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How to tell if brazing has penetrated lugs?

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How to tell if brazing has penetrated lugs?

Old 02-05-20, 10:16 AM
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Miele Man
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How to tell if brazing has penetrated lugs?

In the past I've read about lugged joints failing because the brazing material didn't penetrate far enough into the lugs. I've been think about this again lately and I wonder. How does a frame builder tell if the brazing material has penetrated far enough into the lugs to make a strong join?

Thanks and cheers
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Old 02-05-20, 11:52 AM
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This video gives a really good intro into lug brazing.

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Old 02-05-20, 01:09 PM
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Originally Posted by Cynikal View Post
This video gives a really good intro into lug brazing.

https://www.youtube.com/watch?v=no-OWfC5-RY
Thanks. That was very informative.

Cheers
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Old 02-05-20, 05:55 PM
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Q: How to tell if brazing has penetrated lugs?

A: Feed braze material from top and then use gravity and heat to pull it though and out the lug on the bottom somewhere.

That video is pretty good but I skipped the part where they were talking for a long time. I think the guy got the parts a little hotter than they need to be but not too bad.
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Old 02-05-20, 06:20 PM
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The usual way is to have done enough practice and slicing up to have a personal feedback loop. One starts to get a feel for amount of filler rod/wire to external build up (not that I ever get any) and what the slicing through the finished practice joint shows. These same practice joints can also serve as strength/destruction test samples. Andy
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Old 02-05-20, 07:23 PM
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Originally Posted by Miele Man View Post
How does a frame builder tell if the brazing material has penetrated far enough into the lugs to make a strong join?
There is no "far enough" in my book -- it's all the way or nothing baby! An experienced lug brazer can "see" where the filler is going, almost like X-ray vision. But the main way to know for sure that the filler has bridged the gap at the miter is to flow it all the way across. Filler that entered one side (say the toptube) appears on the other side of the joint (say HT). Even that by itself is not enough to know the joint is sound, but it's a bare minimum.

The other way to know is to saw up a bunch of test joints, and/or break them with long levers and a vise. Don't build a bike for someone to ride until all of your test joints show full penetration when sawed up, and good strength and failure mode when destroyed in the vise.

There was a time when there were cheaply made lugged brazed bicycles, and an apprentice could learn on those. Those days are gone, and anyone reading this forum is interested in top-quality, expensive bicycles. With lugged steel an endangered species, the plethora of self-taught newbies making unsafe frames is not helping. Even a two-week framebuilding course is not going to make you a master framebuilder, so please don't hang out a shingle until you've brazed and broken or sawed up a couple dozen lugs, then tested your own frames on the road and trail. There are a few unicorns, people of exceptional intelligence, gumption and sensitivity to the metal, that have become good without an apprenticeship. But the trouble is everyone seems to think he is that guy. Guess what, most aren't.

That said, lugged steel is pretty forgiving, and some less-than-100% brazed frames have survived an acceptable number of miles. Anyone who does repairs can tell you stories of the Cinelli, Raleigh Pro or whatever that they took apart to find appalling lapses inside. But still, in this market, merely doing better than best frames of the 70s isn't enough anymore. The high-end steel customer expects (and IMHO has a right to expect) near perfection. It's totally achievable, lots of builders achieve it every day, so if you're not at that level then I think it's akin to fraud to sell them, since that's what the customer thinks they're getting.

Some have accused me of not being very encouraging to beginners. To that I say "GOOD!". I want to discourage as many beginners as possible because most people will never be good at this, so they should be discouraged. Those few that know they're good enough will not let themselves be discouraged by what I say. But watch out for the trap of assuming you're good enough to know whether or not you're good enough. Bring your unpainted work to an expert for criticism if possible, including cut-up sample joints -- and don't just show the one best example, if the other 9 out of 10 were worse.

OK that extended rant was not exactly what you asked for... Don't get me started!

Mark B in Seattle
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Old 02-05-20, 09:54 PM
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One reason that I suggested slicing through practice joints is that filler flows follow the heat. So if the inner tube to tube contact (the miter's contact) isn't up to heat the filler won't make that turn from the lug ID/tube ID gap and flow to the tube to tube "root". So a flow through the lug's edges, looking nice by one merit, could lack the complete penetration of the filler.

I think Mark's reference to " An experienced lug brazer can "see" where the filler is going, almost like X-ray vision" is real with brass/bronze and a darkened setting. As the brass/bronze filler enters the lug's gap and travels through the gap the outside color will change, The flow of braze will show by the "shadow" of darker color on the outside of the bright red lug, advancing like a weather front on modern radar images. I don't follow this feedback as the lighting conditions and other visual things going on during brazing are hard enough to track, although I have seen this.

I do agree with the comments about how learning this stuff is different these days. For one thing the fudge factor in steel frames (material thickness wise) has lessened. So strength loss from the longer heat cycles or greater distortion from uneven heating( with thin wall tubing that a newbie is unfamiliar with) to is far easier to happen. I also thing people have higher expectations these days on perceived "quality" then what us old guys grew up expecting.

Still if steel were discovered tomorrow it might be called the wonder material. Low cost to process and fabricate with good lifespan and a slow failure mode. Most lugged frames are far stronger then the riding needs. Typically it's only when a few things have gone wrong that a lugged joint becomes a problem. Andy
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Old 02-06-20, 07:50 AM
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I think beginners nowadays get discouraged from selling right away. The environment isn't like it was 10-15 years ago, where it took a couple of years for people to fail
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Old 02-08-20, 01:49 PM
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Originally Posted by bulgie View Post
Bring your unpainted work to an expert for criticism if possible, including cut-up sample joints -

Mark B in Seattle
Bill was instrumental in my learning. I took him some of my first joints and he just laughed. Wrong flux, wrong filler, wrong torch and something about my momma dressing me funny. Anyway, he was very patient when he could have just said 'bugger off'. Over the first 2-3 years, I was in there bugging him about every other month with more work to critique, questions on tooling, fixtures. He is very generous.

He has very high praise for you Mark. Not to make you blush.
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Old 02-08-20, 02:43 PM
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after a while, you develop a sense if filler has fully penetrated a joint. However, I had one fork where I thought I hadn't gotten good penetration at the crown, just didn't seem right. I carried that thing around for decades and finally got sick of it so I cut it apart. There was a nice fillet around the top of both blades. No regrets, but it's pretty funny. If it had taken the amount of filler I expected, it would have had a big pile in the middle doing nothing. I'm sure I have some joints with extra filler in them, hiding inside of tubes
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Old 02-12-20, 03:56 AM
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You can buy an endoscope camera for about $15 nowadays, which will tell you if there was penetration in the ever important tube to tube miter area. It's possible to non-destructively see if you got an internal fillet, although it might not show if you filled the lug, but cast lugs have less internal volume to fill anyways. If we get into historical bicycle production, several strategies were employed to ensure good penetration. Some are industrial scale processes, others are applicable to the small custom builder. It should be noted that just because these processed existed doesn't mean they were always employed in mass production, but the method for the custom builder that has survived has survived in part because of how much the process can be scaled down and adapted to an individual builder making many designs with minimum equipment.

Starting with joint design, especially on early and/or cheap bicycles, frames and lugs were often designed to not need mitering or tube-to-tube brazing. Anathema to the builder of lightweights, but it happened. These lugs were often very large to give lots of brazing area. Even if the filler didn't penetrate to the end of the tube, it didn't matter much, the joints were designed with the lugs are a major structural component and there was no tube-to-tube miter to braze inside the lugs anyways. The large lugs ensured that even if there wasn't 100% penetration, there was still lots of surface area so the joint was strong even with partial penetration. Pinned lugs gave another point of inspection for braze penetration.

Next came heating, high volume production processes tended to heat the entire joint evenly. This included hearths like Mercian and many older UK builders use, torches with big flames unlike the small OA tips modern custom builders tend to use, furnaces where an entire part of the frame was brought up to temperature, fixtures with a dozen torch heads of various sizes and locations designed to evenly heat a standardized joint, and induction heating. Since the entire joint is brought up to temperature evenly and repeatedly, there is less of a chance that there will be any spots that did not reach temperature and did not get penetration.

Third is how filler was fed into the joints. There have been some small scale builders that have used brazing rings/preforms, especially with plug type joints, but it's more common to feed filler externally. However, preforms were considerably more common for mass production. It was a repeatable process that reduced labor required, and it fed filler from the inside out, meaning the most critical part of the joint, the tube to tube miter, almost certainly had filler. Penetration could be checked if the filler made it to the shorelines, but even then, joints were often considered to have sufficient strength even if there was not full penetration all the way to the shore lines because the critical deep part of the joint had filler. This is the opposite of how most small scale custom builders do it.
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