On 16 May 2018 12:31:49 GMT, David wrote:

snip

I assume that the major issue is keeping everything flush with the
external surface until the distortion of the tube is enough to hold
everything in place. Haven't worked the tools for that out, yet.

On the grounds I would be only doing 'a few' I couldn't justify a good
tool and it probably wouldn't be worth trying to use a cheap / bad
one.

So, I would turn a mandrel that had a reasonably wide foot, with a
close fitting hole for the drawing bolt and a recess turned into the
face that was not quite as deep as the thickness of the rivet nut
outer flange and just a fraction bigger ID than it.

The idea being is that the slightly shallow recess would ensure all
the effort is applied to the rivet nut itself whilst the larger
diameter of the face would help ensure everything was held true.

An HT bolt would be screwed though the mandrel and into the rivet nut
and then drawn back though using a backnut and a few washers or a
thrust bearing. This ensures that the rivet nut is just drawn into
place with little rotational force applied to the rivet nut etc.

Cheers, T i m