Modern Machine-Shop Practice Part 83

[Ill.u.s.tration: Fig. 1210.]

If it be attempted with steel tools to take a very fine cut, as, say, one of sufficient depth to reduce a diameter, say 1/500 inch, the tool is apt to turn an uneven surface. There appears, indeed, to be a necessity to have the cut produce sufficient strain to bring the bearing surfaces of the rest into close contact and to place a slight strain on the tool, because under very light cuts, such as named above, the tool will generally momentarily leave the cut or take a reduced cut, and subsequently an increased one.

It may be accepted that from these causes a finis.h.i.+ng cut taken with a steel tool should not be less than that sufficient to reduce the diameter of the work 1/64 inch. Now an emery-wheel will take a cut whose fineness is simply limited by the wear of the wheel in the length of the cut. Some experiments made by Messrs. J. Morton Poole and Sons, of Wilmington, Delaware, upon this subject led to the conclusion that with corundum wheels of the best quality the cut could be made so fine that a 12-inch wheel used upon a piece of work (a calender roll) 16 inches in diameter and 6 feet long, would require about forty thousand traverses to reduce the diameter of the work an inch, leading to the conclusion that the wear of the wheel diameter was less than one eighty-thousandth part of an inch per traverse.

Now the strain placed upon the work of an emery-wheel taking a cut of, say, 1/1000 inch, is infinitely less than that caused by a cutting tool taking a cut of 1/120 inch in diameter; hence the accuracy of grinding consists as much in the small amount of strain and, therefore, of deflection it places upon the work, as upon the endurance of the wheel itself.

Since both in finis.h.i.+ng and in polis.h.i.+ng a piece of work the object is to obtain as true and smooth a surface as possible, the processes are to a certain extent similar, but there is this difference between the two: where polis.h.i.+ng alone is to be done, the truth of the work or refined truth in its cylindrical form or parallelism may be made subservient to the convenience of polish. Thus, in the case of the stem of the connecting rod that has been turned and filed and finished as true as possible, the polis.h.i.+ng processes may be continued with emery-cloth, &c., producing the finest of polish without impairing the quality of the work, whereas the degree of error in straightness or parallelism induced by the polis.h.i.+ng may impair the degree of truth desirable for a piston rod.

The degree of finish or polish for any piece of work is, therefore, governed to some extent by the nature of its use. Thus a piston rod may be finished and polished to the maximum degree consistent with maintaining its parallelism and truth, while a connecting rod stem may be polished to any required attainable degree.

In finis.h.i.+ng for truth, as in the case of journal bearings, the work, being turned as true and smooth as possible, may be filed with the finest of cut files, and polished with a fine grade of emery-cloth or paper; the amount of metal removed by filing and polis.h.i.+ng being so small as not to impair to any practically important degree the truth of the work: a journal so finished will be as true as it is possible to make it without the use of a grinding lathe.

Instead of using emery-paper, grain emery and oil may be used, but the work will not be so true, because in this case much more metal will be removed from the work in the finis.h.i.+ng or polis.h.i.+ng process.

When it is required to polish and to keep the work as true and parallel as possible, these ends may be simultaneously obtained by means of clamps, such as shown in Fig. 1210, which represents a form of grinding and polis.h.i.+ng clamp used by the Pratt and Whitney Company for grinding their standard cylindrical gauges. A cast-iron cylindrical body A is split partly through at B and entirely through at C, being closed by the screw D to take up the wear. The split B not only weakens the body A and enables its easy closure, but it affords ingress to the grinding material. It may be noted that cast iron is the best metal that can be used for this purpose, not only on account of the dead smooth surface it will take, but also because its porosity enables it to carry the oil better than a closer grained metal. For work of larger diameter, as, say, 2 or 3 inches, the form of lap shown in Fig. 1211 is used for external grinding, there being a hinge B C instead of a split, and handles are added to permit the holding and moving of the lap. The bore of this clamp is sometimes recessed and filled with lead. It is then reamed out to fit the work and used with emery and oil, the lathe running at about 300 feet per minute.

[Ill.u.s.tration: Fig. 1211.]

For grinding and polis.h.i.+ng the bores of pieces, many different forms of expanding grinding mandrels have been devised, in most of which the mandrel has been given a slight degree of curvature in its length; or in other words, the diameter is slightly increased as the middle of the mandrel length is approached from either end. But with this curvature of outline, as small as it may be, it rather increases the difficulty of grinding a bore parallel instead of diminis.h.i.+ng it. When expanding mandrels are caused to expand by a wedge acting upon split sections of the mandrel, they rarely expand evenly and do not maintain a true cylindrical form.

Fig. 1212 represents a superior form of expanding mandrel for this purpose. The length A is taper and contains a flute C. The lead is cast on and turned upon the mandrel, the metal in the flute C driving the lead. The diameter of the lap is increased by driving the taper mandrel through it, and the lead is therefore maintained cylindrically true.

While these appliances are supplied with the flour emery and oil, their action is to grind rather than to polish, but as they are used without the addition of emery, the action becomes more a polis.h.i.+ng one.

[Ill.u.s.tration: Fig. 1212.]

Fig. 1213 represents at A A a wooden clamp for rough polis.h.i.+ng with emery and oil. It consists of two arms hinged by leather at B and having circular recesses, as C, D, to receive the work. At J J is represented a similar grinding and polis.h.i.+ng clamp for more accurate work. G and H are screws pa.s.sing through the top arm and threaded into the lower, while E, F are threaded into the lower arm, and abut at their ends against the face of the upper arm. It is obvious that by means of these screws the clamp may be set to size, adjusted to give the required degree of pressure, and held firmly together. Lead bushes may be inserted in the bores as grinding laps. As this clamp is used by hand, it must be moved along the work at an exactly even speed of traverse, or else it will operate on the work for a longer period of time at some parts than at others; hence the greatest care is necessary in its use.

The best method of polis.h.i.+ng cylindrical work to be operated on entirely in the lathe, the primary object being the polish, is by means of emery paper, and as follows:--

In all polis.h.i.+ng the lathe should run at a fast speed; hence special high speeded lathes, termed speed lathes, are provided for polis.h.i.+ng purposes only.

The emery paper or cloth should be of a fine grade, which is all that is necessary if the work has been properly filed, if cylindrical, and sc.r.a.ped if radial or of curved outline.

In determining whether emery paper or cloth should be used, the following is pertinent:--

The same grade of emery cuts more freely on cloth than on paper, because the surface of the cloth is more uneven; hence the emery grains project in places, causing them to cut more freely until worn down. If, then, the surface is narrow, so that there is no opportunity to move the emery cloth endways on the work, emery paper should be used. It should be wrapped closely (with not more than one, or at least two folds) around a _smooth_ file, and not a coa.r.s.e one, whose teeth would press the emery to the work at the points of its coa.r.s.e teeth only. The file should be given short, rapid, light strokes.

For work of curved outline emery cloth should be used, because it will bend without cracking, and the cloth should be moved quickly backwards and forwards across, and not round, the curve; and when the work is long enough to permit it, the emery paper or cloth should be moved rapidly backwards and forwards along the work so that its marks cross and recross at an obtuse angle.

Now, suppose the grade of emery paper first used to be flour emery, and the final polish is to be of the highest order, then 0000 French emery paper will be required to finish, and it is to be observed that nothing will polish a metal so exquisitely as an impalpable powder of the metal itself: hence, while performing the earlier stages of polis.h.i.+ng, it is well to prepare the final finis.h.i.+ng piece, so as to give it a glaze of metal from the work surface.

When, therefore, all the file marks are removed by the use of the flour emery cloth, the surface of the work should be slightly oiled and then wiped, so as not to appear oily and yet not quite dry, with a piece of rag or waste, then the piece of 0000 emery paper, or, what is equally as good, a piece of crocus cloth, to be used for the final finis.h.i.+ng should be applied to the work, and the slightly oily surface will cause the cuttings to clog and fill the crocus cloth. The cloth should be frequently changed in position so as to bring all parts of its surface in contact with the work and wear down all projections on the cloth as well as filling it with fine cuttings from the work. Then a finer grade, as, say, No. 0 French emery paper, must be used, moving it rapidly endwise of the work, as before, and using it until all the marks left by the flour emery have been removed.

One, or at most two drops of lard oil should then be put on the work, and spread over as far as it will extend with the palm of the hand, when the finis.h.i.+ng crocus may again be applied and reversed as before in every direction; 00 emery paper may then be used until all the marks of the 0 are removed, and with the work left quite dry the crocus for final finis.h.i.+ng may again be applied; 000 emery paper may then be used to efface all the marks left by the 00. This 000 emery paper should be used until it is very much worn, the final finish being laid with the glazed crocus.

If this crocus has been properly prepared, its whole surface will be covered with a film of fine particles of metal, so that if the metal be bra.s.s the crocus surface will appear like gold leaf. If cast iron, the crocus surface will appear as though polished with plumbago or blacklead, while in any case the crocus surface will be polished and quite dry. The crocus should be pressed lightly to the work, so that its polis.h.i.+ng marks will not be visible to the naked eye.

If emery paper be applied to work finished to exact diameter it should be borne in mind that the process reduces to some extent the size of the work, and that the amount under proper conditions though small is yet of importance, where preciseness of diameter is a requisite.

In the practice, however, of some of the best machine shops of the United States, the lathe alone is not relied upon to produce the best of polish. Thus, in the engine works of Charles H. Brown, of Fitchburg, Ma.s.sachusetts, whose engines are unsurpa.s.sed for finish and polish, and which the majority of mechanics would suppose were finely silver plated, the following is the process adopted for polis.h.i.+ng connecting rods.

[Ill.u.s.tration: Fig. 1213.]

The rod is carefully tool-finished with a fine feed. The tool marks are then erased with a fine smooth file, and these file marks by a dead-smooth file, the work rotating at a quick speed, little metal being left, so as to file as little as possible. Next comes _fine_ emery cloth to smooth down and remove the file marks. The lathe is then stopped and the rod stoned lengthwise with Hindostan stone and benzine, removing all streaks. The Scotch stone used with water follows, until the surface is without scratches or marks, as near perfect as possible. The next process is, for the finest work, the burnisher used by hand. But if not quite so exquisite a polish is required, the rod is finished by the use of three grades of emery cloth, the last being very fine.

Sometimes, however, the streaks made by polis.h.i.+ng with emery paper used before the application of the stones are too difficult to remove by them. In this case, for a very fine finish, the lathe is stopped and draw-filing with the finest of files is performed, removing all streaks; and the stones then follow the draw-filing. All stoning is done by hand with the work at rest, as is also all burnis.h.i.+ng.

After the burnisher comes fine imported crocus cloth, well worn, which makes the surface more even and dead than that left by the burnisher.

The crocus is used with the lathe at its quickest speed, and is moved as slowly and as evenly as possible, the slower and more even the crocus movement along the rod, the more even the finish. If the rod has filleted corners, such corners are in all cases draw-filed before the stoning.

The method of polis.h.i.+ng a cylinder cover at the Brown Engine Works is as follows.

[Ill.u.s.tration: Fig. 1214.]

The finis.h.i.+ng cut is taken with a feed of 32 lathe-revolutions per inch of tool traverse, and at as quick a cutting speed as the hardness of the iron will permit. This is necessary in order to have the tool-edge cut the metal without breaking it out as a coa.r.s.e one would do. With the fine feed and quick speed the pores of the iron do not show; with a coa.r.s.e feed the pores show very plainly and are exposed for quite a depth.

After the lathe-tool comes a well oil-stoned hand-sc.r.a.per, with a piece of leather between it and the tool rest to prevent the sc.r.a.per from chattering. The sc.r.a.per not only smooths the surface, but it cuts without opening the pores. It is used at a quick speed, as quick indeed as it will stand, which varies with the hardness of the metal, but is always greater than is possible with a slide-rest tool.

After the sc.r.a.per the cover is removed from the lathe, and all flat surfaces are filed as level as possible with a second-cut file, and then stoned with soft Hindostan stone, used with benzine or turpentine, so as to wash away the cuttings and prevent them from clogging the stone or forming scratches. In using all stones the direction of motion is frequently reversed so as to level the surface. Next comes stoning with Scotch stone (Water of Ayr), used with water; in this part of the operation great care must be taken, otherwise the cuttings will induce scratches. When the Scotch stone marks have removed all those left by the Hindostan stone, and left the surface as smooth as possible, the cover is again put in the lathe and the grain is laid and finished with very fine emery cloth and oil. The emery cloth is pressed lightly to the work and allowed to become well worn so as to obtain a fine l.u.s.tre without leaving any streaks.

[Ill.u.s.tration: Fig. 1215.]

It will be noticed here that the use of the emery stick and oil is entirely dispensed with; but for a less fine polish it may be used, providing it be kept in quick motion radially on the work. The objection to its use is that if there be any speck on the work it is apt to cut a streak or groove following the spot like a comet's tail.

TURNING TAPERS.--There are five methods of turning outside tapers; 1st, by setting over the tailstock of the lathe; 2nd, by the use of a former or taper turning attachment such as was shown in Fig. 508; 3rd, by the use of a compound slide rest; 4th, by means of a lathe in which the head and tailstock are upon a bed that can be set at an angle to the lathe shears on which the lathe carriage slides; and 5th, by causing the cross-feed screw to operate simultaneously with the feed traverse.

Referring to the first method, it is objectionable, inasmuch as that the work axis is thrown at an angle to the axis of the lathe centres, which causes the work centres to wear rapidly, and this often induces them to move their positions and throw the work out of true. Furthermore, the tailstock has to be moved back in line with the live spindle axis for turning parallel again, and this is a troublesome matter, especially when the work is long.

Fig. 1214 shows the manner in which the lathe centres and the work centres have contact, L being the live and B the dead centre; hence C C is the axis of the live spindle which is parallel to the lathe shear slides, which are represented by G; obviously A is the work axis. The wear is greatest at the dead centre end of the work, but there is some wear at the live centre end, because there is at that end also a certain amount of motion of the work centre upon the live centre. Thus, in Fig.

1215, let _c_ represent the live centre axis, _a_ the work axis, D the lathe face plate, and E F the plane of the driver or dog upon the work, and it is obvious that the tail of the driver will when at one part of the lathe revolution stand at E, while when diametrically opposite it will stand at F; hence, during each work revolution the driver moves, first towards and then away from the face plate D, and care must be taken in adjusting the position of the driver to see that it has liberty to move in this direction, for if obstructed in its motion it will spring or bend the work.

[Ill.u.s.tration: Fig. 1216.]

To determine how much the tailstock of a lathe must be set over to turn a given taper, the construction shown in Fig. 1216 may be employed. Draw the outline of the work and mark its axis D, draw line C parallel to one side of the taper end, and the distance A between this line and the work axis is the amount the tailstock requires to be set over. This construction is proved in Fig. 1217, in which the piece of work is shown set over, C representing the line of the lathe ways, with which the side F of the taper must be parallel. D is the line of the live spindle, and E that of the work, and the distance B will be found the same as distance A in Fig. 1216.

It may be remarked, however, that in setting the tailstock over it is the point of the dead centre when set adjusted to the work length that must be measured, and not the tailblock itself.

[Ill.u.s.tration: Fig. 1217.]

Other methods of setting tailstocks for taper turning are as follows: If a new piece is to be made from an old one, or a duplicate of a piece of work is to be turned, the one already turned, or the old piece as the case may be, may be put in the lathe and we may put a tool in the tool post and set the tailstock over until the tool traversed along the work (the latter remaining stationary) will touch the taper surface from end to end.

If, however, the taper is given as so much per foot, the distance to set the tailstock over can be readily calculated.

Thus, suppose a piece of work has a taper part, having a taper of an inch per foot, the work being three feet long, then there would be three inches of taper in the whole length of the piece and the tailstock requires to be set over one-half of the three inches, or 1-1/2 inches.

It will not matter how long the taper part of the work is, nor in what part of the work it is, the rule will be found correct so long as the tailstock is set over one-half the amount obtained by multiplying the full length of the work per foot by the amount of taper per foot.

If we have no pattern we may turn at each end of the part that is to be taper a short parallel place, truing it up and leaving it larger to the same amount at each end than the finished size, and taking care that the parallel part at the small end will all turn out in the finis.h.i.+ng. We then fasten a tool in the lathe tool post, place it so that it will clear the metal of the part requiring to be turned taper, and placing it at one extreme end of said part, we take a wedge, or a piece of metal sufficiently thick, and place it to just contact with the turned part of the work and the tool point (adjusting the tool with the cross-feed screw), we then wind the rest to the other end of the required taper part, and inserting same wedge or piece of iron, gauge the distance from the tool point to the work, it being obvious that when the tool point wound along is found to stand at an equal distance from each end of the turned part, the lathe is set to the requisite taper.