QuangVuong

I know there have been a bunch of threads about using budget torches for frame building, etc. What I have is a Columbus Minimax frame which is mostly welded together. It's got a cracked seatstay joint to the seat tube(probably due to a lack of brazing done there). I would like to have a go at fixing that, and while I'm at it, built up some fillets over the welds for aesthetic reasons.

I have one of these torches around: Hot Devil MAP Pro Trade Torch Kit - 2 Pack - Supercheap Auto

What sort of filler should I be getting(something available in Australia)? From what I've read, 56% Silver looks to be the go. But since what I am trying to do is mainly non structural, is there any alternative(eg the solder used for copper piping, etc?).

And is there anything I should know?

I've got a lot of experience in electronics soldering, but very little in using a torch. Fillet brazing will be an interesting experience, but I am fine with filing and sanding away until I have a nice and smooth fillet.

unterhausen

to build any sort of fillet, you probably need 45% silver. Even if I thought it was a good idea, I think I would find it pretty frustrating. MAPP isn't really hot enough for this, and as a result, you'll probably overheat it. I can't imagine that the first time you put heat to metal you are going to have success. Some people are naturals though.

QuangVuong

I just finished up the piping for the hot water system. Started off pretty messy(too much solder) but got the hang of it after a couple of joints.

I reckon I'm ready to have a go on my frame. I bought it for parts so if I wreck the frame, it's not a loss to me.

45% silver melts at around 700°C, whereas MAPP burns at about 2000°C. I'd imagine it'll be hot enough to melt 45% silver(or even higher percent). Or are you saying that the tubes can't be heated up to ~700°C with a MAPP torch?

unterhausen

you can get it to the right temp, no doubt. The problem is that there isn't enough heat content to properly heat things, and heat control is nearly impossible. The tendency then is to force it, which leads to localized overheating

randomgear

I've used propane (a bit lower in temp than MAPP) to braze up a few racks for myself using 45% silver. Getting the silver to melt isn't the problem, what is the problem is how long it takes to get even small tubes up to temp to be brazed and the amount of tubing that ends up getting heated as a result.

While I haven't tried it, I fully expect that trying to fillet braze a seatstay cluster with propane (and probably MAPP) would cause the adjacent brazed joints at the top and seat tubes to melt.

I believe that there are videos on youtube showing lugged frame construction with propane or MAPP but the bottom bracket seems to be a problem and people end up building little hearths to help reflect heat back into the joint to be able to braze it properly.

Andrew R Stewart 

The practice of a welded steel frame having a second heat cycle of a bronze or silver fillet added onto the joints isn't new. GT did just that years ago with some of their Excell tubed CO built bikes back in the 1990s. I have my old GT sales rep's frame hanging in my garage with a crack with that process. I was never comfy with their rational for the added fillet. Either the weld is the right way and done well or not.

As a repair method I also question the method. The crack would need to be just so, the repair fillet just so. I haven't done much 45% silver work so am unsure of it's merits for this stuff.

What I am getting more and more comfy with is classic fillet in bronze. Having a broad and low temp flame is the opposite of what I would want to have for a well controlled fillet build up. When I do my fillet work I slightly turn down the flame but tighten it up with a tad more OX to narrow the point. Then I play the flame on the previously flowed/sweated joint in a small area till the previous bronze is just shinning up and start adding filler in dabs with the pin point flame coming in and out to control the build up. I would think a broader and cooler flame would cause more area to be heated before the right temp is gotten to. This makes the filler run out and up the joint, not build up in a small spot.

To do this with propane or MAPP sounds like bringing a shotgun to a target range. You might cover the bullseye but... Andy.

QuangVuong

I wont have issues with melting any brazed joints as the frame is mostly welded, other than the seat stays, tips, bidon mounts which are brazed.

The welds on the frame are nice enough, however, there is barely any filler at the fastback seat stays, which is probably why they cracked. Minimal surface area with minimal filler is not a good combination.








Mangled ST top. This will probably need a proper clamp brazed onto the top.


It would be nice to fillet the welds, but I guess it isnt necessary. I plan on adding on some gussets so I could just use body filler to smooth out the welded joints. But I would still like to have a go with repairing the cracked seat stay joint, and add on a seat post clamp. Im taking it I need some sort of way to localise the heat to get the tubes up to working temp quickly. Will wet rags/heatsinks be of any help?

56% silver would be nice to use as it melts at a lower tempurature, but it is expensive. 45% is cheap enough to have a go. Is something like this what Id be looking at? What sort of diameter would be best?
BOC ProSilver 45T Cd Free Brazing Alloy | BOC Australia

Or these which are the same but are flux coated.
BOC SilverCoat 45T Cd Free Brazing Alloy: 2 Rod Pack | BOC Australia

Andrew R Stewart 

QV- Thanks for including some shots.

A few comments- can you see the SS/ST crack inside the ST? Reason I ask is that the ST looks to be very thin at the top. It's a known concern to weld stays to a thin ST, this is why so many ATBs have reinforcing sleeves or butted ST top sections. It's quite possible that the weld it's self didn't fail but the tube cracked right at the weld edge, especially if the ST had been reamed after the welding and further thinned right at the weld's inside the ST spot.

It looks like the ST/post clamp section got pried open a bit. Are the binder ears still well joined to the ST? If so then careful reshaping the clamp in general would be the first attempt to restore this aspect. This reshaping will need to be done if a collar is added anyways. But until I try the existing binder I'd not assume the need for an additional collar.

Silver 56% is not meant to be used in a fillet situation. It looses strength as it's thickness grows. It wants about a 0.005" gap max for full strength. When filleted 56% will cool first on the outside of the fillet, then as the inside cools it contracts a bit. This induces stress and a possible interior crack to the fillet. Additionally in general any silver filler wants a very clean surface to be added onto. Bronze if far stronger with gaps and globs (which is what a fillet really is, a nicely shaped glob) and is far more tolerant of less clean surfaces. Unless you have a sand blaster getting a weld to be fully clean for silver takes a lot of work and often some weld bead removal to get the low points and tiny pocks clean. Silver can be used with far more success as a file scratch filler on the surface of a lug or tube. There's no real stress for any preexisting stress risers to grow from.

I'm no welder but the welds on this frame look pretty lumpy and ragged to my eyes. I know that's no true indication of how well the weld fused the two tubes (can't tell a book by it's cover). But taken with other aspects it does lead me to look at this frame with a questioning eye. Ugly and uneven welds, very thin ST top section, chrome that has easily flaked off (and chroming any frame needs care to prevent additional internal corrosion concerns) (was the chrome even applied over a copper then nickel base?) all are trending in the same side of the quality fence.

It is good that you are taking the time to do your homework before just diving in and doing stuff that isn't likely to work well and might make the frame's integrity worse. Andy.

unterhausen

if it's chromed up there, which it appears to be, then you want to remove the chrome in the immediate area and take steps to avoid heating the chrome that you leave behind. Chrome is not to be messed with https://www.osha.gov/SLTC/hexavalentchromium/

QuangVuong

The top of the seat tube is 0.6mm according to the Minimax specs. I can see the weld penetration of the TT inside the ST, which has been reamed. The seat stays are brazed on, and the crack is right on the joint between the seat stay and ST. I believe this cracked from someone prying open the clamp open, as the seat post was stuck in there. The binder ears look to still be fine. I will stick a 26.8 or whatever smaller seatpost in and give reshaping a go. Then work my way up to a 27.2.

I will be paint stripping the frame soon and see what it looks like underneath. The welds are no where near as nice as the welds on my Litespeed, but they are decent enough compared to other welded frames.

QuangVuong

I did a fair bit of work in the past few days. First off was to fix the seat stays which were cracked at the joint.






So I fired up the MAPP torch.



Using the nickel silver rods from Bunnings, I filled up the joint. Maybe a little too much, but it got the job done…and the seat post binders fell off due to the heat.



Round 2 - Put back the binders as straight as possible, so I stuck the 6mm screw in, then found some random piece of metal to clamp down binders.



Eyed the position and started heating things up.



After some sanding, it's looking better

QuangVuong

After seeing a frame with a gusset, I thought it'll be cool to have one. So why not make my own.



It started out as a trace off the frame. Then I drew in some curves.



Transferred to sheet metal that was 1mm thick.









And, of course I need ones of these too. It started off as the same 1mm thick sheet, drilled and filed to the QV shape. Then using a hose clamp to hold it down to the HT, a hammer was used to bend it to shape.



Then the gussets and head badge were brazed on. It's a bit cleaner now, but the MAPP flame is barely got enough to melt the nickel silver rods. Maybe I should've gone for the bronze rods or proper 45% silver rods which melt at a much lower temp(~600-700°C), rather than ~900°C.



Next step is the filler. This is Septone Metal Tech Filler which has fiberglass and stainless steel mixed into it. A bit harder to use, but will be very solid.

Live Wire 

Ouch. Were you planning on riding this bike?
assuming you can get a seatpost in there, it will be interesting to see if it breaks at the seatstay join or the gusset points first.

himespau 

Why would it break at the gusset points? Stress risers?

unterhausen

you should really do something about the chrome. Hope you haven't injured yourself doing this

QuangVuong

yes, of course I'll be riding it. It'll be reamed, so no issues with getting a seatpost in there.

I hope nothing breaks. But if something does, I reckon it'll be the seatstay joint or the binders. I don't see why the gussets would break? They are cosmetic, so the sharp ends won't really start a crack. Plus I doubt I overheated anything, so the metallurgy shouldn't have changed.

I held my breath…

the chrome was far enough away from where I was heating, so I unlikely got a whiff of anything too poisonous.

unterhausen

the whole frame is chromed, it's just that they only polish it where it's going to show. I don't think that holding your breath is particularly effective

Andrew R Stewart 

Reminds me of my first attempts at squirting Imron, outside. Just stay up wind and you'll be ok... Andy.

himespau 

So what are the appropriate steps to avoid dealing with it when welding SS with chromium? Do you use a respirator? That sounds hot as a mother...

Andrew R Stewart 

No metallurgist here but I would think the answers depend on how the chromium is chemically bound in the metals involved and what gasses/other elements are present to combine with any chromium that might be released. Andy.

QuangVuong

Not all frames are fully chromed. There are chromers out there who polish a particular section, then only dip just a bit past that area. The rest of the frame remains bare steel. Here's a nice example. The rear triangle and HT lugs were polished, and only those areas chromed.




From the the looks of things, my frame was only chromed in the rear triangle. The chrome doesn't pass the brake bridge, so it's far enough from the top of the seat stays.

Anyway, after a lot of sanding(angle grinder + multifunction tool + hand), the initial gusset shape is done. A coat of primer will show what else needs to be filled in or further sanded.

randomgear

Very impressive! Your brazing is better than mine.