sivad360

Howdy, I am interested in building a front rack as a learning project for my commuter. I am planning on silver brazing with map gas blow torch and using 3/8x.028 4130. is silver brazing suitable for this application or should I try and find a oxy/pro setup.

Tandem Tom

As a novice myself I built a rear rack last year after taking Doug Fattic's frame building course. I setup a oxy propane brazing rig. I used bronze/brass to do the fillet brazing of the rack. Then I painted it to match the frame.

Andrew R Stewart 

Silver filler (for most this mean about a 56% silver content, as I will reference to here) is not too fillet friendly for a few reasons. I would only use it for a rack if the tubes were stainless steel. If you do go with silver take care on your miters as silver looses strength rapidly as the gaps grow beyond 0.005". Don't forget vent holes as the silver will blow out of a joint easier then brass does if the hot air has no other escape path.

The not mentioned data point is what torch you are planning to use. The way you stated your questions suggest you'll be using a MAPP gas or some other non oxy added torch system. For the small diameter and thin gage tubes most racks are made from the vastly lower energy output of a non OXY torch will likely do. But this building stuff is addictive and if you come back in a year and seek advice on a "better" torch set up I won't be surprised at all. At that time I suggest looking into a propane/oxy set up. Search for Dou Fattic's postings about this option. Andy

unterhausen

I use stainless .035 and 45% silver filler for racks. I would probably use .035 4130 too. Easier to bend good looking radii.
Might take a better bender though.

Not sure what to say about it being beginner friendly. Probably not. But smaller fillets seem to build more easily with silver than bigger ones.

sivad360

thanks for the advice, i'm going to give it a couple goes with stainless .035 might use smaller diameter cross members. I think you might be right about addiction, but current living conditions are not very conducive to storing/setting up a mixed gas torch. small projects will have to do until i get a bigger place

unterhausen

I was going to suggest that 5/16" is a good diameter for Porteur racks. I think that's what I'm going to be using on my next one unless I have some 3/8" sitting around I want to get rid of.

I don't think 1/4" is strong enough for racks. Especially stainless, which is weaker than 4130

Doug Fattic 

You are going to run into two issues with your choices that will make building a rack more difficult. 1st a mapp gas blow torch has a bigger and lower temperature flame that makes pin point control really difficult. It is going to be much easier to do small fillet brazing with the kind of oxygen/propane torch set up many of us use. 2nd it is usually easier for a beginner to braze small fillets with brass. This is because it is easier to keep its melting temperature in the zone between too runny or not hot enough to flow. Silver goes from solid to running everywhere you don't want it to go in a nanosecond. The higher the silver content, the worse that is. A small pinpoint flame makes heat control in a tiny area much easier. You will have a hard enough time trying to get it right with the proper set up. Adding 2 more levels of difficulty just makes it much easier to get frustrated and discouraged and lowering the chance you will be successful.

As an experiment one time I bought at Lowes on sale a Bernzomatic oxy/propane kit that came with regulators for canisters and a torch handle designed with a recessed .040" tip for .propane. The propane will last but I burned though a bottle of oxygen in about 17 minutes. That means you will use up a number of O2 bottles making your rack. The torch worked ok. I took off the oxygen regulator and hooked it up to my oxygen concentrator. It worked and I could build racks or even a frame with it but I wouldn't want to. My pro set up is much nicer. But the oxypropane Bernzomatic (or the Mag-Torch pictured) is an option.

My frame building class students find deals on used oxygen concentrators all the time. In my last class one student found one practically new for $125. However luck is involved and not everyone has been lucky. You can buy refurbished ones with a 3 year guarantee for $300. You don't need a regulator with a concentrator. And if you decide you want to get into framebuilding, the concentrator is a foundation tool (while the Bernzomatic is temporary).