Beginner fillet braze criticisms?

[Kai Vierstra]

Hi,
My name's Kai and I'm fairly new to the list. Been watching the posts, and more importantly frying some tubes. Not a framebuilder yet, not even close, but I'm ready to put together a rack for myself with the hopes that that road will lead me to my very own hands making me a bike. So, here's some not great pictures of my first decent (I think, I hope) braze using some nice rod I ordered from Henry James. The tubing is 3/8' .035. I've used some really thick plumbers brazing rod to practice with for awhile now, but the difference between that and the "good" stuff was night and day. So, while it seems ok, I was wondering if there's anything you with much experience might notice that I need to consider. Tried not to get too hot, got it just barely red.
First picture is after I soaked it a bit, the second is with a tiny bit of filing and having cut away the tube to see what the inside looked like. You can't really tell from the picture, but the fit was good. What I was hoping to see was some of the bronze having seeped through for a micro fillet on the inside. This micro fillet was something that an expert local framebuilder had showed me on their fillets that I found impressive. Unfortunately it wasn't there. There were no gaps, but there was no extra stuff. How important is that interior bump? Does every good braze have it? Could it be detrimental? Too much heat to attain? 
Thanks for any input
-Kai
Brooklyn, NY

[Mark Bulgier]

Nice work, I think you're ready to make a rack.

Flowing a little into the gap so it shows up inside is a good practice but not strictly necessary when you make the outside fillet large enough.  Your sample fillet is definitely large enough (maybe a bit too large).   If you do get it to flow everywhere on the inside, you can get by with much smaller fillets.

Small fillets have several advantages: Less filler used up, lighter weight final product, usually less filing/sanding needed to look good, and (maybe most important) faster brazing cycle and faster cooling due to reduced thermal mass, so you get some normalizing (strengthening) from the self-quench.  Cooling too slowly leaves the steel more annealed.

Of course cooling too rapidly is bad too -- don't dunk it in water while it's red-hot, it'll be brittle as heck.  Normalized Cr-Mo is tubing that cooled from above the transformation temperature at the normal rate that thinwall tubing cools in still, room-temperature air.  That's a fairly mellow heat-treat state that is stronger than annealed but still with very good ductility.

Thinwall tubing with a big fat fillet cools noticeably slower than just the tubing by itself, so it is left a bit weaker.  Small fillets minimize that.

How small is too small?  Make some samples and break them in the vise with leverage.  The braze joint should not break, ever.  The tubing adjacent to the braze should buckle before the braze pops.  I think you'll find you can go pretty small if you "wet" the miter and get that little inside fillet.

One more tip, it looks like you could stand to use some more flux.  I recommend applying paste flux to the whole joint even if you're using flux-coated rod (which I sorta hate) and/or a gas fluxer (which I love).  Learn to conserve the flux as you heat the joint so it doesn't blow away or drip off.  Strive to have shiny steel with no burnt looking spots.  It's not the end-all, be-all, don't obsess over it, but with practice it's possible to have a shiny non-burnt joint when done. The flux will soak off easier and you'll have less cosmetic cleaning work to do, especially if you're going to have the rack plated.

Mark Bulgier
Seattle

[Andrew R Stewart]

One trick to experiment with heat during fillet brazing is to use a ring of filler inside the tube which is mitered against the cross tube. Then you sweat it out. This does a few things. It makes sure there is internal filler after all is done. It gets you comfy with higher heat needed to melt out the filler which is not directly in the flame. Make sure you clean inside the tube and flux past where the filler ring is . Andy.

 

Andrew R Stewart
Rochester, NY USA

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[Mark Stonich]

To use correct terminology, it's only "Brazing" if the filler is drawn in by capillary action. The industry standard term for what we call "fillet brazing" is "bronze welding". I bring this up because IMHO a good lugless joint involves both processes. After tacking I braze all the way around the joint, sucking bronze into the interface between the tubes and getting some into the interior. The internal fillet is most useful on the sides, where the external fillet is very thin. 

After "brazing", I "bronze weld" the fillet. I do this in 4 short passes, starting in the corners and working out to the sides. (Probably not practical on such small tubing) I suspect not many framebuilders build the fillet this way. But I learned the 4 step way when I took 120 hours of Airframe Welding classes back in the '60s, so that's what seems logical to me.

BTW your joints look pretty good, if a bit large.  And +1 on more flux. Flux the joint, not the rod. A bit of oxidation on the surface of the fillet is desirable as it adds surface tension. Industry practice is to use an oxidizing flame to promote this. But probably not Plan A with tubing as thin as we use.

 Mark Stonich;    BikeSmith Design & Fabrication

   5349 Elliot Ave S. Minneapolis, MN 55417 USA

      Ph. (612) 824-2372   http://bikesmithdesign.com

         https://www.flickr.com/photos/bikesmith/ 

[M-gineering]

Use way more flux, those curly edges show the bronze doesn't really flow
in the metal,
Try feeding in a bit of rod when you doing the ears of the miter, to get
some interior miter going. No point in trying to achieve 100% interior
fillet though (takes to long), but getting the tube edge attached is nice

On 10/23/2015 12:40 AM, Kai Vierstra wrote:
> Hi,
> My name's Kai and I'm fairly new to the list. Been watching the posts,
> and more importantly frying some tubes.

> Thanks for any input


--
mvg

Marten Gerritsen
Kiel Windeweer
Netherlands

[Truls Erik Johnsen]

Hi!

Ok, as the other ones have said: Flux the joint. No need to go crazy with big blobs of it (you'd get flux inclusion that way), but you want a nice layer to cover the area you're working on. I would also point out that you should go in closer with the flame. Get real close with the flame. Perhaps only 3-4 millimeters from the workpiece. Thus no big flame necessary (and use a neutral one, that's what the filler rod manufacturers recommend). Think of how you angle your torch, and your filler. Keep 'em 45 degrees to the workpiece and 90 apart. You want heat _into_ the material, not bouncing off it. And you want even heat on both parts of the joint (reflect upon which tube heats the fastest, then act upon it). Try to keep the workpiece level as much as possible (reposition workpiece when needed).  Heat up 'till flux get transparent (if you're using the correct filler/flux combo that will be a good indication of heat). Melt a wee bit of filler. If it blobs, just retract your filler a wee bit, then continue heating 'till it flows nicely on both parts of the piece and creeps along in the joint. When it flows, dab a small amount of rod into the puddle, and move along with your flame. Keep the rod close to the puddle at all time. If the joint tends to overheat, pull the torch back a wee bit add more rod (or move faster). Good luck!

Illustration of filler and torch position (for welding, but the same goes for brazing):

Cheers,

Truls

Johnsen Frameworks

Hølen, Norway

[Truls Erik Johnsen]

Some inspiration (not the ideal setup imho, but still a nice illustration and result): https://www.flickr.com/photos/planetjohnsen/2859016656

T

[Kai Vierstra]

Thank you all! I'll flux like mad next chance I get. I already bent my racks initial rectangle, not perfectly but close enough to be comfortable. Will post followups as I make my way. I run sculpture shops at a school so I'm always faced with the equipment but rarely get the time to work it myself, between all the big kids I work with at college and the small kids I'm raising at home my progress will be glacial, but ever forward.
Thanks again, and feel free to keep the tips flowing
-Kai

[Jim G]

Just curious -- did you flux inside the joint?  Meaning, did you flux the tubes before you assembled the joint?  If there's no flux inside, then there's no way the brass will flow inside the tube to form the internal fillet.  It probably doesn't matter much -- Hirose seems to flux his rack joints after assembly so there's probably no internal fillet either.

I should do what you're doing -- braze some rack joints and cut 'em open.  :)

-Jim G

On Thursday, October 22, 2015 at 5:33:51 PM UTC-7, Kai Vierstra wrote:

[Mark Bulgier]

Jim G wrote:

> If there's no flux inside, then there's no way the brass will flow inside the tube to form the internal fillet. 

Not quite true, the molten flux will flow into the joint a little by capillary action, and the molten brass pushes some flux ahead of itself I think.  Fluxing inside is recommended, but it's not a clear-cut case of no braze penetration without it.

Lots of cheap lugged frames were made by assembling the tubes and lugs with no flux, tack-welding, then dipping the assembly in a flux slurry before brazing.   Penetration of brass through the lug was not always complete but it was good enough.  I shudder with revulsion at the thought of it, but that's just because I'm a bike snob.  Lots of people are still riding lots of happy miles on frames made that way.  Not knowing that they're not supposed to be enjoying it, the poor ignorant rubes!  ;^)

Still, I hope nobody here makes frames that way - ugh.

Mark Bulgier
Seattle

[Kai Vierstra]

I got more flux inside than I had on the outside. About .25" of a plug in the end of the tube. The outside had a thin coating that didn't extend much if any farther than the bronze weld* does.