Band Sawing Machines

The simplest form of band sawing machine is that in which the work is fed to the saw by hand, a machine of this class, constructed by J. A. Fay & Co., being shown in Fig. 3125. It consists of a standard or frame a, carrying the saw-driving wheel b, and the upper wheel c, the saw being strained upon these two wheels. The lower wheel runs in fixed bearings, while the bearing of the upper wheel is carried in a slide provided in the frame, being operated in the slide by a screw, whose hand wheel is shown at e, so that it may be suited for different lengths of saws.

The bearing of the upper wheel is so arranged that the tension placed on the saw may be governed by a weighted lever f, which enables the upper bearing to lower slightly, so that if a chip should fall between the saw and the lower wheel, it may not overstrain, and therefore break the saw.

At j, is a bar carrying a guide g, which sustains the saw against the pressure of the cut, a similar guide being placed below the table t, at g′. This latter guide is fixed in position, whereas the upper one, g, is adjustable for height from the work table, so that it may be set close to the top of the work, let the height of the latter be what it may. g′′ is a guide and shield for the saw at the back of the machine, and h is a shield to prevent accident to the workman, in case the saw should break.

Band saws are ribbons of steel, brazed together at their ends and having their teeth provided on one edge. The widths of band saws vary from ⁄16 inch to 8 inches, and their thicknesses from gauge 18 to 22 gauge, according to width.

The advantage of the band saw lies in that it may be run at high velocity, may be made thin, and its cutting action is continuous.

As a band saw is weak, it is desirable to have the teeth as short as possible and leave enough room for the sawdust, so that it shall not pack in the teeth.

In a circular saw, the centrifugal force acts to throw the sawdust out, while in a frame saw, the backward motion of the saw acts to clear the teeth of the dust, whereas in a band saw the dust is apt to pack in the teeth while they are passing through the work. The remedy is to space the teeth widely, thus giving room for the dust without having a deep tooth, an ordinary form of tooth being shown in Fig. 3126.

A stronger form of tooth is shown in Fig. 3127, the tooth gullets being well rounded out, and the teeth shallow at the back, while having ample room in front for the dust.

In determining the shapes of the teeth of band saws, we have the following considerations:

One of the principal objects is to have the back edge of the saw bear as little as possible upon the saw guide, and as the feed tends to force that edge against the guide, we must so shape the teeth as to relieve the back guide as much as the circumstances will permit. This may be done by giving to the front faces of the teeth as much rake as the nature of the work will permit. Thus, in Fig. 3128, it will be seen that from the front rake, or hook of the teeth, as it is commonly called, there is a tendency for the cut to pull the saw forward, this tendency being caused by the pressure, on the teeth in the direction of the arrows, and obviously acting to prevent the saw from being forced against the back guide.

For sawing soft woods, such as pine, the teeth may be given a maximum of front rake or hook, whereas for hard woods, the front faces must be made to stand at very nearly a right angle to the length of the blade, and the feed must therefore be lighter, in order to relieve the back edge of the saw from excessive contact with the back guide, which would not only rapidly wear the guide, but acts to crystallize the edge of the saw and cause it to break.

The set of the teeth of band saws is given in two ways, i. e. by spring set, which consists of bending each alternate tooth sideways, as in Fig. 3129, or by swage set (upsetting or spreading the points of all the teeth), a plan that may be followed with advantage for all saws thicker than about 20 gauge.

Spring set is given either by bending, or by hammer blows, and swage set either by blows or by compression. In spring set, each tooth cuts on one side, and there is consequently a pressure tending to bend the tooth sideways, and break it at the root, whereas in spread set, the tooth cuts on both sides equally. As the front faces of band saw teeth are filed straight across, as in Fig. 3129, and are not given any fleam for any kind of woodwork, the set, whether spring or a spread, should be equal in amount for every tooth, and the pitch and depth of the teeth should be exactly alike, so that no one tooth will take more than its proper share of the cut.

The bend or set of the tooth in spring set saws, should not extend more than half way down the depth of the tooth, which will make the set more uniform and save tooth breakage, it being borne in mind, that a tooth hard enough to break if the set extends down to the root, will set easily if it extends half way down only, and that a saw may be soft enough to file, and of a proper temper, and yet break if the spring set is attempted to be carried too far down the tooth.

If as in the case of fine pitched teeth, the teeth are filed with a triangular or three square file but little front rake or hook can be given, without pitching the teeth widely. This is shown in Fig. 3130, in which s, is the section of a saw, and f, a section of a three square file. The front faces have no rake, and the file is shown as acting on both faces.

In Fig. 3131, we have the same pitch of teeth, but as the file is canted over, so as to give front rake or hook to the tooth, the tooth depth is reduced, and there is insufficient room for the sawdust. In order, therefore, to give to the teeth front rake, and maintain their depth while keeping the pitch fine, some other than a three square file must be used.

The principal defect of the band saw is its liability to break, especially in band saws of much width, as say 3 inches and over. A saw that is 6 inches wide will ordinarily break by the time it has worn down to a width of 4 inches. Now for heavy sawing it is necessary that wide saws be used, in order to get sufficient driving power without over-straining the saw.

The causes of this saw breakage are as follows:

In order that the saw may be regulated to run on the required part of the upper wheel, and lead true to the lower wheel, it is necessary that the upper wheel be canted out of the vertical, and band sawing machines are provided with means by which this may be done. If the upper wheel were set level, as in Fig. 3132, the saw itself would be held out of level, and the toothed edge would be more tightly strained than the back edge. Furthermore the middle of the saw cannot bed itself perfectly to the wheel. Furthermore, the velocity of the toothed edge would be greater than that of the back edge because of its running in a circle of larger diameter when passing over the wheels.

This is to some extent remedied by setting the wheel out of the vertical, as in Fig. 3133, in which case the two edges will be more equally strained, and have a more equal velocity while passing over the wheels.

There will still however, be an unequal strain or tension across the saw width, and it is found that unless the saw is made what is known as loose, it is liable to break, and will not produce good work. It is to be observed however, that the above may be to a great extent, and possibly altogether, overcome by means of having the rim face of the wheel, or of both wheels, curved or crowned in their widths, so that the saw will be in contact with the face of the wheel, nearly equally across the full saw width. This would also cause the saw to run in the middle of the wheel width, and thus enable the alignment of the saw to be made without requiring the upper wheel to be set out of level.