someone asked:
>"I've heard this said many times - use the fixture for tacking or
>pinning but don't do the final braze in the fixture. But now a
>question. Would it be possible to maintain accurate alignment while
>still brazing in the fixture if the centering cones holding the seat
>and head tubes were somehow spring loaded?"

the tubes are never straight.
the interference fits are never known until it's "let's-go" time.
the heat cycle wrt expanding and contracting is never "exact".
you need to work though all the possible deviation and evaluate
your net results. only then can you adapt a predictable, repeatable
system that works with your skill set, tooling, and material choices.