There are many times when we need to miter tubes in order to fabricate various things. Motorcycle chassis, bicycle frames, rag and tube aircraft are things that immediately come to mind. My own experience is with bicycles, so the examples will come from that.
Hand mitering small (<1.5”) tubing is not terribly time consuming, but if I were in production environment I would definitely do this with machine tools. In either case, the accuracy and strength of the final joint is dependent on the skill and experience of the person doing the work. When I was learning, I bought several 6’ lengths of mild steel tubing similar in diameters and thickness to bicycle tubing. I cut miters and practiced making fillet joints with low fuming bronze (LFB) filler rod. Cutting up the joints you make is when you get to see if your mitering accuracy is good enough.
Things you will need:
Half round file to fit mating tube (8" for 1" dia, 10" for 1-1/8" dia, etc.)
small flat file
small round file for deburring
tubing blocks or 2 short length of 1” C-channel
hack saw with 32 tooth blade
Marking material: marking pens or fast drying spray paint
Step 1: Get Joint Specs
From drawing or from model, get the following information:
1. Diameter of tube to be mitered
2. Diameter of mating tube
3. Angle of joint
4. Offset, if any. Offset is the distance between tube axes.
Step 2: Get a Paper Pattern
Use a program to generate a paper pattern. I use the tube notching program at Nova Cycle Supply. I like this one because it gives a reference line that is 4 inches from the centerline of the mating tube. That's helpful when making mitered tubes of a specific length. Another program can be found at MetalGeek.
Step 3: Mark a Centerline
Use the torpedo level with the flat file to mark a line on the top of the tube. While holding the torpedo level with the small flat file under it, pull the file along the top of the tube all the while keeping the bubble centered. The mark is pretty handy to have. I mark both ends. You can also use all the usual machine shop stuff if you have a surface plate and vee blocks. In that case I would mark top, bottom, left and right just because it’s so easy to do in one setup.
Step 4: Apply Paper Pattern to Tube and Mark.
Cut out the pattern. Align one edge with centerline you just marked. Make sure the strike up marks on the pattern align. Now mark the edges with a marking pen or spray paint. Fast drying is what you’re going for here. I then remove the pattern, but I know some folks just leave in place.
Step 5: Rough Cut
It is entirely possible to make the entire cut with a file. The guy that taught me how to do it insisted that I do it that way. Now I make rough cuts with a hacksaw. When guiding the cut at the start I recommend leaving your finger in place for longer than you think you need to do it. Those tiny teeth on the saw love to skitter out of the saw kerf and leave extra work at finishing time. Position the work so you can saw close to the lines. I usually make two cuts on either side and then bend the waste back and forth to break the section free. Be diligent in containing the waste pieces. If left on the floor, they can puncture a tire or a shoe.
Step 6: File to Fit
The pattern on the tube should be pretty accurate, but you need a secondary method of making certain the tube fits, it is centered and at the proper angle. Place the tube in the vise so the angle of the tube is equal to the angle of the cut. That is to say, you want to file horizontally.
Check the angle of the tube using the angle finder. Now use the correct size file to file almost to the mark. When you’re close you can begin to check for fit. A piece of tubing of the correct size should sit horizontally, as checked by the angle finder, with no rocking and should have contact along the entire length of the miter circumference. In addition, for miters that have no offset, ears on either side of the cut should be the same height as measured by the square.
It may be helpful when beginning to take the test piece and apply some bluing. Place the test piece in the miter, rub it around and then take it away. After the test piece is taken away it will be easy to see the contact points. Then file away the blue. It’s possible to make accurate fits in this way. With some experience, you’ll be able to tell where the high spots are by rocking the test piece. The pivot point of the rocking is the high spot. And the high spots are always somewhere on the ears. Final fit is achieved by rubbing the file in a rotary motion.
Step 7: Check Your Work
Test tube should be horizontal. There should be no appreciable gaps around the circumference and the test piece should be in the center of the miter.
This is only one way to accomplish this. I've seen grinding wheels and angle grinders used. If I were gas welding the tubes, my requirements for joint accuracy would be much less. Mostly I braze, so keeping the copper alloy layer thin is key to joint strength.