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Metal bars, rods, and tubes are usually cut to length with some kind of saw. Sheet metal may also be sawed to shape, and a hacksaw is used for all such general work. This saw consists of a metal frame, a handle, and a blade, the frame being either of the fixed type taking only one length of blade, or adjustable to take blades of various lengths. An example of this type of saw is shown in Fig. 1. In both cases the blade is help tightly in the frame by screwing up the wing or knurled. nut, thus pulling on the square-section blade holder shown in Fig. 1.
Hacksaw blades are usually designed to cut on the forward stroke only, and the blade must therefore be fixed in the frame with the teeth pointing away from the handle. The blades are made in lengths between 8 and 14 inches, but longer ones may be purchased for special jobs. The number of teeth per inch ranges from 14 to 32.
The blades are generally ½ inch wide and about 1/40 inch thick. The teeth are set so as to make a cut somewhat wider than the thickness of the blade and so prevent tightness of the blade in the cut. The set is obtained either by having alternate teeth bent slightly outward, or by having a "wavy-edge" where alternate sections of the blade are bent outwards. In some types of hacksaw blades, designed for use solely on soft metals, such as copper, the blade is made thinner toward the back. This avoids the necessity for having any set, in the usual sense, on this type of blade.
Types of Blade. The teeth of a hacksaw blade must be extremely hard, and consequently they cannot be sharpened or set when they become dull, as is the case with a saw made for woodworking. Some blades are hardened and tempered all over to the same degree, but the most widely used types are tempered so that the teeth are hard and the rest of the blade comparatively soft. It is particularly useful for sawing in awkward corners, when the saw is more likely to be twisted, and convenient for such jobs as cutting pipes, angle irons, thin sheets, channels, and other awkward sections.
Blades hardened all the way through last longer and cut better than the others, and are generally more suited to the skilled worker, while the soft-backed blades are much less easily broken in the hands of the unskilled worker.
Much of the difficulty encountered in cutting metal is due to improper tension of the blade in the frame. The blade should be tightened in the frame so that it does not whip sideways when cutting. But it should not be made too tight, particularly when new, since there is then a great chance of breakage if the frame is slightly twisted when sawing.
A hacksaw blade, like a file, will give longer service if used on softer metals when it is new and on harder materials after the teeth have become dulled. Also, the cut must not be started against the edge, but downward, as shown in Fig. 2.
It is important not to cut too quickly with a hacksaw, as this will cause the teeth to heat up and lose their correct temper; in addition, the teeth do not bite into the metal, but tend to slide over the surface. Between forty and fifty strokes per minute is a good sawing speed if the saw is held correctly, with the left hand gripping the front of the frame and the right hand used in the same manner as on the handle of a file. Because of the positioning of the teeth in the blade, downward pres- sure should be applied with the left hand on the forward stroke only. ht the end of the stroke this pressure should be relieved. and the saw pulled back to the starting point. The finer-toothed. blade should be used on tool steel, brass and copper tubes, and thin sheet metal where a smooth cut is essential to prevent the blade being broken. Coarser teeth are used for cutting wrought iron. For general work, a blade with 18 teeth per inch is the most satisfactory.
Notes on Cutting. If a blade should be broken when partly through a piece of metal, it is best to begin the cut from the other side rather than to try to work through the old. saw cut. The cut is slightly narrower than that which the new blade will make, and in working down the old cut the set of the teeth is likely to be impaired and the new blade damaged. For anyone doing a good deal of work with metal it is a good. idea to have two hacksaws, one with a new blade for brass, copper, etc., and, one with a slightly worn blade for steel and iron.
Backed Saw and, Piercing Saw. While hacksaws are by far the most generally used, there are two other handsaws for metal; the small backed saw and the piercing saw. The backed saw as shown in Fig. 3 is used for cutting small sections of brass and copper tubes, and is more useful for art metalwork and electrical work than for average home cutting jobs.
The piercing saw, shown in Fig. 4, is used for cutting internal holes, slots, and shapes. It consists of a frame with jaws and a handle; the lower jaw is adjustable so that the saw blades of various lengths may be used. The blade should be set in the frame with the teeth pointing toward the handle
so that the cut is made on the downward stroke. The method of holding the saw is shown in Fig. 4. Anyone interested in doing decorative work in thin metals will find a piercing saw a most valuable tool. To cut a slot or any other enclosed shape, it is first necessary to drill a small hole. The blade is then inserted, and fastened to the saw frame, and the hole is cut to the desired shape.
Care must be taken to keep the saw running true, otherwise the blade is very easily broken. The saw must not be forced, and slow, easy strokes are necessary. While metal saws work very well on small cutting jobs, sawing through a large section of sheet metal is tedious work. For large cutting jobs the home mechanic would do well to take the work, when possible, to a machine shop and let them cut the metal on a shearing machine, as shown in Fig. 5. This machine consists of a fixed blade and a cropper, operated by a long handle so as to obtain the advantage of a long lever. This machine will cut sheet metal of fairly heavy gauge in a fraction of the time it would take with a hacksaw.