much like wood, metal tubing has its imperfections, so before the tubes are cut, i roll each tube on my granite surface plate to find any bowing in the tub. i mark the high point in the tube and i make a scribe line down the entire length of the tube using a piece of 1/4" aluminum angle stock as a guide and a sharpened tungsten electrode to make the scribe line. this gives a nice reference point for cutting the tube and for braze ons and bottle mounts. I do this to all of the front triangle tubes. When the frame is all brazed together, the bows in each tube will be aligned along the vertical axis of the frame.
One final thing before cutting the tubes is finding the depth of the tubes butting. some tubing manufacturers provide the butt length data for each tubeset, but others do not. i use a combination of sight and the depth gage of my vernier caliper to figure out the butting length. fancier measuring tools are out there, but not worth the expense in my opinion. you can generally see the line of where the butting begins when you look inside the tubing. i mark the measurement with a sharpie
now we can cut the tubes
a few methods for cutting miters in tubing
1. there is a program on the internet called Tubemiter, you enter the tube diameter, the angle of the cut and the size of the tube it will be jointed to. then the program give you a flat representation of the cut that you then print out and tape around the tube. then you can mark the cut and use a hack saw and files to make the miter.(ive never used this method by the way)
2. use the lugs as your template for your miter. insert the tub into the lug to the point where you want to make the cut and use your scribing tool to mark the shape of the cut. once you do this, you make the cut the same way you did the cut on with method 1.
4. using a mechanical cutting tool like hole saws on a milling machine or abrasive disc tools. pretty much, this is the most expensive method, but it can also be the quickest and most repeatable. i really recommend cutting your miters by hand to understand the technique. the downside of using machines to miter is that its real easy to mess up a tube by cutting it off center or too short, with hand mitering, it can be much more forgiving. if you miter is a little off and you are taking your time to not file it too short right away, you usually have enough tubing to fix the issue.
with each of these methods, you goal is to get a perfect miter. first check that the angle of your cut is right by using a steel protractor. then use a machinists square to make sure the high points of the cut are square to one another, then use the tube that the mitered tube is to be jointed to, to see if light gaps can be seen when you stick them together, if there are light gaps, use a half round file to finish the cut.
i usually start with the seat tube because only one end of the tube needs to be mitered, so its the easiest tube to deal with. also the main cut is a 90 degree angle. i then make a compound miter for the downtube.
then i make the headtube/downtube junction miter followed by the downtube bottom bracket miter and finally the top tube miters(these are the hardest to get in line with one another, so take your time to get them straight)