It's late and I'm still hyped up from an evening of shop work on the next frame. If i had a blog I'd write there but since I'm a ludite this forum is the most i can handle...
This thread's subject is lug bending. But tonight's work was mitering the remaining of the main triangle. The overlap is I have adjusted the lugs and shell to the angles that the design states and now that the tubes actually are fitted together the adjusted sockets are found to be not spot on. This is not the first time I've dealt with this. Since I'm no pro or frequent builder there is an amount of relearning/discovery with each frame. So tonight as I work the tubes I get to find out how the initial lug bending went.
Well I got 3 out of the 4 joints pretty close. The seat, top and lower head lugs are real close. As I shape their shore lines to the style this frame has (a match to another frame made last year) I will need to do some minor tweaking. Not bad considering that I shifted the angles a bunch on the seat and top head lug (6 and 4 degrees respectively) (the lower head lug needed less then a degree of change). This is well within my expectations.
But the ST and DT angle in the shell need more work. The sockets needed to be closed up by about 4 degrees and I did so using the bending bars i got from Omar Khiel. But the differences between the bars and the tubes, that the DT has a S&S coupler in it, my building secquence with the ST brazed in seperately and my initially not changing the angle past what was wanted combine to make the DT fit up about a degree off (too large an angle). I think much of the bending angle and actual ST/DT angle is due to the ST not being held in the shell during brazing at the same angle, WRT the shell's socket, as was the case with the bending bars. In hind sight if I had bent the shell's sockets a degree or two past the intended ST/DT angle I might be spot on now.
I intend to correct this with a few methods. With the ST brazed in the shell I can't just bend the shell more without the chance of the ST giving (and showing it's bluge from the distorting just above the shell's socket. Been there, done that). So I have done some black smithing to the bottom end of the DT as a first step. WHAT!? Tampering with a tube? Isn't that like a golden no no? Well yes and no. Tubes are bent all the time. Some times we call it raking, sometimes we call it tire clearance. In this case I have slightly bent the lower edge of the DT/shell miter inward. All of this within the overlap of the shell's socket. The second part of the angle shifting will come (not yet done, need turkey dinner first
) from grinding the shell's socket a bit. Mostly on the shell DT's underside. Then some more blacksmithing on the Dt socket shore line to keep the gap manageable. I might have to revisit the DT's miter lower edge shaping, we'll see. In the end the goal is the right ST/DT angle with no pressure to hold the tubes in place as this would close up the gaps and make flowing filler between the tight spot hard.
I always think before these point in the building that i've got it all right. Then I fit togethet the lugs and tubes and have to go round again, manipulating a bit here and there. This makes me, once again, want to build with fillets and not lugs. And my next frames will be so. I just have to figure out how I'll deal with the forks in keeping with the rest of the frame's smooth joints. As usual as i get into the build of a frame i find mself thinking about the next one.
Well i'm a couple of beers later and have slowed down enough to clean up and go to bed. Thanks for reading this and I hope some one finds a nuget of information here. happy turkey to all. Andy.