Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 88

Many workmen think that so long as the tooth of a saw has a sharp edge the shape of the tooth is a matter of small importance, and as a result of this ignorance or indifference they are always in trouble with their saws, and their work becomes much more laborious than if proper attention had been paid to the shape of the teeth. Substances of different texture cannot be cut advantageously with the same tool; in fact, the tool must be adapted to the work if the best all-round results are to be produced. Fig. 253 illustrates a form of tooth suitable for an ordinary hand rip saw. The tooth points number about four to the inch, and the front of the tooth is upright, that is, at an angle of 90°. The face of the tooth should be filed to an angle of 85° to 87°, or 3° to 5° from the square. Some experts contend that the teeth of a rip saw should be filed dead square. The object in filing them a little on the bevel is that the teeth may cut more freely and easily when they have become a bit dull, there being then what is termed a little clearance cut in the teeth. In nearly all timber there is, it is well known, a certain amount of fibre to be cut either directly or obliquely across, and teeth that are filed square will not, whether they are sharp or dull, divide this fibre so easily as teeth that have a slight bevel. Fig. 254 shows a suitable tooth for a hand-saw used for cross-cutting soft wood. The tooth points in this saw may number five or six to the inch. The front of the tooth slopes at an angle of about 105°. The face of the tooth in sharpening should be filed to an angle of from 55° to 60°. The softer the timber that is to be cross-cut, the more acute should be the angle of the teeth, as the keener edge separates the fibres more easily. Fig. 255 shows a form of hand-saw tooth suitable for cross-cutting hard wood. The number of tooth points may be from six to eight to the inch, and the front of the tooth should slope at an angle of 110° to 115°, according to the hardness of the timber to be sawn. The face of this tooth should be the inch, and the front of the tooth should slope at an angle of 110° to 115°, according to the hardness of the timber to be sawn. The face of this tooth should be filed to an angle of 70° to 75°, because the cutting edge must be less acute owing to hard-wood fibres being more compact than those of soft wood. Another form of tooth in a one-way hard-wood cross-cut saw is shown by Fig. 256. If this form of tooth is properly sharpened, it will be found to cut much faster in hard wood than a tooth that cuts both ways. The front of this tooth, as illustrated, slopes at an angle of about 105°. If the wood to be cross-cut is dry and very hard, the teeth should slope at an angle of 110°, or even more. The tooth points of this saw should be about 5/8 in. from point to point, and the face of the tooth should be filed or ground, as the case may be, to an angle of about 70°. The teeth should be parallel with the back edge of the saw, which, in working, should be pulled and pushed perfectly straight. There is not much difference between Fig. 256 and Fig. 257, but the dotted lines in the latter figure show that the angle of the tooth is shaped correctly. By “hook” is meant the lead or rake the fronts of teeth have—that is, the distance the point of tooth overhangs the bottom of front of tooth. It will be noticed that in Fig. 258 the teeth are filed a little hollow in front- and at the back. This is a great mistake, and several reasons could be assigned. One reason is that a correct angle cannot be determined (see dotted lines in Fig. 258). In shaping the teeth of a tenon saw it is wrong to make their cutting faces upright as shown in Fig. 253. This causes the saw to bind in the wood at the forward stroke. The front edges of the teeth being vertical, they bite too much. For a tenon saw to act freely, the front edges of the teeth should make angles of about 80°. There are several peculiar forms of American teeth, though they are no great improvement on the ordinary saw teeth. Figs. 259 and 260 represent American cross-cut saw teeth. Fig. 259 is termed the regulating tooth. By filing the centre tooth to an acute angle, it will be suitable for cutting soft wood; for cutting hard wood the teeth should be filed to a more obtuse angle. Fig. 260 is termed the lightened tooth. The sharpening should be much the same as Fig. 259 for different kinds of wood. A tooth that is least difficult to sharpen when filed to its proper cutting angle is found to be the best form of tooth. For general cross-cutting the peg tooth (Fig. 261) is recommended. This is suitable for a soft wood cross-cut saw with two handles. The faces of this tooth, which cuts both ways, should be filed to an angle of 50°, the space between being 130°. When this form of tooth is used in cutting hard wood it should be filed to a more obtuse angle. If a saw is to cross-cut hard wood only, the gullet or one-way cross-cut tooth (Fig. 256) is recommended. This tooth will be found easy to sharpen, and, if properly sharpened, will cut as fast as any tooth, and faster than most. The more upright the front of teeth (Fig. 257) the faster the cut, but the more difficult to work. The more they incline backward the more easily they will work, but the more slowly they cut. M-shaped teeth (Fig. 262) are now largely used for crosscutting, chiefly in America, and as they have cutting edges both back and front they will cut in both directions. Usually, they are bevelled and set alternately, and they cut rapidly if kept in good order, The teeth of Disston’s American saws (Fig. 263) have less set than those in most English saws. Figs. 264 and 265 represent pit saw teeth. The form of tooth shown by Fig. 264 may answer fairly well for soft wood of ordinary thickness, but for thick timber, and especially the harder kinds, such teeth will cause jarring and bad work. Teeth as in Fig. 265 are the more suitable form. Of course, with all saws, the harder the timber to be sawn the more obtuse the angles of the saw teeth. The teeth in a saw cutting hard wood should be about 5/8 in. apart, and for soft wood, 3/4 in. For cross-cutting logs with a two-handled saw, a saw with teeth similar to Fig. 266, instead of ordinary cross-cut teeth, can be recommended, as it is much quicker and cleaner in cutting. These saws were invented and are largely used in America. The cutting is done by the single teeth, the double ones acting as clearers, and being filed about 1/32 in. shorter than the scoring teeth. The teeth are often formed on both edges of the blade, and when one set are worn down, they are knocked off, and the other edge of the blade used. The saw is made slightly curved; the handles are reversible. Fourteen gauge steel is used. This form of saw is rapid and easy to work, and does not bind in the cut; at the same time comparatively little set is required. It will cut well both hard and soft wood.

Fig. 254.—Cross-cut Saw Teeth.

Fig. 255.—Cross-cut Saw Teeth for Hard Wood.

Fig. 256.—American Cross-cut Saw Teeth.

Fig. 257.—American Cross-cut Saw Teeth.

Fig. 258.—Saw Teeth filed hollow: Incorrect.

Fig. 259.—American Regulating Saw Teech.

Fig. 260.—American Lightened Teeeth.

Fig. 261.—American Peg Teeth.

Fig. 262.—M-shaped Saw Teeth.

Fig. 263.—Disston Saw Teeth.

Fig. 264.—Teeth of Pit Saw.

Fig. 265.—Teeth of Pit Saw.

Fig. 266.—Teeth of Two-handled Cross-cut Saw.