Modern Machine-Shop Practice Part 152

The work should be held as near down to the surface of the jaws of the vice as will allow the required amount of metal to be filed off without danger of the file teeth coming into contact with those jaws, and should be placed so that the filing operation when finished shall be as near as possible parallel with the top of the vice jaws. These jaws then serve somewhat as a guide to the filing operation, showing where the metal requires filing away.

For cutting steel that contains hard spots or places, a second-cut file is more effective than a rough or b.a.s.t.a.r.d file.

Rough files are more suitable for soft metals, the b.a.s.t.a.r.d cut being usually employed upon wrought iron, cast iron, and steel by the machinist. But in any case the edge of the file is employed to remove small spots that are excessively hard. The file should be clean and dry to cut hard places or spots, and used with short strokes under a heavy pressure, with a slow movement.

When a file has been used until its cutting edges have become too dull for use, it may be to some extent resharpened by immersion in acid solutions; but the degree of resharpening thus obtained has not proved sufficient to bring this process into general or ordinary application; hence, the files are either considered useless, or the teeth are ground off and new ones formed by recutting them.

A recut file is of course thinned by the process, but if properly done is nearly, if not quite, as serviceable as a new one, providing that in grinding out the old teeth the file be ground properly true to curve; but, unfortunately, this is rarely found to be the case.

An excellent method of resharpening files, and also of increasing the bite of new files (which is an especial advantage for bra.s.s work), is by the means of the sand blast. The process consists of injecting fine sand against the backs of the teeth by means of a steam jet, and is applicable to all files, from the rasp to the finest of Groubet files.

The action of the sand is to cut away the backs of the file teeth, thus forming a straight bevel on the teeth back, and giving a new cutting edge, and the process occupies from three to five minutes.

[Ill.u.s.tration: Fig. 2248.]

Fig. 2248 represents a machine constructed for this purpose. Steam is conveyed by the piping to the nozzles A, A, which connect by rubber hose H, H to sand pipe K, so that the steam jets pa.s.sing through A, A carry with them the mixture of quartz, sand, and water in the sand box. By means of the overhead guide frame at D, E the file clamp C is caused to travel when moved by hand in a straight line between the nozzles A, A in the steam box, from which the expended sand and water flow down back to the sand box. Thus both sides of the file are sharpened simultaneously, and from the fixed angles of the nozzles and true horizontal motion of the file the angles of all the teeth are equal and uniform.

To distribute the sharpening effects of the sand equally across the width of the file, the carriage has lateral or side motion, as well as endwise, and on the apparatus represented adjustable rollers regulate this side movement. Having the two motions, any part of the file can be presented to the blast.

The following is from _Engineering_:--"A comparative trial of the cutting power of the sharpened files was lately made with the following results: A piece of soft wrought iron was filed clean and weighed; 1200 strokes were made by a skilled workman with one side of a new 10-inch b.a.s.t.a.r.d file, the iron was again weighed, and the loss noted. The other side of this file was then subjected to the sand blast for five seconds, and 1200 strokes were made with this sand-blasted side on the same piece of iron, great care being taken to give strokes of equal length and pressure in both cases. The iron was then weighed, and the loss found to be double as much as in the first case.

"These operations were repeated many times, counting the strokes and weighing the metal each time, and the quant.i.ty cut was found to gradually become less for both sides as these became worn. When the weight of metal cut away by 1200 strokes of the sand-blasted side was found to be no greater than had been cut by the first 1200 strokes of the ordinary side when quite new, a second sand blasting was applied to it for 10 seconds, and in the next 1200 strokes its rate of cutting rose to nearly its first figure. When the cut made by the ordinary side of the file fell to about four-tenths of its cut when new, it was considered by the workman as worn out, and a new file of the same size and maker was used to continue the comparison with the one sand-blasted side; 83 sets of 1200 strokes each and 13 sand-blastings were made on the same side of this file, and in that time it cut as much metal as six ordinary sides. In 99,600 strokes it cut away 14 ozs. avoirdupois of wrought iron, and 16.4 ozs. of steel.

"With an equal number of strokes its average rate of cutting was, on wrought iron, 50 per cent. greater than the average of the ordinary sides, and on steel 20 per cent. greater. As the teeth became more worn, the time of the application of the sand blast was lengthened up to one minute. After the thirteenth re-sharpening its rate of cutting was nine-tenths that of the ordinary side when quite new.

"When the teeth become so much worn that the sand blast ceases to sharpen them effectively, the file can be recut in the usual way, and each set of teeth can be made to do six times as much work as an ordinary file, and to do it with less time and labor, because it is done with edges constantly kept sharp. The time required to sharpen a worn-out 14-inch b.a.s.t.a.r.d file is about four minutes, or proportionately less if sharpened before being entirely worn out. Smooth files require much less time. About 4 horse power of 60 lb. steam used during four minutes, and one pint per minute of sand (pa.s.sed through a No. 120 sieve), and the time of a boy are the elements of cost of the operation."

RED MARKING OR MARKING.--This is a paint used by machinists to try the fit of one piece to another, or to try the work by a test piece or surface plate. It should be composed of dry Venetian red, mixed with lubricating oil of any kind.

Instead of Venetian red, red lead is sometimes used for marking, but it is too heavy and separates from the oil, and furthermore will not spread either evenly or sufficiently thin, and is therefore much inferior to Venetian red.

It is applied to the surface of the test piece or piece of work, and the latter is brought to bear on the surface to be tested, so that it leaves paint marks disclosing where the surfaces had contact, and therefore what parts of the surface require removing in order to make the surfaces have the desired degree of contact.

When either the test piece or the work can be put in motion while testing, one piece is rubbed upon the other or pa.s.sed along the same in order that the bearing marks may receive the marking more readily and show the bearing spots more plainly, the operation coming under the head of fitting. When neither piece can be given motion, one is made to mark the other by being struck with a mallet or hammer, or to avoid damage to the work from the hammer blows, a piece of wood or copper is interposed.

This operation is termed "bedding."

[Ill.u.s.tration: _VOL. II._ =Sc.r.a.pERS AND Sc.r.a.pING.= _PLATE IX._

Fig. 2252.

Fig. 2253.

Fig. 2254.

Fig. 2255.

Fig. 2256.

Fig. 2257.

Fig. 2258.

Fig. 2259.

Fig. 2260.]

The thickness of the coating of marking varies with the kind of work, the finer fit the work requires to be, the thinner the coat of marking.

Thus in chipping a thick coat is applied, for rough filing a thinner, for smooth filing a still thinner coat, and so on, until for the finest of work the coat is so thin as to be barely perceptible to the naked eye. When either the work or the testing piece can be given motion and the surfaces rubbed together, a thinner coat of marking may be used.

Marking is usually applied with a piece of rag doubled over and over, and bound round with a piece of twine so as to form a kind of paint-brush. This will give the surface a lighter and more evenly spread coat than would be possible with a brush of any kind. For very fine work red marking may be spread the lightest and the most even with the palm of the hand, which will readily detect any grit, dirt, or other foreign substance which the marking may contain from being left exposed.

[Ill.u.s.tration: Fig. 2249.]

[Ill.u.s.tration: Fig. 2250.]

THE HACK-SAW.--The hack-saw is employed by the machinist for severing purposes, and also for sawing slots in the heads of screws. The blade should be tightly strained in the frame, which will prevent saw breakage. The ordinary method of doing this is to provide the end of the saw frame with a sliding stud threaded at its end to receive a thumb nut. The studs at each end of the blade should be squared where they pa.s.s through the frame, as at A, B in Fig. 2249, so that the blade shall not be permitted to twist. An improved form is shown in Fig. 2250, in which the end E has a saw slot to receive the blade F. At the handle end of the blade it is held by a stud sliding through the frame, being squared at B; at C is a nut let into and screwed in the handle, and into or through the nut is threaded the end of the stud, so that by rotating the handle the blade is strained. The curve in the back at A gives a little elasticity to it, and therefore a better strain to the blade. A hack-saw should always be used with oil, which preserves the cutting edge of the teeth.

In sharpening a hack-saw it is best to rest the smooth edge of the blade on a piece of hard wood or a piece of lead, and spread the tops of the teeth by light hammer blows, which serves a two-fold purpose, first it thickens them and enables them to cut a groove wide enough to let the blade pa.s.s freely through, and secondly it enables the teeth to be filed up to a sharp cutting edge with less filing.

[Ill.u.s.tration: Fig. 2251.]

The screw-driver to be used in saw slots should have its end shaped as at A in Fig. 2251, which will tend to prevent it from slipping out of the saw slot, as it will be apt to do if wedge-shaped as at B, because in that case the action of the torsional pressure or twist is to lift the screw-driver out of the slot.

Sc.r.a.pERS AND Sc.r.a.pING.--The process of sc.r.a.ping is used by the machinist to true work, and to increase the bearing area of surfaces, while the bra.s.s finisher employs it to prepare surfaces for polishing, applying it mainly to hollow corners and sweeps.

For sc.r.a.ping work to fit it together the flat sc.r.a.per is used, ordinary forms being shown in Figs. 2252 and 2256.

That shown in Fig. 2252 may be made of a flat smooth file, of about an inch wide, and 3/16-inch thick, which is large enough for any kind of work. Two opposite faces, one of which is shown at A, are ground beveled so as to leave the end face B about 1/16-inch thick. This end face is then ground square as denoted by the dotted lines, producing two cutting edges of equal angles, and therefore equally keen. If it were attempted to grind face B at an angle as denoted by the dotted lines G, in Fig.

2253, the lower edge H would cut too keenly, causing the sc.r.a.per to chatter and cut roughly, while the upper one I would not cut sufficiently easily.

For very smooth work the sc.r.a.per may be formed as in Fig. 2256, the front face E being ground slightly out of square as shown, and the bottom face F being given considerable angle to the body of the sc.r.a.per.

For very rapid cutting, however, the front face E may be at an angle of less than 90 to the top of the sc.r.a.per.

The only objection to this form is that the eye lends no a.s.sistance in bringing the edge fair with the work surface. The sc.r.a.per should not exceed about 6 inches in length, exclusive of the handle, for if longer it will not cut well or smoothly, and its end face should be slightly rounded as in Fig. 2254. Its facets should be ground square or straight and carefully oil-stoned after the grinding, the oil-stoning process being repeated for two or three resharpenings, after which it must be reground upon the grindstone.

The sc.r.a.per should be grasped very firmly in the hands, and held as in Fig. 2255. It requires to be pressed hard to the work during the cutting and lightly during the backward stroke.

The strokes should not exceed for the roughing courses, say, half an inch in length, the first course leaving the work as represented in Fig.

2257.

The second course should be at a right angle to the first, leaving the work as in Fig. 2258, and after these two courses the work should be tested by surface plate, or with the part to which it is to fit, as the case may be. Previous to the testing, however, the work must be carefully wiped clean with old rag, as new rag or waste is apt to leave ravelings behind. The surface plate should be given a light coat of red marking, and then moved backward, forward, and sideways over the work, or, if the work is small, it may be taken from the vice and rubbed upon the surface plate, and the high spots upon the work will be shown very plainly by the marks left by the plate. The harder the plate bears upon the work the darker the marks will appear, so that the darkest parts should be sc.r.a.ped the heaviest.

After applying the plate, the sc.r.a.per may again be applied, the marks being at an angle to the previous operation, the testing and marking by the plate and sc.r.a.ping process being continued until the job is complete, appearing as shown in Fig. 2259.

It will be noted that the sc.r.a.per marks are much smaller and finer at and during the last few sc.r.a.pings; and it may be here remarked that the sc.r.a.pings are very light during the last few finishing processes.

The strokes of the sc.r.a.per being made of a length about equal to the acting width of its edge cuts, makes the sc.r.a.per mark approximately square, on which account it is sometimes termed "block" sc.r.a.ping. It gives an excellent finish, while not sacrificing the truth of the work to obtain the finish.

The sc.r.a.per will not remove a quant.i.ty of metal so quickly as a file, and on this account it is always preferable to surface the work with a file before using the sc.r.a.per, even though the work be well and smoothly planed. Not until the file has almost entirely removed the planer marks, and the surface plate shows the surface to be level and true, should the sc.r.a.per be brought into requisition, the first courses being applied vigorously to break down the surface.

It would appear that sc.r.a.ping might be more quickly done by taking long sc.r.a.per strokes promiscuously over the work, but in this case the bearing marks are not well defined and do not show plainly, which leads to confusion and causes indecision as to where the most or heaviest sc.r.a.ping requires to be done, whereas in the block sc.r.a.ping the marks are clearly defined and the high patches or spots on the work show very plainly, and the workman is able to proceed intelligently and with precision.