The"hack saw"illustrated in Fig. 1, No. 3, which should be held as illustrated in Ch. XVI, f. 1 7, is used only on soft metals, and the angle to which the teeth should be filed is shown in Fig. 1, No. 3, but the best method is to sharpen the teeth on an automatic grinding machine which not only grinds the teeth to the correct slope but spaces them as well.
Metalworkers do not always rely on"setof teeth to keep a clear kerf and so prevent the saw from binding, as the saws with thin backs keep their edge better, answer the same purpose, and are more common.
A most efficient saw for cold iron or steel as well as for other metals is shown on the same page (Fig. 1, No. 4). This, a Bradenberg patent, has a very long life; the peculiar"set"should be noticed. Some saws are made with teeth that are"set"and hardened and the backs left soft.
Metals are now sawn either hot or cold, a circular saw running partly immersed in a bath of cold water for hot metal, in a bath of oil for cold metal. Names of the parts of a saw are as follows:Spaceis the distance from tooth to tooth measured at the points;gulletorthroatis the depth of the tooth from point to the root "gauge"is the thickness of the saw generally measured by the wire gauge;setthe amount of clearance given to the saw teeth in either direction to prevent the saw binding and to clear away the chips.
A file is one of the principal tools of a metalworker and the most difficult to master. It consists of a steel blade or body of very variable form and size with a "tang"for fixing into a handle. Teeth of suitable form and size are cut on the blade, and the latter is hardened and tempered. Files are classified and described according to the form, use, and nature of their teeth. Most of the terms used to describe the form require no explanation, e.g. parallel, half round, round (a taper round is called a rat tail), triangular; a knife file has a section like the section of a knife blade; a warding file is a very thin flat file used for filing the wards in keys. Pillar and cotter files are narrow files of rectangular section. A riffler is a bent file used for filing concave surfaces. Feather-edge files have a rhombic section with two very acute and two very obtuse angles. The teeth or cut of a file are spoken of as rough, middle, bastard, second cut, smooth, and dead smooth, according to the number of teeth per inch, which ranges from about 14 to 100 or more. The teeth of an ordinary file form two sets crossing each other at an angle; this arrangement constitutes the double or cross cut. A file with a single set of teeth is termed a float. A safe-edged file is one which has one size or edge without teeth. Rasps may be grouped with files; though their use is chiefly confined to the woodworking trades, their form and process of manufacture are similar to those of files; but the teeth consist of "burrs"thrown up with a diamond-pointed chisel instead of the straight-edged chisel which is used in making the teeth of a file.
The action of filing is similar to that of sawing, consequently it is a scraping action, and if the teeth of the saws in Fig. 1, Nos. 3 and 4, are compared with the enlarged section of the file tooth, Fig. 1, No. 5, on the same page, the similarity is at once apparent. It should be noticed that in all files the front part of the tooth slopes backwards or has what is termed"negative rake". The width of the tooth at the bottom and the height of it have a great deal to do with the life and efficiency of a file, for if the tooth is high and narrow it is liable to chip or wear away more rapidly that if the tooth was of medium height and had a wide base, which gives solidity and length of life owing to having plenty of support under the cutting edge. The angle of the cut in relation to the axis of the file is also important, as by means of this we get a slicing cut which causes the metal to come away in curled chips and with comparative ease.
All files have a small amount of curvature in their length. If a flat file is examined it will be seen to be thickest in the middle, and this allows a nearly flat surface to be obtained by filing, as any high part of the surface can be filed down in that particular place by slightly raising the handle of the file and pressing with the left hand on the front portion of the blade.
A scraper is usually a triangular file ground at the end on all the faces so giving three keen edges which are finished up on the oil stone; of course there are many other forms, but this is very effective. This acts in the same way as a file, only it is like one tooth being used instead of a number, and it has the advantage of being able to be held so that it will cut quickly or slowly according to the angle of the face presented to the work. Another most important point is the height the work should be so as to obtain the best results, and as the work is usually held in a vice this governs the work. The right height for the tops of the jaws of a vice is, when the worker is standing upright with his elbow close to his side and his hand touching his shoulder the tops of the jaws should just touch the point of his elbow.
The most suitable files for use in a general way are 4 in., 6 in., 8 in., 10 in., 12 in.; half round bastards and smooths; safe edge bastards and smooths. A few rat-tail files, and some safe-edge squares might be added as the necessity arose. Files used for iron and steel should be kept separate from those used on soft metals; and when new files that have been used on soft metals will not cut, pass them on to be used on iron and steel which they will cut admirably. New files used on iron and steel get the teeth chipped, and wear out rapidly.
When a file gets clogged it should be brushed with the file cleaner (Ch. XVI, f. 16); if some pieces of metal get jammed in between the teeth of the file and cause scratches on the work being filed, they can be removed by pushing them out with the corner of a flat piece of brass or iron; these small pieces of metal are called"pins". To avoid these when filing iron or steel chalk the file or put a spot or two of oil on it. When filing aluminium use paraffin or turpentine. The proper method of standing and holding a file is shown in Fig. 2.