I just tacked-then-tinned together the front triangle for my first fillet-brazed frame. I’ve discovered that the head tube is now about half a degree steeper than I want it (and how it was set up in my jig) – 73.5 deg vs. 73.0 deg.
I’m wondering if it’s standard practice to slacken the head tube angle in the jig by half a degree when fillet brazing, or if there’s an improvement I can make in my technique? I tried to tack on the centerline, starting with the top/bigger angle of the HT/DT joint, before moving to the underside of the HT/DT joint. When I did my tinning pass, I tried to do it in quarters, starting at the top of the DT, going halfway down one side, then going halfway down the other side, and repeating that starting from the bottom of the DT.
There are a couple of things to look out for here. How tight are the mitres? If you have a gap at all the tube is going to get sucked into that. Do you know if the tacking steepened things up or just the tinning. You’ve probably spotted that the alignment will change more as the joint cools than it does when it gets hot. When I tack, I’ll tack in the centre-line initially but more importantly I’ll tack where the cooling joint will pull the opposite end of the tube into the joint rather than away from it when it cools. For example, on the DT, I’ll tack the top of the DT onto the HT first, that’ll pull the DT up into the BB. Working out a procedure like that by doing a tack at a time and seeing what happens (for you) is important.
I know there are people that tin joints but I just don’t see why. I don’t know what the point is. The enemy of the tube is heat, I want to get in and out fast. It’s heat that weakens the material and heat that makes things move around so I want to spend as little time in there with a torch as I can.
I’d never change the jig set up to compensate for something like this. I think you’ll just end up chasing your tail and second guessing things.
But I can confirm at least that I don’t do anything weird with fixture setup (as a fillet brazer).
When brazing out the head tube I do top of top tube, followed by under the downtube. So top and bottom of the cluster, and the most open angle first (top of TT). Sounds like you’ve done the same…
It’s also the last joint I’ll do in the front end, the BB cluster and then the ST-TT come first. So things are fairly stable by the time I’ve made it to the head tube.
Sorry not much direct help but doesn’t sound like you’ve done anything ‘wrong’. A simple matter of the mitres needing to be a little tighter maybe?
Thanks for that. What certainly did happen is that a big gap (~1/16") opened up on the bottom of the DT at the BB. I initially figured that that was because my tack there wasn’t adequate and I started tinning at the top of the DT/BB joint vs. at the bottom (I rewatched Thrifty Framebuilder’s videos and learned that one). But, maybe it was also because things moved when I tacked at the HT end…?
At any rate, I’m going to try to melt the DT apart, clean everything up, and try again…if for nothing else than a learning exercise. I may write the whole thing off as scrap and start over, which sucks because it’s taken me ~2 years to get to this point.
I’d always been confused why Paul Brodie TIG-tacked his frames. I think I know why now.
Put an edge against the front of the HT and see if it’s bowed. With the top tube ‘in the middle’ of the HT, the tube may bow around that joint. Even heat all the way around the HT, good heat control and tight miters should solve your issue. Also, tinning with Nickel requires a fair bit of concentrated heat.
Over the weekend I attempted to melt apart the tinning to separate the down tube. I got it apart, but it was ugly! It was a lot harder than I thought. I was trying to save the tube, but guess I shouldn’t have removed that tube hot. I ruined the tube anyways - blew a hole in one end and tore out a chunk on the other.
