still leaking from somewhere.
The inspection window in the lid of the wind chest is 5mm Perspex/Lucite. It was cut about 12 mm oversize on the width, set on contact adhesive and again received a fillet of CSS. The lid seal is a 'Raven' brand hollow 'D' section draught excluder strip of about 6mm uncompressed height. I set the hinges to give a compression to about 2mm and I cant detect any air leak. (There is one but I don't think it's from that seal). If using a hollow section seal on the lid don't cut right through it when you cut the mitres as it will leak. Leaving about 3mm in the corner of the mitre seems to stop this.
The Drive Mechanism
The cranked con-rods were each band sawn out of a 10mm hard wood board (I finished by making half a dozen until I got the length and shape I needed) and the connection to the bellows was through a 4mm hole drilled in the bottom end of the rod via a 4mm 'L' shaped rod. This was screwed on below the bellows and allowed the con-rod to rock freely to and fro as the crank went through its 360 degree rotation.
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