spent most of today down in the metals cave.
brazing takes loads of preparation. i made the mistake the other day of not setting up the torch until after i had cleaned and fluxed the parts, so the flux dried as i set pressures and hunted for a decent tip. here's a review of my procedures today, which i'll practice and tweak in the coming months:
set up to clean and flux the parts. run a compressed air hose and nozzle near the sink where you'll be cleaning (bleed the compressor of water beforehand so the air is dry). have near the sink: a respirator with Organic Vapor and Acid Gas cartridges, safety glasses, rubber gloves (i previously tried non-latex, medical-type gloves that i had used with photo chemicals--HCl dissolved them quickly), a solvent (i'm using mineral spirits; denatured alcohol would probably do; acetone is stronger); toilet bowl cleaner (i'm using Lysol Power; see previous posts); something to scrub with (Scotch-Brite pads work well); paper towels or rags for applying solvent; flux; flux brush; and a suitable place to put the dry parts while waiting for flux. for dissolving flux later on, the sink should have hot running water, the hotter the better, and a bucket large enough to hold the work.
set up your brazing area. have handy: torch and related accessories (striker, tip, etc.); gloves; shade 3 brazing glasses; respirator (i use a combo OV/AG cartridge plus a P100 filter, probably overkill); and brazing filler.
set the line pressures. this is an area where there are as many opinions as there are brazers. Paterek says 5 psi both. i brazed with some success today at 7 psi acetylene and ~10 psi oxygen.
first wipe all the parts down with your solvent of choice. gloves, respirator (OV cartridge), and glasses are necessary. be sure to take care of the insides of the tubes by stuffing them with a solvent-soaked rag and a dowel. this can be done well in advance to remove packing grease and grime, although said grease also prevents rust, so re-grease with WD-40 or somesuch as needed, and re-de-grease before continuing with brazing prep.
now go at the parts with some abrasives. steel stock is usually covered in mill scale from being rolled between the dies that made it. it's sorta gray-black, sometimes with lettering indicating what type of metal it is. this has to come off completely for brazing to work. a shoe-shine motion with production cloth works well. 3/4" and 1" diameter grinding bits work well in an air-powered die grinder for interiors where necessary. the Deda tubes i bought only have a light layer of scale, but Aircraft Spruce's 4130 is covered in it. with enough ventilation, a respirator (particulate filter, N95 or better) isn't essential for grinding, but glasses are. and wash up before returning to the civilized world.
now wash off all the abrasive dust you just covered everything with. don gloves, respirator, and glasses. squirt the toilet bowl cleaner on the parts and scrub. HCl is pretty good at removing scale and rust, but only with plenty of elbow grease. wash the acid off with cold water, blow dry with compressed air, then set aside in a dry place.
now for the flux. leave the safety get-up on. apply flux to all mating surfaces liberally--i.e. the inside of the lug, the outside of the tube that goes in it, even the inside if it seems necessary. now the clock is ticking a little: the flux starts to dry up as soon as it goes on. it can be reconstituted by adding water, but i haven't any experience doing so.
the last stage of pinning, as i understand, happens here. i haven't yet practiced this part, as i seem to have misplaced my Fred Parr pinning kit.
now secure the parts--no safety gear necessary, except for rubber gloves when touching fluxed areas. this can be tricky on unpinned practice joints: they'll want to slide around as they heat up and the flux starts to get runny, so they can't be held so that any piece will fall. also, the part you're currently trying to fill with silver needs to be below the point where you're adding silver or it won't want to run in there--capillary action alone isn't enough.
the brazing itself is harder than i had imagined. you have to heat the whole assembly up, then focus the heating in a specific spot and add silver. all while keeping the torch moving.
this was my first real decent attempt, and it went OK. i got good penetration, not perfect, and very sloppy. i think i used about triple the silver i should have. and 56% is definitely the stuff for this type of joint...i switched to 45% for a bit and found it really gummy, not smooth enough to really shoot in. i could see it working well for drops though.
in review, i can see that brazing is easy to learn with enough practice, but hard to do really well.