Originally Posted by
scarlson
Oh for sure! I totally agree. I often prefer cutting threads that with a single point on the lathe, especially large diameter fine threads. You can get mighty fine quality this way, and you do more thinking and less working, which I like. With a good setup I would do it, but fixturing an assembled fork in a lathe is a bit of a pain and the lathe bed needs to be pretty long. A further difficulty is putting a centering cone in the bottom end of the steerer, with the fork blades in there. And then they're spinning around begging to tangle your shirt sleeve or grab your hand or whatever, so it's somewhat dangerous. I can think of a couple ways to do it involving some weird faceplate or a long tailstock extension and turning between centers. Fixturing's a bit of a pain no matter how I think about it, though, so I bought a cheap die. And it was fine quality. As good as any die I've ever used.
A bit nerdy here, but 24tpi is super easy to do on almost any American lathe because it factors into a lot of 2s and one 3, so most lead screws (often 8tpi or some other power of 2) will pick up the thread in multiple spots when doing multiple passes.
Ach I didn't see your post until after I wrote mine (I guess I'm that slow...) Needless to say your post was right up my alley, not too nerdy at all. (Thank you Sir may I have another?)
About the lathe needing to be long: not necessarily. I was able to use a steady-rest to hold the bottom of the steerer (right end of the steerer when laid out horizontally). Or was it a follow rest? So many years ago. I do remember mounting a follow to the right side of the saddle for some odd setup (left side is normal). But I think that was for something else, I think I used a steady for threading steerers. Either way, then you can take the tailstock off completely, if the bed is too short to leave it on.
On a short steerers, the steady could get in the way of the tool post, so then you need a center coming off the tailstock. But with so many crowns having some odd curve at the bottom edge, a cone might not fit and you'd need a semi-custom-sized cylindrical plug to grab the ID of the steerer. Though a slight taper could help, you can't use a normal 60° lathe center, it would only touch in two places on the curve. Adding to the difficulty, on most smaller lathes like mine, the ram on the tailstock isn't long enough to reach the crown while clearing the dropouts, so you need to make a long extension on whatever center you're using. I've done a steerer that way too. I made my extended center with a straight shank to hold in a Jacobs chuck, not as precise as a center with a Morse taper, but good enough — it's far enough away from the cut that it doesn't matter much.
OK NOW we're getting nerdy!
For the top of the steerer (left end horizontally), I used an internally expanding 7/8" collet. Ooh I should make one in 22.0 to do French steerers. My lathe is inch-based but it can make metric threads by taking out some gears and replacing them. By happy coincidence the 1.0 mm thread is made with the
same gears as inch threads, I just have to flip one of them over so it runs on the 120t side of the 127/120 combo gear. Those guys were thinking! 24 tpi × 127 ÷ 120 = 25.4 tpi exactly.
Mark B