Modern Machine-Shop Practice Part 174

[Ill.u.s.tration: Fig. 2561.]

If two eccentrics are used, as in a link motion, the lines for setting one eccentric are equally applicable to both; the lap and lead line F being located on the crank-pin side of E when there is a rock shaft, as is supposed to be the case in Fig. 2561; and on the other side of E when there is no rock shaft; and in this case the eccentric that is to operate the valve to make the engine run forward must have its throw-line following the crank pin, as at J, in Fig. 2561; the eccentric K operating the valve for running backward. Conversely, in the absence of a rock shaft, the throw-line of the forward eccentric leads, while that of the backward eccentric follows the crank pin.

When the line of connection of the eccentric rod is not parallel to the axial line of the cylinder bore, the crank must be placed horizontally level (or if it be a vertical engine, on the dead centre), but instead of the straight-edge being placed parallel to the throw-line of the crank, it must be placed at a right angle to the line of connection of the eccentric rod.

[Ill.u.s.tration: Fig. 2562.]

Thus in Fig. 2562 the engine is supposed to be a vertical one, and the crank is, therefore, placed on its dead centre, its throw-line being vertical instead of horizontal as in our previous examples (which were supposed to be for a horizontal engine). It is also supposed to have a rock shaft A; hence the straight-edge is set at a right angle to the line of connection of the eccentric rod which is denoted by B.

It is obvious that to set the crank throw-line vertical the circle B in Fig. 2509 may be used, the spirit-level being resorted to to discover when the crank stands vertical.

[Ill.u.s.tration: Fig. 2563.]

An example in the erection or setting of framed work is shown in Fig.

2563, which represents a side elevation of a frame put together in four parts, two side and two end frames. A and B are journal bearings requiring to stand parallel and true one to the other, B being capable for adjustment in distance from A by means of the adjusting screws G, H.

The bearings C, D, E, F, are to be parallel one to the other and to A, B. Their proper relative distances apart, and the axes of all the shafts, are to stand at a right angle to the side frames.

[Ill.u.s.tration: Fig. 2564.]

Fig. 2564 represents an end view of the frame, the ends T being bolted to the side frames S and S' at I, J, K, and L.

Now it is obvious that the ways for the bearings A, B, C, &c., may be trued out, ready to have the bra.s.ses fitted before the framework is put together, and that from their positions they would have to be planed out at separate chuckings; supposing, of course, the frame to be too large to be within the capacity of the machine table. It would be difficult to cut all the surfaces of the bearing ways to stand in the same plane, unless there were some true plane to which all might be made common for parallelism.

Furthermore, unless the surfaces where T is fastened to S and S' are properly bedded to fit each other, bolting them up would spring and bend the frames out of their normal planes. To meet these requirements, there are given to the side frames a slightly projecting surface where the feet of T meet them, and furthermore, the feet of T themselves project beyond the sides of T as shown. These projecting pieces may therefore be planed to a common plane without planing the sides of the respective frames; and this plane should be as nearly as can be parallel with the body of each frame surface. The surfaces of the bearing ways may then be planed parallel to those of the projections, and the jaw surfaces true to the side surfaces, and all the bearing ways will stand true if the frames be properly set--when put together with the bolts. But unless the bedding surfaces at I, J, K, L, be made to bed and fit properly, the whole truth of the bearing ways and their distances apart across the framework may be altered. Thus, supposing the feet of T at I and J to meet S as denoted by the dotted lines O R, and whether the fault lie with the feet of T or with the projections on S the result will be that the pressure of the bolts holding I J to S will bend S so that its plane will be a curve as denoted by the dotted line P P, and the distances apart of the journal ways B B and D D respectively will be wrong, being too wide on account of the bend outward of S.

But the feet may touch on the opposite corners, the surfaces of S' or of T being out of true or out of full contact, as denoted by the dotted lines V W on K, L; in this case the frame S' would be bent to the curve Q Q, and the journal ways would be too close together.

On the other hand, the want of fit between these surfaces may be in the direction of the length of the frame instead of the direction of its height, as has been supposed; or it may be in one direction on one foot and in another direction on another foot. But in whatever direction it does exist, it will inevitably bend and twist the frame.

It must not be taken for granted, that because these surfaces have been planed or milled, that therefore they are true; because frames of this cla.s.s cannot, if large, be held without springing them to some extent from the pressure of the bolts or other devices necessary to hold them to be cut.

It is not uncommon to plane the surfaces as true as may be, and put the frames together, bolting them up tight, and then applying the straight-edge trammel and rule to test the truth, correcting any error that may be found by inserting pieces of paper, sheet tin or material of requisite thickness on one side of the surfaces, so as to offset the error in their fit and bring the framing true; but this is not the proper way, because it reduces the area of contact, and furthermore renders a new testing and adjustment necessary whenever the frames are taken apart. It is better therefore to apply a straight-edge to the surfaces and true them to it, testing them vertically as by placing the straight-edge across K L, and longitudinally across S', at K and the corresponding projection at the other end of the frame, filing them until they appear true.

The holes through the frame may be drilled before filing these surfaces, so as to reduce the area to be filed. Since the end frames T do not in this example carry any journals or mechanism, the position of T is not so particular as it otherwise would be; hence, the holes in its feet may be marked off and drilled independently of the frame, the holes being drilled a little too small to allow for reaming with the holes in the frame. The framing will then be ready to put together (all machine work upon them being supposed to be done). The feet of all such frames should be planed true, so that the frame, when put together, may stand true and steady when placed upon a level floor or foundation, and in this case the distance and parallelism of the feet surfaces will be true with the ways or bearings, affording much a.s.sistance in holding the frame while putting it together. The height of the holes may be measured and marked from the feet surface, thus insuring truth as far as height is concerned. Lines may be drawn or marked on each side frame, at the proper distance from and parallel with the jaws of the ways A, B, thus completing on the side frames the marking of the location of the centres of the holes for bolting the end frames on.

If the frames were of a size to be sufficiently easily handled, the end frames might be put in their places, and the whole framework set true, so as to mark the holes in the end frames from those already drilled in the side frame. But if the use of a crane were necessary to lift them, it would be better to mark the holes on the end frames, and drill them before putting the framework together at all, leaving sufficient to ream out of the holes to bring them fair, notwithstanding any slight error in drilling them. In this case, a line denoted by the dotted line X in Fig.

2564, should be drawn across the frame, and the holes at I and J be made equidistant on each side of it, as well as the proper distance apart. X must be at a right angle to the trued foot surfaces at I J, so as to cause the side frames to stand vertical while their feet are horizontal.

Supposing now the holes to be drilled and the frames are to be bolted together, the whole frame may be held temporarily together by bolts pa.s.sing through the side frames at each end, or a bolt may be pa.s.sed through the holes F to steady it. Indeed, if these holes F have been accurately bored, a neatly fitting mandrel pa.s.sed through them should hold the side frames true. The end frames T having been set to stand at a right angle to the side frames, and with their holes at I J, &c., as near fair as may be with the holes in the side frames, two feet, as I J, may have their holes reamed fair with the holes in the side frames, and tightly fitting bolts be driven in and screwed firmly home. Before reaming the other holes (as K L) of each end frame, the jaws to receive the bearing boxes should be tested for alignment one with the other.

Truth, in this respect, being of the utmost consequence for the following reasons:

Suppose the bearing ways on one side frame to stand higher than those on the other, then, the shafts will not stand level in the frame unless (except in the case of the bra.s.ses or boxes in B) the lower bra.s.ses are made of unequal thickness through the crown, to an amount equal to that of the error. In the case of the bra.s.ses in A, C, D, E, the joint faces of all the bra.s.ses of one side frame would require to be made thinner beneath the journal than above it on the high frame, and thicker beneath than above on the low frame, This would entail much extra work in planing, marking, and boring the bearing boxes or bra.s.ses, and be an inferior job when done.

Again, the bores of all the bra.s.ses would not be parallel to the crown or bedding faces, and this error would entail the following extra work: 1. Ascertaining the amount of the error, and allowing for it in marking the bra.s.ses; 2. The setting of the ways of the bra.s.ses out of true with the ways when clinking them for boring; and, finally, extra fitting or filing the bra.s.s bores when fitted with the shafts in place. This extra fitting would be necessary for the following reasons:

When the surfaces of work are to be parallel, they can be measured with calipers. Surfaces to be at a right angle can be tested with a square; those to be in line can be tried with a straight-edge, and in each case the truth or alignment of the surfaces is tested by contact of the testing tool. But in the cases where surfaces at an angle are tested or measured the tools must be set to a line or lines, and the work must be measured or cut to lines, thus: Suppose it were found that the bedding surface of the bra.s.s B was a certain amount out of alignment with the corresponding bedding surface on the other side frame, and, by measurement, this amount determined to be 1/64 inch, then there is a liability to error in measuring this 1/64. The bra.s.ses must be marked (for boring the same 1/64 out of square, inducing another liability of error in marking that amount); this marking being done by lines, there is a liability to error in setting the work to the lines. From these liabilities to error, it is generally found that work not true in alignment requires, when it comes to be put together, to have each piece fitted to its place and corrected for alignment.

But, suppose the ways are made true and in proper alignment, then the bra.s.s bores are simply made of equal thickness at the crown, and on the sides at a right angle to the inside faces of the ways; and truth, in these respects, may be measured by actual contact, with the square or calipers, eliminating the chance of error.

In repairing the machine, or putting in new bearings or bra.s.ses; the measurement and transferring of the error in the ways to the bra.s.ses has all to be gone through with again, and the parts fitted for alignment; whereas, if the ways are true, the bra.s.ses can be made true, and to go together, with but little, if any, adjustment when tried in their places.

[Ill.u.s.tration: Fig. 2565.]

The most accurate method of testing the adjustment of the ways is as follows: Fig. 2565 represents a plan view of the frame; N represents a straight-edge applied to the surfaces of the jaws _a_ _b_. The method of applying this straight-edge is to place one end across a jaw, as _a_, while the other end is elevated above _b_; then, while pressing the end firmly against _a_, lower the other end to the face of _b_; if its edge at that end falls fair with _b_, so as just to touch it, the process may be reversed--one end being pressed to _b_, and the other lowered upon _a_. By this means, it will not only be discovered whether the jaws _a_ and _b_ stand square across the frame, but also whether the frame on either side is sprung. A square _c_ may also be rested against N, and its blade _d_ tested with the side face of the way, as shown. The same process of testing should be applied to the other jaw faces _e_, _f_.

Suppose, however, that the width between the jaws _a_, _f_ was less than that between _e_, _b_, then the straight-edge, when pressed to _a_, would show a s.p.a.ce between its edge and _b_; and also a s.p.a.ce between its edge and _e_, when its other end was pressed to _f_; and, when these s.p.a.ces were equal in amount, the frames would be set true in one direction. To test the truth in the other direction, the straight-edge should be applied after the same manner to the bottom surfaces _g_, _h_.

It will not answer to rest the straight-edge against the two surfaces and observe their coincidence with its edge, because any error cannot be sufficiently, readily, or accurately tested by this means. Nor will it answer to test by the bearing marks of a straight-edge applied with marking, unless the coat of marking be very fine and the straight-edge be moved without any vertical pressure on it; because, under such pressure, the straight-edge will bend.

The ways for all the bearings should be tested in this manner; so that, if from any error in the machine work, some of them will not come fair, the frames may be set to align those that it is of most importance to align truly; or if there is no choice in this respect, then those carrying the largest bearing should be set true; because, if it be decided to correct the error on the other bearing or bearings, there will be less area to file or operate upon. The setting being complete the holes may be reamed and the remaining bolts put in, the testing being repeated after the frame is finally bolted together. If this final test shows that bolting the frame up has altered the alignment by springing the frame, the bolts in one foot, as say I, Fig. 2564, may be slackened and the test repeated; and, if the frame is then found true, it is the bolting at I that causes the spring, on account of the bedding surfaces not fitting properly. If I is not found to be at fault, it may be bolted up again and J tested by loosening its bolts, and so on, until the location of the error is detected. Furthermore, when the frame is bolted up, the width of the bearings, as from _a_ to _b_, should be tested; for in a job of this kind, it will pay to have the framework so true to the drawing that, if the other parts, as the shafts, bearing parts, &c., be also made to the drawings, the parts will go together, thus avoiding the necessity of varying all the other parts from the drawing to accommodate errors in the framework.

[Ill.u.s.tration: Fig. 2566.]

Among the jobs that the erector is often called upon to perform is that of patching or repairing pieces that have cracked or broken. Fig. 2566 represents a case of this kind, the fracture being at D. The principle to be observed in work of this kind is to cause the bolts to force the fractured pieces together, so that the irregularity or crookedness of the crack, as at D in the figure, may serve to lock the pieces together.

[Ill.u.s.tration: Fig. 2567.]

Suppose, for example, we were to put on a patch P, Fig. 2567, and there would be but little to prevent the crack from opening under severe strain, and the patch would stretch, permitting the crack to open and finally causing the bolts to break or sheer off. A preferable plan, therefore, is to put two patches on the sides in the following manner:--

The holes should be drilled through the beam and the plates held against the beam so that their holes may be marked by a scriber pa.s.sed through the holes in the beam. The holes in the plates should be drilled closer together than those in the beam, so that when driven in they will serve as keys to close the two sides of the crack together, as shown in Fig.

2568, where it is seen that one side of the bolt bears against the holes in the patch and the other against the holes in the beam. To facilitate getting the bolts in place the plates may be heated so as to expand them.

In cases in which it would not be permissible to drill so many holes through the beam on account of weakening it, we may use patch bolts with countersunk heads, as in Fig. 2569. Two only of the bolts pa.s.s entirely through, and it is best to let them be taper, as at A in the figure, the head not meeting the patch. The hole in the beam, after being reamed taper, should be filed out on the side B, and that in the patch plates on the other side, as at C and D, so that the bolts will serve as keys.

After these two bolts are in place and their nuts firmly screwed home, the holes for the patch bolts may be drilled through the plates and into the beam. When the countersunk head bolts are fitted they should be turned down behind the head, so as to leave a part weaker than the bolt, and then screwed in until the required end breaks off. The taper bolts should be of steel, but those with countersunk heads may be of iron.

[Ill.u.s.tration: Fig. 2568.]

[Ill.u.s.tration: Fig. 2569.]

ERECTING AN IRON PLANER.--If an iron planer be properly fitted and erected, the table will be quite solid in the [V]-ways in the bed, and will not rock or move even though a heavy vertical cut be taken at the extreme sides of the table, but any error of truth of alignment or fit either in the bed-ways or the table [V]'s will cause the table to lie improperly in the [V]'s and to be apt to rock as it traverses. The author has had planed upon a planer thirty years old, at the Freeland Tool Works, in New York City, a cast-iron surface 12 20 inches, the metal weighing about 60 lbs., and the surfaces were so truly planed that one would lift the other by reason of a partial vacuum between the two.

These planed surfaces were exhibited by the author at the American Inst.i.tute Exhibition in 1877, and were awarded a medal of superiority.

The manner in which this planer was fitted and erected, and the principles involved in such fitting and erecting, are as follows:

While it is essential that the foot or resting surface of a planer bed (whether it stands on legs or rests direct upon its foundation) be as true as it is practicable to plane it, still it is more essential that the [V]'s or ways be true, and as the casting will be apt to alter its form from having the surface metal removed, it is best to plane the side on which the ways are the last.

When the bed is placed upon the machine to have its resting surface planed, the casting being uneven, it will be necessary to place packing pieces of suitable thickness beneath the places where the clamping plates hold it, so that the pressure of those plates may not spring or bend the casting.

These packing pieces require to fill up solidly (without lifting the bed) the hollow places, and it is a good plan to place among them a piece of strong writing paper for reasons which will appear presently.

In planing the bed all the surfaces should be roughed out before any are finished. Before any finishing cuts are taken all the clamping bolts should be loosened and the pieces of paper tried by pulling them, so that if the casting has altered its form it will be made apparent by some one of the pieces of paper becoming loose.

In this case the packing must be readjusted, clamping both as lightly as will hold the work, and all as equally as possible, when the finishing cuts may be taken.

[Ill.u.s.tration: Fig. 2570.]

The best form of template to plane the ways to is that shown in Fig.

2570, in which B is a side and A an end view. A corresponding female template being shown at D to be used in planing the table [V]'s.