PRACTICAL MECHANICS FOR BOYS TEXT

will fit against each other perfectly flat. A pair of such disks, if absolutely true, will hold together by the force of cohesion, even in a dry state, or they will, as it were, float against each other. TRUE SURFACING.--Prior to about 1850 the necessity of true surfacing was not so important or as well known as at the present time. About that period Sir J. Whitworth, an eminent English engineer and mechanic, called the attention of machinists to the great advantage arising from true surfaces and edges for all types of machinery, and he laid the foundation of the knowledge in accurating surfacing. PRECISION TOOLS.--Due to his energy many precision tools were made, all tending to this end, and as a result machines became better and more efficient in every way. It had this great advantage: It taught the workman of his day how to use the file and scraper, because both must be used conjunctively to make an absolutely flat plate. Contrary to general beliefs, shapers and planers do not make absolutely accurate surfaces. The test of this is to put together two plates so planed off. There is just enough unevenness to permit air to get between the plates. If they were perfectly true they would exclude all air, and it would be a difficult matter to draw them apart. TEST OF THE MECHANIC.--To make them perfectly flat, one plate has chalk rubbed over it, and the two plates are then rubbed together. This will quickly show where the high spots are, and the file and scraper are then used to cut away the metal. [Illustration: _Fig. 46. Fig. 47. Hexagon Nut_] In England the test of the mechanic used to be determined by his ability to file a piece of metal flat. It was regarded as the highest art. This is not the most desirable test at the present time, and it is recognized that a much severer test is to file a narrow piece exactly flat, and so that it will not have a trace of roundness, and be square from end to end. TEST SUGGESTIONS.--In a shop which does not have the advantage of a planer or shaper, there are so many articles which must be filed up, that it is interesting to know something of how the various articles are made with a file. To file a hexagon, or six-sided nut will be a good test with a file. To do this a little study in geometrical lines will save a vast amount of time. In beginning the work, measure the radius with a divider, and then step off and make six marks equidistant from each other on the round surface. [Illustration: _Fig. 48. Cutting Key-way_] USE OF THE DIVIDERS.--The distance between each of these points is equal to the radius, or half the diameter, of the round bar. See Fig. 46, which shows this. The marks should be scribed across the surface, as shown in Fig. 47, where the lines show the ends of the facets of the outside of the nut. Do not let the file obliterate the lines at the rough cutting, but leave enough material so you can make a good finish at the line. CUTTING A KEY-WAY.--Another job you may have frequent occasion to perform, is to cut a way for a key in a shaft and in a wheel hub. Naturally, this will be first roughed out with a cold chisel narrower than the key is to be, and also slightly shallower than the dimensions of the key. A flat file should be used for the purpose, first a heavy rough one, for the first cutting. The better way is to have the key so it can be frequently tried while the filing process is going on, so that to fit the key in this way is a comparatively easy task. KEY-WAY DIFFICULTIES.--But the trouble commences when the groove is filed for the depth. Invariably, the mistake will be made of filing the width first, so the key will fit in. As a result, in deepening the groove the file will contact with the walls, and you have a key-way too wide for the key. To avoid this, file the depth, or nearly so, and then with a fine file cut in the corners in the direction indicated by the dart, Fig. 48. A proper key is square in cross section. In such a case the depth of the key-way, at each side wall, is just half the width of the key-way. An excellent key-seat rule can be made by filing out two right-angled pieces, as shown in Fig. 49, which can be attached to the ordinary six-inch metal rule, and this will enable you to scribe the line accurately for the key-way on the shaft. [Illustration: _Fig. 49. Key-seat Rule_] [Illustration: _Fig. 50. Fig. 51. Filing Metal Round_] FILING METAL ROUND.--It is sometimes necessary to file a piece of metal round. This is a hard job, particularly where it is impossible to scribe the end of the piece. Suppose it is necessary to file up a bearing surface, or surfaces, intermediate the ends of a square bar. You have in that case four sides to start from, the opposite sides being parallel with each other, so that you will have two dimensions, and four equal sides, as shown in Fig. 50. The first step will be to file off accurately the four corners 1, 2, 3, 4, so as to form eight equal sides or faces, as shown in Fig. 51. If you will now proceed to file down carefully the eight corners, so as to make sixteen sides, as in Fig. 52, the fourth set of corners filed down will make the filed part look like the illustration Fig. 53 with thirty-two faces. [Illustration: _Fig. 52. Fig. 53. Making a Bar Round_] This may be further filed down into sixty-four faces, and a few cuts of the finishing file will take off the little ridges which still remain. By using emery cloth, and wrapping it around the bearing portion, and changing it continually, while drawing it back and forth, will enable you to make a bearing which, by care, will caliper up in good shape. KINDS OF FILES.--Each file has five distinct properties; namely: the length, the contour, the form in cross section, the kind of teeth, and the fineness of the teeth. There are nine well-defined shapes for files. These may be enumerated as follows: [Illustration: _Fig. 54. Cross Sections of Files._] No. 1. The cotter file. The small kind is called a verge or pivot file. No. 2. Square file, which may be tapering from end to end, or have parallel sides throughout. No. 3. Watch pinion file. This may have its sides parallel or tapering, to make a knife-shaped file. No. 4. Clock-pinion; which may be used for either nicking, piecing, or squaring-off purposes. No. 5. Round, with parallel sides for gulleting purposes, or rat-tail when it tapers. No. 6. Triangular, or three equally-sided body for saw filing. No. 7. Equalizing file. This is parallel when used for making clock-pinions or endless screws; or for slitting, entering, warding, or making barrel holes, when the body of the file tapers. No. 8. Cross, or double-round, half-file. No. 9. Slitting file; which has parallel sides only. A cant file. CHARACTER OF THE FILE TOOTH.--Files are distinguished principally by the character of the oblique, or cross grooves and ridges which do the cutting and abrading when the file is drawn across the surface. This is really more important than the shape, because the files, by their cuttings, are adapted for the various materials which they are to be used upon. The files are classified as _Double Cut_, of which there are the _rough_, _middle_, _bastard_, _second cut_, _smooth_, and _dead smooth_. The _Float Cut_, which is either _rough_, _bastard_ or _smooth_; and The _Rasp Cut_, either _rough_, _bastard_ or _smooth_. Several types are illustrated in Fig. 55, which show the characteristics of the various cuts. The rasps are used principally for soft material, such as wood or for hoofs, in horse shoeing, hence they need not be considered in connection with machine-shop work. [Illustration: _Fig. 55. Files._] HOLDING THE FILE.--The common mistake on the part of the beginner is to drag the file across the work at an angle. The body of the file should move across straight and not obliquely. Note this movement in Fig. 56 where the dash shows the correct movement of the file with relation to the work. Also observe that the file cutting ridges are not straight across the file, but at an angle to the direction of the dart. [Illustration: _Fig. 56. Correct File Movement_] INJURING FILES.--Now the frequent practice is to use the file as shown in Fig. 57, in which case it is moved across obliquely. The result is that the angle of the file cut is so disposed that the teeth of the file do not properly aid in the cutting, but in a measure retard the operation. File teeth are disposed at an angle for the purpose of giving them a shearing cut, which is the case when the file moves across the work on a line with its body. To use a file as shown in Fig. 57 injures the file without giving it an opportunity to cut as fast as it would when properly used. [Illustration: _Fig. 57. Incorrect File Movement_] DRAWING BACK THE FILE.--In drawing back a file it is always better to allow it to drag over the work than to raise it up. It is frequently the case that some of the material will lodge in the teeth, and the back lash will serve to clear out the grooves. This is particularly true in filing copper, aluminum, lead, and like metals, but it is well to observe this in all cases. CHAPTER V HOW TO COMMENCE WORK The question is often asked: Where and how shall the novice commence work? When the shop is equipped, or partially so, sufficient, at least, to turn out simple jobs, the boy will find certain tools which are strangers to him. He must become acquainted with them and not only learn their uses, but how to use them to the best advantage. FAMILIARITY WITH TOOLS.--Familiarity with the appearance of tools, and seeing them in the hands of others will not be of any value. Nothing but the immediate contact with the tool will teach how to use it. FILE PRACTICE.--The file is a good tool to pick up first. Select a piece of metal, six or eight inches long, and follow the instructions laid down in the chapter relating to the use of the file. Practice with several kinds and with different varieties of material will soon give an inkling of the best kind to use with the metal you have. Use the straight edge and the square while the filing process is going on, and apply them frequently, to show you what speed you are making and how nearly true you are surfacing up the piece. USING THE DIVIDERS.--Then try your hand using the dividers, in connection with a centering punch. As an example, take two pieces of metal, each about a foot long, and set the dividers to make a short span, say an inch or so, and step off the length of one piece of metal, and punch the last mark. Then do likewise with the other piece of metal, and see how nearly alike the two measurements are by comparing them. You will find a variation in the lengths of the two measurements at the first trials, and very likely will not be able to make the two pieces register accurately after many trials, even when using the utmost care. Sooner or later you will learn that you have not stepped paths along the two bars which were exactly straight, and this will account for the variations. In order to be accurate a line should be drawn along each piece of metal, and the dividers should step off the marks on that line. FINDING CENTERS.--By way of further experiment, it might be well to find the exact center of the ends of a square bar, putting in the punch marks and then mounting it in the lathe centers to see how accurately this has been done. If either end is out of true the punch marks can be corrected by inclining the punch, so that when it is struck it will move over the point in the direction of its true center. This may be followed up by centering the end of a round bar so as to make it true. This will be found to be a more difficult job, unless you have a center head, a tool made for that purpose. It is good practice, however, to make trials of all this work, as it will enable you to judge of measurements. It can be done with the dividers by using care in scribing the centers. HACK-SAW PRACTICE.--Practice with the hack-saw should be indulged in frequently. Learn to make a straight cut through a bar. Try to do this without using a square to guide you. One of the tests of a good mechanic is ability to judge a straight cut. The following plan is suggested as a test for the eye. Use a bar of iron or steel one inch square, and make a cut an eighth of an inch deep across it; then turn it around a quarter, so as to expose the nest face, and continue the cut along the side, the same depth, and follow this up with the remaining two sides, and see how near the end of the first cut and the finish cut come together. The test will surprise you. CUTTING METALS TRUE.--When you saw off the end of such a bar for trial purposes, use a square, after the cut is made, and note how much it is out of true in both directions. It is a curious fact that most mechanics are disposed to saw or cut crooked in one direction, either to the right or to the left. In tests made it is found that this defect is persisted in. It is practice only which will remedy this, and it would be well for the boy to learn this for himself as early in his career as possible, and correct the tendency to veer in either direction. The test of sawing around a round bar is also commended. After a few trials you will be surprised to see how your judgment will improve in practice. LATHE WORK.--Learn the uses of the chuck. As you have, probably, economized as much as possible, a universal chuck is not available, hence the first experience will be with an independent chuck, where the three dogs move independently of each other. This will give you some work to learn how you can get the job true. Now, before attempting to cut the material, thoroughly learn all the parts of the feed mechanism, and how to reverse, as well as to cross feed. Learn the operation of the operative parts so that your hand will instinctively find them, while the eye is on the work. FIRST STEPS.--See to it that your tools are sharp, and at the first trials make light cuts. Practice the feeds by manually moving the tool holder, for surface cutting as well as for cross cutting. SETTING THE TOOL.--Set the cutting tool at various angles, and try the different tools, noting the peculiarities of each, at the different speeds. Do not, by any means, use refractory metals for your first attempt. Mild steel is a good test, and a light gray iron is admirable for practice lessons. METALS USED.--Brass is good for testing purposes, but the difficulty is that the tendency of the boy, at first, is to try to do the work too rapidly, and brass encourages this tendency. Feed slowly and regularly until you can make an even finish. Then chuck and re-chuck to familiarize yourself with every operative part of the lathe, and never try to force the cutting tool. If it has a tendency to run into the work, set it higher. If, on the other hand, you find, in feeding, that it is hard to move the tool post along, the tool is too high, and should be lowered. THE FOUR IMPORTANT THINGS.--Constant practice of this kind will soon enable you to feel instinctively when the tool is doing good work. While you are thus experimenting do not forget the speed. This will need your attention. Remember, you have several things to think about in commencing to run the lathe, all of which will take care of themselves when it becomes familiar to you. These may be enumerated as follows: First: The kind of tool best to use. Second: Its proper set, to do the best work. Third: The speed of the work in the lathe. Fourth: The feed, or the thickness of the cut into the material. TURNING UP A CYLINDER.--The first and most important work is to turn up a small cylinder to a calipered dimension. When it is roughed down ready for the finish cut, set the tool so it will take off a sufficient amount to prevent the caliper from spanning it, and this will enable you to finish it off with emery paper, or allow another small cut to be taken. TURNING GROOVES.--Then follow this up by turning in a variety of annular grooves of different depths and widths; and also V-shaped grooves, the latter to be performed by using both the longitudinal and transverse feeds. This will give you excellent practice in using both hands simultaneously. The next step would be to turn out a bore and fit a mandrel into it. This will give you the opportunity to use the caliper to good advantage, and will test your capacity to use them for inside as well as for outside work. DISCS.--A job that will also afford good exercise is to turn up a disc with a groove in its face, and then chuck and turn another disk with an annular rib on its face to fit into the groove. This requires delicacy of measurement with the inside as well as the outside calipers. The groove should be cut first, and the measurement taken from that, as it is less difficult to handle and set the tool for the rib than for the groove. LATHE SPEEDS.--Do not make the too common mistake of running the mandrel at high speeds in your initial tests. It is far better to use a slow speed, and take a heavy cut. This is good advice at all times, but it is particularly important with beginners. CHAPTER VI ILLUSTRATING SOME OF THE FUNDAMENTAL DEVICES There are numerous little devices and shop expedients which are desirable, and for which the boy will find uses as he progresses. We devote this chapter to hints of this kind, all of which are capable of being turned out or utilized at various stages. LACING BELTS.--To properly lace a belt is quite an art, as many who have tried it know. If a belt runs off the pulley it is attributable to one of three causes: either the pulleys are out of line or the shafts are not parallel or the belt is laced so it makes the belt longer at one margin than the other. In Fig. 58 the lacing should commence at the center hole (A) of one belt end and lace outwardly, terminating at the hole (B) in the center of the other belt end, as shown in Fig. 58. In Fig. 59 the lacing commences at A, and terminates at the hole (B) at the edge. This will be ample for all but the widest belts. Fig. 60 is adapted for a narrow belt. The lacing commences at one margin hole (A), and terminates at the other margin hole (Z). Fig. 61 shows the outside of the belt. Fig. 62. GEARS.--This is something every boy ought to know about. Fig. 62 shows a pair of intermeshing bevel gears. This is the correct term for a pair when both are of the same diameter. [Illustration: _Inside Fig. 58. Belt Lacing_ _Outside Fig. 58. Belt Lacing_ _Fig. 59. Belt Lacing_ _Fig. 60. Belt Lacing_ _Fig. 61. Belt Lacing_] MITER GEARS.--In Fig. 63 we have a pair of miter gears, one being larger than the other. Remember this distinction. Fig. 64. CROWN WHEEL.--This is a simple manner of transmitting motion from one shaft to another, when the shafts are at right angles, or nearly so, without using bevel or miter gears. Fig. 65. GROOVED FRICTION GEARING.--Two grooved pulleys, which fit each other accurately, will transmit power without losing too much by friction. The deeper the grooves the greater is the loss by friction. Fig. 66. A VALVE WHICH CLOSES BY THE WATER PRESSURE.--The bibb has therein a movable valve on a horizontal stem, the valve being on the inside of the seat. The stem of the handle has at its lower end a crank bend, which engages with the outer end of the valve stem. When the handle is turned in either direction the valve is unseated. On releasing the handle the pressure of the water against the valve seats it. Fig. 67. CONE PULLEYS.--Two cone pulleys of equal size and taper provide a means whereby a change in speed can be transmitted from one shaft to another by merely moving the belt to and fro. The slightest change is available by this means. Fig. 68. UNIVERSAL JOINT.--A wheel, with four projecting pins, is placed between the U-shaped yokes on the ends of the approaching shafts. The