Fig. 2410Fig. 2410.
Fig. 2410.
This is often the case in repairs, and sometimes on new rods, in which slight inaccuracies of workmanship are apt to occur. In this case it is best to mark a line, asg, inFig. 2410, representing at each end of the rod the centre of the spacefin that figure. Then set apair of trammels to the correct length of the rod, and with one point of the trammel resting on the point at the intersection of linecwith lined(the latter being the linegtransferred to the centre of the bore) at the small end of the rod, we mark a line at the other end. If the linesdare too far apart, making the rod too long, the trammels will mark a liner, and the distance between linesranddat 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 brass if the error of length is to be equally divided between the two ends of the rod, in which case a linet, midway betweendandr, must be marked, the trammel then being rested ont, and the linesmarked. These two lines,sandt, are then the centre lines for the bores of the brasses.
If it is determined that one pair of brasses shall be central in its spacef, 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 lined, and putting behind the back brass at the other end a liner whose thickness is equal to the distance betweendandrat the large end of the rod. It is obvious that the measurement for rod length must be taken on the linec.
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 brass, 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 brass, and then, after keying up the brasses, the rod length may be trammelled.
As the liners placed behind connecting-rod brasses require to be very finely bedded, the facility with which their forms permit them to be fitted is an important consideration.
Fig. 2411Fig. 2411.
Fig. 2411.
InFig. 2411is 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 sectionabeing thicker than the sides or wingsb. This is a troublesome form of liner to fit as well as to make. If it be made of wrought iron, the wingsbmust 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 brass 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 pass 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.
Fig. 2412Fig. 2412.
Fig. 2412.
To fit up a link motion, assuming 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 templatet,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 passed 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 finishing and polishing, 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 from1⁄64inch under the prussiate of potash process to1⁄16or1⁄8inch 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 assisted 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 quantities, 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 of3⁄8ths 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 scrap 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 11⁄2inches 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:
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.
No. 6 is raw limestone (not burned).
Take of No. 1, 45 lbs., and of No. 2, 12 lbs. Pulverize finely and dissolve in as much water as will dissolve it and no more—say 14 gallons of water in a tight barrel; and let it be well dissolved before using it.
Then take three bushels of No. 3, hardwood charcoal broken small and sifted through a No. 4 sieve. Put the charcoal in a wooden or iron box of suitable size made water-tight.
Next take of No. 4, 5 lbs., and of No. 5, 5 lbs., the rosin pulverized very fine. Mix thoroughly No. 4 and No. 5 with the charcoal in your box.
Then take of the liquid made by dissolving No. 1 and No. 2 in a barrel as stated, and thoroughly wet the charcoal with the whole of said liquid, and mix well.
The charcoal compound is now ready for use.
A suitable box of wrought or cast iron (wrought iron is preferable) should next be provided, large enough for the work intended to be steelified.
Now take No. 6, raw limestone broken small (about the size of peas), and put a layer of the broken limestone, about 11⁄2inches thick, in the bottom of the box. A plate of sheet iron, one-tenth of an inch in thickness, is perforated with1⁄4-inch holes one inch apart. Let this plate drop loose on the limestone inside the box. Place a layer of the charcoal compound, two inches thick, on the top of said perforated plate. Then put a layer of the work intended to be steelified on the layer of charcoal compound, and alternate layers of iron and of the compound until the box is full, taking care to finish with a thick layer of compound on the top of the box. Care should also be taken not to let the work in the box come in contact with the sides or ends of the box. Place a suitable cover on the box and lute it with fire-clay or yellow mud. The cover should have a quarter-inch hole in it to permit the steam to escape while heating.
The box should now be put in an open fire or furnace (furnace preferred), and subjected to a strong heat for five to ten hours, according to the size of the box, and the bulk of iron to be steelified. Remove the pieces from the box one by one and clean with a broom, taking care not to waste the residue, after which, chill in a sufficient body of clear, cold water, and there will be a uniform coat of actual steel on the entire surface of the work to the depth of1⁄16or1⁄8of an inch, according to the time it is left in the fire. The longer it is left in the fire the deeper will be the coat of steel.
Then remove the residue that remains in the box, and cool with the liquid of No. 1 and No. 2, made for the purpose with 20 gallons of water, instead of 14 gallons, as first used with the charcoal compound.
The residue must be cooled off while it is hot, on a piece of sheet iron or an iron box made for the purpose. Turn the residue into the supply box, and it will be ready for use again. The more it is used, the better and stronger it will be for future work.
There is nothing to be renewed for each batch of work but the limestone, and that, after each job, will be good burned lime.
A process used at the Elevated R.R. shops in New York city is as follows: The materials used are: leather, 1 part; bone dust,5 parts; salt, 1 part. Heat for 48 hours to a red heat in a box sealed with fire clay, and quench in a solution of 3 pounds of potash to 30 gallons of water.
The wrought iron thus treated is impervious to a new smooth file at a depth of1⁄16of an inch.
The potash water is said to prevent both warping and the formation of blister marks on the work.
The durability of work case-hardened is greatly enhanced, but it is an expensive process; not so much by reason of the cost of it, but because it involves resetting and a refitting of the parts. The resetting is necessary because the work warps under or during the process. This warping can be prevented to some extent by placing the heaviest pieces in the bottom of the box, and so packing the same that the weight of the top pieces shall not tend to bend those beneath them when the hardening material has burned away, and so placing the upper pieces that they shall not be bent by their own weight. Thus both in packing and locating the work in the box the utmost care is necessary.
Setting Work after hardening it.—Work that has been hardened or case-hardened usually swells during the hardening process, and therefore requires refitting afterwards. This swelling usually occurs in all directions, thus holes and bores become of smaller dimensions, while the outside dimensions also increase, bolts become of larger diameter and sometimes increase in length.
In very exceptional cases, however, the dimensions of a piece of work will not alter.
This renders it usually necessary to refit the work after it has been hardened, thus holes which are ground out by laps or bolts may be ground to diameter in a grinding lathe.
In some practice, however, the work to be hardened is made a somewhat too easy fit, the holes tapped out and the bolts ground in by direct application of the bolts to their holes in connection with flour emery and oil. This latter plan is also adopted for forms not easily ground out in a machine, as, for example, a die in a link of a link motion.
Fig. 2413Fig. 2413.
Fig. 2413.
Fig. 2414Fig. 2414.
Fig. 2414.
To prevent surfaces or forms of this class from altering their shape or dimensions during the hardening process, slips of iron are sometimes fitted to them before they are placed in the hardening box. ThusFig. 2414represents a double eye, andFig. 2413a link having thin pieces fitted in as shown atain both figures.
The heating for the hardening process is also apt to impair the alignment of the work, causing it to require resetting by the aid of parallel strips and straight-edges.
Fig. 2415Fig. 2415.
Fig. 2415.
The faces of the link having been set, the width of the link slot must be set, for it may open or close in places. If it opens it may be closed by the jaws of a powerful vice, while if it closes it may be opened by a pair of inverted keys, inserted as shown inFig. 2415, and driven in by the hammer. At each trial, however, a mark should be made on the driven key, so that it may be known how far to drive it at the next trial.
Fig. 2416Fig. 2416.
Fig. 2416.
Fig. 2417Fig. 2417.
Fig. 2417.
Fig. 2416represents a link that is supposed to have been case-hardened, and to therefore require resetting. The stem fromatobshould first be straightened to a straight-edge on both its side and edge faces. It should then be tested for winding with the winding strips,c,d, placed as inFig. 2416, and then as inFig. 2417.
Fig. 2418Fig. 2418.
Fig. 2418.
Fig. 2419Fig. 2419.
Fig. 2419.
To test the alignment of ende, press a straight-edgesfair against its side face, as inFig. 2418, and measure the distanceh. Then place the straight-edge on the other side face ofeand measure the distancei,Fig. 2419, and these distances both measuring alike,ewill be true providing that the jaws at endfhave not altered from their proper width apart.
Fig. 2420Fig. 2420.
Fig. 2420.
Fig. 2421Fig. 2421.
Fig. 2421.
To test the alignment of the jaws at endf, press a straightedge against the outside face of the hub and measure thedistancej,Fig. 2420, then apply it on the other side and measure distancek,Fig. 2421, and when distancesjandkare equal and the widthlbetween the jaws is correct, endfis in line in one direction. To test it in the other direction, apply a pair of parallel strips, placing one on endeas inFig. 2417, and the other across the face of the hub of endfto see if there is any twist.
Suppose, however, that distancesj kare unequal, then if distancelis too narrow (when tested by the piece that fits between the jaws) then the jaw atfthat gives the widest distance ateis the one that requires correction, or if distancelis too wide, the jaw that shows the least distance at endeis the one requiring correction.
The link should be warmed to about 300°, or nearlyblackhot, and pieces of sheet copper placed between the work and the anvil, and between the blacksmith’s tools and the work, so that the latter may not be bruised by the blows delivered to effect the straightening.
After the process has been performed at each end individually the testing should be repeated, because setting the endfmay have impaired the setting of ende, in the alignment tof.
It is obvious that the same setting or aligning process would be required in the case of a large link, where the ends were forged separately and welded to the body after the machine work and fitting had been done to them.
Fig. 2422Fig. 2422.
Fig. 2422.
Fig. 2423Fig. 2423.
Fig. 2423.
Fig. 2424Fig. 2424.
Fig. 2424.
Fig. 2425Fig. 2425.
Fig. 2425.
Fig. 2426Fig. 2426.
Fig. 2426.
Fitting Brasses to Boxes or to Pillar Blocks.—In the operation of fitting brasses to their boxes or to pillar blocks there are two things to be especially guarded against: First, having the brass let down one-sided, as shown inFig. 2422; and next, aslant, as shown inFig. 2423. The first depends on taking the proper amount off the two side faces, and the second in cutting the inside of the flanges fair. To cut the side faces fair, grip the brass in the vice, as shown inFig. 2424(the brass being shown in section), in which a isablock of wood. Take the measure of the box, down where the brass will come when home, and, if there be any taper to the box, set the inside calipers to the top of the location for the brass, and after the brass is in the vice place a square under one side-face, as atbinFig. 2426, and see how much there is to come off. This saves the use of outside calipers, and is better because, not only is the trouble of setting the latter avoided, but the inside calipers can be tried to the box and the work in an instant, and a correction can at once be made if the calipers have got shifted. The cape chisel, or cross-cut, as it is sometimes termed, should first be used, taking a cut close to the flange, and making it half as deep as the calipers (applied as shown inFig. 2426) show there is metal to come off. Then a similar cut should be taken close to the other flange, especialcare being taken to take both cuts equally deep, and leaving as much to come off the other side face of the brass; otherwise, the brass will come atwist. Then take a straight-edge, and, placing its edge fair with the two chisel-grooves, while holding it firmly against the joint face of the brass, mark a line running from one chisel groove to the other; this line serving as a guide for the depth of all the other cape-chisel grooves. Now cut off the intermediate spaces with the flat chisel, using a straight-edge as a guide. If the box is taper, chip the side face to a corresponding taper, using a bevel-square, or estimating the amount by the eye if it is not too much. Now file the chipped surface flat and true, and then turn the brass upside down, gripping it with the wood as before, and dress the other side face (applying the inside calipers as inFig. 2426), and bring that face down to within about1⁄64inch of the size to which the calipers are set. If the block of wood is made a little shorter than the length of the brass, the calipers can be applied without moving the brass from or in the vice. The method of applying the square to these side faces is shown inFig. 2425, in whichais the brass in section,ba straight-edge, andca square.
Fig. 2427Fig. 2427.
Fig. 2427.
Fig. 2428Fig. 2428.
Fig. 2428.
Fig. 2429Fig. 2429.
Fig. 2429.
Fig. 2430Fig. 2430.
Fig. 2430.
We now turn our attention to the flanges, and apply a square to the crown of the box, bringing the edge of the blade fair with the edge of the box, as shown inFig. 2428,arepresenting the box in section, andbthe square. Supposing the crown of the box to stand square, as shown in the engraving, and as it should do, we set the brass upon a truly-surfaced iron plate and square up the joint face, as shown inFig. 2427, in whichais the surfaced iron,bthe brass, andcthe square. Since, however, the joint face of the brass may not be parallel with the crown face, we may place the square so that its blade edge comes fair with the crown face—that is, as shown atdinFig. 2427—and set the brass crown (by means of inserting a wooden wedge under its face) truly perpendicular or parallel with the square blade edge. Now try the square with the side face of the brass, setting the latter true with the square blade, as inFig. 2430;abeing the iron plate andbthe square; and, supposing the box to be true, as it usually is, we may set a scribing-block, as shown inFig. 2427, and mark off how much is to come off the flanges by scribing a line around the flange, sufficiently depressing the scriber-point to allow an equal amount to come off each of the flanges. Sometimes, however, the inside faces of the box are not true with the outside face. To test this, we place a straight-edge across the outside face, place a square on it, and apply it to the inside face of the box, as inFig. 2429, which is a plan view of the box,abeing the straight-edge andbthe square. If the square thus applied shows a want of truth in the box, we may set the brass over when adjusting it (as inFig. 2427) to a corresponding amount, and thus mark off the flanges to suit the box.
Fig. 2431Fig. 2431.
Fig. 2431.
Fig. 2432Fig. 2432.
Fig. 2432.
To hold the brass while operating on the flanges, we resort to the device shown inFig. 2431, in whichais a bolt,bthe brass,ca piece of hard wood, andpa clamp fastened down by a nutd. To sustain the platep, so that it shall not fall down on the piece of wood every time the brass is taken out to try it in the box, we may insert the spiral springs, shown in the separate view of the bolt, nut, and plate. One such holding device will do for different sizes of brasses, by either gripping the bolt lower down in the vice jaws or putting washers between the nut and the plate. This will hold the brass very firmly, and at the same time leave the whole of the flange easily got at. When the flanges are dressed, we may try the brass in the box, putting red-lead marking on the box to mark where the brass binds. While letting the brass down, however, we must be careful to let it down fair, to avoid the state of things shown inFigs. 2422and2423. A ready method of doing this is (supposing the box to be true, as it should be, and making the necessary allowance if it is not), to set a pair of inside calipers to the joint face of the brass and the top of the box, as shown inFig. 2432, trying the calipers (in the two positions there shown) on both sides of the box. This should be done every time the brass is tried in the box, until such time as the brass begins to bed against the bottom of the box.
We now come to the bedding of the brass to its seat in the box. This requires skillful treatment; for one mistake will involve a great deal of extra work to rectify it.
In fitting the brass to the box care must be taken to leave it a rather tighter fit to the box than it requires to be when finished, that is after the bore has been made, because in the boring operation the sides of the brass are apt to close and loosen the fit of the brass to the box.
Fig. 2433Fig. 2433.
Fig. 2433.
When the side faces and flanges are so far fitted as to render probable the brass driving home at the next trial, the bed of the box should be given a coat of red-lead marking, and small pellets of stiff red lead or putty should be stuck on the bottom of the box, two at each end of each bevel, and two at each end of the bottom, with one in the middle of the bottom and each bevel, as shown ata,b,c,d,e,f, inFig. 2433, by the black spots. Then when the brass comes home, it will flatten these pellets, and their thickness (when the brass is taken out) will show how much the bevels are out, and how much to take off the brass to make it bed. These pelletsmustbe restored to their original shape every time the brass is tried; otherwise, they may mislead. To insure their sticking to the box, and not coming out with the brass, the bottom of the box must have red-lead marking kept upon it. The chipping should continue until the pellets flatten out equally on the two bevels, but are left a little thicker on the bottom. If this is not done, the bottom will bed first, causing a great deal of extra filing, because filing the side bevels will let the bottom down too far.
In driving the brass in and out of the box while fitting it, a piece of wood must be used to strike on, otherwise the brass will stretch during the fitting and come loose in the box during the boring.[33]
[33]See remarks on Pening,p. 68.
[33]See remarks on Pening,p. 68.
Fig. 2434Fig. 2434.
Fig. 2434.
Fig. 2435Fig. 2435.
Fig. 2435.
The patterns from which the castings for brasses are moulded should not be made of the same angle or sweep on the bedding part or bottom as the bottom of the box, pedestal, or pillar block, because the brass casting, in cooling in the mould, contracts across the bore; thus if inFigs. 2434and2435the full lines denote the shape of the pattern the dotted lines denote the shape the casting will be.
Fig. 2436Fig. 2436.
Fig. 2436.
The result of this is that when the brass is let down in the box it will bed on the crown and not at the sides. Thus inFig. 2436,ais a pedestal, andba brass which beds atc, but not atdore. InFig. 2437is shown an example of a brass, with a circular bottom, which would bed at the crownc, but not at the sidesd e, until the metal was cut down to the dotted circlef.
Fig. 2437Fig. 2437.
Fig. 2437.
The amount to which this contraction in the mould occurs varies with the size of the brass, the difference in the thickness at the crown and at the face joint, the composition of the metal of which the casting is made, and the temperature of the metal when poured into the mould. It should always be allowed for, however, for the following reasons. Referring again toFig. 2436, it will be noted that it requires a heavy cut offcto bringe,dto a bearing, while it is apparent that if the brass met the box ate,dbefore it did atc, but little filing ate,dwould let the brass down a long way. It saves work, therefore, to so make the pattern as to insure that the brass casting shall have bedding contact atdandebefore it does atc. As an example of the allowance to be made for this purpose, it may be stated that in brasses of 6 inches bore and 9 inches long, the hexagon of the brass pattern atd,e,Fig. 2436, would require about1⁄16inch put on them to compensate for the contraction, supposing that the hexagon on the brass pattern were made at first to fit the hexagon of the pedestal or axle box.
To originate a true flat surface we proceed as follows: In the absence of a standard plate to go by, we must have three plates, and one of them must be accepted as a provisional or temporary standard. This we will call No. 1, and we fit Nos. 2 and 3 to it and then try them together, and if they also fit it is proof that No. 1 was true, and that all three are therefore true. It will very rarely happen, however, that this is the case; but Nos. 2 and 3 merely serve to show how much No. 1 was out of true.
Fig. 2438Fig. 2438.
Fig. 2438.
Fig. 2439Fig. 2439.
Fig. 2439.
Fig. 2440Fig. 2440.
Fig. 2440.
Suppose, for example, that No. 1 is concave in its length, and we fit No. 2 to it, as inFig. 2438, and then fit No. 3 to it as inFig. 2439, and when we come to put Nos. 2 and 3 together, as inFig. 2440, we find that they are out of true to twice the amount that No. 1 is, and that all the work that has been done to them to fit them to No. 1 has been thrown away, and possibly to make them worse instead of better. It becomes important therefore to select the most true plate for No. 1, and this we may do asfollows:—
Fig. 2441Fig. 2441.
Fig. 2441.
Fig. 2442Fig. 2442.
Fig. 2442.
If we have a straight-edge that is known to be true, we may lay it on the face of a plate and move it laterally from each end alternately, and if it swings from the centre the plate face is rounding, while if it shuffles across moving first at one end and then at the other the face is hollow; but if it glides as it were across, the surface is nearer true. The straight-edge must not be pressed to the plate, but merely touched laterally to make it move laterally, for if we take a true straight-edge and press it vertically to a true surface while moving it, it will show the marks of contact the most plainly immediately beneath the parts where it is pressed. Selecting by this means the two plates that appear to be the most true we proceed to test them further as follows: We give to one of them which we will call No. 1 a light coat of red marking, and placing it upon the other or No. 2, we move it about in all directions and then take the two apart to examine the bearing marks. Suppose then that No. 1 shows the bearing marks to be at the shaded places,aandb, inFig. 2441, while the bearing marks on No. 2 are as at the shaded partsaandbinFig. 2442, the two endsahaving been placed together; then we know thatbis a high spot on No. 1, andaa high spot on No. 1 for the following reasons. The marks ata, No. 2, have been made by the marking ataon No. 1, and will extend across No. 2, a distance depending upon how much No. 1 has moved across No. 2, for if corneraof No. 1 had only moved half-way across No. 2, it could only have marked half-way across it. Similarly spotbon No. 1 has marked spotbin No. 2, because it has been moved all the way across, it being evident that the marking onb, No. 1, can only mark plate 2 as far across its width as it is moved across it. From this it follows that the higher or more prominent a spot is the less will be the area of the bearing mark at that spot.
Fig. 2443Fig. 2443.
Fig. 2443.
Fig. 2444Fig. 2444.
Fig. 2444.
Now suppose that the two plates were curved to an equal degree as inFig. 2443, and the bearing marks would extend all over both surfaces; but we may discover this error by turning one plate at a right angle, as inFig. 2444, in which case the bearing marks would show along the edges of No. 1 and along the middle of No. 2, and we may correct each with the file until both plates mark all across and from end to end when tried together lengthways as inFig. 2443, and one across the other as inFig. 2444.But the plates may be curved to a different degree, as inFig. 2445, and it then becomes necessary to know which to file the most in correcting them and fitting them together, which we may discover asfollows:—