Stephen Gardiner was kind enough to complement the quality of my incorrect filed angle and although I think he was just trying to make me feel better I did consider that perhaps describing my technique might be useful to someone. Having never done an instructional type of post, it will be a good exercise for me. All are encourage to ask questions if I have left something out!
So, you’ve got a piece of rail and you want to file the end to a particular angle.
I start by marking the angle on the top of the rail. I also sometimes mark the bottom as well or just the bottom depending on what seems necessary. If you are prone to getting an unwanted vertical angle as you file, do both so you can catch yourself at it. Machinists typically use a blue marking die which one either paints or sprays on. It is lacquer based so it smells and it takes a solvent to get it off again. For small jobs, I use a big Sharpie maker; an alternative I learned about in Simon Bolton’s books. The marker will do a 4″x10″ sheet of brass in less than a minute of vigorous scribbling if you need to.
I then use an engineer’s protractor to get the proper angle and a machinist’s scriber to make a mark in the blacked area on the rail.
The marker makes the scratch from the scriber easy to see.
After that, we get to the actual filing. I use a 12″ single cut file for most of my filing. This is less aggressive than default double cut file you get at the hardware store. You can usually find the single cut ones if you look. As far as actual technique, I grip the rail in my fingers and rub it back and forth on the file which I either lay on the bench or my lap. I find it easier to see and check the angle mark as opposed to clamping the rail and moving the file.
I check every dozen strokes or so to see how I am doing and adjust as necessary to try and keep the filing parallel to the mark on the rail. Eventually, I usually get down to the mark. If I mess it up, I either remark the end and take some more off or start on a new piece if I can’t spare any more length.