Framebuilders - Lugs, Cast vs Stamped

Lugs: an egghead's observation

background: Invested casted lugs were considered to be the touch of class. Massed produced frames often had lugs that were stamped from sheet & welded. This was often offered as the distinction between great and OK.

Now, not that I am any expert but basic metalurgy is very clear that a casted part has a granular structure that is without direction, sort of like partical board. meanwhile a sheet has distinct directional grain structure, like a board of wood. therefore shouldn't a lug that is formed from a sheet is stronger that one that is casted????

OK, there are other factors involved. An investment casted lug can be made to very exacting tolerances, therefore the fit to the tubes being joined can be very consistant and therefore a lower temp higher silver brazing material can be used. A stamped part is at the mercy of the dies used and the amount of bend yu can put in the material, this and springback and other metal forming issues will result in a part that is at a looser tolerance and will therefore require a hotter more gap filling brazing material.

So how does it all stack up? which is better? casted or stamped or what????

Generally, cast lugs are better due to the improved dimensional accuracy as you noted. Strength is not really a factor either way since the lug is not a highly stressed part. There is one major exception to this rule though, some of the true master framebuilders, like Brian Baylis, often use stamped lugs as the starting point for one of their creations. The lug is filed and shaped, stamped lugs are softer and file much easier, and in the end you wind up with a classic frame reminiscent of days gone by. Brian Baylis has a 2-3 year waiting list so he must be doing something right.
http://www.classicrendezvous.com/USA/Baylis_main.htm
http://www.vintagecyclestudios.com/index.html

Generally, cast lugs are better due to the improved dimensional accuracy as you noted. Strength is not really a factor either way since the lug is not a highly stressed part.




agreed. cast lugs raise the bar of quality, tolerance-wise.
they also allow one to keep in step with the times by using
tubing with O.S. dimensions, thereby opening up the options
for all involved. that said, the lug is a "holder". the quality
of the joint is dependent on the interefernce fit and the
worker's skill. the lug material is not part of the equation.
ps: some people think of the cast quality of modern (btw,
hardly "modern" at all; they've been around since the 70s)
lugs as a shortcut which eliminates "handwork". i prefer
to think of cast parts as a starting point with which to make
a better frame.
e-RICHIE©™®

Speaking of nice starting points... :D

We're currently testing CNC'd lugs.

If they end up being safe, we think they'll change the landscape of lugged construction:


angles will be variable instead of fixed
any look will be possible, if you like feature cut 49 and nozzle cut 162 then no problem
very low development cost - no more need to spend $$$$$$$$$$$$$$$$$$ to produce lugs
just-in-time production efficiencies - no need to produce the lugs until they're needed


I'm guardedly excited about this.

One thing that CNC'd lugs would do is it make it possible to bring classic designs back. As a designer I look at the original work of the designers at places like Nervex and I am floored by their sensibilities. I don't think we do enough to recognize them for that work and I think it would be a fitting tribute to them to bring their designs back as affordable components aimed at the world of production bikes.

It would seem that lugs could be made also by sintering metallic powders, the same way precision parts are made for automotive use, as a savings over more costly casting and forging.