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Fig. 43 External lap holder

Fig. 43 External lap holder

External laps are used in the form of a ring with an outer band or holder and an inner shell, which forms the lap proper, made of copper, cast-iron or brass. The lap is split and screws are provided in the holder for adjustment. Length of an external lap should be at least equal to the length of the work; a little longer does no harm. Figure 43 illustrates one of these.

Grading Abrasives for Lapping—For high-grade lapping, abrasives may be evenly graded as follows: a quantity of flour emery or like abrasive is placed in a heavy cloth bag which when gently tapped causes fine particles to be sifted through. After a sufficient quantity has been obtained it is mixed in a dish with sperm or olive oil. The larger particles will sink to the bottom in about one hour and the oil should then be decanted into another dish. Be careful not to disturb the sediment at the bottom. The oil is allowed to stand for several hours longer, after which it is poured again, and so on until the desired grade is obtained.

Charging Laps—To charge a copper or cast-iron lap, spread a little of the prepared abrasive over the lap and roll it between two hardened steel blocks; do not rub, but roll the lap with even pressure. On laps for external work, a hardened steel plug smaller than the hole should be used to roll the abrasive into the ring. For breaking, when there is a considerable amount of material left on the gauge, I mix coarser emery with sperm oil, charging and using the copper lap with it until there are only two to three ten-thousandths left to finish with the finer specially prepared abrasive. When a lap is once charged it should be used without applying more abrasive until it no longer cuts. If a lap is overcharged with abrasive, a rolling action between the lap and work takes place, which results in uneven distribution. A properly charged lap should never develop bright spots. On the contrary, the surface should be a uniform gray. Use plenty of sperm or olive oil to wash the fine steel cuttings away from the lap, keeping the same adjusted to prevent the work from becoming bell-mouthed.

Dry Lapping—Use the finest optical emery by lightly sprinkling it over the fast-revolving lap; wash off with gasoline and test for size often. To finish, use powdered rouge in the same manner; when completed you have one of the most highly finished surfaces possible.

Laps jor Flat Surfaces—Laps for plane surfaces are made from cast-iron and lead. Figure 44 illustrates a small cast-iron lap for small work. In order to secure the most accurate results, the lapping surface must be a true plane. On the cast-iron lap the surface is checked or scored by narrow grooves located between % and V2 inch apart across the full length of the plate, forming a series of diamonds as on a stock. The lead lap is constructed in the same manner except for the grooves, its surface being a true plane. The latter is used as a roughing lap and the cast-iron one for finer finishes. After a lap is charged, all loose abrasive should be washed off with gasoline to determine if the surface., has a gray appearance. Repeat until all bright spots are charged, if this is not the case. When lapping, the surface should be kept moist with kerosene; gasoline will cause the lap to cut a little faster, but it evaporates so rapidly that the surface becomes glossy in spots. Loose emery should never be applied while lapping, for if well-charged with the fine abrasive in the beginning, kept well-moistened with kerosene, and not crowded too much, it will last a long time. The pressure applied to the work should be just enough to insure uninterrupted contact. A lap can be made to cut just so fast, but with too great pressure it will strip in spots. The causes of scratches on the work are: loose abrasive on the lap, too much pressure, or poorly graded abrasive

Diamond Laps—I use diamond laps and diamond dust extensively, especially on very fine holes, charging laps for lapping the spindles of anvils of micrometers, and charging cast-iron lapping plates

Fig. 44

Ccut-iron lopping plate. Made In various il»s to suit requirement»

Fig. 44

Ccut-iron lopping plate. Made In various il»s to suit requirement»

to this condition, will last for years, provided you exercise reasonable care in its use.

All reamers are ground lengthwise in a special tool grinder made for them, and, like wood-working tools, they must first be put into sharp condition before reliable work can be expected. All reamers when new are up to size or three to four ten-thousandths over. Copper-plate each flute, and with a medium or fine -inch oilstone, stone the flutes up to cutting edge by starting from the rearmost portion. If you are experienced in the stoning process, it is possible to stone lengthwise, an operation in which the high points left by the grinding wheel are immediately discovered. They must be stoned until the flute has a bright evenly polished surface, at the same time retaining size. To avoid a chattered hole, see that each flute cuts, with the others, an equal 'amount of chips. Particularly with tapered reamers is it more or less common to encounter chatter, which, naturally, is fatal to a satisfactory recess. Important to remember is—never chuck a hand reamer in a lathe or drill press, as hand reamers are made to ream by hand only; the same may be said for taper-pin reamers. Machine reamers are specially made; included in this type are the taper-pin reamers. It is a grave mistake to allow more than three to five thousandths of stock for the reamer to remove. When a rough drilled hole is encountered it does require more stock. A clean-bored hole need have only two or three thousandths of metal to be finish-reamed.

Screw-drivers and Bits — This subject has been more or less neglcctcd, because every one looks upon a screw-driver as a tool intended either to remove a screw or to place one in position. On gun work, owing to the screw slots being much smaller than the standard recommended by manufacturers, the quality of the screw-drivcr is of major importance. It would be impossible to picture a high-grade shotgun made with screw slots from .025 inch wide for screw to .057-inch width for a inch screw; they would look and be entirely out of proportion. For this reason the makers have adopted small-slot ted screw heads for both rifles and shotguns. In order that the gunsmith can cope with this situation he must have screw-drivers constructed in a way quite different from that of the ones usually purchased in a hardware store. The latter are, as a rule, made only from cheap material; and when ground to fit a gun screw, they twist out and mar the head, a fault which is inexcusable even in an inexpensive arm.

Figure 21 will show the correct way to shape screw-drivers for gunsmithing purposes; unless the bit fits the slot in every dimension, there results, as just mentioned, a distorted or damaged head. This fact makes it imperative, for really fine work, to have a driver for every slot size.

In attempting to remove a tightly imbedded screw that does not yield to ordinary effort, I obtain the best results with correctly sized driver bits that fit a brace, exerting steady pressure on the brace with the left hand, and with the right gently turning the screw out. This method will start any screw no matter how firmly set, and may also be employed to seat them tightly. Have you ever noticed that on a high-grade gun, resting on its butt plate, all slots point in the same direction, north and south, according to the points of the compass on the map? Well, it's true, and the same applies, in a continued line, to the butt-plate screws. Any other arrangement is considered unsightly and a detraction, marking the difference between precision and indifference. 'Be as particular in the care of good screw-drivers as of any other valued tool by refusing to submit them indiscriminately to the grinding wheel for every odd-sized screw, but rather have a generous selection, well filed, tempered, made from the best spring steel or drill rod, and fitted to serviceable handles.

Soldering — Years of experience in this line have led me to believe that nine out of ten men who have occasion to resort to this art could still, with advantage to their work, improve their technique. Soldering is sometimes erroneously referred to as "sweating," but there is a vast difference in strength between a well fitted and sweated union and one soldered in the ordinary way. An important point, frequently overlooked, is the proper cleaning of surfaces to be joined, and this operation is too often left for the flux to correct. While nearly all metals can be joined by use of the same flux, a proper selection is at times necessary. The result following improper preparation and cleaning overshadows in every instance the effects of good soldering and is particularly noticcablc in gun work. For strength, adapt the parts to one another perfectly, because the more accurate the fitting, the stronger the union. Use a solder with as high a melting point as possible. The temperature of the work to be joined should be brought as near as possible to the melting point of the solder to insure better fusion and flow.

There are a number of fluxes or soldering salts on the market that are satisfactory. Their action in soldering is to remove and prevent the formation of oxids during the operation, to allow the solder to flow freely, and to unite firmly the surfaces to be joined.

For sheet tin, rosin or colophony may be used, but owing to the ease and rapidity of applying, a solution of zinc chlorid (zinc dissolved in HC1) is more generally employed.. Beeswax or tallow can be used, and so can almost any of the pastes, fats, or liquids prepared for that purpose.

For lead, a flux of almost any oil and rosin in equal parts is satisfactory.

Lead burning, now almost a lost art, is a different operation from soldering. Here the surfaces must be bright and free from all oxid, while instead of solder, which is tin and lead, the operation is done with pure lead, rosin and oil.

Fluxes — Hydrochloric acid, rosin, turpentine, tallow, and especially chlorid of zinc or soldering liquid, are among the common fluxes used in soldering.. For a complete list see Chapter XXVI.

A very good acid mixture for cleaning work to be soldered is equal parts nitric and sulfuric acid and water. Never Pour the water into the acid!

By laying a piece of tin foil, covered on both sides with a flux, between two perfectly fitted and clamped parts and heating until the foil js melted, a satisfactory union is effected. This is very good in joining broken parts of brass and bronze work. If their apposition is good, they can be joined in this manner so that the joint is not only strong, but almost imperceptible.

A solution of copper for copper-plating steel or cast-iron before soldering is easily made and applied. It is identical with the one recommended for coppering the flutes of reamers before stoning, and is composed of copper sulfate 3% ounces, sulfuric acid 3 Y2 ounces, distilled water 1 gallon. Dissolve the copper sulfate in water and add the acid.

The best solder for gun work such as soldering ramps, rear-sight bases, sling-swivel bases, etc., is 75 per cent lead and 25 per cent tin, which has a melting point of 432 degrees Fahrenheit. Solder having a low melting point is not recommended for gun work because of the heat a rifle or shotgun is subject to in rapid fire. The tropical sun of Africa has been known to melt off soldered parts of firearms where no attention was given to this feature.

The soldering copper for ordinary use should be about IV2 pounds in weight, length 2% to 3 inches, octagonal in form, and pyramidal point with square edges. It should be fixed to a straight or angulated iron rod about 8 inches long with an ample wooden handle. When heating for use, the best way to ascertain correct temperature is to hold it near the face. By virtue of heat emanation one soon learns to know if the copper is heated to the right degree, and if a bright warm glow is felt, it is hot enough for use.

Taps and Tapping — When the amateur first finds use for taps he is likely to become discouraged, since most tapping troubles are caused by the use of drills that are too small in diameter. Tap drill sizes made specially for machine screws should be varied according to the material to be tapped and the depth of the tapped hole. Soft, tough material such as copper, Norway iron, stainless steel, aluminum, etc., should have a larger hole for the tap than harder metals. If the recess is a trifle too small when tapping soft material, the thread is in danger of being torn off, the effectiveness of the thread depth is decreased, and is not nearly as perfect as it would have been had the tap drill been larger. When tapping soft material, the metal at the top of the thread is somewhat drawn, thereby increasing the depth of the threads, particularly if the keen edge of the tap has been dulled from use.

For the beginner it is best to check from a table of double depth of threads. As an illustration, suppose we wish to tap for a 6 x 48 screw which is a U. S. standard form of thread. Let us turn to the table of double depths of thread, where it will be observed that for a number 6 tap measuring .138 inch in diameter, .027 is double depth of a 48 thread. But because the metal expands as a tap is used in a hole, we can drill a hole only 75 per cent of that depth of thread. Therefore the correct drill would measure 0.111 inch ; this being too small we must select a size larger, which must measure 0.120. The nearest drill to that size is selected, which is #31. When only a shallow hole is to be tapped, two drills are needed—one for starting and one for bottoming. The latter may be accomplished by use of a drill ground off square at point, or a flat-bottom drill, originally constructed for the purpose, permitting installation of the greatest number of threads. In the use of small taps, be particular to grind them with the correct leed, which is the cutting point of the tap. Most taps are furnished with a correctly ground leed, but after they become dulled, it is necessary to grind the end back and regrind the leed. Before grinding a tap, study a new one for the purpose of learning how the manufacturer first ground the leed, and follow that pattern. Even the best mechanics are inclined to grind too much clearance, which causes a tap to break easily. Satisfactory results are obtained only when using a tap wrench, proceeding slowly with small taps, and never trying to complete a full turn of the tap. The secret of success is to back it out a good half-turn, starting again slowly, for taps are brittle and easily broken. If you are conscious that a tap springs in the least, reverse and start over.

To Remove a Broken Tap—When broken near the surface it is an easy matter to remove it by driving on both sides with a round punch, ground in the form of a round-nose chisel. Two persons can do this much better than one by striking light blows; when they are striking evenly the force of the blows can be increased as may be required. By driving on both sides, the tap is not wedged against one side of the hole as when using a single drift, but is forced to turn out. This is an old method, and one of the first I was taught.

Another method, which has proven a life-saver many times, is to inject into a hole a little nitric acid, diluted in the proportion of one part acid to three parts water. The action of the acid upon the broken part and steel loosens the tap so that it can be removed easily. The remaining acid should be washed out so it will not destroy the threads in the steel, which then should be coated freely with oil.

Lubricants for Tapping—The breaking of a tap is caused by: improper lubrication, the tap not being square with the hole, ignorance, or carelessness. Frequency of this misfortune can be reduced greatly by proper lubrication alone. Ranking first and best among the greases I use is a mixture of white lead and sperm oil. Another reliable combination is 10-per-cent graphite, 30-per-cent tallow, 40-per-cent white lead, and 20-per-cent sperm oil. Machine and lard oil class with the poorest of lubricants for tapping; however, they are probably used more extensively than either of the two good formulas just mentioned. When tapping in cast iron, a small amount of kerosene helps materially. In high-power firearms a full, fine, smooth thread is imperative to obviate the dangers of the sudden, violent, and repeated shocks, which have a cumulative effect upon every part of the mechanism.

Makeshift Taps—I have often made a tap by threading a piece of steel in a lathe or with a die, filing three flutes in the end, then backing off the clearance, and after hardening and tempering I had a tool that lasted for many operations. If the need is urgent, file a flat on the end, on a taper; then remove a small segment of the remaining arc, constructing thereby a cutting edge; harden and temper and finish the hole with this. You will find very often in gun work that the manufacturer has not used a standard screw or thread and it will be necessary to ease out a tapped hole for just one particular screw. A tap made in the above manner has a considerable amount of clearance, answers the purpose nicely, and is not easily broken. When it becomes dull, all that is necessary is to grind the face or flat; if it should then cut too large, narrow the face by further grinding.

Turning Tools and Tool Grinding — Turning tools are of varied shapes adaptable to the many different purposes they are intended for. There are certain principles governing the form of turning tools which apply generally. When grinding lathe tools or any other turning tools there are three points to remember: first, the cutting edge of the tool as viewed from the top must have a specific shape or contour; second, there must be a certain amount of clearance below or behind the cutting edge; and third, tools are given a backward shape, so to speak, and a side slope, or a combination of the two 011 the part against which the chip bears when the tool is in use. However, experience is our best teacher in the different ways of grinding lathe tools. It is much more economical to buy the standard high-speed tool bits and an Armstrong tool holder than forged tools. These are designated by different names, such as parting, thread, side, turning or round nose, boring, etc. The shape when viewed from the top is suggestive of other uses than plain cylindrical turning. The exact form can be best determined under working conditions. This is illustrated by reference to the parting tool which is used for cutting grooves and parting work at given intervals over specified distances. Naturally it must be widest at the cutting edge to prevent binding while feeding into the work.

Aside from the contour, in relation to the cutting edge, there remains to be determined the proper clearance, amount, and direction of the slope of the top of the tool. The word "top" is used to designate that surface against which the chips bear when severed from the work. For most tools it should slope away from what is the working part of the cutting edge and be set a trifle above center. To use this type in turning a diameter of, say J/i g of an inch, it would be impracticable; so we grind one with a more pronounced point and greater clearance at the top. On small work the metal must be removed with an even free cutting effect. In thread tools the clearance must be less decided, but for really good work it is advisable to stone the point on center. If the reader familiarizes himself with the above he will at least have a working basis to start from; but, as already stated, success will be commensurate with the amount of painstaking practise which has been expended.

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