c_booth

I've been doing a fair bit of fillet brazing recently, and while I'm pretty pleased with my progress on the whole, I'm struggling to get perfect distortion free results.

Firstly, am I asking too much? Is a small amount of distortion simply an inevitable part of the process, and if so, what sort of tolerances are deemed acceptable?

Obviously there are a number of variables involved when discussing possible causes Eg Mitre tolerance/torch temperature/heat distribution/length of heat cycle/tack and build sequence etc and any distortion is likely the result of a combination of small errors in a number of these areas, but does any one of these (or something I haven't mentioned) stand out as critical?

Heatsinks are always an option, but I'm not convinced they're necessary for brass fillets, and I'd far rather get the technique nailed watertight than end up relying on the heatsink 'crutch' (I appreciate they have their uses in certain circumstances though)

Greatly appreciated as ever

sannerbikes700

If you post a few pics of your raw fillets I bet we can give you some pointers on where you can improve.

Doug Fattic 

When you say "distortion" are you saying the frame went out of alignment (all the tubes are not in the same plane) or do you mean the tubes didn't remain either round or became curved?

c_booth

Hi Doug - I have bookmarked somewhere an old post of yours from the frameforum detailing the fillet brazing process, which has been a huge help in getting me to where I am, so i owe you a thanks already.

Re. distortion, I'm primarily talking about 'ovalising' the tube, eg the headtube of my last frame has a variation of about 0.25mm in it's ID. It's wider on the axial plane, which leads me to think my mitre is a touch on the loose side? This will obviously ream out when finishing the headtube, so I kinda wonder if I'm being overly pedantic?

I did have some alignment issues (headtube was out by about 0.6mm) which I put down to ill fitting mitres again, this time not getting the top tube mitres perfectly in line (they were done by hand, but that should be no excuse)

I'm using oxy propane - not sure if the lower heat therefore longer heat cycle vs acetylene has any bearing? Eg although I'm not burning the flux, it is apparent that by the time I've finished, the flux away at the edge of the heat zone is really quite dark with the buildup of oxides, which suggests too lengthy a heating cycle?

I'll maybe do a couple of fillets on some scrap tube and post up some photos.

Actually another question I have which is relevant here is how critical is it to build an internal fillet, or is simply getting good penetration across the butt/mitre wall adequate (assuming in both scenarios there's a good buildip of brass on the outside of the join)?

unterhausen

I am not sure .25mm is going to be fixed by better mitering, you might just need to more evenly heat the tube including around the front. See "witch wanding"
Head tube was out .6 mm over what length? Might well be ok.

In my experience, if you get good penetration there will be at least a small internal fillet. The internal fillet adds very little strength, as does the external fillet. The real strength is in the area between the tubes.

c_booth

Ah unterhausen - I was looking at this old thread the other day - https://www.bikeforums.net/showthread...llet%20brazing - did that bike ever get finished?

Witch wanding - is that what he's up to at 3.18 in this clip - https://www.youtube.com/watch?v=p7DGb...eature=related - that looks like a whole world of potential disaster . Be interesting to experiment with some scrap tubing though.

It's quite a long headtube, I think 22cm or thereabouts. I did manage to cold set it to about 0.3, but it felt like a horrible, abusive process and I doubt I've done the frame any good whatsoever, so it's something I really hope to avoid in future.

You're prob right re. poor heat distribution to the front of the headtube - I'm possibly a tad neglectful in that respect in my hastiness to get the fillet built.

unterhausen

no, I've been busy. Need to finish it.
yes, that's exactly what he's doing. I didn't realize anyone did it in quite that organized a fashion, pretty neat. That might be a good use of a junk frame.