The spinning wheel test yesterday was... half successful. I took the measurements for the new whorls from my previous prototype hack of the original flyer and whorl, and in theory, there should have been next to no pull on the thread, thus resulting in a lot of twist being delivered while the yarn winds on to the bobbin very slowly. I had significant pull on the thread and thus by far not enough twist to suit my taste. Due to the temporary fixings of the new flyer, it was also not possible to just hold the thread tight until enough twist was delivered. I've tested the flyer whorl before, with the hacked old spool from the old flyer, and it did work quite well, so the excess winding speed is probably due to too much friction between the bobbin and the axle.
The new flyer and bobbin arrangement sits differently on the axle than the previous one, so the wheel and the whorls are out of proper alignment. This, in turn, means that I have to adjust the alignment; I fiddled with it yesterday, but did not arrive at the best final solution yet.
The orifice end of the axle also needs some kind of stopper. The old flyer had a snap ring to prevent it from going too far through the leather bearing, and the new one will need something similar. That, too, is on the list of things to do.
And finally, the bobbin itself was in need of some smoothing out of the insides, to reduce the aforementioned friction on the axle. Because it's difficult as whatnot to drill an absolutely straight, absolutely centered hole through a round dowel, I decided to laser-cut a lot of small rings and glue them on top of each other, stacked on the axle. That did work, mostly, with a little leftover snaggyness. Taking a round file to the insides, though, did take care of it, and it now spins much nicer.
The next steps in the process are thus clear. Now I only have to do the drilling, clamping, glueing and smoothing connected to them...