There are lots of kinds, Bastard, Flat, Second Cut, Warding etc., but for all intents and purposes, the general hobbyist only needs needle files. Needle files can come in many shapes, such as round, half round, flat, square, three-square, but what you really need to understand is the way they work, and the two types of teeth.
Teeth:To start with, the teeth on a file looks something like this up close. (view pic below).
The teeth will only cut one way, and if you try and use them in reverse, they risk curling over and blunting. Pretty straight forward eh?
The Type of Cut:There are two types of cut, single and double. A single is like this pic below;
The double cut is the exact same again, however, the teeth criss-cross. The main difference for you to understand is that single cut takes off less material, but looks prettier, whilst double cut files tear away much faster.
The Cut Angle:The file will have all the teeth arranged into rows, or a direction of cut. This is the angle of the teeth, and it is important to understand which direction you can cut in and why.
The two best directions to use are straight up with the file (left side image) and in the opposite direction to the teeth (right side image). If you go in any other direction, you will actually cut gouges OR not cut at all.
Single and Double Cut Files:
In this image you can see that the teeth all are cut in the same direction, making this a 'single cut' file.
However, you can see here that this file has two sets of teeth. I have marked the cross over point in chalk where the teeth transition.
The results of directional filing:As you can see here, a straight push on this chalk leaves a smooth flat finish. This is the simplest method to use, and the result speaks for itself.
However, if you move in the direction of the teeth, you end up scoring gouged lines across the surface of whatever you are filing. Believe me, resin WILL do this, as will plastic. I have seen many a miniature ruined by gouging.
This image is where I have filed AGAINST the direction of the teeth. Again, the finish is very smooth, and the damage from the gouging is removed. This is my preferred method of filing.
The below example is why we tend to avoid double cut files. The teeth leave a series of small marks that waver and zig zag across the surface. This is best only used for very rough work.
Although the files used here are fulls sized files (from when I did my apprenticeship, back in the dark ages), they are honestly just scaled up versions of what we use. This is a single cut needle next to a single cut straight file, and as you can see, they are almost identical.
So, next time you want to get a nice straight line on a tank, or you're filing the side of a marine torso so you can get the arm to sit flat against the joint, remember these tips,