Thanks Mark for wishing me in on this one!
There's as many opinions about what bi-laminated construction really is as there are ways of achieving the effect.
Basically bi-lams are "dummy lugs" formed by brazing a decorative sleeve over the end of the frame tubes and then bronze-welding the tubes together. The "sleeves" are sometimes tubes with an internal diameter matching closely the external diameter of the frame tubes to be joined. The "sleeves" would have the decorative shapes cut into them, before being added to the frame tubes. An easier way to achieve the same look was to take flat pieces of mild steel, fret and file the decorative shapes in them -- far easier to file flat sheet than round tubes, and sometimes it was possible to cut two or three sheets at the same time - then roll the sheets over a mandrel to form a tube. Some builders would fusion weld the joint and dress off the weld to produce a cylindrical sleeve, resembling a proper tube, others would simply slip the very tight but not-welded sleeve over the frame tubes.
At this stage methods vary again. Some builders would then braze the sleeves to the ends of the frame tubes and then cut the mitres, then bronze-weld everything together, while others would mitre the frame tubes, slip the sleeves over and then bronze-weld the frame joints, while at the same time brazing the sleeves into position, thereby possibly needing a larger fillet of weld material to cover up any gaps. Others would slip the sleeves over the frame tubes but not into place, then bronze-weld the frame joints THEN slide the sleeves over the finished joints and braze the sleeves into place. No doubt there are other ways too. Each method has some advantage over the others. I always use the first method, using a slightly higher temperature rod for the brazing of the sleeves in place, then follow up with a slightly lower temperature rod for the bronze-welding of the frame joints
Opinions vary as to why the bi-laminated system was used. In truth I suspect it was due to a shortage of lugs after WWII, and builders, forced to use bronze-welding to produce their frames simply decided to use decorative sleeves to make certain frames less austere. Given the time and a customer who is willing to pay am little more I would probably produce all my "randonneur" and touring frames using this method. Even though it sounds a bit complicated itm can produce highly decorative "lugs" at a far lower price - in my experience and opinion - than fretting and filing lugs "in the round". As any lug-cutter will tell you, the first half of the lug is relatively easy..the real difficulty and skill is in cutting the other side to match perfectly
However the "bi-lam" system is very useful in that builders can produce frames that look like lugged ones while at the same time producing frames angles that would be impossible using the standard lugs available to them. It was also possible to produce the "lugged look" on frames eg tandems, that used over-sized tubes. Remember also that it is far easier to produce complex decorative shapes on flat sheets of steel than on ready pressed or cast lugs, as the sheets are easier to hold and manipulate. If you look at the Hetchins section of Claasicrendezvous, turn to the "Historic Hetchins" site, then look for "manufacturing methods " or " techniques" or some such phrase you will find an interview with Ken Janes, the craftsman who cut most of the decorative sleeves for Harry Rensch at Paris Cycles. He has a display on those pages of fretted plates and sleeves that are worthy of the name "filigree" as in silversmithing. I suspect that your States-side builder Dave Bohm, who produces wonderfully intricate lugs, and who started out in the jewellery trade, would have "cut his teeth" as an apprentice, using a piercing saw and needle files to hone his filigree skills.
If you need more info I think you will find pages of it in the Archivrs as this subject and explanations crop up fairly regularly.
Norris Lockley..from an usually sunny and hot Settle UK