Segmented forks (ie, the kind I build a lot of) have been a hot topic on Frameforum this week, and so I'm following a (newish) FF tradition and doing a "Friday show and tell". This is oriented toward fellow framebuilders moreso than casual readers, so be warned that I'm skipping some of the more basic steps in the description.
First off, a little background. I'm building this fork for Steve Garro at Coconino Cycles, so it's going on a bike for a customer of his, and I'm not involved with the frame or fork design process. Steve wants a 440mm long, 45mm rake, disc specific fork for a 180 pound rider doing all-around XC. Fair enough!
The dropouts are from Ceeway, in the UK. I buy these in huge batches because I use them a lot - and I'm happy to sell a set or two to anyone who wants to build a fork like this but doesn't want to pay $20 worth of shipping to get them from the UK. The part number is LE13-1. There's a version that will fit 25.4mm tubes as well (these are for 28.6mm) but I seldom use them. They are intended to be plugged in and brazed in place, but I simply cut the plugs off and weld them on, because it's quicker for me to do. They're cast, so the metal isn't super happy about being welded, but it works well enough.
I mount the legs and steerer up in my spiffy Anvil fork fixture - the legs are 1.2/.7/.9mm double butted (with the thick section at the top) and are made for me by Fairing Industrial. Steerer tube is a True Temper MSRDLT OX platinum steerer tube that's butted 1.6/1.1mm.
Next, I machine up some crown pieces on the mill. These ones are made from 1.125"x.049" 4130 purchased from Wicks aircraft. The fixture I made to cut every crown section at a 15 degree angle. I can make 1" pieces as well, by using a little shim shoved into the fixture. It's certainly possible to make these by hand as well, but it's a big waste of time for me, since I've got the milling machine. The crown pieces always require some deburring (as you can see) and a bit of hand mitering at the leg side to fit perfectly, but the total time required to finish them after they come out of the mill is probably <2 minutes.
I fit everything together, make sure the miters are tight, and cut off the legs at a 30 degree angle (I like the look, and this also helps make sure they don't bump into the downtube of the frame). I also drill some small vent holes in the legs, which the crown will cover up. This is to prevent the problems caused by trying to weld an enclosed space (the pressure of the hot gases inside the crown as I finish welding it would cause difficulties with blowing through as the pressure tried to escape through the molten weld puddle), and also to allow argon into the crown while I weld it.
Next, I fit some purge plugs to both the steerer and the legs, crank up the argon backpurge, and tack the fork together. I tack in 4 places, evenly spaced around each joint (for a total of 16 tacks) to make sure everything is going to stay put when I take the fork out of the fixture.
Welding up the fork (steerer joints first, followed by the crown/leg joints) comes next. The backpurge runs constantly at around 5 cfh. I weld each joint in (approximately) quarters, doing one half of each joint before swapping to the mirroring joint on the opposite side. I don't really know if this sequence is mandatory - it was the first welding sequence I tried with these forks, and since it seems to work well to keep everything straight, I'm not about to start experimenting.
I miter a disc tab (which I buy from Paragon Machine works, and use for both frames and forks) on the mill and align it using my sweet Anvil disc tab fixture, then tack (with the backpurge going once again) and finish weld. I find that it's helpful to finish weld one side (I do the left side first), wait for the metal to cool down a bit, then wire brush the opposite side to remove the nasty crud that has built up from the atmosphere reacting with the hot metal. I'm sure you could flux up the back side too, but the flux is (IMO) harder to remove, so I stick with this method.
Next I've gotta cap the legs, which is a fairly crude process. Since I'm too cheap to get a big run of caps laser cut, I just use square pieces of .032" sheet metal and braze them on, then cut/grind away the excess material. Someday I swear I'll get my act together and have some better caps made, but these work fine for now. I braze a bit of bronze onto the fork leg before putting the caps on, then hit 'em again with the torch to bind the cap solidly to the fork leg. Notice that I also braze on the crown race at the same time. The caps are held in place for brazing with spring clamps, with a little extra piece of place to prevent the clamps from dimpling them. I love how gnarly and grungy things look with flux all over them...
Finally, I file/grind off the excess cap material (you can also look at the crown/leg weld in this picture if you're one of those folks who likes to stare at welds all day) and cut the steerer to a nice even 10" (254mm, for those of you who aren't down with the SAE).
Here's a final shot of the fork. Final weight (before paint, or cutting the steerer to length): 889 grams. I'll ship her out to Steve after the holidays.