Thursday, May 27, 2010

More Fork

Lots more incremental progress in the shop this week. Famebuilding is a very low-speed activity when you're as picky as I am. But I'm pretty happy with how things are turning out.

The first step this week was trimming the blades so that the wheel sits both exactly half-way between the two blades and gives the correct wheel clearance underneath the crown. This was an important but non-photogenic process, involving many calculations and much careful hacksawing. Eventually I got everything right.

Normally the next step would be to braze the blades into the fork crown and complete the fork. But I'm using brazed-on Mafac bosses on this bike. Since these bosses are much higher-up on the fork blade than cantilevers, and since I want to fillet-braze them in place with brass, I thought I ought to attach them before doing the blades/crown. If I were to brass-braze the bosses after silver-brazing the crown, the heat would draw silver out of the crown.

So I proceeded to brazing on the Mafac bosses. The whole thing was a very inexact process. The miter was tricky to get, for one thing, since the fork blade is oval and its radius is always changing. Then you need to have the bosses end up between 60 and 64mm apart for the Mafacs. It took a lot of fiddling, and eventually I settled for pretty loose miters. But I thought the brass would fill them.

Here are the bosses jigged in a rack tang (another element of inexactness!):

Unfortunately the first try was a total failure. I tacked the bosses in two spots each, but when I let everything cool and unscrewed the bosses from the jig, the screws only turned a few times and then stuck in place. I torqued one screw so hard that the tack broke. On the other side, the bolt broke off in the boss. I heated up the boss on that side and un-tacked it. I figure what happened was that things got very hot inside the boss and the screws oxidized and wouldn't come loose. I'm sure I didn't get brass on the threads, because they did loosen a few turns.

Anyway, I had to do everything over again, including the tricky miter. This time I used much shorter bolts (indeed, I used the same bolts, as they snapped off, with three threads or so left) and greased them. When I tacked them this time, there was a nice little grease fire on each bolt, but they unscrewed just fine. Then I fillet-brazed them properly. They came out looking like this before filing:

And with some filing, like this:

I then stuck the blades in the fork crown to see if the bosses were more or less lined up. They were. I won't know for sure until the blades are properly brazed in to the crown, but it definitely seems like things will be fine. (The crown is backwards here; the photo below shows the proper, smooth-sided orientation).

The pads even ended up in the right place!

Originally I had planned on brazing little tubes on each of the bosses to act as spring retainers. But I mitered the bosses so deeply that there isn't really room for this anyway. And the little tab-retainers that came with the bosses are actually very nice. I'll just use them:

Interspersed with all this Mafac stuff has been lots of dropout filing. My "slotless" method is definitely a gigantic headache in terms of filing. Without eyelets it would be fine; with eyelets it's endless. This would be a totally impractical method for a production shop. But for me, it's more good practice. Here is how things look so far:

And here is a shot that shows what a long process this has been. I taped this drawing of my fork to the workbench last week when I was trying to figure out where to trim the blades. It is now very tattered, and covered with the aforementioned calculations. But it does show that I was able to successfully do what I intended.

I would have finished the fork today, but the inside of the fork crown is very dirty, and unfortunately inaccessible with emery cloth. I'll need to get Olivier to sandblast it or else dunk it in acid.

Otherwise, the next step is brazing the front triangle! Things are coming along.


Jim said...

It is a trial and error process,of course,but things are coming along quite well. Your work looks very much like what Mike did for me on my Bertin randonneuse. You will be impressed by the solidity and responsiveness of the braze-on Mafac 2000s once you try them.


AH said...

Thanks Jim! I don't mind a few missteps here and there -- it is indeed all a learning process. The nice thing about starting with a randonneur is that pretty much every possible complication that might arise WILL arise; they're complicated bikes. When I built Niles in Doug Fattic's class, I chose about as easy a frame as possible, which was a sensible route for a bike to be built in a limited time. Now I have the luxury of time, and so have gotten very complicated.

I'm very excited to try out the brakes! I still haven't ridden a bike with brazed-on Mafacs. I was pretty excited when I mounted them on the bosses today, and everything was in the right place!