FISHMOUTH CUTTING TUTORIAL
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.
The cutting disc (also known as a "zip disc") is a very thin grinder disc that can quickly cut through hollow steel tubing of various diameters. Round tubing as well as square tubing can easily be cut by the zip disc, although a hacksaw can also be used if you don't mind the added effort. The zip disc used in this tutorial is a 5 inch diameter disc with a 3/64 inch thickness.
In these tutorials, the safety guard has been removed from the grinder so that you can see the action more clearly. It is not advisable to work with the safety guard removed from the angle grinder as a disc fracture could cause injury. Proper face and eye protection such as safety glasses and a high impact face shield are necessary for grinding safely.
When cutting a line around a tube using a zip disc, keep the grinder disc at 90 degrees to the tubing and cut from the top so that you can see the line. If the tubing that you are cutting is long enough so that you can hold it securely with one hand, it is easy to make the cuts while rolling the tube along your workbench, keeping the cutting disc along the top of the tube.
Cut along the line as you roll the tube and you will make a very accurate cut around the circumference of the tube. This method of cutting off a tube can be more accurate than the results form an inexpensive chop saw if you take your time and follow the line. All of of our Bike, Trike & Quadcycle Plans were initially built using only an angle grinder for cutting and a basic AC welder for welding, so you certainly don't need a garage full of expensive tools in order to carve out your own home built vehicles.
After cutting a steel tube, the edges that remain will be razor sharp, especially on the inside. Be careful. These edges are so sharp that they could easily slice through your work gloves, so they need to be filed after cutting. It's a good habit to file the edges after every cut, even on the tubing you don't plan to use or may discard.
A half round file is perfect for removing the sharp edges on a round steel tub after cutting. Use the round side for the inside of the tubing, running the file at an angle while working around the entire inside circumference.
The flat side of the file can be used to take off the outer edged and to remove any high spots around the cut area. Sometimes there will be a small amount of error where the start of the cut meets the end. This is easily filed down using the flat side of the file.
The end of the tubing that has the electrical tape as a guide is cut using the same method, but instead of following the marker line, the cutting disk or hacksaw is run along the edge of the tape on the side you want to cut. Electrical tape or masking tape will both work as a cutting guide.
Using the same method of rolling the tube along your bench, cut with the disc held at 90 degrees to the tubing and stay along the top so that you can easily see where you are cutting. Keep rolling the tube until the last cut meets up with the start.
After cutting, the razor sharp edges are once again filed down on the inside and outside of the tubing using the half round file.
A fishmouth cut will typically require a pair of semi circular cuts of equal depth at the end of the tubing. This will allow another round tube to connect at 90 degrees, forming a T shape. Although huge precision is certainly not necessary, the fishmouth cuts do need to be close in depth and adjacent to each other on the end of the tubing. A good eye and a marker are all you need to mark out a fishmouth for cutting.
If you are only making a single fishmouth tube and have done this a few times, then the circular cut can be traced by placing a piece of the required diameter tubing over the marled line so that you can trace a line from corner to corner where the half circle meets the end of the tubing. The paper template method shown next is certainly easier if you do not have a good eye for detail. However, after making a lot of these cutouts, you will be able to make them with decent precision by hand.
If you have a lot of fishmouth cuts to make or do not feel confident enough to simply draw them manually, a simple paper template can be made that will allow you to trace them out with great precision. A pair of fishmouth cuts are made in a piece of paper that will be wrapped around the end of the tube so that you can trace the lines, making a perfect cut line every time.
The paper template can be made by tracing an existing accurate fishmouth cut from another tube, or by making a drawing in a computer graphics or CAD program for printing. Marking the tube for cutting is as simple as wrapping the paper around the area to be cut and then tracing out the fishmouth areas.
The goal is to grind out the entire fishmouth cut, but it would take much too long to try to grind out the entire area one little it at a time, so a rough triangular cut will first be made. This triangle will be made inside the fishmouth area, removing most of the material so that the fine grinding of the curved area can be made.
With the triangular section removed, there is very little material left to grind out of the fishmouth, making it easy to fine tune the curved section with the rough grinder disc. The small triangular slivers are extremely sharp, so toss them in your garbage bin right away so they don't find their way onto the floor and then into the soles of your boots.
To fine tune the fishmouth cut around the curved section, a rough grinder disc will be used. You cannot grind using the face of the thin cutting disc as it was only designed to cut on its edge. The rough grinder disc can be pressed into the work on its edge or face, so it is good for shaping and grinding.
Hold the grinder so that the disc is hitting the work area at about 45 degrees, and grind the disc along the work so that it grinds evenly along the line. Only slight pressure is needed when pushing the rough grinder disc. The material will come away quickly. Try to grind the fishmouth out so that only a slight bit of the traced line remains around the entire area.
A 90 degree square placed between the two tubes is used to check the angle, as it may not be obvious by eye if the error is slight. Fine tuning of the angle is done by grinding the fishmouth on the high side of the angle ever so slightly to bring the tube deeper into the fishmouth. A hand file may be the best for very slight adjustments because it is very easy to grind away too much metal using the grinder disc.
Many of our Bike, Trike & Quadcycle Plans make use of square tubing, and since the head tube and bottom bracket will be a round tube, it is often necessary to cut the fishmouth into the square tubing. Cutting a fishmouth into a square tube is much easier than doing the same on a round tube because you are working with a flat surface. A square and marker will be needed to trim off the end so that it is straight before marking the fishmouth lines for cutting.
Finish grinding the rounded area using a rough grinder disc. A used grinder disc that has a worn edge may actually work better for this job than a brand new disc because it had a slightly rounded edge. Run the disc along the edge, working away the material until it is ground just ahead of the traced line.
Use a square to verify the 90 degree angle between the two tubes as you fine tune the fishmouth cutouts. If the angle is high on one side, use the half round file to take down a small amount of material on the fishmouth and then check the angles again until they are correct. Since many of our DIY Bike Plans include square tubing frames, this fusion between round and square tubing is quite common.
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