avhed

There should be an advantage at doing this so all the tubes/lugs align as well as they can at the seat lug and bottom bracket.

Dancingbear

Won't help with the DT/HT lug, Top tube length and BB drop will affect that one. Think of all the lugged frames that were built prior to when investment cast lugs became prevalent. Proper miters and good brazing are likely more key than an exact lug fit. The old pressed lugs were of much weaker material than what is used now, yet failures were always the result of poor miters or poor brazing.

unterhausen

if poor miters result in broken frames, then that would be a surprise to a lot of people riding classic frames right now. The mitering on even high end European bikes was often horrible

There is about a half degree of slop in an investment cast lug. Wouldn't help to make the angles exactly match the lugs.

Andrew R Stewart 

+1. I'll add that it's up to the builder to fit the parts together. After a "few+" frames one begins to understand what matters and what can be played with. Andy

avhed

Performance is in mind, not reliability.

Andrew R Stewart 

Please explain this statement. I can't tell if you are agreeing with Dancingbear's post or not. What he said is much along what I did. Andy

unterhausen

these sorts of concerns will totally fade away if you actually build a frame.

avhed

Meaning I know about the first sentence and am not concerned about the second. The OP is about alignment, I change one word in the OP to clarify.

unterhausen

I fail to see a distinction. They should line up exactly as the builder wants

avhed

The gap for silver brazing is <.01 inches. Will a factory produced lug angle be more likely to stay within this or a bent lug?

unterhausen

where did you get that number? But it sounds about right. Remember, the whole lug/tube/filler assembly doesn't have to be full strength, it just has to be "strong enough." If you ever try to disassemble a lugged joint, you will see that it doesn't take much to hold it together. So even if a bent lug has gaps larger than your tolerance in some percentage of the socket area, it will definitely have areas where the gap is within tolerance.

When a builder gets full penetration, a filler failure in lugged frame construction is exceedingly rare. I have not had access to any, but I have seen one example on the internet. I doubt it was actually due to gap tolerance.

This discussion seems to stem from a mistaken notion that there are a lot of lugs available in a lot of different angles. Unless you have money for engineering and tooling of a new lug, you get to bend lugs. I generally use 73 degree seat tube and head tube, so that's ok, but the downtube head lug is always a problem. But even then, the gaps don't get very large.