Modern Machine-Shop Practice Part 213

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.

[Ill.u.s.tration: Fig. 3131.]

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 princ.i.p.al 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.

[Ill.u.s.tration: Fig. 3132.]

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 pa.s.sing over the wheels.

[Ill.u.s.tration: Fig. 3133.]

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 pa.s.sing over the wheels.

[Ill.u.s.tration: Fig. 3134.]

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,[48] 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.

[48] See page 69, Vol. II., for what is technically known as looseness in a saw.

RE-SAWING BAND SAW MACHINE.

A re-sawing machine is one used to cut lumber (that has already been sawn) into thinner boards. Fig. 3134 represents a band saw machine, constructed by P. Pryibil, having a self-acting feed motion, consisting of four feed rolls, all of which are driven, and two small idle rolls, which are so arranged as to guide the last end of the stuff or work after it has left the driven rolls.

Four rates of feed are provided, and the upper wheel can be set at the required angle from a perpendicular while the machine is in motion.

The upper guide wheel, and the mechanism by which it is carried, is counterbalanced by a weight that hangs within the column or main frame, and is therefore out of sight.

[Ill.u.s.tration: Fig. 3135.]

The construction of the parts by means of which the upper wheel is adjusted in height to regulate the tension of the saw, and which also cants the wheel out of the vertical, is shown in Fig. 3135, which represents a portion of the main frame or column, on which is a slideway B, for the slide C, which carries the bearing for the upper wheel.

The method of moving the slide C for moving the upper wheel to adjust the saw tension is as follows:

By means of the handle H and the worm and worm wheel at W, the shaft S is revolved. The upper end of S is threaded into the nut N, which is capable of end motion in its bearing at _e_, and which abuts against the lever L, the latter ab.u.t.ting against the end of the screw M, and acting at its other end on the rubber cushion P. Now suppose that S be revolved in the direction denoted by the arrow, and the effect will be to raise the nut N. This effect will be transferred through the screw M to the slide C, which will rise up on B, carrying with it the upper wheel bearing and wheel.

When the upper wheel receives the strain of the saw, then the continued revolution of shaft S will cause the nut N to lift endways in its bearing _e_, the screw M acting as a fulcrum to cause the lever L to compress the rubber cushion P. The amount of tension on the saw is tested by springing it sideways with the hands. Now suppose the saw to be properly strained, and that a piece or chip of wood accidentally gets between the saw and the lower wheel, and the result will be that the slide C will (from the extra strain caused by the chip) move down on its slideway B, which it is capable of doing, because the long arm of the lever L can move down, compressing P, and this will prevent the saw from breaking.

To cant the wheel for leading the saw true to the lower wheel, the following means are provided:

The upper wheel bearing rests on the fulcrum at _a_, and is guided sideways by the screws _c_ and _d_. At _f_ is a stud threaded into the bottom half of the upper wheel bearing, the wheels _g_ and _h_ threading upon _f_. The weight of the upper saw wheel endeavors to lift the end J of the wheel bearing, and wheel _h_ determines how much it shall do so, while wheel _g_ acts as a check nut to lock the adjustment.

[Ill.u.s.tration: _VOL. II._ =BAND SAW WITH ADJUSTABLE FRAME.= _PLATE XXII._

Fig. 3139.]

[Ill.u.s.tration: _VOL. II._ =BAND SAW MILL.= _PLATE XXIII._

Fig. 3140.

Fig. 3141.

Fig. 3142.]

The feed rolls are carried in slides which are operated in slideways by means of screws, and the two back rolls, or those nearest to the column are maintained vertical. The two front ones, however, are provided with means by which they may adjust themselves to bear along the full depth of the work, notwithstanding that it may be taper. The construction by means of which this is accomplished is shown in Figs. 3136 and 3137, in which A is front and B a back feed roll. The bearings of feed roll A abut against rubber cushions C, C, whose amount of compression is regulated by the set screws D, D.

[Ill.u.s.tration: Fig. 3136.]

[Ill.u.s.tration: Fig. 3137.]

[Ill.u.s.tration: Fig. 3138.]

The construction of the saw guides is shown in Fig. 3138, which is a plan view partly in section. S S are hardened steel plates set up to the saw by means of studs whose nuts are shown at N N. W is a friction wheel which supports the saw against the thrust caused by the work feeding to the saw. The adjustment of the wheel W to the saw is obtained by means of the wheel H.

The hand wheel H operates the screw _r_ _r_, that adjusts the wheel W to the saw, the wheel J serving to lock the screw in its adjusted position.

Fig. 3139 represents Worssam's band saw machine, in which the standard may be set at any required angle for cutting bevels.

When the work is heavy and not easily handled it is preferable to set the standard and saw at the required angle, rather than to set the table at an angle and have the saw remain vertical. In Worssam's machine this is accomplished as follows:

A is the main frame carrying the work table T, and having circular guideways B, B', which carry the standard C having guide C' for working in the circular guideways B, B'.

The saw-driving wheel D, is carried in bearings provided in C, and, therefore moves when the standard C is moved.

At the upper end of C, is the slide E, which carries the bearing for the upper wheel F, this slide being adjusted to regulate the saw tension by the hand wheel O, whose screw threads into a nut in the slide E. H carries the front guide G, for the saw, the back guide G' being carried by a bracket bolted to C. The back guide is fixed in position, but the front one is adjustable to suit the height of the work by raising or lowering it.

The means for setting the saw at the required angle to the work table are as follows:

At the back of the standard C is a rack J, whose pitch line is an arc of a circle of which the axis of the guideway C' is the centre.

Into the rack J fits the worm wheel K, at the bottom of the shaft of which is a pair of bevel gear wheels L, which are operated by the hand wheel M.

A band saw machine constructed by Messrs. London, Berry & Orton, is shown by Figs. 3140, 3141 and 3142, in plate XXIII. The saw-driving wheel D, has wrought iron arms turned true and screwed into the wheel hub. The wooden segments have their grain lengthways of the rim, and between them are placed pieces of soft wood with the grain across the rim. This acts to keep the joints tight, notwithstanding the expansion and contraction of the wood.

The upper wheel is adjusted for straining the saw, and for leading the saw true, by the following construction. It is carried in a U-shaped frame F, which is pivoted at _y_ to a slide that is gibbed to the main frame, and by operating the screw shown at X, the frame F is set to the required level.

To regulate the tension of the saw, the hand wheel K is operated, which drives the pair of bevel gears J and I, the latter of which operates the threaded shaft H, whose upper end G connects with the slide which carries F. Within G is a spring to act as a cushion to the slide, and thus prevent saw breakage should a chip pa.s.s between the saw and its driving wheel.

The saw guide frame is secured to the main frame at _m'_, _m'_. Upon the face of _m_, is a slideway for the saw guide arm _n_, which may thus be adjusted as closely to the upper face of the work as possible.

The weight of arm _n_ is counterbalanced by a rope pa.s.sing over the pulley V, and supporting the counterbalance weight _w_. The feed motion is constructed as follows:

On the same shaft as the main fast and loose pulleys A, B, is the feed pulley L, which by belt connection drives pulley M, which is on the shaft W, upon which is a friction disc N, by means of which the rate of feed is regulated. The feed disc N drives the wheel O; the degree of contact between these two (N and O) is regulated by means of the weight T, on the lever U.