Modern Machine-Shop Practice Part 208

[Ill.u.s.tration: Fig. 3080.]

The centrifugal force created by the great speed of a circular saw is found to be sufficient to cause it to stretch and expand in diameter.

This causes the saw to run unsteadily unless it is hammered in such a way as to have it rim bound when at rest, leaving the stretching caused by the centrifugal force to expand the saw and make its tension equal throughout. The saw obviously stretches least at the eye, and the most at its circ.u.mference, because the velocity of the circ.u.mference is the greatest, and the amount of stretch from the centrifugal force is therefore the greatest.

It is obvious that the amount of centrifugal force created will depend upon the speed of the saw, and it therefore follows that the hammering must be regulated to suit the speed at which the saw is to run when doing cutting duty, and in this the saw hammerer is guided solely by experience.

A circular saw may have its tension altered and impaired from several causes as follows:

1. From the saw becoming heated, which may occur from the arbor running hot in its bearings, or from the work not being fed in proper line with the saw.

2. From the reduction in diameter of the saw by frequent resharpening of the saw, this reduction diminis.h.i.+ng the amount of centrifugal force generated by the saw, and therefore acting to cause the saw to become loose at the eye.

3. From the saw teeth being allowed to get too dull before being sharpened, which may cause the saw teeth to heat, and thus destroy the tension.

4. From stiffening the plate at the throats of the teeth when gumming the saw, an effect that is aggravated by using a dull punch.

5. From the saw teeth having insufficient set, and thus causing the saw to heat.

The methods of discovering the errors of tension in a saw, and the process of hammering to correct them, have already been explained with reference to the use of the hammer on pages from 68 to 70 of volume 2 of this work.

Before hanging a saw on a mandrel, it is necessary to know that the mandrel itself runs true in its bearings or boxes. In a new machine this may be a.s.sumed to be the case, but it is better to know that it is so, because if the mandrel does not run true several very improper conditions are set up. First, the saw will run out of true circ.u.mferentially, and therefore out of balance, and the high side of the saw will be called upon to do more cutting duty than the low side.

Second, the centrifugal force will be greatest on the high side, and the saw will be stiffer, thus setting up an unequal degree of tension.

Third, the saw will run out of true sideways, cutting a wider kerf than it should, thus wasting timber while requiring more power to drive.

The collar on the saw arbor should be slightly hollow, so that the saw will be gripped around the outer edge of the collar, and the arbor or mandrel should be level so that the saw will stand plumb. The boxes or bearings of the arbor should be an easy working fit to the journals, and there should be little, or what is better, no end play of the arbor in its bearings.

If a saw arbor becomes heated enough to impair the tension of the saw, it has been hot enough to impair its own truth, and should be examined and trued if necessary.

The most important point in this respect is that the face of the collar against which the saw is clamped should run true, bearing in mind that if it is one hundredth of an inch out of true in a diameter of, say 3 inches, it becomes twenty hundredths or one-fifth of an inch at the circ.u.mference of a saw that is 60 inches in diameter.

In cases of necessity, a saw that wabbles from the collar face of the mandrel running out of true, may be set true by means of the insertion of pieces of paper placed between the saw and the face of the collar.

The first thing to do in testing the saw is to take up the end motion of the saw arbor, or if this cannot be done, then a pointed piece of iron or wood should be pressed on the end of the mandrel so as to keep it from moving endways while the saw is being tested.

The saw should be revolved slowly, and a piece of chalk held in the cleft of a piece of wood should be slowly advanced until it meets some part of the face of the saw just below the bottom of the saw teeth.

As soon as the chalk has touched and the saw has made one or two revolutions the chalk should be moved a trifle farther on from the teeth, and another mark made, and then moved on again, and so on, care being taken to notice how much s.p.a.ce there is between the high and low sides of the saw. It will be found, however, that the shorter the chalk marks are the more the saw is out of true.

A more correct method is to chalk the face of the saw and use a pointed piece of iron wire of about one-quarter inch in diameter, but in any case the saw should only be touched lightly.

The pieces of paper should be portions of rings or segments, and should extend an equal distance below the circ.u.mference of the collar, because the same thickness of paper will alter the saw more in proportion, as it is inserted farther in toward the eye of the saw.

If it should happen that two thicknesses of paper are necessary to true the saw, one should be made about half the length of the other, and the long one may extend farther in toward the eye of the saw. Thus one ring of paper may be an inch deep and the other one-half inch deep.

If but one piece of thin paper is needed, it may be simply a straight piece inserted half way down the collar and trimmed off level with the collar. In placing the paper, the middle of its length should be on that side of the saw that is diametrically opposite to the marks left by the chalk on the face of the saw.

When the saw is trued and is started it will be loose on the outside, but as its speed increases it should stiffen up so as to run true and steadily when running at its working speed.

If the saw is to be tried by actual work, it must be borne in mind that the tension of the saw must be right for its speed when in actual use, and not when running idle. If the machine has belt power enough to maintain the same speed whether the saw is cutting at its usual rate of feed, or whether it is running idle, the tension will not be altered by putting on the feed, but if the saw has been hammered to run at the full speed of the machine when not cutting and the feed is heavy enough to slacken the speed, then the tension of the saw will not be correct for its working speed.

[Ill.u.s.tration: Fig. 3081.]

The eyes of small saws are either made to fit the mandrel an easy sliding fit, or else the mandrel is provided with cones to accommodate various sizes of holes, an ordinary construction being shown in Fig.

3081, in which A is the saw arbor, fast on which is the collar B, S representing a section of the saw, W a washer or loose collar, and N the nut for tightening up W. The cone _c_ is screwed upon A and pa.s.sed through the saw until it just fills the hole, and thus holds the saw true.

In putting on the saw, it should be pa.s.sed up to the collar, and _c_ screwed home until it binds in the saw eye with enough force to bring the threads of _c_ fairly in contact with those on the mandrel A, but if screwed home too tightly it may spring the saw, especially if the saw is a very thin one.

As _c_ must be removed from the arbor or mandrel every time the saw is changed, the wear on its thread is great, and in time it becomes loose, which impairs its accuracy.

[Ill.u.s.tration: Fig. 3082.]

This objection is overcome in the construction shown in Fig. 3082, which is that employed by the S. A. Woods Machine Company. It is seen in the figure that the cone _c_ fits externally in a recess in the collar B, and at the coned end also upon the plain part _e_ of the arbor. The cone is hollow and receives a spiral spring _s_, S. When the saw is put on it first meets _c_, and as nut N is screwed up, the saw S and cone are forced along arbor _e_ until the saw meets the face of B, and the clamping takes place. The strength of the spring _s_ is sufficient to hold the saw true, and as the motion of cone _c_ is in this case but a very little, therefore its wear is but little, which makes this a durable and handy device, while the saw cannot be sprung from over-pressure of the cone. Circular saws of large diameter, as from 40 inches upwards, are made a fair sliding fit upon their arbors or mandrels, and are provided with two diametrically opposite pins that are fast in the arbor collar.

The pins should be on diametrically opposite sides of the arbor, and an easy sliding fit to the holes in the saw, but they should not bind tight. Both pins should bear against the holes in the saw, and if both the pins and the holes in the saw are properly located, the saw will pa.s.s up to the collar with either side against the arbor collar, or in other words, the saw may be turned around upon the arbor.

If the pins, or either of them, bind in the holes of the saw, and the latter is forced on the arbor, it will spring the saw out of true, and when this is the case care should be taken in making the correction to discover whether it is the pins or the holes in the saw that are wrongly located. If it is the pins, the error will show the same whichever side of the saw is placed next to the arbor collar, while if the error is in the holes, the error will show differently when the saw is reversed on the arbor.

When a saw becomes worn, and its teeth require sharpening, the first thing to do is to _joint_ it, that is to say, bring down all its teeth to the same height, which may be done by holding an emery block or file against it while the saw is running, care being taken to hold the block or file firmly, and to continue the process until the tops of the teeth run true.

The next operation is to gum and sharpen the teeth. Gumming a saw is cutting out the throats, or gullets between the teeth, so as to maintain the height of the tooth, and it follows that on saws that have sharp gullets (or in other words, saws in which the back of one tooth and the face of the next tooth join in a sharp corner), the sharpening process with the file may be made to also perform the gumming.

In the case of teeth of coa.r.s.e pitch, however, this would entail too much labor in filing, and furthermore, as the height of the teeth increases with the pitch or distance apart of the teeth of circular saws, and as the higher the tooth the weaker it is, therefore coa.r.s.e pitched teeth are given round gullets so as to strengthen them as much as possible. The gumming of a saw should always be performed before the sharpening, and the sharpening before the setting.

When the sharpening is to be done with the file, the cutting strokes of the file should be in the same direction as the teeth lean for the set, as this leaves a sharper cutting edge, and it follows that the proper plan is to file every other tooth first, going all around the saw, and to then turn the saw around in the vise, and file the remaining teeth.

The height of the teeth and the diameter of the saw will be best maintained by filing the front face of the tooth to bring it up to an edge, but in filing the front face the s.p.a.cing of the teeth should be kept as even as possible.

If the front face has been filed until a tooth is as widely s.p.a.ced as those already filed, and the edge is not brought up sharp, then the edge may be brought up by filing the back of the tooth.

[Ill.u.s.tration: Fig. 3083.]

A saw gumming, gulleting or chambering machine to be operated by hand, and constructed by Henry Disston & Sons, is ill.u.s.trated in Fig. 3083. It consists of a frame spanning the saw, and having screws B B, B B, to adjust to the saw thickness; 4 and 5 are two saw teeth, and 6 the cutter, K is a wheel for the feed screw G, and C and D gauges for regulating position and depth of the gulleting.

The cutter 6 is driven or revolved by means of the handles H H, but an important point in the construction is, that a pawl and ratchet wheel is used to drive the cutter, so that if the handles H H were revolved in the wrong direction, the cutter would not be revolved. This saves the cutter teeth from breakage. The machine is operated as follows:

Run the cutter back by means of screw G as far as necessary, then place the machine on the saw, with the cutter close up in the chamber of the tooth to be gummed.

If the teeth are regular and the same distance apart, start the cutter in any chamber; but if they are irregular, make them even by commencing in the smallest s.p.a.ce. After gumming the saw a few times the teeth must become regular. F is a set-screw to regulate the depth of gullet. Fasten the machine to the saw by means of the screws B B, and proceed to gum the first tooth, one of the points of the star being struck at each revolution by a projection on the handle, steadily feeding the cutter until arrested by set-screw F. Remove the machine to the next tooth towards you, after having run the cutter back, and proceed as before until the whole of the teeth are gummed.

The cutter is so arranged as to slide on its axis, and when one portion becomes dull, remove a washer from back to front, and thus present a new sharp cutting surface; and so continue changing the washers until the whole face of the cutter becomes dull.

Set is given to saw teeth in two ways: first, by what is called _spring set_, which is applied to thin saws and to cross-cut saws; and second, _swage set_, which is given to thick saws and to inserted teeth. Spring set consists of bending the teeth sideways so as to cause the saw to cut a pa.s.sageway or _kerf_, as it is termed, wide enough to permit the saw to pa.s.s through the timber without rubbing on its sides.

Swage set consists of upsetting the point of the tooth with a swage, thus spreading it out equally on both sides of the body of the saw plate, as shown at A, Fig. 3084.

The set of the teeth, whether given by swaging or upsetting, or by spring set, should be equal throughout the saw, so that each tooth may have its proper share, and no more, of duty to perform.

If spring set is employed, it should not extend down more than half the depth of the teeth, and this point is one of considerable importance for the following reasons. The harder the saw is left in the tempering the easier the teeth will break, but the longer they will keep sharp. Now a tooth that is hard enough to break if it is attempted to carry the set down to the root or bottom, will set safely if the set is given to it for one-half its depth only.