Basic Benchwork for Home Machinists. Les Oldridge
5
Files and Filing
Filing is a skill which must be developed by the model engineer, but, unfortunately, a lot of practice is required before the average beginner can become really proficient and be able to file a flat surface. Nevertheless, once the knack has been acquired, the operation becomes automatic and one files flat without thinking about it.
Files are classified according to their length, section, and cut. There are many different types but the model engineer only requires a few. Exactly what he needs depends on the type of work on which he is engaged. I have found it a good policy, with all hand tools, to buy them as the need arises, rather than purchase several at the same time on the off-chance that they will come in handy in the future. This policy spreads expenditure evenly and ensures that only essential tools are purchased.
FILE LENGTH. The length of a file is measured from the shoulder above the tang to the point. Needle files are the exception to this rule, the total length being measured.
FILE CUT. A file may have a single or double cut. In single cut files, the teeth are parallel to one another and at an angle to the centerline of the file. The double cut file has a second cut over the first. This produces small pyramid-shaped teeth that have more cutting edges. It is the double cut file which is in common use. See Fig. 5.1.
Fig. 5.1 Single and double cut files.
Fig. 5.2 Common types of files.
Files may be cut with teeth of the following grades: rough, bastard, second cut, smooth, and dead smooth. Those in general use and likely to be found in the model engineer’s workshop are the bastard, for the heavy removal of material but leaving a fairly rough finish; the second cut, a general purpose file for light removal giving a fair finish; and the smooth for fine finishing work.
Two rather special cuts should be mentioned, the dreadnought for the rapid removal of soft metal such as aluminum, and the rasp, which will deal, in a rough and ready sort of way, with wood.
FILE SECTIONS. There are several different file sections available to cope with different kinds of work. Fig. 5.2 shows the shape and section of a variety of files.
The HAND FILE is parallel throughout its length, viewed from the cutting face, but its thickness tapers toward the end. Both faces are double cut and one edge is single cut. The other edge is left smooth and forms a “safe” edge. This allows cuts to be made into corners without damaging the side against which the safe edge is in contact.
The FLAT FILE is tapered in both width and thickness and is double cut on both faces and single cut on its edges. It is a very useful general purpose tool.
The HALF ROUND is another useful shape, but misnamed; its section is not half round but only a segment of a circle.
SQUARE FILES are double cut on all four faces and are tapered for the first third of their length. They are useful to clear small corners and slots and to form square holes.
THREE SQUARE, or triangular files, are the specialty of the “saw doctor” but are also useful for shaping holes with less than right-angle corners, and for producing really sharp corners.
The ROUND FILE, colloquially known as the “rat tail,” for obvious reasons, is tapered for the first third of its length. It is used for enlarging holes and comes in a variety of sizes. Buy one to suit your particular needs; I find one with a diameter of about ¼ in. (6mm) to be the most useful size.
WARD FILES are similar in shape and cut to the flat files but are much smaller. They are of uniform thickness throughout their length. Originally intended for use by locksmiths for producing wards in keys and locks, they are useful for dealing with the small components commonly found in the model engineer’s workshop.
The KNIFE FILE is wedge shaped and not in general use but is useful on occasions in entering and enlarging slots where standard files are unsuitable.
NEEDLE FILES are very small files of various shapes and sections which are used for very fine and delicate work. The tang is formed into a thin cylindrical shape, and the pitch of the teeth range from 40 to 200 teeth per inch.
RIFFLER FILES are specially shaped to meet special requirements. They are, for example, used when tuning internal combustion engines to smooth the exhaust and inlet ports.
USE OF FILES
Files must never be used unless the tang is protected by a handle. The handle must be firmly attached to the tang and be a comfortable fit in the hand. Failure to fit the handle properly, or using a file without a handle, can lead to the tang being forced into the palm of the hand, causing a nasty injury. To fit the handle, the file should be held upright on a wooden bench or block of wood, tang uppermost, and the handle tapped firmly in position. The tang should extend well into the handle. The file itself must not be hammered. Files are dead hard and, in consequence, rather brittle. Any hammer blows are likely to cause chips to fly off the file, causing injury to anyone nearby and damage to the face of the hammer.
The beginner should learn the art of filing by practicing on odd pieces of scrap metal, mild steel, or cast iron, about 2 in. square in section being most suitable. A partly worn flat file should be used at first, as a new file is too sharp at this stage and in any case it is better reserved for use on brass or bronze before being taken into use on iron or steel.
The correct stance at the bench is just as important when filing as it is for sportsmen when playing golf or cricket. The handle of the file should be held comfortably in the right hand, and the left hand should grip the extremity of the blade with the fingers pointing downwards. The left foot should be placed well forward and the right foot turned slightly outwards. The body should be well balanced and one should feel comfortable. If you are left-handed, reverse the above instructions.
The file should be placed firmly on the work and moved forward with a firm, steady stroke. Every endeavor must be made to keep the movement in a horizontal plane, keeping even pressure on both ends of the file. Beginners nearly always rock the file so that the surface produced becomes curved instead of flat.
If the upper arm is considered as a lever, pivoted at one end at the shoulder joint and the other end at the elbow, it will be seen in Fig. 5.3 that if the shoulder is kept rigid, the elbow moves in an arc. Unless a positive effort is made to correct the movement of the file, it will rock, giving rise to a convex surface on the workpiece, which must be avoided.
A full stroke should be made with the file, in fact the stroke should only finish as the handle approaches the work. The file should then be drawn back, the pressure having been released. The file only cuts one way, the teeth being formed to cut on the forward stroke only.
The jaws of the vise in which the work is held should be level with the user’s elbow. This is helpful in keeping the movement of the file horizontal. The work should be held securely in the vise. If the workpiece has a finished surface, or is soft, the clamps described in Chapter 1 should be used to prevent the serrations in the jaws of the vise causing damage.
Choose the right file for the job, use as big a file as is practicable, and do not nibble at the job with a small file. Use a bastard file if a lot of metal has to be removed, then a second cut one. A smooth file may be used, as the work is brought to size, if a fine finish is required.