A recurring problem when modelling in OO9 is wheels. There are actually two problems. Firstly there are only a few different sizes of wheels available which limits the locomotives and wagons you can accurately model. Even if you are lucky enough to find wheels of the right size the second problem is likely to bite in that they will be of the wrong design. Fortunately many of the available wheels are actually metal tyres around plastic centres, and so as an experiment I've had a go at producing an alternative design by 3D printing replacement centres. I actually started this a while back but managed to misplace the printed parts.
To make life as complicated as possible I've tried to produce some wheels that could take crank pins for modelling inside frame locomotives. Specifically I've produced centres for a Deutz diesel locomotives.
My best guess (from that photo and some other drawings) was that the Parkside Dundas 5.1mm wagon wheels where the closest size wise but having seven curly spokes would usually rule them out. Fortunately, like many other wheels, the plastic centres push out. Some careful measuring with a set of digital callipers allowed me to determine the main dimensions of the centre which I could then use to design a Deutz style replacement. Probably more by luck than design the centres are a perfect fit inside the tyres and are a tight push fit to the original axle -- I've painted the centres red as the FUD prints are difficult to photograph.
Unfortunately I designed the centres to take a 16BA screw as the crank pin only to find I'm actually out of stock so I haven't been able to test that aspect. The plan though would be to fit a screw into the centre and then cut the head off, slide the coupling rod on, and then use a 16BA nut to hold everything in place. Alternatively you could slide the coupling rod onto a screw and screw it into the wheel so the screw head holds it in place. You'd then cut the screw flush with the back of the wheel and file the screw head flat. Either should work, but I'll have to wait until I get some 16BA screws before I know for certain.
One other interesting thing I've discovered is that the Parkside Dundas wheels, and hence my replacement centres, use the same diameter axle as the O14 axles available from KBscale. This is great as it means that you could replace the axle and model outside frames with flycranks. Usually you can't replace the axles as each manufacturer uses a slightly different size so this is very useful. Of course if I'm designing my own plastic centres then I could design them to fit any diameter axle.
For a simple experiment it shows promise although until I take the next step and build at least a rolling chassis I won't know for certain how well it will work.