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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 07-31-10, 04:00 AM   #1
c_booth
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How red........

.........is too red?

(Firstly hello to everyone from a newbie).

Simple question (or perhaps not?) - how red is it acceptable for the steel to turn during brazing before losing integrity?

I've just started learning to braze (we're talking pre kindergarten here) and want to be sure I'm on the right track before I even consider moving on to building a full frame, and I've come accross some conflicting 'opinions' on the subject.

I'm aware that heat spots where the steel starts pitting are clearly undesirable, but aside from that I'm unsure as to whether I'm overheating. Is there a set 'rule'? If I can see red through my goggles have I taken it too far? Should the silver flow without direct heat from the torch? (Very basic stuff here as you can see)

Any help greatly appreciated.

(If anyone knows of a comprehensive brazing guide to be found onlide that'd be great)
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Old 07-31-10, 05:48 AM   #2
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For silver, the steel will barely have changed color. Use your flux (assuming you're using the proper flux) as a temperature indicator. I don't wear anything beyond plain old safety googles for silver brazing, but do wear sunglasses for brass brazing. If you silver braze at too high of a temp, the brazing material's alloy components (especially if you're using stuff with cadmium) will start to come out, weakening the joint.
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Old 07-31-10, 09:28 AM   #3
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Thanks for the reply mudboy - not what i was hoping to hear though ha.

The flux I've been using is SWP CJ301 - ebay linky - http://cgi.ebay.co.uk/ws/eBayISAPI.d...=STRK:MEWNX:IT - along with sif silver no. 43 cadfree.

I have a feeling this is possibly the wrong flux, as I'm struggling to get the solder flowing before the steel starts glowing red. Once it flows I'm getting good penetration, but there's always the sense that I'm using more heat than is necessary.

Re. goggles - I was under the impression the goggles protect your eyes from the bright white of the oxygen rich flame? Is this not the case?

Thanks again.
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Old 07-31-10, 09:56 AM   #4
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For silver, you want the parts to still look silver with a bit of a light peachy tint.

I use ace glasses with a #2 brazing shade for brazing. I have some #3 welding glasses, but they don't allow you to have a good view of the color.

You appear to be using flux for brass/bronze. That will become active at too high of a temperature for silver.
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Old 07-31-10, 10:44 AM   #5
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And 43% cadmium free isn't going to flow all that easily until you get it hot. Use 56% for lugs. My guess is that you're using pretty generic flux like what plumbers use, when you should be using flux specifically for your brazing material.
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Old 07-31-10, 11:11 AM   #6
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Thanks unterhausen. I'm definitely going beyond a light peachy tint so it looks like some new flux is required. I've had a bit of a search online and am finding it all mildly confusing - what are the recognised brands for silver brazing flux? Want to be sure I get the right one this time.
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Old 07-31-10, 11:23 AM   #7
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And 43% cadmium free isn't going to flow all that easily until you get it hot. Use 56% for lugs. My guess is that you're using pretty generic flux like what plumbers use, when you should be using flux specifically for your brazing material.
So it appears I'm making life hard for myself on all fronts ha! Actually, I've just checked a couple of sources and both have the sif 43 @ 55% silver (confusing to say the least) so I think I'm alright on that front. Looks like the flux is the problem.
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Old 07-31-10, 12:45 PM   #8
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However you asked how red is it possible to get the steel without loosing integrity, so to cover off the steel part, it varies, but fairly red. Red is a hard thing to pin down since the degrees of red you will see vary as to eyewear, background lighting etc... Basic rule of thumb is that by the time it gets hot enough to oxidsize it black (where you not using flux), you will have drawn the temper in most steels and have nothing much left to loose. There are some steels that do not loose temper when red hot. Most bike steels are not heat treated in the first place, so it's a moot point. The reason for operating at lower temps is that some tube are very thin, so burning becomes a possibility, and less eat makes alignment issues easier to deal with, and is always good.

The whole point of silver is to have a lower heat braze, for what that is worth. But in a pinch, particularly with insuficiently hot torches, and with low silver brazes, you may end needing to go red. But this is not regular practice, and is more a repair on the road, or other craft thing, of for people without the right tools.
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Old 07-31-10, 02:04 PM   #9
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Good stuff Peter. You're right, 'red' is a hard thing to pin down as there are so many variables I s'pose. The oxidisation 'rule' you mention is exactly the sort of tip I'm after - excellent. You mention burning - what does this entail with steel exactly?
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Old 07-31-10, 03:16 PM   #10
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So it appears I'm making life hard for myself on all fronts ha! Actually, I've just checked a couple of sources and both have the sif 43 @ 55% silver (confusing to say the least) so I think I'm alright on that front. Looks like the flux is the problem.
Oh, OK, you should be fine with 55% then.
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Old 07-31-10, 05:54 PM   #11
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Burning would be at the upper end extreme heat, where the metal actually burns, as it will during unshielded welding. This could happen with lack of care with points etc... since the oxy torch and setting used for brazing isn't all that different than welding, just how long you hold it there. But another sort of burning is just extreme oxidation relative to very thin structures. Again flux is you friend here (on the outside). Basically if you are using the right products and tools and not hanging about excessively you should be ok. Another thing is that high heat in the torch is your friend, or also low heat. The middle may not be as good. So with an Oxy torch, if you keep it moving you can pop the structure up to heat and get in and get out. Try to do it on a lower heat, and by the time you get everything where it needs to be, you can end up having overheated, or for far too long your stuff, and it is very difficult to keep it from getting red hot. On the low heat side of things if you had a propane hearth, not a torch, or a bunch of mechanized torches that heat all parts of the joint at once, then you could use lower heat yet because of heat volume get everything toasty none the less
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Old 07-31-10, 09:24 PM   #12
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Linky to a page listing steel temps by color;

http://www.sizes.com/materls/colors_...ted_metals.htm

For silver, you only need the parent metal to reach the liguidus temp. of the brazing alloy, for 55-56% (BAg7) that's 1205 F.
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Old 08-02-10, 09:19 AM   #13
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i would recommend getting a pound of bronze and practice with that.
for one, its cheaper. also bc you need to get the metal to a higher temp it gives you a little longer to screw around and practice. with brass there are a lot more visual indicators that the temp is right than there is with silver. once you figure out brass, silver becomes a little easier.

another indicator that the metal is overheating is when little blue flames spark out of it. also try a couple little experiments-- burn a hole in a tube with not flux, gives you an idea of what it takes. burn the hell outta some flux too, just to see how much heat it takes (which is not a lot, that is why you keep the torch moving constantly).
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Old 08-02-10, 10:46 AM   #14
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Here is someone at framebuilding class, check out the colour:

http://www.bikeforums.net/showthread...mebuilding-101

post 9
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Old 08-02-10, 12:27 PM   #15
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yes with brass it seems the joint is at a proper temp when a good chunk of it is orange-red (that's the colour I see under dim light). Also with brass you can tell you are overheating locally (or the rest of the joint is too cold) when the brass itself starts fizzing.

In my limited experience with silver it seems you mostly read the flux (bubbling then translucent and fluid), but with my 50% and 45% (with no Tin) I do have to bring a light red colour, peachy describes it well.
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Old 08-02-10, 08:16 PM   #16
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Yes, the flux is your best guide to temperature. For silver brazing I use Harris "Safety-Silv" rod and GasFlux Type "U" paste flux. I find glasses with didinium tint (aka "glassblower's lens") help visualize when the metal is hot enough for the silver.

For brass I use generic brass rod, but GasFlux Type "B" paste, which blows the socks off any other brass flux I've tried.
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Old 08-02-10, 08:35 PM   #17
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I gave up on didymium and as I stated above, I'm using ACE now combined with a #2 tint. The didymium alone doesn't protect against some of the problematic radiation. I have enough eye problems that I don't want to cause more. They actually recommend ACE with a #3 tint, but in my experience you can't judge color w/ a #3 tint.
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Old 08-03-10, 04:05 AM   #18
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Just like to say thanks to everyone who's contributed to this thread thus far - all really fascinating, and of course hugely helpful. Please keep it coming.

The chart cassave posted looks really useful, from looking at that I'd say I've prob been heating the steel up to about 1650/1700f which I believe is brass territory, so I think it's safe to say I've been using the wrong flux (or the wrong solder depending on which angle you take!)

I like the suggestion of practicing with brass (not least because it won't kill my wallet) and just experimenting, probing the limits of the materials I'm working with - I think this is a good base to build from.

Even though I was't by any means anticipating an easy ride, I'm a little surprised by the 'nuance' involved, and even though it can be hugely frustrating at times, I feel this makes it even more exciting - after all where would be the fun in learning if it was all straight forward?

(actually the most frustrating part is waiting for the postman he he)
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Old 08-05-10, 11:49 PM   #19
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I found being a self-taught TIG welder pretty challenging compared to a number of other crafts I have picked up over the years. I came across a blog from another TIG framebuilder, and in it he showed 100 or 200 trial joints he had mitered and welded as part of his learning curve. All a sudden I had an insight into why I was still struggling...
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Old 10-19-10, 09:11 AM   #20
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So, after a bit of an enforced break (work commitments etc pah!) this last fortnight I've been getting down to some serious brazing practice. Using the correct flux has, of course, made all the difference. Done quite a few cheap practice lugs and the repetition seems to be paying off - post mortem dissections reveal improved penetration. So I'm fairly pleased with my progress..........

.......however, there is one thing in particular I'm a bit worried about. Despite achieving what appears to be decent penetration (clear silver line visible through cross sections right through the lug) I'm finding that I'm able to (where I can get a grip with a pair of pliers on a corner that hasn't properly flowed), with a bit of effort, peel the lug/tubing apart (I'm talking thin 3 - 4mm cross section) even where the silver has clearly flowed???

So either I'm expecting too much of the silver filler, or more probably I'm still doing something wrong. Is this a case of fried filler?

Also, should I be looking to flow the mitre?

As always, advice greatly appreciated.
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Old 10-19-10, 09:27 AM   #21
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Also, should I be looking to flow the mitre?
you should be aiming for being able to feed on one side of the miter and getting it to come out on the other side of the miter. This has the extra added advantage of being the best way to make a no-file joint. If you are using pressed lugs, it isn't likely you are filling the whole void at the miter with silver.

As far as being able to pull thin sections of steel off away from silver, that doesn't sound good but it probably isn't that bad. In general, if you are cooking anything the silver doesn't flow.
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Old 10-19-10, 10:51 AM   #22
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you should be aiming for being able to feed on one side of the miter and getting it to come out on the other side of the miter. This has the extra added advantage of being the best way to make a no-file joint.
Hi unterhausen - sorry, I don't quite get what you're saying here. Are you talking about the lug edge or the tube mitre (ie where one tube butts up against the other - sorry just clarifying vernacular)?

I've been attempting to feed the lug at one point and drawing the filler through the joint as suggested, and have been fairly successful, but I'm not managing to flow filler into the mitre - is this 'doable' with silver at all, or does it only apply to brass? As you suggest, the void at the mitre in a cheap pressed lug would be too large for silver to fill - are better quality lugs significantly tighter that silver will flow in this area?

I suppose what I'm getting at is does not having the tube mitre filled seriously effect the strength of the join if the lug around it is well filled?

Last edited by unterhausen; 10-19-10 at 01:19 PM. Reason: fixed tags
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Old 10-19-10, 10:55 AM   #23
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When went through the frame of my first road bike: I had it s components swapped to a new Surly frame.. I owe the shop mechanic a case of beer; who pleaded with Surly management to find me a fire engine red frame as I wanted..... Any other shade of red would not do..... It have a fire engine red Surly. I'd buy him a case of beer. But, he does not drink..
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Old 10-19-10, 01:34 PM   #24
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What I meant was feeding the filler into the lug on one tube and flowing it so that it comes out of the lug on the other tube, i.e. past the miter.

Investment cast (IC) lugs don't have a void at the miter like pressed lugs do. I'm thinking that very few frames built with pressed lugs had any filler between the tubes, certainly the ones I've seen cut apart have not had anything there. So my conclusion is that it is not absolutely required. If you are pulling the filler through from one lug edge to the other past the miter, you are doing much better than a lot of production bikes of years past. With IC lugs, there is no reason to expect an internal fillet, but you should be able to get some filler in the miter.

There was one time I tried to see how much silver I could get to go into a pressed lug. Turns out you can get too much

Last edited by unterhausen; 10-19-10 at 01:51 PM.
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