Harrison gearbox parts

Ah, perfect! I'd assumed that the splines were something to do with functioning of the gearbox, not just assembly.

Yep, making an arbor to hold the gears for boring is the right way to do it.

However... your gears are odd diameters, being "profile shifted" 15DP gears. If the ones you've found from the other parts of the machine don't match those measurements, they're not going to mesh correctly. Measure very, very carefuly before proceeding.
 
Ian
This was the point my friend made and In a wood for the trees way I hadn't thought about.
The layout is that this shaft sits between the input and output shafts. The tumbler meshes with the input shaft and one of ten gears on this shaft. The output shaft has two gears which slide on splines and are controlled by levers on the front of the box so that one or other of them comes into mesh with either the same gear as the tumbler, or one of the other 9. In this way you have umpteen different relative speeds between the input and output.
The important thing being that once these ten gears are installed they can't move sideways along the shaft, the splines just ensure that when one gear is turning they all do, which his sleeve idea accomplishes. Fresh pair of eyes and all that.
I will check carefully to see that the new gears mesh correctly as you suggest.
Once again thanks for all your help and advice.
Fergie
 
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