Bicycle Tube Center Line JIg

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This is a center line jig used to find the center line in all four quadrants of a 4130 Chromoly tube from McMaster Carr with 0.035" wall thickness. The purpose is to use the lines during the phasing of the tube, ensuring precise alignment during the mitering process of bicycle frame building. There are two mirrored jigs designed for stepped accommodation of 1", 1.125" and 1.25" 4130 Chromoly tubing with a 0.035" wall thickness. The jigs are inserted into the ends of the tube until they bottom out on the correct step, secured by a 0.5" square hole that accepts 0.5" square steel tubing to keep them phased correctly. Once the jigs and the square tube are seated into the round tube, a long straight edge is used to mark the center lines, utilizing the mirrored notches in the jigs as a guide. A straight edge can be pushed up against them to create accurate markings. This jig resulted in moderate success during the construction of a straight gauge 4130 chromoly bicycle frame. Further improvements could involve creating metal jigs with a locking mechanism or spring mechanism to secure them in place during marking, reducing the need for tight-fitting square holes that may require filing to fit correctly.