I thought I’d talk a little about the mysterious sequential welding that you may have heard about. Alignment on high-end frames is quite important. Traditionally when a builder completed the frame they would put it on an alignment plate and use a lever bar to bend it into alignment, a process called “Coldsetting”. When working with the newer lightweight thin wall steel alloys you get very little movement when Coldsetting before the tube will buckle so it is necessary to get the frame alignment as close as you possibly can during the welding process.
When you weld a frame the material expands and contracts with the heating and cooling. You may be surprised to learn that it moves a lot more than you might ever think. In the beginning of this video you can see the Headtube slowly move down as the weld cools. It moves over 0.040″, far greater amount than we consider an acceptable tolerance for the completed frame. The first image of the dial was taken after the bottom bracket to Downtube was welded. Most of the front triangle alignment is accomplished when welding the Bottom Bracket to the Downtube and Seatube to the Toptube. Once that is complete, in my case, I measure the Headtube twist as seen in the video and my fixture is setup to give me about 0.015″ of twist. Ideally once the Headtube is welded to the Downtube and Toptube the twist should be gone. In all these shots the measurements are taken off the Headtube which is a relatively short distance and does have some runout due to heat distortion. Later in the build after machining, indications will be taken again from the center line reference and very small correction can be made. During the welding process it’s important to make frequent checks quickly in order to watch what is happening as it happens. Through experience I know how preliminary checks as seen in the video will translate to the final checks taken later. Ultimately little to no final alignment will be needed in about 90% of the frames and when it is, it’s usually only a couple thousands here and there.