This article first appeared in Practical Engineering 1940 Vol1 No22. The information that appears in this article is accurate as of 1940. This article provides the enthusiast with notes on techniques employed in Engineering and Workshops at the time.
Although filing is one of the most important hand processes carried out in the workshop, it is often only half understood. If the very best use is to be made of files it is desirable that a clear understanding should be obtained of the various patterns and their purpose. Some idea of the principles of manufacture also help to ensure more effective use.
The types of file most widely used are: hand, flat, half-round, square, triangular or three-square, round. These, it will be seen, are described according to their shape; other types are named according to their use. For example, a saw file is a particular kind of triangular file used for sharpening saws, ward or warding files are used for filing the wards of locks and for other similarly small work, and riffling files are curved and intended for riffling or filing the insides of curved pipes and the like. Another type which is widely used by fitters is the swiss file. This is similar in many respects to a fairly small hand file, but the teeth are extremely fine. It is useful when filing bearing caps, for instance, when a very small amount of metal must be removed and when the surface should be left reasonably smooth.
Round files may be parallel or tapered; in the latter case they are generally described as rat-tail files, for obvious reasons. Some of the very short round files are drawn out to a sharp point, when they are called needle files. Similarly, there are a number of slightly modified names which describe files of more or less standard shapes, but which are intended for special jobs.
Hand and flat files are best known and are made with similar cuts. Whereas a hand file has parallel edges, the flat file is parallel for about half its length, after which the edges are slightly curved and tapered toward the tip. These files, as well as triangular and round files, can be obtained with up to six different “cuts"—the "cut" refers to the number of teeth or cuts per inch of length.
The six cuts referred to are: rough, 22 teeth to the inch; middle, 26 teeth to the inch; bastard, 32 teeth to the inch; second cut, 44 teeth to the inch; smooth, 68 teeth to the inch; and dead smooth, 120 teeth to the inch. The number of teeth is not absolutely standardised, but the figures given are good averages and can generally be applied.
Hand and flat files are usually cut on two faces and one edge, the other edge being left smooth. The last-mentioned is described as a safe edge, since it does not cut when filing the inside of a rectangular hole. There are two sets of cuts made across the face. One of these, known as the over-cut, is at an angle of 55 degrees to the centre line of the file, while the second Course, or up-cut, is made afterwards and is at an angle of approximately 80' degrees to the centre line.
In manufacture, the blank of crucible cast steel is forged, annealed and ground smooth, and then the cuts are made, the angle of the cutting tool to the face of the metal being about 10 degrees for the coarser files, and only about three degrees for dead smooth. After cutting, the file is generally curved, so it is heated to a dull red and straightened; to protect the teeth during this process it is customary to dip the file in a sticky substance and then to sprinkle it with hoof parings and common salt.
When it has been straightened, hardening is necessary, and this is done by heating to cherry red and immersing in water until the temperature has dropped and then immersing in oil until quite cold. The oil gives some protection against rusting as well as being part of the tempering process. The tang must, of course, be soft to prevent breakage while the file is in use, so this is heated by dipping in molten lead and allowing it to cool. Better-class files are finally finished by sand-blasting, which improves the appearance and sharpens the teeth.
Half-round files are double cut on the flat face, but a series of overlapping single cuts are given to the curved surface. The same treatment is given to round files, whilst square files are either double cut on all faces or double cut on two and single cut on the others. Triangular files are double cut, but in the case of saw files another light cut is given to the corners to round them over slightly. There is a type of hand file with only one set of cuts. This is known as a float, and is used for dealing with hard steel.
Most files are made in lengths from about 6in. to 16in., but swiss files are generally up to l0in. maximum, and ward and similar files are made in sizes up to about 6in. only. The length is measured from the shoulder to the tip.
To ensure the longest possible life, files should be used on different metals in a certain sequence. For example, when new it is best to use them for zinc; then they can be used on copper, aluminium, brass, iron and steel in that order. A file which has been used for steel cannot normally be used satisfactorily on copper because of the blunting of the teeth.
Especially when used for the softer metals, file teeth become choked or "pinned," due to filings becoming packed between the cuts. If filing is continued after this has happened the surface of the metal will be scratched, whilst cutting will be inefficient. That is why a wire brush or scratch brush should always be kept handy on the bench. Any filings not removed by this should be picked out with a scribe. With a new file, and before using a file for soft metals, it is an excellent plan to rub a piece of chalk over the faces; this helps to prevent "pinning."
When files have been neglected, with the result that the cuts are badly pinned and dirty, it is sometimes possible to extend their life by boiling them for ten minutes in a strong caustic soda solution. After that they should be well cleaned with a scratch brush, rinsed in paraffin and dried. Worn files can be sharpened —to a very limited extent—by immersing them for an hour or so in dilute hydrochloric acid.
There are two principal methods of filing: cross or ordinary filing and draw-filing. The purpose of the latter is to form a smoother surface or, at least, one on which there are fewer scratches. Cross-filing is employed to remove the bulk of the excess metal and also when shaping arid fitting.
Throughout almost every filing operation the most important point is that the file should be kept perfectly horizontal. Even when filing an edge or surface which will not be horizontal later it is better to tilt it in the vice.
Most filing is carried out with the work gripped in a vice, and for convenient working it is essential that the vice should be at the correct height. This is determined by the very simple method of standing with the right arm straight down the side. If the elbow is then bent, the tip of the elbow should be exactly in line with the top of the vice jaws. When the vice is too high it is an easy matter to arrange a couple of footboards to raise the body. Should the vice be too low the only method of working comfortably is by placing the feet well apart.
Standing with the left foot slightly forward, the file handle should be held with the right hand and the tip with the left. With regard to the right hand, it is best to place the end of the handle into the palm, to lay the thumb on top of the handle and point the first finger down the side. The method of holding the file when a fair amount of metal is to be removed is best done by ensuring that the fingers of the left hand are bent round the end of the file to grip it. When making lighter cuts on a more delicate job it is generally found better to hold the tip lightly between the finger and thumb.
If a fairly wide surface is to be smoothed down, ensure that the fingers and thumb of the left hand are stretched out and pressure applied through the fingertips and the tip of the thumb. The advantage of this method is that any "rock" of the file can be more easily detected and rectified. Throughout all filing operations it is best to keep the body steady and to allow the right arm to pivot about the shoulder; this is to ensure that the file is kept in a horizontal plane. Downward pressure should be applied on the forward stroke only, but it is not good practice to lift the file off the work on the backward stroke; simply remove the pressure and draw the fib toward the body.
When dealing with a long edge or surface it is often found easier to keep the face true by holding the file at an angle of about 45 degrees to the edge of the work and moving the file forward and sideways at the same time. The direction of the file should be reversed after every few strokes.
Before passing the work, make a careful test with a straight edge. When the surface has been smoothed down in this manner it will be found that the extreme edges have been slightly burred over to form a sharp projecting line. This wire edge can be removed by using the tip of the file as a scraper and, with the face at 45 degrees to the metal and the safe edge down on the vice jaw, running the file forward about twice on each side. Do not, however, remove so much metal that there is any risk of rounding-over the corners.