Graham Singleton wrote:
>>>Why not introduce some grinding paste into the gap and waggle the stick?
><<
>
>Not keen on that, it would tend to embed in the Tufnol and stay there for
>ever.
I would heartily endorse Graham's caution. Even if you could wash it all
out afterwards, using grinding paste (no matter how fine) between mating
parts is much more likely to result in a poor sloppy fit. This is partly
because you can hardly give it little enough waggling. Just consider that
you are forcing the mating surfaces apart with a layer of extremely hard
abrasive and you will see that when you stop & clean it up you will be left
with air space where the abrasive layer was. When grinding valves, it's not
an issue since the mating faces are not cylindrical but a very shallow cone
and so the airspace is taken up by moving the faces closer to each other.
One engineering approach to this would be to use a copper (or sometimes
lead, but that tends to stick) lap (preferably adjustable). It should be
made to almost mate with the HARDER surface of the two (hollow copper
cylinder for a shaft or piston; solid copper cylinder for a bearing or
engine cylinder). The abrasive then embeds itself into the copper in
preference to the workpiece. (However, if the workpiece is anything much
softer than steel, it's probably going to be difficult to clean up enough
after lapping.) The lap should be either much shorter or much longer than
the workpiece - having it the same length tends to result in a
barrel-shaped workpiece. The procedure is to rotate either workpiece or lap
slowly but steadily while sliding them over each other (cover your lathe
bed to avoid it picking up abrasive too!). At intervals, dismantle and
clean up the workpiece very carefully, and offer it up to its mating part
to check for fit. If more material needs to be removed, adjust the lap
(expand for an internal lap; tighten for an external lap) and carry on
lapping.
Another possibility would be to use an adjustable reamer, if one was
available in an appropriate size. This would be a quite expensive approach.
I must say that I'm not (yet) familiar with these parts and so my
suggestions may be totally impractical for reasons I'm not aware of.
cheers
Rowland
... that's Rowland with a 'w' ...
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