This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
When a saw is not round, the defect may be corrected by adopting the following directions: Take a piece of grindstone or a cobblestone and hold it against the points of the teeth while the saw is revolving, and thus reduce or wear down the most prominent teeth; or a piece of red chalk may be held against the points, which will mark them in proportion as they are long or short, when the long teeth are reduced by filing. Circular saws sometimes burst from what appear as unknown causes. There can be no doubt when a saw does fly in pieces that a thorough investigation would trace the occurrence to one of the following causes: (1) Square corners at bottom of tooth; (2) Out of round, with the backs higher than the points, so that instead of cutting, they scrape the dust off with the back; (3) Undue strain put upon the saw by the plate rubbing against the timber, causing it to heat, which takes the life out of a saw. In a recent report of the French Society for Preventing Accidents from Machines, a recommendation is made for the avoidance of the use of circular saws in workshops where practicable.
The following are the reasons for this recommendation: (1) Circular saws are dangerous to workmen; (2) they require more power than other saws; (3) they cut a broader line, and are consequently more wasteful. The speed of circular saws varies with the size, approximately as follows: - 8 in. diam., 4500 rev. per minute; 12 in., 3000; 16 in., 2200; 20 in.. 1800. The speed for cross-cutting can be increased with advantage 1000 ft. beyond those used for ripping, say to 10,000 ft. per minute. Never cut stuff that measures more than 1/3 the diameter of the saw. The manner in which a circular saw is hammered has much to do with the speed at which it can be run, and often when a saw becomes limber and "runs," it is the fault of the hammering instead of the speed. When slack on the periphery, it will not stand speed, and becomes weaker and bends more readily when in motion than when it is still; on the contrary, if it is properly hammered, a little tight, as it is termed, on the periphery, it becomes more rigid when in motion up to a certain limit. The theory of this is that the steel is elastic, and is stretched by the centrifugal strain in proportion to the speed, which is greatest on the line of teeth, and diminishes to the centre.
If saws evince a tendency to spring and a want of rigidity, have them renammered at once, before changing the speed in an endeavour to remedy the defect.
(8) The band-saw is never used for cross-cutting, except when cutting scroll-work, and may generally be treated as a rip-saw. It requires special regularity in shape and set of teeth to prevent it from breaking and from running into the work. In order to set it up, or join the 2 ends together, the 2 tongues are introduced simultaneously into the 2 corresponding openings, and the ends of the saw are pressed together laterally in such a manner as to cause the snugs on the tongues to engage with or hook on to the bevelled edges in the openings, and the thin ends of the tongues then lie in the inclined recesses in the sides of the saw. When the parts are in this position, the 2 extremities of the saw cannot be separated either by a considerable strain in the direction of its length or by a diminution of the tension. To disconnect the ends of the saw, separate the hooked and bevelled edges by applying lateral pressure, and at the same time draw the ends apart in opposite directions. The junction of the 2 extremities is effected by means of a hook or interlocking joint. A portion of the saw near each extremity is reduced in thickness in such a manner that, when the ends are laid together, the two combined do not exceed the thickness of the remaining part of the saw.
Portions of the back and front of the extreme ends are also cut away, so as to leave narrow tongues at each extremity of the saw, and these tongues are provided on opposite sides relatively to each other with snugs or hooks. In the thin portions at the extremities of the saw there are formed, at equal distances from the tongues, 2 longitudinal slits or openings, presenting bevelled or inclined surfaces at the edges nearest the ends of the saw, corresponding exactly to the snugs on the tongues. The opposite edge of each opening is also bevelled or inclined, but at a much more acute angle, so as to form a recess in the side of the saw for the reception of the extreme end of the corresponding tongue, which is suitably reduced in thickness towards the extremity, in order to enable it to be well within the said recess. Where gas is used for lighting purposes, it is often employed for brazing band-saws, and nearly in every case where this is done, the blade of the saw operated upon deteriorates, and breakages gradually increase. As these breakages do not occur exactly at the joint, no blame is attached to the use of gas, and the cause of continual failures is rarely discovered.
A gas flame not only scales steel deeply, but also destroys its nature by burning the carbon out, and this occurs especially at the edge of the flame. Band-saws brazed by gas almost invariably break again at a point some little distance from the previous fracture, at the point where the outer edge of the flame has damaged the metal. The only really satisfactory way of repairing is to make a thick, heavy pair of tongs bright red-hot, and clamp the joint with them. The heat melts the spelter instantly, and makes a good joint without scaling or damaging the steel.
For a joint which has to stand constant heavy strains and bending, it is better to use an alloy of equal parts of coin-silver and copper, melted together and rolled out thin. This alloy never burns, cannot be overheated, and makes first-rate joints, which will stand hammering and bending to almost any extent. The working action of a band-saw is, generally speaking, similar to the working action of a circular saw, - continuous. " Owing chiefly to the thinness of the gauge, the small area of the blade which operates on the wood at one time, and the constant cooling action which is going on, as the saw passes through the air, a comparatively small amount of heat is engendered; the saw therefore can be run at a considerable speed without detriment. On machines in which the saw-wheels are of small diameter, say below 36 in., and where the are of contact of the saw on the wheels is necessarily more acute, the speed of the saw-blade should not much exceed 4500 ft. per minute for all ordinary kinds of sawing.
With saw-wheels above 3G in. diameter, this speed may safely be increased up to G000 ft. per minute; this is, however, on the supposition that the top wheel is of the lightest construction, and is mounted clastically, i.e. has a spring or other adjustment to allow for the expansion and contraction of the saw-blade. There is no advantage in running band-saws beyond 6000 ft. per minute, as the risk of breakage is increased without affording any corresponding gain. In sawing hard woods, the speed should be reduced. The band-saw may be said to have a blade of superior thinness, capable of tension in varying degrees, moving in right lines through the material at a speed that is almost unlimited and can exceed that of circular saws, operating by machinery consisting only of rotating parts and of the most simple construction, the sawdust all carried down through the timber and offering no obstruction in following lines and peculiar adaptation to curved lines.