This is a seatlug for a 29er.  It's almost like building the frame twice.  I first lathe-turn the tubes to fit the OD of the tubes, then miter the lug parts and fillet braze them. The lug "blank" is coated with some layout dye so that the cut lines can be drawn.This is pretty close to ready for final build.  It's been sculpted and trimmed, and the binder boss is tapped.  This style of building allows me to make the lugs in unique angles and just as importantly, it allows me to build the frame by silver-brazing. This is the head-lug assembly for the same 29er.  Since the tubes are so close, it's better to make the TT and DT lug all in one.  Again, this gets silver-brazed along with the mainframe tubes.  The lower temp of silver maintains the tube's original strength, hardness, and resilience.
it all starts with a conversation about how the bicycle will fit and handle.  then a full size drawing is made.  from this, i'll transfer all exact tube lengths and angles and start cutting the steel.I use a 1956 Bridgeport milling machine to miter the tubes.  The cutter is a precision hole-saw specifically for cutting thin-wall steel tubing.  In this pic, I'm setting the angle of the mill head using Browne and Sharp  bevel protractor.This is a mitered down tube and head tube.  To me, this is the foundation----this angle has to be perfect in order for the rest of the frame to come out right.After the tube is cut, it goes onto the framebuiding table.  Here, I'll adjust the angle of the fixture and mark the centerlines of the 2 mating tubes.This is the brazed lug that joins the downtube and the headtube.  The 'sockets' are for internal derailler cables.  The cable fitting is cut to the profile of the downtube and the hole is tapped for an adjuster.  The hole drilled in the downtube is slightly larger than the diameter of the cable.This is the same joint as in the previous 3 it's been painted.The measurement from the BB center to the intersection of the DT and Head Tube is critical.  Now there are points in reality that are used to datum the rest of the frame.With this set up on the framebuilding table/fixture/jig I can measure up the seattube to the intersection of ST and TopTube.  Then, I set the dimension from that point back to the DT and HeadTube intersection. Much easier to measure lengths than angles.