Filed In: Process
Father/Daughter Project - Part 7
Monday, December 9, 2019 | Chris Blandford
All right, time to tack and braze this little frame together.
9. Tack Frame in Jig
10. Quick Alignment Check
11. Braze Frame Free of Jig
To tack the frame together, I begin by applying blue flux to the tubes and loading them into the jig. I apply a full coat of flux--enough to last through the tacking and the brazing. I also flux about an inch up into the ends of the mitered tubes. Using the same 203 tip, a neutral & rumbling flame, and 1/16th bronze rod, I tack the tubes together along the frame’s centerline. Here’s my tacking sequence:
In short: obtuse angles first--from the BB up and around to the seat tube, and then acute angles--back down and around. The general thinking here is that tacking the obtuse angles first will minimize any pull-apart on the opposing side of the miter, as the bronze will pull the tubes more “into” one another. For chain stays, however, I tack the outside (acute) sides first, then the obtuse sides. This is done to minimize suck-in at the rear dropouts and to keep the rear spacing as designed. (For this little bike, I’m waiting to connect the top tubes along with the seat stays, mixte-style. So, I skipped tacks #3-6.)
I let the tacks cool in the jig, then pull the frame. At this point, I haven’t touched the BB with a facer but I go ahead and throw the frame onto my small alignment surface anyway. This is mostly just to check that nothing went super awry during tacking and--less importantly at this point in my learning, I think--to inform the brazing. I use a handy little alignment gauge (available from Compass) to quickly check the head tube and dropouts against the centerline of the frame. (I’ll go over how I check alignment more thoroughly in my next post.) In this instance I found that the head tube and the dropouts both sucked ~1mm toward the non-drive side of the frame’s centerline. This has been a trend with my frames. I think it has to do with the fact that access to the non-drive side of my jig is limited, and it takes me a few seconds longer to get those chain stay tacks in place compared to the drive-side ones. I’m also right-handed and tend to tack looking “over” the head tube of the frame, so the heat is coming from the non-drive side of the frame. I’ve made a mental note to try and work quicker and direct my heat more uniformly into the centerline of the frame. Small progress with each frame, I think.
Thankfully, the head tube itself showed no twist at this point. I was very pleased about that. Given that this frame--in its current state--only has the down tube to support the head tube during tacking/brazing, I was a little worried things wouldn’t work out so great. So far, so good.
Next, I make a Sharpie mark on the outside of the drive side chain stay as well as on the drive side of the down tube, near the BB. When I go to fillet, I’ll start with these sides of the frame, hopefully sucking things back to center along the way. I’m still learning how to inform my brazing via this alignment check, but I think that’s the idea. I’ll make notes when I’m done and try to improve on the next frame.
Finally, I pull the frame from the table, throw it into a Park stand, and fillet braze all the joints.
For the main fillets, I use the same 203 (sometimes a 205) tip, but use the larger (3/32nd) diameter Gasflux bronze rod. I definitely prefer the larger rod for these joints. As mentioned, I don’t apply any additional flux; I just reheat the flux that was there during tacking. I'd love to try a Gasfluxer, but that'd probably be a silly purchase for someone who brazes a few times a year!
I know there are many schools-of-thought when it comes to fillet brazing. No pre-heat vs. pre-heat. Small and tidy ala tig welding vs. broad and entire-joint-focused. One pass vs. multiple passes. Etc, etc, etc. I’ve read everything I can get my eyes on, and have (mostly) settled on a process. I like to pre-heat my entire joints (including the “back” sides of the tubes) to temperature and then lay down as broad and even a fillet as I can manage, in a single as-fast-as-I-can pass. I’ll scrounge together--and post later--a few links to the writings of Dave Kirk, Steve Garro, etc. that describe this method. Super insightful. This was also how I was shown to fillet braze by Tony Pereira in UBI course #1, so it’s how I started out in my practice and how I’m most comfortable. (Interestingly, Ron Sutphin showed a no-preheat method in UBI course #2. Just different strokes, I think.)
A couple beginner’s insights. First, I braze the harder joints up front! I used to start with the “easy” head tube joints, but have since moved to beginning with the bottom bracket and chainstays, then moving to the head tube and finally finishing with the seat tube. Second, I like to take a quick break between joints. Set the torch down, take a swig of beer, change the album that’s playing, stretch. I try to remember that I don’t need to sweat and stress while I braze. And finally, I try to figure out how I’m going to clamp and move the frame--for each joint--before I light the torch. Gameplan it a bit.
For what it’s worth, I’m completely in awe of the professionals’ fillet brazing, as shown in forums and elsewhere. Garro, Kirk, Estlund, Bilenky, Steve Rex, on and on. Pretty ridiculous. What a gulf between those that actually know what they’re doing and someone just learning. I can’t imagine how long it would take me to get to that point. A lifetime. Maybe next time.
That’s it for now. Back to work. More soon.