Modern Machine-Shop Practice Part 212

Fig. 3118 represents a tubular saw machine. The saw runs in fixed bearings, the work feeding on the table B, running on ways on A. The work is here obviously sawn to a curve corresponding to that of the circ.u.mference of the saw.

CROSS CUTTING OR GAINING MACHINE.

In Figs. 3119 and 3120 is represented a machine constructed for either cross cutting or gaining, the gaining head shown on the machine being replaced by a cross-cut saw when cutting off is to be done.

[Ill.u.s.tration: Fig. 3119.]

It consists of a vertical column or standard, upon the face of which a slideway A for the arm B, on which is a slideway C, along which the head for carrying the saw arbor traverses.

When the saw is to be used, the carriage or work table must be locked in position and adjusted so that the saw will come fair in the groove, provided in the table, but it is not necessary to dog or fasten the work to the table, because the saw itself draws the work over fair against the fence.

When the machine is used for gaining, the work must be dogged fast to the table, so that the work and table may be moved accurately together and the widths apart of the gains kept accurate.

Joshua Oldham's combination saw for grooving or gaining is shown in Fig.

3121. It consists of two outside saws, such as shown at the top of the figure, and having spur teeth between the ordinary cutting teeth. The tops of the spur or cross-cutting teeth are a little higher than the other teeth, so that they sever the fiber before the ordinary teeth attempt to remove it, and thus produce very smooth work. The inside pieces, shown at the bottom of the figure, go between the two outside saws, if necessary, to make up the required width of gain. They are made 1/8 inch thick, with an odd one 1/16 inch thick, and will thus make gains advancing in widths by sixteenths of an inch.

SCROLL SAWING MACHINES.

The scroll sawing machine derives its name from the fact that it is particularly fitted for sawing scroll or curved work by reason of the saw (which is a ribbon of steel with the teeth cut on one edge) being very narrow.

The princ.i.p.al points in a scroll sawing machine are to have the saw held under as nearly equal tension as possible throughout the whole of the stroke; to render the machine readily adjustable for different lengths or sizes of saws, to provide it with means of taking up lost motion, and to avoid vibration when the machine is at work.

A scroll sawing machine constructed by the Egan Company is shown in Fig.

3122, a sectional view of the saw straining mechanism being shown in Fig. 3123. A, A, is a casting having slides for the head B, which is adjustable upon A to suit different lengths of saws, and is secured in its adjusted position by the bolt C and nut D. To the ends of the springs S, a strip or band of leather is secured, the other end pa.s.sing around the small step F of a roller R, and being secured thereto. The roller R is so supported that it may rise and fall with the strokes of the saw. A second leather band G is secured at T, pa.s.ses over the large step of R, and at its lower end hooks to the saw, which is strained by the springs S. This reduces the motion of the springs, and thus serves to equalize their pressure throughout the saw stroke.

The lower end of the saw is gripped in a slide or cross-head that is driven by the connecting rod and crank motion shown in the general view Fig. 3122. The lever shown at the foot of the machine moves the belt to the fast or loose pulley to start or stop the machine, and operates a brake to stop the machine quickly.

[Ill.u.s.tration: _VOL. II._ =GAINING OR GROOVING MACHINE.= _PLATE XXI._

Fig. 3120.

Fig. 3121.]

Fig. 3124 represents a scroll saw constructed by H. L. Beach. This machine is provided with a tilting table, which can be set at any angle up to 39 degrees, either to the right or left, the exact angle being indicated by a graduated arc.

[Ill.u.s.tration: Fig. 3122.]

[Ill.u.s.tration: Fig. 3123.]

[Ill.u.s.tration: Fig. 3124.]

The straining device, including the springs, air pump, guide-ways, cross-head and steel bearing, are all attached to the vertical tubular shaft, which is secured to the heavy cast back support by the box E and eccentric lever F. By raising the lever F, the shaft, being balanced, is free to move up or down to suit any length of saw.

At the same time, the steel bearing L forms a support for the back and sides of the saw, and can be raised or lowered to suit any thickness of work.

The under guide-ways are so constructed that their expansion by tightening does not tighten the cross-head. Instead of the ordinary tight and loose pulleys, the crank shaft carries a friction pulley and combination brake by which the saw is stopped or started instantly, by a single motion of the foot.

This leaves the hands entirely free, and saves considerable time in stopping and starting.

The lower end of the saw is held by a steel clamp; when the saw breaks it can be used again by filing a notch. Both ends of the saw are arranged to take up lost motion and wear.

Any desired strain from 10 to 75 pounds can be given to the saw, and the strain is equal at all points of the stroke.

BAND SAWING MACHINES.

[Ill.u.s.tration: Fig. 3125.]

The simplest form of band sawing machine is that in which the work is fed to the saw by hand, a machine of this cla.s.s, 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 1/16 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.

[Ill.u.s.tration: Fig. 3126.]

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 pa.s.sing through the work. The remedy is to s.p.a.ce the teeth widely, thus giving room for the dust without having a deep tooth, an ordinary form of tooth being shown in Fig. 3126.

[Ill.u.s.tration: Fig. 3127.]

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:

[Ill.u.s.tration: Fig. 3128.]

One of the princ.i.p.al 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 circ.u.mstances 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.

[Ill.u.s.tration: Fig. 3129.]

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.

[Ill.u.s.tration: Fig. 3130.]