Modern Machine-Shop Practice Part 82

[Ill.u.s.tration: Fig. 1209.]

When the pressure between the tool and the work is sufficient, from the proportions of the work, to cause the work to spring or bend, the length of acting cutting edge on the tool should be reduced.

As the diameter and rigidity of the work increases, the length of tool cutting edge may increase. The cutting edge of the tool should be kept as close as the work will conveniently admit to the slide rest tool post, 1/4 _inch even_ of this distance being _important_.

The slide rest tool should always be resharpened to take the finis.h.i.+ng cut, with which, for wrought iron or steel, soapy water with soda in it should be used, the soda serving to prevent the dripping water from rusting the parts of the lathe.

Cast iron will cut with an exquisite polish if finished at a _very slow_ rate of cutting speed, and turned with a spring tool, such as was shown in Fig. 974, and water is used. But being a slow process it is not usual to finish it in this manner, though for round corners, curves, &c., this method is highly advantageous.

For cast iron the tool should be as keen as the hardness of the metal of the work will permit. If an insufficiently keen tool, or too deep a cut, or too coa.r.s.e a feed be taken, the metal will break out instead of cutting clean, and numerous fine holes will be perceived over the whole surface, impairing that dead flatness which is necessary to an even and fine polish.

To remove these specks or holes in cylindrical work, the file may be used, but for radial faces hand-sc.r.a.pers, such as shown in Fig. 1295, are used, the work rotating in either case at high speed. Such sc.r.a.pers are oilstoned and held with the handle end above the horizontal level.

The _rest_ should be so conformed to suit the shape of the work, that the sc.r.a.per will be supported close to the work, which will prevent chattering, and a piece of leather should (as a further preventive of chattering) be placed beneath the sc.r.a.per. A very good method of using a sc.r.a.per is to adjust it to the work, and holding it still on the rest, traverse the slide rest to move the sc.r.a.per to its cut.

After the sc.r.a.ping, three methods of polis.h.i.+ng radial faces are commonly employed; the first is to use emery paper only, and the second is by the use first of grain, emery, and oil, and the subsequent use of emery paper or cloth, and the third is by the use of emery wheels and crocus cloth.

If the work is finished by emery paper only, and it requires much application of the same to efface the sc.r.a.per marks, the evil will be induced that the emery cuts out the metal most where it is most porous, so that the finished surface is composed of minute hills and hollows, and the polish, though bright and free from marks, will not have that dead flat smooth appearance necessary to a really fine polish and finish; indeed, the finish is in this case to some extent sacrificed to obtain the polish.

It is for this reason that stoning the work (as hereafter described) is resorted to, and that grain emery and lead is employed, which is done as follows:--

For a flat radial face, a flat piece of lead, say 3/8 inch thick, and of a size to suit the work, may be pivoted to the end of a piece of wood of convenient length and used with grain emery and oil, the work rotating quickly. To afford a fulcrum for the piece of wood, a lever or rest of some kind, as either a hand rest or a piece fastened in the tool post, is used.

The rest should be placed a short distance from the work surface and the lever held partly vertical until the lead meets the work surface, when depressing the lever end will force the lead against the work. The lever end must be quickly moved laterally, so that the lead will approach and then recede from the work centre; this is necessary for two reasons.

First, to prevent the emery from cutting rings in the work surface, and secondly, to prevent the formation of grooves behind any hollow spots or specks the work may contain. The reason of the formation of these grooves is that the emery lodges in them and works out from the contact of the lead, so that if on working out they move always in the same line they cut grooves.

When a lathe is provided with belt motion to run both ways, it is an excellent plan to apply the lead with the lathe running forward and then with it running backward.

When by this means the sc.r.a.per marks are removed, the next object is to let the marks left by the lead be as fine and smooth as possible, for which purpose flour emery should be used; but towards the last no emery, but oil only, should be applied, the lead being kept in constant lateral motion, first quickly and then slowly, so that the marks on the work cross and recross it at different angles.

For round or hollow corners the lead need not be pivoted to the stick, but should be spherical at the end, the marks being made to cross by partly rotating the lever first in one direction and then in the other.

Sometimes the end of the lever is used without the addition of lead, but this does not produce so flat a surface, as it cuts out hollows in the pores of the metal.

For polis.h.i.+ng to be done entirely in the lathe, emery paper and crocus may follow the lead, being used dry and kept also in constant lateral motion. Each successive grade of emery paper must entirely remove all marks existing on the work at the time of its (the paper's) first application, and, furthermore, each successive grade should be continued until it is well worn, because of two pieces of emery paper of the same grade that most worn will cut the smoothest and polish the best. For the final polis.h.i.+ng a piece of the finest emery paper should be prepared in the manner hereafter described for polis.h.i.+ng plain cylindrical surfaces.

The radial faces of wrought iron must be finished as smoothly and true as possible, because being harder than cast iron the emery acts less rapidly upon it. For radial faces on bra.s.s the surfaces should be finished as smooth as possible with the slide rest tool, which should be round nosed, with the round flattened somewhat where the tool cuts, and the tool should not, under any condition, have any rake on its top face, while the feed should be fine as, say, 32 revolutions per inch of tool travel. Under skillful manipulation sc.r.a.ping may then be dispensed with, although it may be used to a slight extent without impairing the truth.

Very small radial surfaces of bra.s.s may best be finished by the sc.r.a.per and polished with emery paper, while large ones may be finished with dry emery paper.

Round corners on bra.s.s work should be finished with a spring tool, such as shown in Fig. 974, but having negative top rake; but if the corners are of small radius a well oilstoned hand-sc.r.a.per is best.

To enable the smooth and true turning of all radial faces of large diameter, the lathe head should, when it is possible, be steadied for end motion by placing a rod between the lathe centres, but if the radial face is solid at the centre so that such a rod cannot be put in, the end motion adjusting device of the lathe should be adjusted. The slides of the lathe should also be set up to have good firm contact, and the tool should be brought up to the work by putting the feed motion in gear and operating it by hand at the cone pulley, or gear-wheel on the feed spindle. If the lathe has no compound rest, the cut should be put on by this means, but otherwise the tool may be brought near the work by the feed motion and the cut put on by the compound rest, the object in both cases being to take up all lost motion and hold the rest firmly or steadily on the lathe shears, so that it shall not move back as the cut proceeds.

Work of cast iron or bra.s.s and of small dimensions and irregular or curved outline should be finished with sc.r.a.ping tools, such as shown in Figs. 1303 and 1310, polished with emery cloth or paper. But whenever sc.r.a.pers are made with curves to suit the form of the work, such tool curves should be so formed (for all metals) as not to cut along the whole length of cutting edge at once, unless the curve be of very small length as, say, 1/4 inch. This is necessary, because if the cutting edge operates on too great a length it will jar or chatter.

For convex surfaces the curve on the sc.r.a.per should be of greater radius than that of the work, while in the case of concave curves the tool should have a less radius. In both cases the tool will require a lateral movement to cause it to operate over the full width of work curve.

If the work curves are sufficiently large, and the same is sufficiently rigid that a slide rest tool may be used, the length of cutting edge may be increased, so that under very favorable conditions of rigidity the tool edge may cut along its whole length without inducing either jarring or chattering, but the best results will always be obtained when with a broad cutting edge the tool is of the spring tool form shown in Fig.

974.

Work of wrought iron or steel of small dimensions and of irregular form, must also be finished by hand tools, such as the graver shown in Figs.

1285 and 1286, and the finis.h.i.+ng tool shown in Figs. 1289 and 1292, the shape of the tool varying to suit the shape of the work.

Round corners or sweeps cannot on any kind of work be finished by a file, because the latter is apt to pin and cut scratches in the work.

For the final tool finis.h.i.+ng of lathe work of plain cylindrical outline, no tool equals the flat file if it be used under proper conditions, which are, that the work be turned true and smooth with slide rest tools, the marks left by these tools being exceedingly shallow and smooth.

A dead smooth file that has been used enough to wear down the projecting teeth (which would cut scratches) should then be used, the work rotating at as fast a speed as the file teeth will stand without undue wear. The file strokes should be made under a light pressure, which will prevent the cuttings from clogging its teeth, and the cuttings should be cleaned from the file after every few strokes. Under these conditions work of moderate diameter may be turned to the greatest degree of smoothness and truth attainable with steel cutting tools, providing that the work makes several revolutions during each file stroke, and it therefore follows that the file strokes may be more rapid as the diameter of the work decreases, and should be more slow as that diameter increases. Allowing the greatest speed of the filed surface permissible, without too rapid destruction of the file teeth, to be 200 feet per minute, and the slowest speed of file stroke that will prevent the file teeth from being ground away or from becoming pinned (when used on wrought iron) to be one stroke in two seconds, the greatest diameter of work that can be finished by filing under the condition that the work must make more than one rotation per file stroke, is about 25 inches in diameter, running about 30 revolutions per minute. The same diameter and speed may be also taken for cast iron, but bra.s.s may be filed under increased speed, rendering it practicable to file it up to a diameter of about 36 inches under the above conditions of work rotation and file stroke speed.

Supposing, however, that from hardness of the metal or from its increased diameter the work cannot make a rotation per file stroke unless that stroke be more slowly performed, then the cuttings gather in the teeth of the file, become locked and form projections, termed pins, above the file teeth, and these projections cut scratches in the work, and this it is that renders it impracticable to hold the file still while the work rotates. But suppose the file be applied to work of such a diameter that, with a stroke in two seconds and the work surface rotating at 200 feet per minute, each stroke acts on a fraction of the circ.u.mference only, then there can be no a.s.surance that the filed surface will be cylindrical, because there is no means of applying the file equally over the whole surface. But it is to be noted, nevertheless, that the file acts with greater effect in proportion as the area filed is decreased, and that as the tool marks are filed out the area of surface operated upon is increased. Suppose, then, that starting from any point on the work circ.u.mference a file stroke be taken, and that it extends around one-third of the circ.u.mference, that the second file stroke extends around one-third also, but that there is an unfiled s.p.a.ce of, say, two inches between the area of surface filed by these two strokes, and that at the third file stroke the file starts on the surface filed at the first stroke, pa.s.ses over the two inches previously unfiled and terminates on the surface filed by the second stroke; then the conditions will be as follows:--

Part of the surface filed at the first stroke will have been filed twice, part of the surface filed at the second stroke will also have been filed twice, while the two inches will have been filed once only.

But this latter part will have had much more taken off it during the third stroke than did the rest of the surface filed at that stroke, because it operated on the ridges or tool marks where, being unfiled, their area in contact with the file teeth was at a minimum. This condition will prevail until the tool marks are effaced, and tends to preserve the truth of the work up to that point, hence the necessity of leaving very fine tool marks becomes obvious.

Apart from these considerations, however, there is the fact that filing work in the lathe is a very slow operation, and therefore inapplicable to large work; and furthermore, on large work the surface is not needed to be so smooth as in small work; for example, tool feed marks 1/1000 inch deep would upon work of 1/2 inch diameter leave a surface appearing very uneven, and the wearing away of those ridges or marks would destroy the fit of the piece; but in a piece, say, six feet in diameter, tool marks of that depth would not appear to much disadvantage, and their wearing away would have but little effect upon the fit of the piece.

Finis.h.i.+ng with the file, therefore, is usually applied to work of about 24 inches in diameter, and less, larger work being finished with the cutting tool or by emery grinding, where a greater degree of finish is required.

Small work--as, say, of six inches, or less, in diameter--may be finished with the file so cylindrically true, that no error can be discovered by measurement with measuring tools of the calipering cla.s.s, though the marks of contact if made apparent by gently forcing the work through a closely fitting ring-gauge may not appear to entirely cover the surface.

To produce filed work thus true, all that is necessary is to set the cutting edge of the finis.h.i.+ng tool at the horizontal centre of the work, properly adjust the live spindle of the lathe for fit to its bearings, adjust the slides of the slide rest so that there is no lost motion, and follow the rules already given with reference to the shape of the tool cutting edge, employing a cutting speed not so fast as to dull the tool before it has finished its cut, using a fine feed except in the case of cast iron, as already explained.

The requirement that the tool shall not become dull before it has finished its cut, brings us to the fact that the length of work that can be thus accurately turned is limited, as the diameter of the work increases.

Indeed, the length of the work in proportion to its diameter is a very important element. Thus, it would be very difficult indeed to turn up a spindle of an inch in diameter and, say, 14 feet long, and finish it cylindrically true, parallel, and smooth, because

1st. The slightness of the work would cause it to spring or deflect from the pressure or strain due to the cut. This may to some extent be remedied by steadying the work in a follower rest, but the bore of such follower itself wears as the cut proceeds, though the amount may be so small as to be almost inappreciable.

2nd. The work being better supported (by the lathe centres) at the two ends than in the middle of its length, the duty placed on the follower rest will increase as the middle of the work length is approached, hence the spring or deflection of the follower rest will be a disturbing element.

3rd. The tool gets duller as the cut proceeds, causing more strain from the cut, and, therefore, placing more strain on the follower rest; and,

4th. It would be necessary, on account of the length of the cut, to resharpen the tool before the cut was carried from end to end of the spindle, and it would be almost impracticable to set the reground tool to cut to the exact diameter.

The second, third, and fourth of the above reasons operate together in causing increased work spring as the tool approaches the middle of the work length; thus the deflection of the follower rest, the increased weakness of the work, and the comparative dullness of the tool would all operate to cause the work to gradually increase in diameter as the cut proceeded towards the work centre (of length).

Suppose, for example, a cut to have been carried from the dead centre, say, five feet along the work; at the end of this five feet the tool will be at its dullest, the shaft at its weakest, and supported the least from the dead centre and follower rest.

Suppose, then, that the reground tool be placed in the rest again and set to just meet the turned surface without cutting it, then when it meets the cut to carry it farther along the work the cut will produce (on account of the tool being sharper) less strain on the work, which will therefore spring or deflect less. Precisely what effect this may have upon the diameter to which the tool will turn the work will depend upon various conditions: thus, if the top face of the tool be sufficiently keen to cause the strain due to bending the shaving cut or chip to pull the work forward, the tool would turn to a smaller diameter. If the depth of the cut be sufficient to cause the work to endeavor to lift, and the tool edge be above the centre of the work, it would be cut to smaller diameter. If the tool cutting edge were below the centre, or if its top face be at an angle tending to force the work away from the tool point, the diameter of the work would be increased.

From these considerations it is obvious that the finis.h.i.+ng cut should be started at the centre of the work length, and carried towards the lathe centres, because in this case the tool will be sharpest, and therefore will produce less tensional strain on the work at the point where the latter is the weakest, while the resisting strength of the work would increase as the cut proceeded, and the tool became dull from use.

Furthermore, if it were necessary to regrind the tool, it would be reset nearer to the lathe centres, where the work would be more rigidly held; hence the tool could be more accurately set to the diameter of the finis.h.i.+ng cut.

By following this plan, however, it becomes necessary to have the shaft as near true and parallel as possible before taking the finis.h.i.+ng cut, for the following reasons:--

Let the diameter of the spindle before the finis.h.i.+ng cut be 1-1/32 inches, leaving 1/32 inch to be taken off at the finis.h.i.+ng cut, then the ring in the follower rest must be at starting that cut 1-1/32 inch bore, and if the rest is to follow the cut the bush must be changed (so soon as it meets the finis.h.i.+ng cut) to one of an inch bore. But if the spindle be turned as true and parallel as possible before the finis.h.i.+ng cut the rest may lead the tool, in which case the bush need not be changed. There are differences of opinion as to the desirability of either changing the bush or letting the tool follow the rest, but there can be no dispute that (from the considerations already given) a spindle turned as true and parallel as may be with the tool started from the dead centre and carried forward can be improved by carrying yet another cut from the middle towards the dead centre. In any event, however, work liable to spring or too long to be finished at one cut without removing the tool to grind it, can be more accurately finished by grinding in a lathe, such as was shown in Figs. 676 or 679, than by steel-cutting tools, and for the following reasons:--