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.
These subjects have been so ably discussed in such works as Grimshaw on 'Saw Filing'; Holly on the 'Art of Saw Filing'; and Hodgson on ' Hand Saws,' that it is difficult to attack them without in some measure traversing the same ground.
A saw tooth consists of 4 parts - face, point, back, and gullet or throat. Teeth vary in spacing, length, angle, rake, set, fleam, and form of gullet. A saw blade may contain several kinds of teeth in succession; but all teeth of a kind must be either quite uniform or arranged in a regular order of change.
The ordinary spacing of saw teeth is as follows : Hand-saws, 5-12 points in an inch; rip, 3-5 at the heel and 6-8 at the point; panel, 8-12; tenon, 11-15; mitre, 10-11; bandsaw teeth should have a tooth space equal to 1/2 the width of the blade for soft wood, and 1/3 for hard, while the depth of the tooth in each case should be 1/5 the width of the blade.
The length of tooth is governed by the hardness of the wood, the longest teeth being best adapted for wet, fibrous, and soft woods, as giving greater clearance; but more care is needed in having a moderate and regular set.
The angle of saw teeth may vary between about 60° and 40°. The fundamental angle is 60°. This may be in the form of an equilateral triangle for hard and knotty wood, but for soft wood it is better that all the pitch should be on the cutting face, - an upright edge with sloping back. For varied work the usual angle is 40°, the pitch being equally divided. Teeth of any angle but 60° are not so readily filed with an ordinary file.
The degree of rake may increase in proportion to the softness of the wood; in hard woods it causes a tendency to spring in. It may also be greater in a circular saw on account of its greater speed. Fig. 304 (from Grimshaw) shows various degrees of rake, the arrows indicating the direction of the strain.
The set of a tooth may be either "spring" (bent) or "swaged" (spread). The former cut only on one side, have more tendency to spring in, and are more subject to side strains : the latter cut on both sides, unless they are sheared, and they are less liable to spring in and suffer from side strains. The more gummy the wood, the greater set is needed. Circular saws require more set than straight ones.
The fleam or side angle of the teeth varies from 80° or 90° horizontally for hard woods, to C0° or 70° horizontally and 30° or 35° vertically for soft. It is most effective in the case of soft woods free from knots; and should not accompany a bent set, as both tend to aggravate the tendency to spring in.
The gullet or throat should always be rounding and never square, as the latter condition gives a tendency to crack. Fig. 305 (modified from Grimshaw) shows when the gullet requires deepening, by a process known as " gumming." The tooth a is in perfect order; b is still capable of doing good work; but c demands gumming. The higher the speed and the faster the feed, the greater the necessity for rounding the gullet, especially in band-saws. Spaulding's rule for finding the amount of gullet in sq. in. per tooth for circular saws is to double the number of cub. in. of wood removed at one revolution, and divide by the number of teeth. Insufficient gullet causes choking, heating, and uneven running.
The depth, fleam, hook, and rake of teeth may increase in direct proportion to the softness of the wood; the spacing and depth of gullet should be augmented for fibrous and porous wood; thin blade and slight set are desirable for costly wood; a thick blade is demanded for hard wood.
The operations entailed in keeping a saw in working order are threefold - filing, setting, and gumming. These will be described in succession.
First of filing. It is a great deal easier to keep a saw sharp by frequent light file-touches, than to let it get so dull as to need a long-continued filing down, after it gets so dulled as to refuse to work. The saving in power, by using a sharp saw, is very great. Thinner blades may be used than where the teeth are dull; because the duller the saw, the more power required to drive it through the wood, and the more strain on each tooth separately, and on the blade as a whole. For the same reason, longer teeth may be used where they are sharp, than where they are dull. The advantage of using sharp teeth is greatest in those saws in which the strain of cutting tends to deform the blade - as in all "push-cut" straight saws and in circulars. (Grimshaw.)
The saw, secured in a proper clamp, should be placed where a strong light will fall on the teeth, so that the filer can have the full advantage of all the light he requires. Should there be a deficiency of light, the filer should provide a good lamp, and place a dark shade between the light and his eyes, so that he can see at a glance when every tooth is filed to a complete point. One careless thrust of the file, when a tooth is filed enough, will do a saw more harm than can be repaired by 1/2 hour's filing. A beginner should always take a try-square and the sharp point of a small file, and make a hair-mark from the point of every tooth at a right angle with the teeth on the sides of the blade. This should be done when the points of the teeth are all at a uniform distance apart. Such marks will enable the filer to keep the face of every tooth dressed at the most desirable angle. These directions, however, are only applicable to saws intended for cross-cutting. Beginners must always exercise unusual care when filing the back of each tooth that has been finished.
After the teeth are filed to complete points, it is an excellent practice to go over them carefully with a half worn-out file, for the purpose of bringing the points to a more perfect cutting edge. (Hodgson.)