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.
Many appliances for bending and spreading teeth are described in Grimshaw's large work on ' Saws.' The crotch-punch of ordinary form is shown in Fig. 315. It is made of steel and case-hardened in the fork, where it comes into contact with the points of the saw teeth. There is much difficulty in making crotch-punches of a satisfactory character, as the tempering has to be extremely hard just for the jaws, while if it runs back too far they have a tendency to split. They should be fitted with a side guard to prevent the operator's hand being injured by the punch slipping off a tooth. This guide may be made to serve also as a means of keeping the punch central or of giving it an inclination to either side. Crotch-punches have been introduced which are claimed to act on the teeth behind the cutting edge as well as at the edge, spreading the teeth without reducing their length and consequently the diameter of the saw (circular). Fig. 316 is a diagram of the end of the punch with part of the covering sleeve removed to show the form.
If a tooth is struck with the convex-sided lower angle, the resulting tooth is as shown at a; a second blow with the upper angle produces the flattened and double set tooth b.
Disston's revolving saw set is shown in Fig. 317. Its price is 1s. 6d. or 2s., according to size. Among the advantages claimed for this useful little tool are the following:__ it is portable, simple, effectual, and cheap; it can be readily adjusted to any size tooth, from a 14-point back-saw to a 4-point rip-saw. The tooth in front of the one being set forms a guide for the tool, and the operator can readily and with certainty slide the set from tooth to tooth. The different bevels on the disc are in accord with the different slots for the various-sized teeth. The screws on each side determine the amount of set. The implement is sold by Churchills.
Trickett's lever saw set, sold by Melhuish, at 3s. 6d., is represented in Fig. 318.
For use, place the set on the saw as indicated, holding it in the right hand; place the punch in line to tooth requiring to be set, then grasp the lever and handle together; the punch in lever forces the saw tooth over on the bevelled head of the bolt, and the tooth is set.
Morrill's saw sets for hand, band, scroll, cross-cut, circular, and mill saws, are sold by Churchills at prices ranging from 3s. 8d. to 16s. Fig. 319 illustrates the application of the implement. Hold the saw on any level place, teeth upwards. Place the set on the saw as shown. The anvil b is movable up and down, and must be regulated to suit the distance that the operator desires to set his saw teeth down from their points. Care must be taken not to have the angle or the point where the bend is made below the base of the tooth. The nut or screw a fastens the anvil in any desired position. The guard e, when moved forward, increases the amount of set to be given; when moved back, decreases it. The guard is made fast by the screw d. The set is operated by compressing the handles /, which carries the plunger g forward, and takes effect on the tooth of the saw c, as shown. Great care should be taken against setting saws too wide, as, with too much latitude, they will chatter and tear rather than cut, at a great cost of power and waste of lumber.
All saws should be set or pressed into line 3 times to 1 filing, as by constant use the teeth wear off on the outside at their points, causing them to heat and spring out of true, thus spoiling the saws, burning the wood, consuming power, and retarding the work, besides rendering it dangerous to the operator.
A spring set with a slightly shearing tooth performs its cutting in the easiest manner, but as only the corners of the teeth operate, twice as many teeth are required to do the same amount of work in a spring set saw as in a fully swaged one; the latter is generally preferred as being more easily kept in order. In bent setting, care must be taken that it is only the tooth and not the plate of the saw that is operated upon, or there is a risk of distorting or cracking the blade.
Gumming consists in deepening the throat or gullet of a saw, and is effected by means of punches, or preferably by rotating steel cutters or emery wheels. Too often the gumming is neglected, more of the face of the tooth being filed away instead, thus reducing the diameter of the saws and causing waste. Grimshaw illustrates several efficient machines for gumming.
According to Duncan Paret, the simplest method by which solid emery wheels can be applied for saw gumming is by placing them on the spindle of the circular saw. The saw to be gummed can then be laid on the saw table, or supported in any convenient way. A simple way is to pass the end of a rope with a small cross stick on it through the eye of the saw, and thus suspend the saw so that it swings evenly balanced just in front of the emery wheel. The weight being thus carried, the operator only has to use his hands to guide the saw against the wheel. Where expensive machinery is scanty, and where people are slow to introduce the latest improvements, there is a steady demand for saw-gumming wheels 14-24 in. in diameter. Where the latest improvements are quickly added, regardless of price, nearly all the emery wheels used for saw gumming are 12-8 in., none of the machines specially designed for saw gumming being intended to carry anything above a 12-in. wheel. Emery wheels are unfavourably contrasted with grindstones as causing a heating of the saw, but this can be obviated by using the wheel under a small constant stream of water.
One advantage of a rotating steel-cutter gummer over an emery wheel is that, whereas an inexperienced hand can ruin a saw by case-hardening with an emery wheel, this cannot be done with a steel cutter or "burr gummer." Most of the emery gummers for circulars require that the saw shall be taken off its arbor to be gummed; all burr gummers work with the saw in position. (Grimshaw.)