i tried a brass joint yesterday with (i think) pretty good results. the joint definitely helped me relax and take my time with heating. also i turned out all the lights for the first time, which helped as well. there was still some ambient light coming from other parts of the shop, and plenty from the torch. i'm _starting_ to get the hang of this.
this was the first test of the aforepictured tube holder, and it worked well. there's a fair amount of slop between the pipe attached to the pliers and the vise, which was only bad when i wanted to precisely position a tube.
the reason i wanted to do that was kinda silly. i'm running low on lugs, only 3 Rivendells left, so i tried brazing two tubes together without a lug, just letting one rest on top of the other. i tried to pin it, by drilling down through the top of the mitered tube, but the nail wouldn't go in right and the flux was drying so i just let one tube rest on the other. this required positioning the bottom tube horizontally, as i discovered when the top tube slid off when the flux heated up. i re-positioned and tried again, and the brass seemed to flow pretty well, except in one spot where i think the miter was off and the tubes were too close.
today i cleaned the flux off a bit and tried some destructive tesing. i held the unmitered tube in the vise and stuck a bar of solid stock into the mitered tube. it pried off without a whole lot of effort. i guess that's the reason for a fillet.

things seem good here.

that big black blotch there isn't so nice. i could tell when i was heating that i overdid it in one spot; i think that was it.

note the hole in the upper tube where i had tried to pin.

you can see a little line of brass around the inside there.

the weird red coloration appeared after soaking in diluted toilet bowl cleaner for a few minutes. today i tried soaking in pure T.B.C. for about an hour with better results. the flux came off quickly with some convincing from a file, although i had to emery off a lot of rust.


Kristin said...

Whoa there, I tripped across this from a framebuilding site. "blaze-n-braze" seems to be the operative word here. Spark one up AFTER you braze please.

First why is there flux 2" away from where your brazing and inside the tube? Try using a little less.

I don't know what type torch or fuel you are using but you need to clamp/jig/fixture your stuff and heat it slowly and evenly so when it is ready to braze you can work around the joint and flow an even fillet. Looks like you were overheating some parts and not enough in others.

If your using paste borax flux I would try heating the end of the tube, dipping it into the flux and clamping it in position, this will give you a cleaner joint with less crud to chip off later, perhaps a little smear on the side of the tube your joining to.

A welders soapstone is good to have to mark the area while your doing the "dry fit"

I am not trying to be critical, I am about to embark on my own frame building for a recumbant trike and looking for suggestions myself as to torches, brazing alloys etc. I have been brazing about 40 years and patched a few frames and other things so I am only trying to suggest you fine tune the technique before you mess up a lot of expensive material.

It's been a few years since I last had a torch in hand but one of the things I remember was either with flux coated rod or plain one dipped in flux, heating the joint and touching the flux to the joint away from the flame, the flux would get sticky and string out just before it was hot enough.

Brazing is nothing more than high temp soldering so you need to heat the parts and let the brass flow into the joint, lead the torch and follow with the filler rod. depending on the angle you may want the flame towards the filler or away. If at all posible position your work so the joint is horizontal all the way round as it's nearly imposible to get a good braze upside down unless your miter is .010-.020 it's the capillary action that cannot happen unless the gap is very small.

Well that was fun.

Hail Bozonia

Bob said...

I've got to disagree with Kristin, use more flux. It is simply not possible to use too much flux in brazing a bicycle. The flux should protect the metal and allow the filler to flow. The heat of the torch and gravity are the only things that should control where the filler goes if you're doing it right.

Flux up both tubes inside and out, anywhere there is going to be heat, and go at least 4-6" back from the joint in all directions. The flux also protects metal that isn't being brazed from the heat of brazing. Heat up the whole joint very evenly until the flux is getting glassy all around, then once the whole thing is nearly at temp you can start to work your way around adding filler and creating the fillet.

Fillet brazing is tough, so practice a lot, and be sure to move the joint around to let gravity help you, not hurt you.

Bob Brown

Rick Guggemos said...

A couple of thoughts...
1) Use a flux that washes of in warm water. It's faster and easier to use.
2) TBC is something Freddy Parr always argues against. He says it never completely washes out and can lead to weaking the tubes.
3) IMHO, flux is about the worst thing to which you can apply a file. It tends to build up and cause other materials to build up in the teeth.
4) Have you looked at the Paterek manual and the section on filet joints? It might help with you lugless practice.

Kristin said...

Sounds like your supplier is trying to sell you the whole case of flux he has. If you look at commercial production brazing they will use only enough flux to clean the joint. Applying with a brush, syringe or use a preform. Makes cleaning up easier.

Bob said "Flux up both tubes inside and out, anywhere there is going to be heat, and go at least 4-6" back from the joint in all directions. The flux also protects metal that isn't being brazed from the heat of brazing."

Flux does not protect the metal from heat, if it did it would inhibit the process. There are gells and other compounds that are applied in some instances to protect surrounding areas from heat, but these are quite a different animal.

This is from Aufhauser's web site
A coating of flux on the joint area, however, will shield the surfaces from the air, preventing oxide formation.

Also note: Post Braze Cleanup
After completing the brazed assembly, it must be cleaned, the flux residues must come off. Fluxes are corrosive. If not removed they can eventually weaken a braze joint. The quickest and most economical method is a water quench. Once the filler has solidified, place the warm assembly in a hot water bath. This will normally "crack" the residue off. For more tenacious residues, agitate the water bath or use a jet spray to knock the flux off. Or simply wire brush the assembly while submerged in the bath. If the flux has been saturated during the heating cycle, the assembly will have a blackish discoloration. In most cases, an acid bath will be needed to assist the flux removal. Care must be taken in choosing a mild acid to avoid etching the joint.

The web page is:

Keeping your files clean, take chalk and grind into the file before filing soft materials like brass, aluminum. lead. Get a "file card" a wire brush made to clean files.