Friday, April 9, 2010

"Investment Stamped" Lugs

Here is the first fruit of the fire I announced in my last post: "Investment Stamped" lugs.

I'm using stamped lugs on my first frame. They have nice long points, they're easy to work with, but they're not easy to use if you want to braze with silver. (Which I do: a practice crown done with brass yesterday reminded me why—it's hard, and things get hot!) 

The reason is that stamped lugs leave large gaps in the transition between the lugs. The usual thinking about silver is that it's very strong, but only when it's operating in narrow gaps. (I don't know if that's strictly true, but I don't want to test it!) Thus investment cast lugs, with tight transitions from tube to tube inside the lug (see these IC Cinellis), are a good match for silver brazing, because they leave no large gaps for the silver to fill. Stamped lugs, with their spacious transitions (see these Prugnats), are thus not a good match for silver.

Unless, of course, you're willing to spend two whole days filling in the gaps with brass and filing them in to shape. This is what I've done, and the results are above. Everything that's silver is steel and was part of the original stamped lug; everything that's gold is brass and was added. These transitions are every bit as tight as an IC lug, so they'll be fine for silver brazing. Plus all the filing was good practice.


johnb said...

Whether silver solder is stronger than brass is not really the point; depending on the alloy and intended use, it probably goes both ways. However, silver solder generally is is intended for small gaps, whereas brazing rod is for larger gaps and can be used to build fillets. I have no doubt that mass produced brazed frames using stamped lugs had gaps inside where the brass didn't reach.

AH said...

Thanks for the comment, John!

Do you think that means that the actual tubes were not directly attached by brass—i.e., at the miter—and that the lug therefore held them together indirectly?

johnb said...

Adam, I think that's what I meant. I doubt if the mass market bikes of the bike boom era even had mitred tubes, in which case the the junctions of the lugs would have been all that provided the strength in the joints. However, the lugs you show that you had to fill are probably not typical of those used on popular lightweight frames back then. The ubiquitous Nervex Pro lugs had pretty tight junctions and short enough sockets that mitring of tubes was pretty much mandatory.

Have you seen anyone else fill stamped lugs with brass to get a snug fit, or is this something you've done on your own?

AH said...

Wow, unmitered tubes! That sounds dangerous! It seems to me that the little tiny fillet of brass or silver between the miter and the tube provides most of the strength.

I've "heard" of people doing this but never seen it. It's good practice when you're not crunched for time, and it's nice to bring old lugs "up to date." I have some stamped Cinelli lugs on their way, and I'm eager to see what the transitions are like on them.

The Prugnats were used on some pretty nice bikes, too -- surely plenty with nicely mitered tubes!