The file is one of the most basic and essential tools found in the modeler’s armamentarium.  The proper file can rough out a shape rapidly or refine and smooth the finest of details.  Having a good selection of files will enhance your creative potential.  While files seem so common, they are actually quite amazing tools.  The first files were made completely by hand with each tooth formed by striking a chisel at the proper angle and interval.  The first successful file cutting machines came into use in the mid eighteenth century.  This, along with improvements in the refining of iron ore into steel and better heat treating processes, led to the development of the modern files.

The basic parts of file are shown in Fig. 1.  Files are sized by length.  American and Swiss files are measured differently.

Files are graded according to the degree of fineness or coarseness and whether the teeth are single or double-cut, Fig. 2.

Single cut files have rows of teeth cut parallel to each other at an angle of 65 degrees from the centerline.  Sometimes referred to as “Saw Files,” Single cut files are used for sharpening tools, finish-filing and draw-filing.  They are also the best tools for smoothing the edges of sheet metal.

Double-cut files have rows of teeth crisscrossed so the teeth become diamond-shaped.  The first row of teeth is called the overcut.  On the top of these rows, a second set of teeth is cut at a different angle to the file axis.  This row is known as the upcut and is finer than the overcut.  Double-cut files are generally used for rough work and fast removal of material.  They are sometimes referred to as "Machinists’ Files".

Curved-cut is an arrangement of file teeth in curved contours across the working face of the file.  The teeth are milled with a decided undercut (or positive rake).  These files are used mainly for automobile body repair.

Rasp-cut is a pattern by which each tooth is formed individually by a single-pointed tool or punch.  These files are generally used for wood.

Some files have edges that are uncut or smooth.  This edge is often referred to as a “safe edge.”  These files are extremely useful for highly precise detail work, Fig. 3.

The fineness or coarseness of a file indicates the spacing of the teeth and the number of teeth per inch, Fig. 4.  There are four different grades for American Pattern Files: Coarse Cut, Bastard Cut, Second Cut, and Smooth Cut.  They range in that order from fewest to most teeth per inch.  The differences from Coarse to Smooth are relative to the length of the file because coarseness of a file increases as the length from point to heel increases.  In other words, a 6” file, either single or double-cut, has more teeth per inch than a 12-inch file.

Swiss pattern files are graded from 00 – 6, with 6 being the finest.  They are commonly sold in sets of grades 2, 4, and 6.  Swiss pattern files are always double-cut.

It is easier to learn how to operate a lathe than it is to learn the techniques of proper filing.  The ability to achieve precision with a file is an art and the mark of a true craftsman.  In the old days, an apprentice would be given exercises that would require him to file a perfect cube or file two pieces that were to fit together perfectly.  The only way to master these techniques is to practice.

There are three basic filing operations:

Straight-forward filing: in which the file is pushed straight ahead across the work.  The teeth of the file only cut in the forward stroke.  The file should be carried forward in an almost straight line with the pressure first applied on the tip in the beginning of the stroke; on the tip and heel while in the middle of the stroke; and finally on the heel at the end of the stroke.

If pressure is applied on the return stroke, the teeth are dulled and the file can be quickly ruined.  Except when working with soft materials, the return stroke should be made with the file lifted clear of the material.  If too much pressure is used on the forward stroke, the teeth are liable to clog or even shell off (the breaking of teeth caused by excess pressure, back raking, or filing sharp edges or corners).

A typical beginner fault is rocking the file or trying to remove too much material at one time.  Both actions usually produce a convex surface.

Make sure the file is clean before starting.  The file can be easily cleaned with a file card, Fig 5.  One trick is to rub chalk into the teeth of a new file to prevent the teeth from clogging.  Chalking also helps to prevent pinning (tiny particles of fillings that wedge between the teeth) that can scratch the work.

Draw-filing: in which the file is grasped at each end and with an even pressure, alternately pulling and pushing over the work.  The file is held perpendicular to the direction of motion, Fig. 6.  One advantage of draw-filing is that the file can be held steadily.  As a result, a fine surface finish can be obtained.  Draw filing is usually done with a single-cut mill bastard file or a long-angled lathe file so that the metal is cut with a true shearing action that avoids scoring.  In draw-filing it is important to balance the pressure over the entire stroke so that a hollow spot does not develop in the center.

Lathe-filing: in which the file is stroked against the work that is revolving in the lathe, Figs. 7, 8.  Lathe filing is used to smooth out small flaws from the cutting tool, create a free form shape, or make small corrections.  With the work turning toward you, hold the file at a slight angle and use a long forward stroke, moving laterally about half the width of the file.  Do not hold the file rigidly or stationary, but keep continually stroking.  Clean the file frequently as it will clog quickly.

Take care of your files.  New files should be broken in on flat surfaces of soft metals.  Use only light pressure.  Don’t just throw your files in a drawer.  Care should be taken to keep each file separate.  Keep them in a rack or drawer with partitions, Fig. 9, 10.  Be sure to keep files away from water or moisture and avoid getting them oily.  Oil makes the file slide across the work surface without cutting.  Get in a habit of cleaning your files with a file card before putting them away.  With care, a quality file will last a long time.

Here is a chart of the more common file shapes.