Basic Fishmouth Cutting - Page 1 of 13
This basic tutorial will cover cutting a simple 90 degree fishmouth into round and square tubing. A fishmouth is a semi circular cut taken at the end of a tube so that it can be joined to another round tube. In bicycle frame building, it is often necessary to cut several fishmouths into the ends of the steel tubing where the head tube or bottom bracket will be installed. Before cutting a fishmouth, the end of the tubing must be cut marked to be cut off square. This can be done using a piece of paper and marker.
Wrap the paper around the end of the tube where you want to make the cut so that the ends meet, forming a straight line around the tubing. For best results, make sure the paper is at least 3 inches tall and wide enough to completely wrap around the tubing. When there is no slack in the paper, the ends will meet to form a straight line all the way around the steel tubing.
Electrical tape can also be used as a guide to mark off the end of the steel tubing, as long as you have a few points marked around the tube as a guide. Mark three or four points around the tube and then use the tape to connect the dots, creating a straight edge that can be used as a cutting guide.
With the tubing marked all the way around, a grinder cutting disc or hacksaw can be used to make a very accurate cut. If you have a decent steel chop saw, then you do not need a line around the entire circumference of the tubing, but this method certainly works well when manually cutting the end of a steel or round tube.
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