Originally Posted by
mudboy
I don't understand this mentality. How are you supposed to ensure correct frame angles, etc. if you don't at least tack braze a frame in a jig? I do the bulk of the brazing "free", but I tack my lug points while it's still in the jig.
Pete
Pete, you're right; that's the way most builders do it.
They use the jig to ensure everything is accurately aligned, then tack braze the tubes to the lugs and dropouts while the frame is in the jig. The tack brazes keep the tubes in place relative to one another while the builder finish brazes each major cluster after removing it from the jig. After brazing each major cluster, the frame is cold set if necessary on an alignment table to ensure there are no cumulative stresses built into the frame.
Reynolds, in the technical data sheet for 531 recommends this brazing sequence to avoid stress buildup:
"When the tubes are fitted into the lugs, care should be taken that the assembly is not in a state of stress. Brazing should be carried out in the following clockwise sequence:
• bottom bracket
• down tube/head tube joint
• head tube/top tube joint
• top tube/seat tube joint
This sequence obviates the danger of a stress raiser being created at a major shock point.
When brazing frames made from Reynolds 531 Tubing, the joints should be pre-heated, and after brazing the cooling should be controlled and not forced. Brazing must be carried out in a shop free from draughts.
Great care must be taken not to overheat the material, or to heat too large an area of the tube. Overheating will lead to burning or to brass inclusion (where the molten brass or bronze enters the grain of the steel), either of which will make the tube brittle.
Setting, when necessary, should always be done cold."
Frames can be and are built without jigs, but for most builders it takes more time and patience to do it that way.
To quote Richard Sachs:
"Tacking a frame in the jig allows me to take a frame design from paper and make it into a three-dimensional pattern. You have to be careful to tack it and not braze it. When I tack a joint, only ten percent of the area is covered in brazing material. By not fully brazing the joints, you can check the alignment and make changes if you have to."
Here's a photo of Richard from a 1990 issue of
Bicycle Guide with a frame in his jig:
