Kinds of files
Files in common use are made by chiseling grooves at an angle on the surface of a piece of steel (called a blank) which is later hardened. These grooves are called teeth; and though generally they run diagonally, they may run crosswise. Not only the surfaces, but the sides or edges of files are toothed. Some files, though, have plain edges and are called plain-edged files. They are excellent for use where the filing must be done in corners or cramped places and care must be taken not to mar or scratch adjacent surfaces.
A single-cut file has teeth slanting only in one direction, from left to right toward the point, or end of the file. This type of file is used with light pressure, and produces a fine, smooth finish on metal surfaces or a sharp edge on such cutting tools as scissors, knives, and saw teeth.
A double-cut file has one set of teeth slanting as in a single-cut file with another set, finer than the first, crisscrossing over it in the opposite direction — from right to left toward the point. Greater pressure is used with this type than with the single-cut. Metal is removed f aster, and, therefore, the finish is not so smooth.
A rasp-cut file does not have grooves as does a single-cut file. Rather, it consists of rows of individual, separated teeth made by a short, narrow, punch-like cutting chisel. It provides an extremely rough cut and is used chiefly on wood, leather, aluminum, lead, and other such soft materials. It removes material very fast, but leaves an extremely rough finish.
A curved-cut file has symmetrical curved grooves extending across its face. It has a rather coarse cut, and is used principally on lead and babbitt.
A file may be single-cut, double-cut, rasp-cut, or curved-toothed; and the distance between the cuts or teeth may also vary. Thus the coarseness of the teeth — that is, the number of teeth per inch — is designated as rough (least number of teeth per inch), coarse, bastard, second-cut, smooth, and dead-smooth.
Note that double-cut refers to the kind of teeth and second-cut refers to the size of the teeth. Coarse and bastard cuts are used for heavier work, to remove material speedily where it does not matter if the finished job is coarse or rough. Second-cut and smooth are used for finer work and for finishing off the rough results given by the coarse and bastard files. Rough and dead-smooth are seldom used, except for special work.
Coarseness varies according to the length of the file, the distance from the tip to the tang. In comparing the coarseness of two files, therefore, they must be of the same length. A 6" file cannot be compared with a 12" file, since they are of different lengths.
Shapes of files
Another factor that helps in describing the type of file is the cross section, or shape of the file if cut across just as bread is sliced. The most common shapes are flat, square, triangular, round, and half-round. There are many other kinds.
The flat file, which comes in a variety of cuts and coarsenesses, is the one most commonly used. Generally it tapers and is a double-cut bastard type, although it is also made second-cut, smooth, or even dead-smooth.
The mill file is single-cut and square-edged. Some have one or two round edges. Since fine work can be done with this file and the resulting finish is smooth, the mill file is used for sharpening mill or circular saws, cross-cut saws, and mowing-machine knives, and for lathe work, draw filing, and smooth-finish filing in general.
Machinists' files are used throughout industry when the chief purpose is to remove metal quickly and the resulting rough finish is not of special concern. These files include flat, round, half round, square, triangular, and knife shapes. Except for small sizes of round and half-round shapes, they are all double-cut.
The equaling file is not of ten used, except for fine-tool manufacture. It ranges from 6" to 12" in length and is generally a double-cut bastard type.
The round file is used for enlarging round holes and shaping curves. It tapers, and varies in length from 4" to 18". The cut is generally single bastard. The round file is sometimes called a rattail or a mousetail.
The slotting file (sometimes called slitting) is used for filing grooves for cotters (tapered flat rods or pins), wedges, or keys. It is double-cut, usually bastard, and its Bides are at right angles to each other.
Care of files
Mechanics should take good care of their files for three simple reasons: avoiding personal injury, lessening labor by using sharp instead of dull instruments, and reducing expense of tools and materials.
Files should be kept clean and free from chips of the material they are used to file. After each few strokes tap the handle (no other part) gently and firmly on the bench to loosen the chips. Files should be frequently cleaned with a file card. By brushing in the direction the teeth run, the file card removes material that may be clogging the teeth.
Usually resting in the handle of the file is a pick, which is nothing but a piece of soft-iron wire of heavy gage. It is used for removing stubborn burs or pins jammed between the teeth which cannot be removed by the card alone. A stiff-bristled brush is used for finishing the cleaning job. First the file is brushed in a direction parallel with the teeth, and then along the length of the file. The card, brush, and pick may come as separate pieces or as a combination in one tool.
Pinning is the result of scratching a surface while filing because the file teeth were clogged with the material filed. Carding a file prevents this. Chalking a file before it is used on soft metals helps prevent it from getting clogged.
New files are usually coated with oil to prevent rusting. On most jobs this does not give the mechanic any trouble, but in certain instances the oil must be removed before filing. A simple way is to cover the file with chalk and then card off the oil-soaked chalk. An oiled file fails to cut, or bite, properly on hard-surfaced material such as cast iron. Such a file may glaze the surf ace of the material, thus blunting the file teeth. Then, too, dust and filings may pack down in the oil of the file and eventually become so caked and hardened as to render the file useless.
Sometimes a file is so hard-caked that the card cannot help. In such cases it may help to hold the file for a few moments in boiling water. This may soften the caking for easier removal with the card. The heat obtained by the file from the boiling water helps to evaporate any film of water remaining and so prevent rusting.
Rusting in storage may be prevented by covering the file with a thin coat of oil. However, as we have noted, the oil should be removed before the file is used. If stored for some time, the file should not only be covered with a thin coat of oil, but should also be wrapped in an oiled paper.
It is most surprising to compare the hard work done by a file with the delicate teeth it has. If possible, examine these teeth with a hand magnifying glass and see for yourself. Throwing a file into a tool drawer on top of other tools or throwing other tools on top of a file can only cause damage to the teeth of the file. Handles should be properly placed on all files. Files should be hung up by their tangs on a rack where they cannot rub against each other or be damaged by other tools. The best file handle is the old-fashioned wooden type. It should be of proper size and should reach almost to the shoulder of the file.
The teeth of a file slope at such an angle as to provide for pressure and the resultant cutting while the file is being pushed. If the pressure is maintained on the return stroke, the teeth are broken off.
Filing of sharp edges and hard surfaces should be begun with an old file and then continued and finished with a new file. If the new file is used in such cases from the beginning, its teeth may be damaged.
Use of files
To one who has not done it, filing looks very simple. However, to hand file a perfectly straight flat surf ace is one of the most difficult operations in the metalshop. On the other hand, with sufficient practice, and by conscientiously following several simple rules, the skill can be mastered.
Selecting the file
In general, the selection of a file depends on the type of metal, the shape of the piece, the shape it is to be filed, and the degree of accuracy required.
Begin with a bastard-cut file for filing brass, bronze, or cast iron and finish with a second-cut file, or smooth-cut file for the first two materials.
For filing aluminum, lead, or babbitt, use a bastard file or a float-cut file.
Use a file of proper length for the work: a short file for a short piece; an 8- or 10-inch file for medium-sized work; and a file as large as can be conveniently used for larger work. Large areas should be smoothed with large, rough files. Small areas should be filed with small files with bastard or second-cut teeth.
It is logical that when a square edge is required, a round file should not be used. For slots, a slotting file should be used; for enlarging circular holes, a round file should be used; and so on. For general use, where great accuracy is not required, the bastard file is used. For close fits and preparing surf aces for polishing, the final filing should be done with a second-cut or smooth file.
General filing instructions
Flat filing (sometimes called smooth filing) is a means of giving a fine, smooth finish to the surf ace of metals. Hold the file handle in the right hand, palm down. Hold the point in the palm of the left hand, palm down, with the fingers curved around it. Do not rely only on the motion of the arms while filing. Move the body forward on the cutting stroke; straighten the body on the return stroke.
Hold the file flat; avoid rocking it. At the beginning of the stroke put pressure on the file with the left hand. As the stroke moves forward, reduce this pressure and replace it with pressure from the right hand. Alternate pressure in this manner for each cutting stroke.
Draw filing makes a smooth and level surface. It consists of grasping the file firmly at each end, the handle in the right hand, and alternately pushing and pulling the file sidewise across the work. Note how the hands are held and the position of the thumbs. Pressure should be even between the two hands and should be applied during the cutting motion. On the return motion, the file should be lifted away from the work. Ordinarily a standard-mill bastard file is used, but a single-cut smooth file will do. If some chalk is rubbed on the file, the finish is improved.
A still smoother surface can be obtained by wrapping fine emery cloth around the file and then proceeding as if for draw filing.
Round surfaces should be filed with a rocking motion, but following the general directions for flat filing.
Emergency file substitute
In an emergency, when no file is available, metals can be "filed" by rubbing them with a hard, flat, smooth stone. This will not produce the desired effect, but it can help greatly.
To avoid accidents and to prevent loss of tools and materials, certain safety precautions should be followed: