In an earlier contribution to this discussion,
Mathew J. Newton wrote:

On Wed, 11 Nov 2009 23:13:37 +0000, Roger Mills wrote:

Does anything have to slide along the D-shaped shaft in
operation, or only in assembly. If the latter, can't you: * Start
with a length of 8mm mild steel rod * Rotate it in a drill chuck,
and use a file and emery cloth to reduce it to 7.6mm

Hmmm... interesting. Do you think that would work? I haven't got a
pillar drill - or were you just talking about a suitably-clamped
standard drill?

I had in mind using a horizontal drill stand. Even better if you can find a
bearing which will fit over the other end end of the shaft to support it
while you 'machine' it. [You might find a bearing on a router bit which
would do the job].

* Bore out one end of the coupling - thus avoiding having to reduce
the end of the shaft to 6mm?

I hadn't considered boring out the coupling either! I suppose though
if I can form the rest of the shaft with my 'homemade lathe' as per
your method above then I can just go a bit further with the small
end..?

I would have thought that it would be much easier to bore out the coupling
than to turn a 8mm shaft down to 6mm concentrically without a proper lathe
(whereas going from 8mm down to 7.6mm - if you really have to - is less of a
problem).
--
Cheers,
Roger
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