Fig. 3106Fig. 3106.
Fig. 3106.
The segments are made of varying thicknesses at the cuttingedge, and are tapered for a distance for from 6 to 8 inches inwards from the teeth points. Thus in the figure there is shown atpan edge view of a segment, fromatobbeing parallel, and frombtocbeing ground off taper.
The segments are held to the disc by the two sets of screws,r,s, and are further secured at their edges by pieces of copper, as shown atw. Between the edges of the segments there is left a space or opening of about1⁄16inch, which is necessary to insure that the segments shall not bind together edgeways, as that might prevent their seating fairly against the face of the discd.
The seats for these pieces of copper are shaped as shown in the face views atw, and in the edge views atw′, the mouth of the slot being widened on each side, so that riveting up the pieces of copper will prevent the segments from moving sideways.
In fitting in these pieces of copper, it is essential to take care that they do not completely fill the slots, but leave a small opening at each end of the slot, as atfandgin the figure, and in order to do this the copper must be left about1⁄8inch narrower than the width of the slot.
If the copper is, in riveting up, brought to bear against the end of the slot, it will twist the segments out of line one with the other, causing the saw to drag, cut roughly and produce bad work.
Left-Hand.Right-Hand.Fig. 3107 and 3108Fig. 3107.Fig. 3108.
Left-Hand.Right-Hand.
Fig. 3107.Fig. 3108.
Figs. 3107and3108represent portions of segmental saws for cutting veneering. In some of these saws the screw holes are so arranged that the segments can be moved out to maintain the diameter of the saw as it wears.
For dressing the edges of planks parallel and to width what are called gang edgers or gang edging machines are employed.
A gang edger consists of an arbor driving two or more circular saws, through which the boards to be edged are fed. Means are provided whereby the distance apart of the saws may be rapidly adjusted while the saws are in motion, so that if a board will not true up to a given width, the saws may be set to cut it to a less one without delay.
Fig. 3109Fig. 3109.
Fig. 3109.
Fig. 3109represents a self-feeding gang edger, constructed by J. A. Fay & Company, and in which the left-hand saw may be fixed at any required position on the left-hand half of the saw arbor, while the two right-hand ones may be adjusted independently along the arbor, while the machine is running.
At the back of the saw is a feed roll, and above it a pressure roll, whose pressure may be regulated by means of the weight and bar shown at the back of the machine. The object of placing the feed and pressure rolls at the back of the saws, is, that if a board is found to be too narrow for the adjustment of the saws, it may be withdrawn without stopping or reversing the machine, and the saws may be drawn together sufficiently to suit the case.
Fig. 3110is a plan andFig. 3111an edge view of the work table, and show the means of adjusting the saws.ais the saw arbor, and 1, 2, 3, the circular saws. Saw 1 is carried by the sleeveb, which is secured in its adjusted position by the set screwc.
Fig. 3110Fig. 3110.
Fig. 3110.
Fig. 3111Fig. 3111.
Fig. 3111.
The mechanism for traversing saws 2 and 3 corresponds in design, and may be described as follows:
The arborahas a splinesto drive the sleevesd,d′, which hold the saws and are carried by armse,e′, which operate in slideways and have racksf,f′, into which gear pinions whose shaftsg,g′, are operated by the hand wheelsh,j.
It is obvious that by means of the hand wheelsh,j, saws 2 and 3 may be regulated both in their distances apart or in their distances from saw 1, while the machine is in full motion, the bushes or sleevesdandd′being carried by and revolving in the slide pieces or sliding bearingseande′respectively. Now suppose thate′be moved to the left by hand wheelj, until it abuts against the end ofd, at the slide end, and a further movement ofd′will also moved, causing it to operate its pinion and revolve the hand wheelh, hencedandd′may be simultaneously moved without disturbing their distances apart by operating hand wheelj. On the yoke above the saws is a coarse-figured register plate to enable the setting of the saws to accurate widths apart.
This machine is employed for the purpose of reducing balks or logs into planks of any thickness required. The machine is fixedon the floor of the saw mill, all the gearing being underneath the floor, so that the table may be set level with the floor, which is a great convenience when heavy logs are to be operated upon. The machine consists of a substantial bed plate or framea,Fig. 3112, carrying the saw and the feed works. The carriage runs on rollers, some of which are fixed to the framea, and others to the framing timbersb, which are long enough to support the carriage throughout its full length, when the carriage is at either end of its traverse.
The driving pulley for the saw arbor is shown atc,Fig. 3112, in dotted lines and inFig. 3113in full lines. Upon the saw arbor is a cone pulleyd,Fig. 3113, for operating the carriage to the feed, the construction being as follows:
Fig. 3113Fig. 3113.
Fig. 3113.
Referring toFigs. 3112and3113, cone pulleydconnects by a crossed belt to cone pulleye, on whose shaft is a pulleyewhich drives the pulleyf, on whose shaft is the pinionf, which drives the gearg. On the same shaft asgis a piniong, which drives the gear wheelh, which engages the rackj, on the carriage, and feeds the carriage to the cut. The diameters of pulleyse,f, and off,g, andg, are proportioned so as to reduce the speed of the cone pulleyd, down to that desirable for the carriage feed. But, as there are four steps on the conesd,e, therefore there are four rates of cutting feed or forward carriage traverse, which varies from 15 to 30 feet per minute.
The speed of the saw varies in practice, some running it as slow as 9,000 feet per minute at the periphery of the saw, and others running it as high as 16,000 feet per minute. The latter speed however, is usually obtained when the saws are packed with fibrous packing, which will be explained presently.
The quick return motion for the carriage is obtained as follows:
Referring toFigs. 3113, and3114,kis a fast andk′a loose pulley on the shaftk, and receiving motion by belt from a countershaft.
The speed of the fast pulleykis such as to give a returnmotion to the carriage of about 50 or 60 feet per minute, being about twice as fast as the carriage feed motion.
We have now to explain the methods of putting the respective carriage feed motions into and out of operation, and insuring that both shall not be in gear at the same time.
Fig. 3114Fig. 3114.
Fig. 3114.
Referring therefore toFigs. 3113and3114, suppose the carriage to have completed a feed or cutting traverse, and the operator pushes with his knee the lever or handleh,Fig. 3114, which revolves shaftm, on which is an arm that moves the belt-shifting rodn, thus moving the belt from fast pulleyfto loose pulleyf′, thus throwing the feed gear out of engagement and causing the carriage to stop. He then presses down the foot leverl,Fig. 3113, which operates the belt-shifting rodp,Fig. 3114, and moves the belt from loose pulleyk′, to fast pulleyk, which having a crossed belt, operates the pulleyfin the reverse direction and traverses the carriage backwards, or on the return motion.
Upon releasing the foot from the leverl, the weightwoperates the foot leverl, and the belt is re-shifted from fast pulleykto loose pulleyk′, and the carriage stops.
The carriage is formed of iron plates with an open space of about1⁄2inch between them, as shown inFig. 3114, this space forming a race to permit the carriage to travel past the saw. The only connection between the two sections or parts of the table, is a wide plate at the rear end which secures them together, and causes the lighter portion of the table, which is merely driven by the friction of the rollersc, to always travel with the lower or under portion, which is driven by the rackj. In larger machines for the heaviest work, both sections are driven by a rack motion.
The guide motion for the carriage is constructed as follows:
a,a, are brackets placed at intervals along the whole frame work.
These brackets support rollersc, which have flanges on them to prevent any side motion of the carriage, the construction being most clearly seen inFig. 3113;bbeing a bearing for the shaftvof the rollers. Each section of the carriage, it will be seen, has two ribs or ways which rest on the rollers, which are arranged four on each shaftv(i.e.two for each section of the carriage).
The fence or gauge against which the face of the work runs is very simply arranged as is shown inFigs. 3113, and3114, being secured to the shaftq, by a long boltt, threaded into the top of the fence, and at its lower end abutting against a shoe fitting partly around the top of the shaftq. It is squared at the top to receive a wrench or handleu, and it is obvious that unscrewing the handle releases the fence from shaftq, so that the fence may be moved rapidly by hand across the table to approximate the adjustment of the fence from the saw. The fence having been thus approximately adjusted, and locked to the shaft by means of the handleu, the final adjustment is made by means of the hexagon nutw, on the bed of the shaft nutx, serving as a lock nut, to holdqin its adjusted position.
Fibrous Packing.—The fibrous packing before referred to is composed of hemp, plaited in a four strand plait and inserted in an open-top trough, along the sides of the saw for a distance about two inches less than the radius of the smallest saw the machine uses.
This packing steadies and stiffens the saw, and also affords a means of adjusting its tension, while the saw is running.
Suppose for example, that the saw is rim bound,[47]and the fibrous packing may be rammed tightly to the saw, as near to the saw rim as possible, and less tight as centre of the saw is approached.
[47]For the principles involved in hammering saws to equalize the tension seepage 69 (Vol. II.)et seq.
[47]For the principles involved in hammering saws to equalize the tension seepage 69 (Vol. II.)et seq.
This warms the saw, but makes it warmer at the circumference than at the centre, expanding the circumference, and by equalizing the tension, enables the saw to run straight.
Fig. 3115Fig. 3115.
Fig. 3115.
When the packing is to be adjusted, the carriage is run out of the way, and the packing operation is performed by hand, with a caulking tool. The packing and its box, as applied to a rack saw bench is shown inFig. 3115, by the dark rectangles. By thus packing the saw to guide it and keep it straight, thinner saws may be used, saws 52 inches in diameter, and having a thickness of but 7 or 8 gauge being commonly employed, and in some cases of 9 gauge.
Saws that are thus packed, produce much smoother work.
The packing, it may be observed, is kept well lubricated with oil, and the following is the method of adjusting it.
The side of the saw on which the operator stands is the last tobe packed, the packing on the other side being inserted so as bed fairly but lightly against the saw, so as not to spring it, which may be tried with a straight-edge. The packing on the other side is then inserted to also bed fairly against the saw, without springing it, and the saw is run until it gets as warm as it may, from the friction of the packing. If, then, the saw flops from side to side, the outside (circumference) isloose, and the packing is rammed together on both sides of the saw and near the saw arbor or mandrel, care being taken that in ramming the packing the saw is not unduly pressed on either side.
Expert sawyers generally change the packing when the saw is changed, and thus keep for each saw its own packing. The slot or pocket in which the packing lies is about 11⁄4inches deep, and1⁄2inch wide.
Fig. 3116Fig. 3116.
Fig. 3116.
In ordinary circular saw benches or machines the packing comes about up to the level of the table, as shown inFig. 3116, in whichais a hand hole for putting in and lifting out the plateb, so as to put in or remove the wooden piecesc,d, upon which the packing rests.
Fig. 3117represents a saw mill constructed by the Lane & Bodley Company. In this machine two circular saws are employed, the upper one being of small diameter and revolving in the same direction as the log feed.ais the driving pulley for the main saw arbora, andbthe driving pulley for the upper saw arborb. The carriage feed is obtained by belt from cone pulleycto cone pulleyd, on whose shaft is a friction pulleye, which, for the feed motion, is moved by levereinto driving contact with pulleyf, whose shaft drives the piniong, which gears with the rack of the carriage. The three steps on the conesc,d, give three rates of feed, and a quick return motion is given to the carriage by engaging the friction pulley with a wheel not shown in the engraving.
The log to be sawn rests upon the slideways s’, and is secured thereon by the dogsj,j, which are capable of sliding up or down upon the headsh,h′. When the handleskare raised the slides carrying dogsjare free to be moved up and downh,h′, whereas when handleskare depressed the dogsjare locked and hold the log. The operation is to raise the dog slides to the top ofh,h′, set the log up to the faces ofh,h′, and then by raising handlesk, let the dog slides fall, their weight forcing the dogs into the log, and the depression ofklocks the dog slides uponh,h′, respectively.
The log feed is obtained from the leverl, which operates the ratchet wheelt, which drives bevel gearsvandw, which operate the screws that slide the headsh, andh′, along the slidewayssands′.
Three rates of log feed are obtained by regulating the amount of motion that can be given to the leverl, the construction being as follows:
In the leverlis a slot in which a stoprcan be secured at different heights, and the piecemhas four notches. The limit to whichlcan be moved to the left is until it comes against the stopx, but the limit to which it can be moved to the right is governed by the height of the stoprin the slot inl. If stopris set at its highest position in the slot,lcan be moved to the right until the stoprmeets the right hand step on the circumference ofm, and a maximum of log feed is given.
Fig. 3118Fig. 3118.
Fig. 3118.
Fig. 3118represents a tubular saw machine. The saw runs in fixed bearings, the work feeding on the tableb, running on ways ona. The work is here obviously sawn to a curve corresponding to that of the circumference of the saw.
InFigs. 3119and3120is 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.
Fig. 3119Fig. 3119.
Fig. 3119.
It consists of a vertical column or standard, upon the face of which a slidewayafor the armb, on which is a slidewayc, 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 inFig. 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 made1⁄8inch thick, with an odd one1⁄16inch thick, and will thus make gains advancing in widths by sixteenths of an inch.
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 principal 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.
Fig. 3122Fig. 3122.
Fig. 3122.
Fig. 3123Fig. 3123.
Fig. 3123.
A scroll sawing machine constructed by the Egan Company is shown inFig. 3122, a sectional view of the saw straining mechanism being shown inFig. 3123.a,a, is a casting having slides for the headb, which is adjustable uponato suit different lengths of saws, and is secured in its adjusted position by the boltcand nutd. To the ends of the springss, a strip or band of leather is secured, the other end passing around the small stepfof a rollerr, and being secured thereto. The rollerris so supported that it may rise and fall with the strokes of the saw. A second leather bandgis secured att, passes over the large step ofr, and at its lower end hooks to the saw, which is strained by the springss. 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 viewFig. 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.
Fig. 3124Fig. 3124.
Fig. 3124.
Fig. 3124represents a scroll saw constructed by H. L. Beach. This machine is provided with a tilting table, which can be setat any angle up to 39 degrees, either to the right or left, the exact angle being indicated by a graduated arc.
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 boxeand eccentric leverf. By raising the leverf, the shaft, being balanced, is free to move up or down to suit any length of saw.
At the same time, the steel bearinglforms 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.
Fig. 3125Fig. 3125.
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 class, constructed by J. A. Fay & Co., being shown inFig. 3125. It consists of a standard or framea, carrying the saw-driving wheelb, and the upper wheelc, 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 ate, 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 leverf, 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.
Atj, is a bar carrying a guideg, which sustains the saw against the pressure of the cut, a similar guide being placed below the tablet, atg′. 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, andhis 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 from1⁄16inch 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.
Fig. 3126Fig. 3126.
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 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 inFig. 3126.
Fig. 3127Fig. 3127.
Fig. 3127.
A stronger form of tooth is shown inFig. 3127, the tooth gullets being well rounded out, and the teeth shallow at the back, while having ample room in front for the dust.
Fig. 3128Fig. 3128.
Fig. 3128.
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, inFig. 3128, it will be seen that from the front rake, orhookof 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 contactwith 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.
Fig. 3129Fig. 3129.
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 inFig. 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 inFig. 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.
Fig. 3130Fig. 3130.
Fig. 3130.
If as in the case of fine pitched teeth, the teeth are filed with a triangular orthreesquare file but little front rake or hook can be given, without pitching the teeth widely. This is shown inFig. 3130, in whichs, is the section of a saw, andf, a section of a three square file. The front faces have no rake, and the file is shown as acting on both faces.
Fig. 3131Fig. 3131.
Fig. 3131.
InFig. 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.
Fig. 3132-3133Fig. 3132.Fig. 3133.
Fig. 3132.Fig. 3133.
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 inFig. 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 inFig. 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 acrossthe 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]Seepage 69, Vol. II., for what is technically known as looseness in a saw.
[48]Seepage 69, Vol. II., for what is technically known as looseness in a saw.
A re-sawing machine is one used to cut lumber (that has already been sawn) into thinner boards.Fig. 3134represents 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.
Fig. 3134Fig. 3134.
Fig. 3134.
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.
Fig. 3135Fig. 3135.
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 inFig. 3135, which represents a portion of the main frame or column, on which is a slidewayb, for the slidec, which carries the bearing for the upper wheel.
The method of moving the slidecfor moving the upper wheel to adjust the saw tension is as follows:
By means of the handlehand the worm and worm wheel atw, the shaftsis revolved. The upper end ofsis threaded into the nutn, which is capable of end motion in its bearing ate, and which abuts against the leverl, the latter abutting against the end of the screwm, and acting at its other end on the rubber cushionp. Now suppose thatsbe revolved in the direction denoted by the arrow, and the effect will be to raise the nutn. This effect will be transferred through the screwmto the slidec, which will rise up onb, carrying with it the upper wheel bearing and wheel.
When the upper wheel receives the strain of the saw, then the continued revolution of shaftswill cause the nutnto lift endways in its bearinge, the screwmacting as a fulcrum to cause the leverlto compress the rubber cushionp. 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 slidecwill (from the extra strain caused by the chip) move down on its slidewayb, which it is capable of doing, because the long arm of the leverlcan move down, compressingp, 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 ata, and is guided sideways by the screwscandd. Atfis a stud threaded into the bottom half of the upper wheel bearing, the wheelsgandhthreading uponf. The weight of the upper saw wheel endeavors to lift the endjof the wheel bearing, and wheelhdetermines how much it shall do so, while wheelgacts as a check nut to lock the adjustment.