Ken,
>From both a technical standpoint and a standpoint of beauty, I believe lugs should be filed thinner as opposed to left thick. Super thin lugs are not acceptable to me for a few reasons, but are structurally proper. Super thin lugs often don't survive a tube replacement because sometimes the lugs are filed through to the braze, leaving edges gone or holes in the lug. Also the lug can come apart during extraction of the old tube, especially if there is a cutout in it. If the lug is brazed with brass, you're really asking for trouble. That's just my opinion from my personal taste and experiences repairing frames. I personally prefer a lug thinned to perfect consistant thickness all around the edge, but not super thin and NEVER filed down to the tube.
>From a technical perspective, a lug is a "stress distribution collar". Therefore the "crotch" of the lug where the tubes intersect should be gently curved from edge to edge relieving the crotch as much as possible in the process. I perfer the look of lugs like this and they do their job of stress distribution (lugs DO NOT hold frames together) perfectly, look great, and are easily repairable if you get tagged by a car. To me, that is perfect, anything more or less is less than perfect. Ironically some builders actually accentuate the crotch area as a style. Mario Confente did this, and looking at the seat lug crotches is the best place to look to see if Mario filed the frame. I learned lug filing from Ron Smith at Masi, who understood the look and the technical aspects of lug filing better than Mario did! All the lug filers at Masi did it Rons' way, only bikes built by Mario show sharp crotch style filing.
Brian Baylis La Mesa, CA
Please note: message attached