There are a number of ideas that exist in the framebuilding world that most likely have their origins in logic rather than scientific testing and just plain old marketing. If a framebuilder does something a certain way, he might as well promote it as superior to his competitors. My opinion is that thinning lugs is mostly about aesthetics and how to do it is mostly about what works best for the builder with the equipment he has. For example, I know of no studies that have compared old style pressed lugs to investment cast ones although there is no limit of opinions on the benefits or disadvantages of each. That doesn't mean that observations leading to these conclusions don't have validity.
I'll take off the gross thickness of a lug before brazing with a bench belt sander and/or Dyanfile (the name brand of a hand held belt sander) but I still recommend to my framebuilding students that they do most of their lug thinning after the frame is brazed. For one thing a lug can be held very stably as part of a frame and for another, I can control the precise angle I hold the file by comparing it to the tube. That is really hard to see when it just stands alone in some kind of lug vise (which I have several very nice ones). Furthermore, nether a lug's hole nor a tube are perfectly round so when they are joined it ends up there are some roundness variations that filing can correct.
What is challenging when filing a lug after it is brazed is making sure your file doesn't nick into the tube. It is very easy for a file to slide off a lug. I like to show off a well filed lug before it is painted where there are no marks on the tube at all.
Doug Fattic, discussing the myths of framebuilding in Niles, Michigan, USA
From: "Rich Rhodes" <richlanguageelk(AT)gmail.com>
Nuttin' against a Bob Jackson, or for fixing something that ain't broken, but a frame builder told me that thinned lugs help distribute and redirect the stress to where it needs to go. But maybe Mr. Fattic would care to comment.
Rich Rhodes
From: Tom Dalton <tom_s_dalton(AT)yahoo.com>
Isn't there a practical benefit to very thin lugs that is not realized if you wait until the bike is brazed to thin them out? Don't thin lugs provide better heat soak in less time, or something like that? Or is the benefit independent of when the thinning is done, such as getting a more resilient joint irrespective of whether is it thinned before or after?
Or, are we talking exclusively (rather than primarily) about asthetics?
Isn't filing lugs after the fact about a thousand times more difficult? I always thought a good frame was made by first prepping the lugs, then brazing it very neatly, and then doing almost no filing or sanding as touchup. Isn't tidying up a frame that nobody could be bothered to do carefully in the first place a bit of a stockings on a pig scenario?
When is a Rensho no longer a Rensho?
And, really, isn't Garth just pulling our collective leg?
Tom Dalton