This diagram shows the steps used for manually notching a typical sized handrail pipe used in industry. Normally, pipe would be notched by a mechanical tool designed  for this purpose. However, that's not always possible, it has to be cut by hand using a grinder. Shown above is a method that works well, and, with practice, the pipe will fit almost as good as a mechanically notched pipe.

     Place the pipe in a vise, or have someone hold it for you, and you can do a good job on this. Use a scrap pipe of the same size to place into your cut out area to check the fit. This method  can be used on almost any size pipe, within reason. Just change the depth of the first  cut accordingly, and the distance in from each side of the pipe in proportion to the pipe.

     Safety wise, you must use both safety glasses AND a face shield.  The glasses alone do not stop grindings from going up UNDER the  glasses. Notching pipe in this manner will send grindings to that very spot. Use the shield, and pull it in close to your neck.