Bicycle Tube Center Line JIg

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Summary 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. Purpose The purpose is to use the lines during the phasing of the tube during the mitering process of bicycle frame building. Function There are two mirrored jigs. They are stepped to accommodate 1", 1.125" and 1.25" 4130 Chromoly tubing of 0.035" wall thickness. The jigs are to be inserted into the ends of the tube until they bottom out on the correct step. The ID of the jig is a 0.5" square hole that can accept 0.5" square steel tubing to hold the jigs in the tube and keep them phased correctly. Once the jigs and the square tube are inserted and seated into the round tube a long straight edge can be used to mark the center lines. This is the purpose of the mirrored notches in the jigs. A straight edge can be pushed up against them to mark the line. Results This resulted in moderate results during the construction of a straight gage 4130 chromoly bicycle frame. Further improvements might be to create them out of metal and create either a locking mechanism or a spring mechanism to keep them in place during the marking process. I found that the tight fit of the square hole tended to keep them in place. At first it was too tight and filing was needed to get the square tube to fit correctly.