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Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

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Old 05-07-12, 01:12 AM   #1
veryredbike
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Tolerances for voids

I've been doing more practice lugs with silver, and can consistently get everything just about perfect, but not quite.

My question is: if I've got pretty much full penetration, but there's a void or two of 2-4 mm, is that still structurally sound?

I'm noticing that those spots tend to be where there is the largest gap between the two crappy tubes that I'm cutting up to make these, which makes me think that it might be just too big of a gap for the silver to bridge all of the way, but I'm not sure and would love to know where you guys draw the line between "solid, but not perfect" and "pretty paper-weight"

Thanks,
Jim
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Old 05-07-12, 11:55 AM   #2
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Well obviously a void of 2-4 mm isn't a total deal breaker since there are plenty of lugs that have cut-outs far larger than that. Of course that assumes the void is just where you want it.

I would start with what you know, which is apparently that there are bad gaps in your joint.
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Old 05-07-12, 12:16 PM   #3
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Hadn't thought about it like that, good point!

As far as the gaps go, it's the result of the tubes I'm using. The ID of the larger sleeve is smaller than what they claimed, so I have to put a slit in it and open it up a bit, which makes for an uneven fit. I think I can avoid that when I go back to using actual lugs.

I'm still curious if anyone has a rule of thumb for where they draw the line.
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Old 05-07-12, 01:38 PM   #4
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are you talking about a void on the shoreline that can be seen from the outside, or are you talking about voids that you only find when you cut into the joint? I would never leave a void on the outside, I don't really have a way to measure what's going on in the inside.
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Old 05-07-12, 02:38 PM   #5
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Instead of slicing the outer tube, try sanding to reduce the OD of the inner tube so that a tighter clearance can be achieved for your test joint. Bronze will span much bigger gaps than silver if you do need to fill a sloppy loose lug joint (or use a silver composition better suited to gaps).
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Old 05-07-12, 05:11 PM   #6
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are you talking about a void on the shoreline that can be seen from the outside, or are you talking about voids that you only find when you cut into the joint? I would never leave a void on the outside, I don't really have a way to measure what's going on in the inside.
These are just internal voids, the shorelines look pretty great.

As for sanding... I think you're underestimating how far off these tubes are ;-) I'm going to go back to practicing with real lugs to see if clearance solves this problem.
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Old 05-07-12, 07:51 PM   #7
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Maybe slitting the inside tube will give you a different fit up. Do a practice joint with the outside tube not fully wrapped around the inside tube. Like a patch. Play around to see how different gaps and fits work for flow brazing.

One of the issues with silver is that it will contract during cooling. So any fillet of thickness will mean that the silver cools from the outside to the innermost spot and this spot will have had the silver trying to contract away from it. To me that's a stress riser in the making.

This is why so many builders use brass. Andy.
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Old 05-07-12, 10:08 PM   #8
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Maybe slitting the inside tube will give you a different fit up. Do a practice joint with the outside tube not fully wrapped around the inside tube. Like a patch. Play around to see how different gaps and fits work for flow brazing.

One of the issues with silver is that it will contract during cooling. So any fillet of thickness will mean that the silver cools from the outside to the innermost spot and this spot will have had the silver trying to contract away from it. To me that's a stress riser in the making.

This is why so many builders use brass. Andy.
Good point, I'll definitely minimize the gaps when fitting my lugs to avoid as much stress as possible. As far as doing the practice lugs differently, I'll give that a shot too.

That being said, a while back you mentioned that you'd cut apart a few of your old frames, and in a couple of cases were a bit disappointed in the fill. I'm getting a ton of good advice on this thread, but I'm still curious about where the line is for an acceptable fill and an unacceptable fill.

If I get to the point where I can do 10 lugs in a row, and cross sections are totally filled except for a gap a few mm wide (with the shorelines fully filled), can I take that as an indication that my joints are solid, or would you be disappointed if you found any voids whatsoever? To be clear, I'm not talking about "the silver never reaches the actual joint" just "the silver fills the actual joint 95% of the way around, but there's a little spot where it took an alternate route.

MassiveD is saying that a gap like that isn't a big deal, necessarily, and that makes sense since (as he says) lots of people do cutouts that are larger, but I guess I'm just curious what different builders have to say about where "the line" between disapointed and happy is when they cut open one of their joints.

Thanks!
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Old 05-08-12, 12:06 AM   #9
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My point was just to the question of what is acceptable void wise. There is a whole other quality or skill side to the thing. You should be able to fill the lug at least the areas that have close contact.

There is also a process aspect, are you drawing the filler in from different directions and getting and air lock.

Another way of looking at it is that if you are using the lugs that have holes and are missing 75% of their surface area already, then you don't want to have voids to boot. Sorta depends on your baseline.
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Old 05-08-12, 12:13 AM   #10
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These are just internal voids, the shorelines look pretty great.

As for sanding... I think you're underestimating how far off these tubes are ;-) I'm going to go back to practicing with real lugs to see if clearance solves this problem.
That is probably a good idea, another option is a reamer.
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Old 05-08-12, 01:47 AM   #11
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My point was just to the question of what is acceptable void wise. There is a whole other quality or skill side to the thing. You should be able to fill the lug at least the areas that have close contact.

There is also a process aspect, are you drawing the filler in from different directions and getting and air lock.

Another way of looking at it is that if you are using the lugs that have holes and are missing 75% of their surface area already, then you don't want to have voids to boot. Sorta depends on your baseline.
Gotcha, yeah, I see what you mean. I'm only introducing filler from one side of the lug. Is air lock what it sounds like (trapping bubbles of air so that you can't fill)?

I'm using the slant six lugs for the final thing, and won't be carving anything into them, so I should have a lot of surface area.

Basically you're saying that there are situations where small voids won't compromise the structure, but there are a lot of other variables that go into it, so you can't wave a magic wand and say "your voids are forgiven"? ;-) Plus, they can indicate flaws in procedure that can cause other issues.
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Old 05-08-12, 06:10 AM   #12
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VRB- I have cut apart some old frames of mine and found voids/lack of fill. One had about 30,000 miles on it. While dissapointing I don't think that a 100% fill rate is needed, and if over heating or too long a heat cycle was needed to achieve such, maybe a less then full fill is actually better. A lug is already such an overkill as far as the joint strength (why do you think they're used in production anyway). I have decised to use brass as I can for frame joints (and canti bosses and other high stressed braze ons) for a number of reasons but fill rate and strength with lugs is not the highest reason. My comments about silver's cooling and contracting was more aimed at fillet, not flowing under lugs. Andy.
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Old 05-08-12, 10:24 PM   #13
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Thanks!
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