notching tube sounds like a simple process however requires some experience and most of all patience. you can notch tube with several types of tooling to include bridgeport mill, belt sander notcher, horizontal mill, various styles of hole saw tooling and last but not least an angle grinder.
the first thing i do is locate where the tube will be place. once this is established, some info needs to be required -what size notches will be done? to what relative degree will the notches be made? does the tube require a bend? are both notches on the same plane?
when determining the degree, i place a mark on the outside of the tube on both ends. you can then pull a string line or straight edge from mark to mark. once this is done i use a starret protractor (there are three different sizes and i use the correct one depending on the amount of room at the junction) and determine the degree on both sides. now i pull a dimension from the center of the tubes on both sides of the tube junction to determine the tube cut length. at this point, choose the correct size tube od and thickness for the application. now i cut the tube either using a bandsaw, cold saw, chop saw, or even a grinder. at this point you are ready to notch. depending on the angle will determine which tool i will use to notch. the preferred choice in my shop is a clausing horizontal mini mill in which i will use a roughing end mill to make the notch. this seems to be the most convenient machine and keeps the tube cool, not over heating the potential weld surface potentially changing the molecular component of the material. finally i will make notches to both sides of the tube. i will make a complete notch on one side and make about a 60% notch to the other side. at this point i will make several notches taking off .020″-.050″ at a time until i get the perfect fit. the key is to continue checking the tube after every notch and take off a very small amount in order to get the optimal fit. sometimes it will take me 6-8 times to get it right. You can save time by getting aggressive with both notches initially, however if you cut too short you can waste material which in turn equals a waste of money. once the tube is fit within the correct tolerance, i take it over to the belt sand and put a slight bevel all the way around the tube on both notched sides, removing the burr as well as making the welded portion of the tube slightly thicker. i then place the tube in the lathe at high rpm and use an emory cloth to clean off any debris, grease and mill scale. i also use a drum sander to clean the inside of the tube. a final acetone wash inside and out and now the tube is ready to be tacked into place.