On Tuesday, September 15, 2020 at 6:48:01 AM UTC-4, charliebrown wrote:
I'm thinking of making a tool to be slid up inside a bicycle frame down
tube that would expand and push out dents from the inside.

I have been thinking about two different inter-acting problems.

1. how to manipulate the tool - so far the best solution is to have it
have one part a curved shoe with two bowden cables attached; both cables
go in from one side of the BB shell if the dent is on the side, and one
from each side if the dent is top/bottom. The cables outers should be
stiff enough the push/pull and twist it. Each cable will go out to a
handlebar (what else?) with two stout brake-levers mounted.

2. how to expand and contract the other part of the tool - I am not sure
if contracting it will be a problem but want a positive method anyway,
don't want the bloody thing left up there; not sure I can trust a spring.
I can't decide whether to use a toggle, or a screw at 90 degrees to the
long axis of the tube. The tube wall will be at most 1.5mm thick, often
less, dents might be as much as 5mm deep.

A toggle might be tricky to fit inside a tube, so I'm leaning toward a
screw with a ratcheting cross-lever operated by the two bowden cables.
That'd be a bit of a bother as I think the ratcheting will be insert -
place - push - remove - advance ratchet - repeat. A toggle might allow
all the push to be done at one go, but getting positive contraction is
not a simple as with a screw - and as well I am not sure the cable would
take the strain needed to push the dent out. Toggle mechanical advantage
is best at the angle most constrained by tube diameter, there is probably
less than an inch to work inside.

I also thought about a screw-operated pair of wedges - but the
manipulation arm would have to be able to take torque as well, yet be
flexible enough to bend through the 90 degrees of the bb shell/tube
joint. A pair of wedges could be operated by cables, but again would the
advantage be sufficient given the limit of the cable.

Don't know whether this is going to help, but this:
https://youtu.be/zEL-qJuaMN8 is how they get dents out of brass musical instrument tubes. Of course you'd have to scale up the sturdiness of the tools and use a hammer on the outside, but it ought to work.