Filed In: Process
Father/Daughter Project - Part 13
Tuesday, January 28, 2020 | Chris Blandford
Moving on to the fork. Steps:
1. Fit and braze steerer to crown
2. Shape crown, if desired
I begin by fitting and brazing the steerer tube to the crown. The crowns I’ve worked with are nicely machined post-cast; to this point, they haven’t required much fiddling to fit nicely around the steerer. (I was taught that the steerer-crown interface should have no play, but should be loose enough to allow the crown to slide down the steerer under gravity.) In this case, I merely cleaned up the outside of the steerer and the inside of the crown with 80-grit emery. (I was also taught to orient the emery "scratch marks" in the direction one wants the silver to flow while brazing. So, I do that with everything I silver braze, as shown here.)
Once cleaned up, I drill a hole for a pin that will hold the crown to the steerer while brazing. I use regular hardware store nails for the pinning, tapering them slightly on my belt sander so that they fit tightly when I tap them into the assembly-to-be-brazed. Also worth noting is that I allow just a small lip of the steerer to protrude past the bottom of the crown. When I go to braze, I’ll do so “upside down”. Having a little lip there allows easier feeding of the silver filler.
Finally, I drill vent holes if appropriate. This crown is hollow in the shoulders. I vent fork blades top and bottom, so these holes will allow the blades to be vented directly through the steerer. Some crowns I’ve used don’t allow this; those forks’ blades just get a simple hole, somewhere up near the crown.
Once everything is ready, I clean with isopropyl, flux the steerer/crown/pin, assemble, and then apply more flux. Ready to braze.
Because silver brazing requires lower temperatures than bronze brazing, I--previously--associated silver brazing with smaller torch tips and smaller flames. Made sense, I thought. I have since found, however, that using a larger tip and a bigger, broader flame is actually more appropriate for lugged joints. The temperature control--as I’m now starting to grasp--is handled by the movements of the user, not the size of the flame. As such, I now braze these joints with my largest torch tip, a Smith 207 (and 56% silver filler).
As mentioned, I braze this joint upside down. I also braze it out of any jig. Hence the pin. At UBI, we were taught to braze the steerer and blades to the crown in one shot. Even the slightest imperfection in the fit up of all those (four) parts, however, can result in the steerer/crown being under (even a tiny bit of) tension. I've found that this little bit of tension can make brazing this joint, in particular, difficult. So, I now braze lugged forks in two steps. Attaching the steerer to the crown first also makes carving/cleaning up the crown easier. It’s an extra heat cycle, yes, but it’s a tradeoff I know some well-respected builders make. Good enough for me.
I fire up the torch. I use a big, rumbling, 2-3x reducing flame. I preheat the entire circumference of the steerer. I feed silver into the “lip”, and draw it through to the race seat. Finally, I don’t skimp on filler. I make sure these silver-brazed joints are good and full. Eventually I’ll get the hang of how much silver fills a given joint; for now I just overshoot the hell out of it. “Shorelines”--or whatever--be damned.
Once brazed, I allow the assembly to cool completely. Then I soak off the flux, trim the pin, and shape the crown. In this case, I simply rounded off the sharp corners and also removed the brake mount... things (nubs? faces?). I thought this would look nice with the dropout treatment and swoopy curves of the frame. Finally, I touch up the blade sockets’ edges with a file. (I don’t bother with all the casting marks just yet; I’ll take care of those after the fork is done.)
That’s it for now. Blades next!