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Old January 10th 17, 07:20 AM posted to rec.bicycles.tech
Jeff Liebermann
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Posts: 4,018
Default Need advice on bottom bracket repair

On Mon, 9 Jan 2017 22:24:41 -0000, "Benderthe.evilrobot"
wrote:

"Theodore Heise" wrote in message
...
On Sun, 08 Jan 2017 17:01:01 -0800,
Jeff Liebermann wrote:
On Sun, 08 Jan 2017 16:31:15 -0800, Jeff Liebermann
wrote:
(...)

Yet another untested idea...

Use an induction heater on the steel cup.
https://www.youtube.com/watch?v=MyZEaPQinO0
https://www.youtube.com/watch?v=YJElT9xK3bk
https://www.youtube.com/watch?v=uimEZKrVNO0
https://www.youtube.com/watch?v=3u1IBgefNDs [1]
http://www.theinductor.com
You'll need various size coils for different bolts, cups, seat posts,
etc. There are induction heater kits on eBay:
http://www.ebay.com/itm/1000W-ZVS-Low-Voltage-Induction-Heating-Board-Module-Flyback-Heater-Brass-Coil-/282317713643
The steel cup gets hot and not so much the rest of the bicycle,
including the aluminum bottom bracket which sucks away much of the
heat. However, you can get the steel hot enough to melt the aluminum,
so please use an IR thermometer to keep the temperatures down.
Aluminum melts at 660C.


Yikes!


Is there a problem?

An electric heat gun is safer than a blowtorch - but I think you still have
to be a bit careful.

The heavy duty paint strippers aren't too bad, but some heat guns are
designed for lighting solid fuel cooking ranges.


The problem with external heat generators is that the heat affected
zone is rather large and will surely creep into undesirable areas,
such as painted parts. With an induction heater, you only heat the
steel parts. Aluminum only gets hot as far as what it conducts away
from the steel.

If you happen to have an induction cooktop stove, you can demonstrate
how this works. Put a steel pot on top of the induction heater, and
only the steel will get hot. Put an aluminum, glass, or copper pan on
the stovetop, and they stay cold. Toss a coin with stainless
(magnetic stainless mostly works, non-magnetic does not).

It's the same with using an induction heater on the bottom bracket.
The steel components get hot, while the aluminum bottom bracket and
frame do not.

I don't propose heating the cup to red hot:
http://www.hearth.com/talk/attachments/metal-color-temp-chart-png.100306/
which is why I suggested an IR thermometer be used to monitor the
temperature. Melting the threads in order to release the cup would be
rather counter productive. The problem is that I don't know what
temperature is appropriate, so there will need to be some trial and
error. As always, one needs to sacrifice an old bicycle frame in the
interest of moving cycling technology forward.

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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