Show and Ask Sunday: Internal Fillets

[Michael Slater]

I asked about merging the BB/CS and BB/ST junctions last week.  I got a lot of helpful answers on- and off-list.   Thanks

One idea that came from those many answers was to to experiment with  improving the internal fillet of the brazes.  I reckon if I can get a decent internal fillet, then I reduce the need for big glob tacks outside the joint, and it probably gives a all-around grip on the joint anyway.

I swear I heard that some people put rings of brass inside the tubes to assist building a (big) internal fillet.  Definitely it's a technique using silver for fork ends, for example.  

I turned down a mandrel and used it to wind lengths of Cycle Design brass into springs that fit into the interior of the seat tube and expand against the edges.

I fluxed it up and fitted it to a BB shell (good tight miters).  I heated from the outside, but didn't get any brass flow-out.  I heated the tube longer and hotter than I would have tolerated if I was brazing normally. Still no flow.

So I cut the tube off (it was tacked to the shell) to inspect.  No brass melt at all.  I probably should have expected this. Even with the brass touching the walls of the red-hot tubes, there simply isn't enough heat transfer to bring the brass to melting temperature.  I guess a scientist would say that I was only heating the internal brass via conduction. Missing out on convection and radiation which I'd get on an outside joint.

Still curious if I heard right that people do manage to make this work.  

Anyway, so that experiment is over.  And now I'm left with a  shell and a tacked-on ST stub.  So I worked on deliberately building an internal fillet (from the outside).

Remember these were quite tight miters.  What I did was get a close, hot flame, get it all cherry, dab (much less than normal) amount of brass, and then heat it and walk it along the miter edge.  I notice the edge sucking in the brass immediately.  And it appeared that after that first drink, trying to jam subsequent amount of brass in, failed.  All I managed to do was cook some of the outer flux. (I was using a harsher, closer flame, like when I tack than when I braze a fillet).  Then I did the other side, and this time tried to avoid forcing in more brass after the golden moment at the start.

Results was a small, even fillet inside the circumference of the tube.  I +think+ this is probably good. I can see how I could manage to cram a whole lot more brass in there.  You can see on one side of the tube that the overheating drew some of the brass up the seat tube.  This was from me trying to coax more brass into a reluctant, plugged miter joint.  The other side doesn't show this.

Attaching a picture of those results.  

[omar-...@cox.net]

Hi Michael--

If you want to get more brass inside the tube you can try to alternate putting the flame outside the joint then inside the BB shell.  The brass "stopped" because the surface of the BB shell in there was not hot.  It looked fine to me the way you had it.  Also, I would recommend venting those joints by putting a hole in the BB shell where each tube intersects the shell.  I do 3/8" holes here at least.

Omar Khiel

Oasis Custom Cycles LLC.

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[Eric]

Try working on a more even pre-heat. You want both tubes to be up to
temp, and neither needs to be a glowing red hot mass.

If you are using pre-forms your will generally need to heat it up a
bit more, but you shouldn't have to nuke it to see results. Getting
things overly hot in a localized way will also stop flow out. Make
sure you have a good amount of flux inside the tube for this as well,
and again-preheat both tubes. Get the BB shell up to temp and ready to
"accept" the bronze as it flows from the seat tube. Planning a pre
heat and braze heating cycle will help.

For your own budget, I'd recommend using pieces of tube cut off for
brazing practice rather then actual BB shells.

This one is a face fed internal fillet- http://farm4.staticflickr.com/3075/5806734950_bb40f96aaa_z.jpg

Best,

Eric


--
Eric Estlund
Winter Bicycles
541.556.5156
winterbicycles.com
Eugene, OR

[jon norstog]

Omar. list:

+1 on that. Also, I think a little bit of heat is OK, and way better
than a cold joint. One of the reasons for butted tubes is that any
method of joining is going to draw the temper from the steel
(air-hardening steels excepted). Michael's joint looked like he got
it up there for heat, but nowhere near damaging the steel by "cooking"
the joint.

jn

"Thursday"

[Harold Bielstein]

If I'm not mistaken, Andy Newlands uses brass rings inside the tubes. Maybe he can chime in on this subject.

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Hal Bielstein
hkbie...@rap.midco.net

[Alistair Spence]

Andy does use bronze pre-forms, a la the Bill Davidson method. This is
for lugs though, not fillets, IIRC.

As for the OP's question, I agree with the answers already given. The
filler will only go where you've created an environment for it to
flow. ie. if the metal's not up to temp on the inside of the fillet,
the filler's not going to wet out there.

With a BB shell, sink some heat into it, all over, and on the inside
too. Be patient until the flux is getting nice and syrupy all over
it's surface. Then go to town with the fillet. I do a tinning pass
first, but I know that not everyone does this.

The best fillet brazer I ever saw kind of tinned and filleted at the
same time. I cut one of his test joints apart and saw a nice internal
fillet. Slick.

Alistair.

P.S. Here's a video of the Davidson pre-form method (lugs),
http://www.youtube.com/watch?index=10&feature=PlayList&v=ztXzoyYa45M&list=PLB35A1BBC0E9BBF76

[Mike Giannico]

I have always put a nice size tack btween the st and bb directly in the rear and then working the brass to one side then the other. You can add heat to either side and watch it run in the miter in either direction adding more
From the outside if you feel it nessecary. If I were to use internal rings I would make sure they were the exact shape of the miter inside with a lot of physical contact between both of the tubes, this way you get direct heat transfer from parent to filler and not relying on internal air temps and cast of heat. And plenty of flux will help the filler melt easier. I have never used an internal ring and I have brazen perfectly fine joints with an acceptable internal fillet. Kudos to those who use rings

Mike g

[Mark Bulgier]

IMHO brass (or silver) rings inside the joint are not useful for fillet-brazed frames, they are only good for lug brazing. The idea is that when the filler melts, it flows everywhere by capillary action, very rapidly, if the entire joint is up to flow temperature when the ring melts. It's beautiful to see when it happens -- the filler flows right up to the lug edge everywhere, and stops right there -- zero cleanup.

Although you have to spend a little time soaking in the BTUs to melt the ring, the joint is done so soon after that point that the overall joint brazing time is usually less than for a lug brazed by feeding rod by hand from the outside. I have brazed a lug in 1:15 (one minute fifteen seconds) from sparking the torch to turning it off.

The other key to brazing lugs fast (besides pre-placed filler inside) is two oxy/propane torches, one in each hand, and big ones at that. We used one really big torch and the other bigger still -- a rosebud. It can be done with oxy/acetylene too, but not as quickly, propane is ideal.

Because the entire joint is hot at once, you pretty much need to pin. Brass tacks will likely melt before the ring inside, so you risk letting the joint moving around. It's really hard to pin a lugless joint.

BTW I don't recommend this technique to amateurs or even pros who build in small numbers. The amount of development time and testing you put into it will probably not pay off.

Back to the subject of inside fillets -- I don't think they are important at all, though they are a by-product of good prep and good brazing. Put another way, a joint with an internal fillet all the way around is probably a good joint -- but not because the fillet. The inside fillet adds near nothing to the joint strength or longevity, if the outside (fillet or lug) is done well.

Mark Bulgier
Seattle, WA
USA