Fillet brazing chainstays

[Michael Slater]

I followed the typical sequence on my latest frame: built the front triangle, filet-brazed it together, and then added the chainstays.  Of course the chainstays don't fit cleanly against the bottom bracket. The filets of the ST/BB junction get in the way.  

I can miter away the material from the chainstay to fit against the filet.  But I dislike this kind of freehand mitering nonsense.  What's worse is brazing the stays onto the old ST/BB filet bronze requires a more heat to remelt the bronze.  It's a large lump of metal compared to the thin chainstays.  Moreover, it's presumably lost some of the zinc during the first braze. This is the element that lowers the melting point in the first place.  Means higher heat, yet worse flow. Just wholly gross compared the main triangle fillets.

I'm thinking that on the next frame that I'll tack up both the chainstays and the seatube in the same operation.  I'll not lay too much filet around the rear side of the seat tube, just from 8pm to 4pm looking down from the seat (head tube at 12:00) .  Then I'll proceed with finishing the main triangle.  I don't want to lock-down the chainstays with a full braze until the main triangle  is done because I prefer to align them using head tube datum rather than bottom bracket datum.  But at least this time they'll be stuck against the BB with a nice clean miter.  Plus this will involve none (or at least much less) remelt of the ST/BB fillet.

Another open question is how to execute the brazing of the two chainstays.  A lot of variables to control simultaneously:  ST/CS angle,  dropout distance from centerline of the frame, dropout distance from bb center.   I managed to get my dropouts nicely aligned finally, but it felt like I was herding cats, and I wasn't thrilled that I put the frame on the alignment table probably a half dozen times during the course of the braze. (not very feasible to braze on the table -- only two positions out of eight are suitable).  

I intentionally let the chainstays float, rather than keeping a dummy axle holding them together. I thought, perhaps foolishly, that this would mean there was less stress brazed into the system as a result.  But maybe the downsides (more fussing around, heat cycles, ?warm?-setting) means it's a lousy tradeoff.  

I don't think brazing entirely on the Sputnik jig makes sense either.  Seems like almost for sure I'll end up brazing stresses into the stays which, when i yank it out of the fixture, will spring the stays out of alignment. And they'll be a lot harder to fix after they're locked on with a full braze.

I definitely need a better process for final brazing/alignment of the chainstays.  

[Steven Shand]

HI Michael,

for what it's worth, I tack everything (except seatstays) in the jig
at the same time.. It comes out of the jig and onto the alignment
table (I also reference from the headtube not the bb shell). If it
needs tweaked, it gets tweaked. Then onto the stand to braze free.

One thing I think helps a lot is if the stays are going to get a
bridge, tack that in place in the jig too. In fact that's the first
part that gets brazed in the stand. I also always leave the dummy axle
in place throughout the full brazing process.

As much as possible, all braze-ons are also done before the tubes are
cut and tacked.

Heading back to the alignment table so often in the process doesn't
make much sense to me. The only time the alignment matters is after
it's tacked and after it's brazed. If I checked alignment that often I
think I'd also feel like I was herding cats!

Try and let the sequence of the brazing help even everything out. I
think that this is just one of those things where a process will
eventually appear that works for you.

Cheers

Steven

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[jon norstog]

Michael, list:

I assemble the chainstayswith their bridge and usually the dropouts as
well separately. I used to use a plate jig I made up myself but have
lately been using my Anvil chainstay mitering fixture for this job.
The Anvil isn't exactly right for the job and brazing on it leaves a
lot of scorch marks. I'm thinking of having Don make up a second
fixture just for stays.

I use an old HJ universal jig. The setup for locating the dropouts and
shays is not its best feature. Which means take it slow on the join.

Anyway, the chainstay assemble is pretty much together and checked for
alignment already. On my BMX bikes there is plenty of room for
welding on the stays. On fillet brazed road bikes I usually grind
away any of the ST-BB fillet that gets in the way of the stays. I
don't have an alignment table, so this is an operation I take pretty
slowly, checking as I go.

That's one way to do this job. Others have their own systems.

Good luck!

jn


"Thursday"

[M-gineering]

On 13-5-2012 9:02, Michael Slater wrote:
> I followed the typical sequence on my latest frame: built the front
> triangle, filet-brazed it together, and then added the chainstays. Of
> course the chainstays don't fit cleanly against the bottom bracket. The
> filets of the ST/BB junction get in the way.
>
> I can miter away the material from the chainstay to fit against the
> filet. But I dislike this kind of freehand mitering nonsense

Mitering the stays with a slight offset towards the bottom will also get
you out of the area of the seattube fillet
--
mvg

Marten Gerritsen
Kiel Windeweer
Netherlands

[Mike Giannico]

I build the entire front triangle by tacking in the fixture, checking alignment, then then remove it from the fixture and braze it all up in the vice and blocks or a park stand if you have one then recheck for straightitude.  at the seat junction I braze halfway around the seat tube. While  tacking the back side of the seat tube bb junction where the stays are gonna go I have a good strong tack weld and I try to internally fillet the rest of the way around to hold nice and strong against all the fillet in front pulling in that direction.  Some times you might have to sand off a little filler if you overdo it. 

Once I finish brazing the chain stays on I have a habit of adding general heat all the way around that entire finished junction dt/st/cs in order To get rid of some of the stresses. This might be going overboard but it couldn't hurt unless you just roast it.

 Then cool and align chain stays and lock it all together with your seat stays..

I can definitely see doing the front tri and the chain stays at the same time. If your awesome. Something to work up to. I'm a little slower due to years of abuse to all vital brain function. 

Hope this helps

Mike g

[omar-...@cox.net]

Hello Michael--

Interesting questions, and equally interesting answers so far.  I do this whole thing a bit differently, and I will share that with you in a moment.  For the particular issue in your post, I think you should consider either grinding away some of the fillet that is in the way, or attach the chainstays low on the shell and out of the way. 

I tack the front triangle and chainstays in the jig at one time.  For fillet frames I put two brass "tacks" on each tube, usually top and bottom, and I tack the chainstays all the way around with an internal fillet.  I have found that anything less and the stays will pop off during alignment.  Remove from the fixture and check alignment.  I use a dummy axle in the rear drops during brazing.  I start the brazing process at the BB, front to back.  That is down tube, seat tube, chainstays, in one operation.  That way it only gets hot once.  I use a Park stand and rotate the frame to keep the brazing area vertical.  Then I do the head tube area, down tube first, then seat tube/top tube joint last.  I check alignment again, then do the seat stays, then bridges and braze ons last.  Many, over the years, have said that this is bad, but I have been doing it this way for 24 years with no issues.

I can see that you have spent quite a bit of time thinking about all of this.  I do my frames in the same sequence whether fillet brazed or lugs.  As someone mentioned, when you get your process nailed down, it will become more second nature for you.  It took me quite a while, when I started, to get my process down. 

One more thing, don't worry about re-melting the brass.  It won't mind.

Omar Khiel

Oasis Custom Cycles LLC

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