Modern Machine-Shop Practice Part 160

To line up the bra.s.ses for length we proceed as follows: One of the liners adjusts the length of the rod and the other simply serves to set the key back to its proper height, so that it shall not pa.s.s too far through the keyway, as the wear of the bra.s.ses lets it down. Which of the liners will be the one by which to alter or adjust the length of the rod depends upon the design of the rod itself; but, in the case of all solid-ended rods, or those in which the position of the strap is fixed by means of bolts, it is the liner behind the end bra.s.s, as D, in Fig.

2402, as stated in the opening of this discussion, and it is the first one, therefore, to be fitted. The s.p.a.ce at E is where the second liner requires to be placed, its thickness being that necessary to lift up the key from its bottom or lowest position, as shown in the cut, to the highest position.

[Ill.u.s.tration: Fig. 2403.]

In strap-ended rods in which the strap is not bolted to the rod, but moves farther upon the rod as the key pa.s.ses farther through the keyway, it is the bra.s.s next to the rod end, as B, in Fig. 2403, by which to adjust the length of the rod, and its liner L is, therefore, the one to be fitted first; the s.p.a.ce E is, in this case, the one to be fitted with a liner of sufficient thickness to lift the key up. It will now be noted that the thickness of L in both figures requires to be exact, so that the rod may be of correct length, which is necessary, so that there may be the same amount of clearance or s.p.a.ce between the piston head and the cylinder cover when the piston is at the respective ends of the stroke.

But the liners to fill the respective s.p.a.ces E need not necessarily be of the exact thickness (although it is better that they should be), because if too thin the only effect will be that the key will pa.s.s farther through the keyway than otherwise. In considering in any form of rod which is the liner to be put in first to bring the rod to length, we have the general rule that the bra.s.s that moves in the strap or rod end when the key is moved farther through the keyway is the one to be lined last. The method of obtaining the proper thickness of the liners L, Figs. 2402 and 2403, are as follows: If the rods have been correctly made at first, the centre of the bra.s.s bores will be midway in the s.p.a.ces for the bra.s.ses (denoted by F in the two figures). If the oil-holes in the strap or rod end (as the case may be) have been drilled in the centre of this s.p.a.ce F as they should be, then the line _g_ will represent the centre of F and the centre of the oil-holes, and all that will be necessary will be to place behind D and B respectively a liner of sufficient thickness to bring the joint face of these bra.s.ses (D and B) even with the line _g_. To ascertain the thickness of liner necessary for this purpose, suppose the case of a rod end of the design shown in Fig. 2402, then, with the strap off the rod, drive the bra.s.s D down until its crown face beds fairly against the strap C, and with a scriber mark on the inside face of the jaw of the strap a line coincident with the joint face of the bra.s.s, then set the bra.s.s up the strap until its joint face comes fair with the centre of the oil-hole or the central line _g_, and then mark a second line so that on taking the bra.s.s out of the strap there will appear two lines, and the distance between these two lines is the necessary thickness of liner. In the case of the form of rod end shown in Fig. 2403, the process would be as follows: Let the strap have placed in it the bra.s.s B only, place it upon the rod, and set it so that it binds the gib and key, when the key is lifted up to its required position, then, with the bra.s.s B bedding fairly against the rod end, mark on the strap a line coincident with the joint face of the bra.s.s as before. Then move the bra.s.s in the strap until its face comes fair with the centre of the oil-hole or line _g_, and mark another line, and the thickness between these lines is the thickness of liner required at L.

[Ill.u.s.tration: Fig. 2404.]

[Ill.u.s.tration: Fig. 2405.]

[Ill.u.s.tration: Fig. 2406.]

If the bra.s.s is to be lined sufficiently to merely bring the key up without respect to the length of the rod we may drive the key home as in Fig. 2404, and mark on it a line coincident with the edge A of the strap. We then lift the key up to its proper height and mark a second line, so that when removed from the keyway the key will have on it the two lines shown in Fig. 2405, A being the first and B the second line; and the difference between the width of the key at A and its width at B will be the thickness of the liner necessary to be placed behind the bra.s.s nearest to the key. To ascertain the precise amount of this difference (because a very small error as to this amount causes a great deal of extra labor), we set a pair of outside calipers to the width at A; and then pa.s.sing the caliper points down to B, we keep one of the points even with the line B, and insert a wedge until it just fills the s.p.a.ce between the other point and the side of the key, as shown in Fig.

2406, C being the wedge, which should be chalked along its surface so that, when inserted until it touches against the caliper point, the latter will leave a mark on the wedge, denoting exactly how far the wedge entered, and hence the exact required thickness of liner.

[Ill.u.s.tration: Fig. 2407.]

[Ill.u.s.tration: Fig. 2408.]

It has thus far been supposed that the joint faces of the bra.s.ses are made to come bra.s.s and bra.s.s, that is to say, b.u.t.t close together from the key pressure, when the bra.s.s bores properly fit the journal.

Suppose, however, that the joint of the bra.s.s is left open as in Fig.

2407, and in that case a strip of metal F, whose diameter equals that of the journal, may be inserted between the bra.s.ses as shown, and at its centre should be provided a small centre-punch mark, denoting the centre of the bore. A piece of this kind should be inserted in the bra.s.ses at each end of the rod and placed in the middle of the length of the bore, the centre-punch marks being to apply the trammels to. Or if the rod was made of correct length when new, and the bore of the bra.s.ses, therefore, requires to stand central in s.p.a.ce F, Fig. 2403, then the pieces F, Fig.

2407, may be dispensed with by marking a line B, Fig. 2408, central to s.p.a.ce F, Fig. 2403. Then put the strap on the rod (with the bra.s.ses, gib, and key in place), and pull the strap back to hold the key up to its proper height.

[Ill.u.s.tration: Fig. 2409.]

The two bra.s.ses should then be placed as far apart as possible in the strap, each bedding fairly against its back or crown. Then, using the joint face of the back bra.s.s as a straight-edge or guide, a line should be marked on the side face of the strap, this line representing the position of that face when the bra.s.s is bedded fairly home, and being shown in Fig. 2408 at A. This bra.s.s should then be moved forward until the bore of the pair of bra.s.ses at D, Fig. 2408, measures equal to the diameter of the journal (of the crank pin or of the cross-head pin as the case may be) and a second line B, also coincident with the joint face of the bra.s.s, should be marked upon the strap, and the strap will then have marked on it the two lines shown in Fig. 2409, in which it is shown removed from the rod; the distance apart of these two lines will be the thickness of the two liners combined, hence half this thickness will be the thickness necessary for each liner. Suppose, however, that it is not known whether the rod has been correctly made, and therefore it be unknown whether, in order to have the rod of the correct length, the bra.s.s bore should stand in the centre of the s.p.a.ce or not.

This is often the case in repairs, and sometimes on new rods, in which slight inaccuracies of workmans.h.i.+p are apt to occur. In this case it is best to mark a line, as G, in Fig. 2410, representing at each end of the rod the centre of the s.p.a.ce F in that figure. Then set a pair of trammels to the correct length of the rod, and with one point of the trammel resting on the point at the intersection of line C with line D (the latter being the line G transferred to the centre of the bore) at the small end of the rod, we mark a line at the other end. If the lines D are too far apart, making the rod too long, the trammels will mark a line R, and the distance between lines R and D at the large end will be the amount the rod is too long, while half this distance will be the thickness of liner to go behind each bottom bra.s.s if the error of length is to be equally divided between the two ends of the rod, in which case a line T, midway between D and R, must be marked, the trammel then being rested on T, and the line S marked. These two lines, S and T, are then the centre lines for the bores of the bra.s.ses.

[Ill.u.s.tration: Fig. 2410.]

If it is determined that one pair of bra.s.ses shall be central in its s.p.a.ce F, all the error being thrown on the other pair, this may be done by lining one pair up so that its bore is true to line D, and putting behind the back bra.s.s at the other end a liner whose thickness is equal to the distance between D and R at the large end of the rod. It is obvious that the measurement for rod length must be taken on the line C.

Having thus determined what thickness of liner is necessary to bring the rod to its proper length, it remains to find the thickness of liner necessary for the other half bra.s.s, to bring the key up to its proper position, the process for which has already been explained. After, however, the various liner thicknesses have been found, and the sheet metal selected to cut them from, it is well to try if the thickness is correct by cutting off a small piece of the metal, putting it in place behind the bra.s.s, and then, after keying up the bra.s.ses, the rod length may be trammelled.

As the liners placed behind connecting-rod bra.s.ses require to be very finely bedded, the facility with which their forms permit them to be fitted is an important consideration.

[Ill.u.s.tration: Fig. 2411.]

In Fig. 2411 is shown the forms commonly given, the requisite form of liner being shown beneath each. Form 1 will bed very firmly to its seat, but it will be observed that its liner is a difficult one to make, the bottom section A being thicker than the sides or wings B. This is a troublesome form of liner to fit as well as to make. If it be made of wrought iron, the wings B must be either forged or filed to their reduced thickness.

In the form at 2 in the figure we have the same defect, while in addition the liner will not adjust itself so readily in position to its bed.

This latter is an easier form to make in the moulding pattern, and easier to mould, and somewhat easier to fit, but it is not so firm as the first. To cause this form of bra.s.s to bed easily to its proper position it is sometimes given a lug on the bottom, as at 3 in the figure, the lug extending part of the width across only, because if it extended fully across, the liner would require to be in two pieces, causing trouble both in fitting them and in getting them into their places. When the lug extends partly across, the liner must have a slot to pa.s.s over and admit the lug, and this causes trouble in bending the liner to the required curve.

In the form shown at 4 in the figure all these difficulties are avoided, while, if the lower corners are made square instead of rounding, a simple piece of sheet metal will serve as a liner requiring but little fitting and bedding if it be of the proper thickness.

[Ill.u.s.tration: Fig. 2412.]

To fit up a link motion, a.s.suming the machine work to be done, the first thing to do is to face up the side faces of the links, making them parallel, and true to a surface plate. The slot is then filed out square to the side faces, its curve being filed to a template T, Fig. 2412, which is provided with a piece of wire for a handle. It is supplied with red marking, and is rubbed upon the slot to mark the high spots. The same template may be used to prepare the link block or die; but as soon as the block can be moved in the slot with slight hammer blows (using a mallet or a block of wood) it should be used instead of the template, the bearing marks serving to correct and finish the block as well as the slot. In filing up the block care should be taken to make it of even thickness on each side of its hole and with its sides parallel to the hole, the latter being of great importance. When the block is a sufficiently easy fit in the slot to permit it, a round stick of wood may be put through it and used to move it up and down the link slot for the marking process.

The next operation is to fit the eccentric rod eyes to the link, and to then ream out the holes in both the link and the eyes while they are put together. The block may then be placed in the link, and the rocker pin pa.s.sed through the block and into the rocker arm, so that the working fit of these parts when put together may be tested and adjusted if necessary. The link hanger may then be fitted to the saddle pin, when the whole will be ready for the file finis.h.i.+ng and polis.h.i.+ng, after which it may be case-hardened.

CASE-HARDENING.--Case-hardening consists in the conversion of the surface of wrought iron into steel, or in converting the grade of a low steel into a sufficiently high grade to render it capable of hardening.

The depth to which this conversion occurs depends upon the material used to produce it, and the length of time the process is continued, varying from 1/64 inch under the prussiate of potash process to 1/16 or 1/8 inch in the case of long-continued box case-hardening.

Work that is thoroughly case-hardened has a dull white, frosted-looking surface. If the surface of the work is mottled, or has patches of fancy water-mark colors, it may be hard, but it is not so to the highest attainable degree.

To thoroughly test this, take a new dead-smooth file and apply its corner edge under heavy pressure to the work on an edge where the fancy colours are, and then on an edge where the surface is white, and the latter will be found to be the hardest as well as hardened the deepest.

The simplest method of case-hardening is by the prussiate of potash process, for which it is essential that the prussiate of potash be finely powdered, and contain no small lumps. The piece being heated may then, if small, be dipped in the prussiate of potash, or if large have the same spread upon it. In either case, however, the work must be hot enough to cause the potash to fuse and run over the work surface, and this action may be a.s.sisted by using a piece of iron wire, spoon-shaped at the end, wherewith to apply potash to the work and rub it upon the work surface.

After the potash has thoroughly fused and run over the entire surface of the work it will usually have become somewhat cooled, and will require reheating before quenching in the water.

If this reheating be done in the blacksmith's fire, it is not well to put the blast on; it is better to let the blast on gently while applying the potash to the work, so as to have a live clear fire to put the work in, and reheat it with the blast turned off.

While the work is in the fire it should be constantly rotated, not only to heat it evenly, but to let the adhering potash run over the entire surface, and as soon as the required heat is attained the work should be removed from the fire quickly and quenched in water.

It may be added, however, that if after the potash has been applied and fused more potash be added, so that it will adhere to the work and not fuse until the work is put into the fire a second time, then, after the work is quenched and taken from the water, there will be found on it a thick white and closely adhering fur of melted potash, and the work will be a dead white, with no fancy colors on it, and as hard as it is possible to make it.

The prussiate of potash process is, of course, from its expensiveness, both in material and labor, too costly for work to be done in quant.i.ties, and box-hardening is therefore resorted to.

In box case-hardening the work is case-hardened all over. It consists in packing the work in an iron box containing the hardening material, and subjecting the whole to a cherry-red heat for some hours.

A very common process is to fill a sheet-iron box with the work closely packed about with bone-dust, the pieces of the work having at least a thickness of 3/8ths of an inch of bone-dust around them. The seams of the box are well luted with clay to prevent the gases from the consumed bone-dust from escaping, and to exclude air.

Various ingredients are used to effect case-hardening. One process is as follows: 20 lbs. of sc.r.a.p leather and 15 lbs. of hoofs (cut into pieces of about an inch square), 4 lbs. of salt, and one gallon of urine are prepared, and a wrought iron box with a lid capable of being fastened on is obtained. The fastenings must be capable of ready unfastening when hot. A layer of leather and pieces of hoofs about 1-1/2 inches thick is first laid in the box, then a layer of salt, and then a layer of work.

Leather and hoof are then packed closely around the work and above it for a thickness of about an inch, and a second layer of work added, and so on, the last layer being of leather, &c., completely filling the box; the urine is then added, and the box well sealed with clay.

The box is placed in a furnace and kept at a red heat for about fourteen hours, and is then taken to a deep tank, and the work quickly immersed, so as not to be exposed to the air after the box is opened.

If the pieces are of solid proportions, so as not to be liable to bend or warp in the cooling, the contents of the box are simply dumped into the tank, the water being allowed to flow freely in the tank to keep up a circulation and cool the work quickly; some work, however, requires careful dipping to prevent it from warping. Thus a link or a double-eye would be dipped endwise, a plate edgewise; but all pieces should be immersed as quickly as possible after the box is opened.

Sheehan's patent process for box case-hardening, which is considered a very good one, is thus described by the inventor:

DIRECTIONS TO MAKE AND USE SHEEHAN'S PATENT PROCESS FOR STEELIFYING IRON.

No. 1 is common salt.

No. 2 is sal soda.

No. 3 is charcoal pulverized.

No. 4 is black oxide of manganese.

No. 5 is common black rosin.