The hooked tool, Fig. 40, is shown to be considerably above the center line, and at that point it will do the most effective cutting on steel.

If, on the other hand, bra.s.s is operated on there should be no top rake, as ill.u.s.trated in Fig. 41, thus a.s.suring a smooth job.

LAPS.--This is a tool which is very useful, particularly for grinding and truing up the cylinders of internal combustion engines, as well as for all kinds of bores of refractory material which cannot be handled with the cutting tool of the lathe.

It is made up of a mandrel or rod of copper, with lead cast about it, and then turned up true, so that it is but the merest trifle larger than the hole it is to true up.

USING THE LAP.--The roller thus made is turned rapidly in a lathe, and the cylinder to be trued is brought up to it and the roller supplied freely with emery powder and oil. As rapidly as possible the cylinder is worked over on the roller, without forcing it, and also turned, so as to prevent even the weight from grinding it unduly on one side.

More or less of the emery will embed itself in the lead, and thus act as an abrasive. The process is called "lapping."

SURFACE GAGES.--Frequently, in laying out, it is necessary to scribe lines at a given distance from some part of the work; or, the conditions are such that a rule, a caliper, or dividers will not permit accurate measurement to be made.

For such purposes, what is called a surface gage was devised. This is merely a heavy base, provided with a pivoted upright on which is mounted a scribe that is held by a clamp so it may be turned to any angle.

[Ill.u.s.tration: _Fig. 43.--Showing uses of the Surface Gage._]

SURFACE GAGE.--The clamp holding the scriber is vertically movable on the pivoted upright. By resting the base of the surface gage on the line to be measured from, and swinging one point of the scriber to the place where the work is to be done, accuracy is a.s.sured. One end of the scriber is bent, so it can be adapted to enter recesses, or such places as could not be reached by the straight end.

CHAPTER IV

ON THE USE OF THE FILE

The most necessary tool in a machine shop is a file. It is one of the neglected tools, because the ordinary boy, or workman, sees nothing in it but a strip or a bar with a lot of cross grooves and edges, and he concludes that the only thing necessary is to rub it across a piece of metal until he has worn it down sufficiently for the purpose.

THE FIRST TEST.--The fact is, the file is so familiar a tool, that it breeds contempt, like many other things closely a.s.sociated in life.

Give the boy an irregular block of metal, and tell him to file it up square, and he will begin to realize that there is something in the handling of a file that never before occurred to him.

He will find three things to astonish him:

First: That of dimensions.

Second: The difficulty of getting it square.

Third: The character of the surface when he has finished it.

FILING AN IRREGULAR BLOCK.--To file a block of an irregular character so that the dimensions are accurate, is a good test for an accomplished workman. The job is made doubly difficult if he is required to file it square at the same time. It will be found, invariably, that the sides will not be parallel, and by the time it is fully trued up the piece will be too small. See Figs. 44 and 45.

Then, unless the utmost care is taken, the flat sides _will not_ be flat, but rounded.

FILING A BAR STRAIGHT.--The next test is to get the boy to file a bar straight. He has no shaper or planer for the purpose, so that it must be done by hand. He will find himself lacking in two things: The edge of the bar will not be straight; nor will it be square with the side of the bar.

[Ill.u.s.tration: _Fig. 44. Rounded Surface_ _Fig. 45. A Winding Face_]

FILING BAR WITH PARALLEL SIDES.--Follow up this test by requiring him to file up a bar, first, with two exactly parallel sides, and absolutely straight, so it will pa.s.s smoothly between the legs of a pair of calipers, and then file the two other sides in like manner.

SURFACING OFF DISKS.--When the foregoing are completed there is still another requirement which, though it appears simple, is the supreme test. Set him to work at surfacing off a pair of disks or plates, say one and a half inches in diameter, so that when they are finished they 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 sc.r.a.per, 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 sc.r.a.per are then used to cut away the metal.

[Ill.u.s.tration: _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.

[Ill.u.s.tration: _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.

[Ill.u.s.tration: _Fig. 49. Key-seat Rule_]

[Ill.u.s.tration: _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.