Modern Machine-Shop Practice Part 149

Clearly when the chisel has arrived at position 2, the moulding will be finished to shape, and it is therefore plain that it takes a length of cutter-edge from C to F to cut a moulding whose depth is S, or what is the same thing, C E.

But to solve the question in this way, we require for every different depth of moulding to make such a sketch, and the square bar that drives the chisel is made in various sizes, each different size again altering the length or depth of chisel edge necessary for a given depth of moulding.

[Ill.u.s.tration: Fig. 2195.]

[Ill.u.s.tration: Fig. 2196.]

But we may carry the solution forward to the greatest simplicity for each size of square bar, and for any depth of moulding that can be dressed on that size of bar, by the following means:--In Fig. 2197 we have the circle and the line A as before; the depth from C to E being the greatest depth of moulding to which the square bar is intended to drive the chisels; while point C is the nearest point to the square bar at which the top of the moulding must be placed. Line _a_ represents a chisel cutting at its highest point; line _b_ a chisel cutting the moulding to final shape at 1/4 inch below C, on the line A; line _c_ a chisel cutting the moulding to final shape at a distance of 1/2 inch below point C and measured on the line A; lines _d_, _e_, _f_, _g_, _h_, and _i_ represent similar chisel positions, the last meeting the point E, which is the lowest point at which the chisel will cut. Suppose, now, we set a pair of compa.s.ses one point at the centre A of the circle, and strike the arc _j_; this arc will represent the path of motion of that part of the chisel edge that would finish the moulding to shape at C; similarly arc _k_ represents the path of motion of that part of the chisel edge that cuts the moulding to final shape on the line A, and at a distance of 1/4 inch below C, and so on until we come to arc _r_, which represents the path of motion of the end of the chisel. All these arcs are carried to meet the first chisel position C _a_, and from these points of intersection with this line C _a_ we mark lines representing those on a common measuring rule. The first of these from C we mark 1/4, the next 1/2, the next 3/4, and so on to 2, these denoting the measurement or depth of chisel necessary to cut the corresponding depth of moulding. If, for example, we are asked to set a pair of compa.s.ses to the depth of cutting edge necessary to cut a moulding that is an inch deep, all we do is to set one leg of the compa.s.ses at C, and the other at line 1 on the line C _a_; or if the moulding is to be 2 inches deep, we set the compa.s.ses from C to 2 on line C _a_. We have here, in fact, constructed a graduated scale that is destined to be found among the tools of every workman who forms moulding cutters, and if we examine it we shall find that the line that is marked 1/4 inch from C is not 1/4 inch but about 5/16 inch; its distance from C being the depth of chisel edge necessary to cut a moulding that is 1/4 inch deep.

[Ill.u.s.tration: Fig. 2197.]

Again, the line marked 1 measures 1-3/16 inch from C, because it requires a chisel edge 1-3/16 deep to cut a moulding that is one inch deep. But if we measure from _c_ to the line marked 2 we find that it is 2-1/4 inches from C, and since it represents a chisel that will cut a moulding two inches deep, we observe that the deeper the moulding is the nearer the depth of cutting edge is to the depth of moulding it will produce. This occurs because the longer the chisel the more nearly it stands parallel to the line A, at the time when its point is crossing the line A. Thus, line _i_ is more nearly parallel to A than line _a_ is, and our scale has taken this into account, for it has no two lines equally s.p.a.ced; thus, while that marked 1/4 is 5/16 inch distant from C, that marked 1/2 is less than 5/16 inch distant from that marked 1/4, and this continues so that the line marked 2 is but very little more than 1/4 inch from that marked 1-3/4. Having constructed such a scale we may rub out the circle, the arcs, the line A, and all the lines except the line from C to _a_ and its graduations, and we have a permanent scale for any kind of moulding that can be brought to us.

If, for example, the moulding has the four steps or members _s_, _t_, _u_, _v_, in the figure, each 1/2 inch deep, then we get the depth of cutter edge for the first member _s_ on our scale, by measuring from _c_ to the line 1/2 on line C _a_. Now the next member _t_ extends from 1/2 to 1 on the moulding, and we get length of cutter edge necessary to produce it from 1/2 to 1 on the scale. Member _u_ on the moulding extends from 1 to 1-3/4; that is to say, its highest point is 1 inch and its lowest 1-3/4 inch from the top of the moulding, and we get the length for this member on a scale from the 1 to the 1-3/4; and so on for any number of members.

After the depth of cutting edge for each member has thus been found, it remains to find the exact curve of cutting edge for each step, and, in doing this, the same scale may be used, saving much labor in this part also of the process, especially where a new piece of moulding must be inserted to repair part of an old piece that needs renewal in places only, as is often the case in railroad cars.

In Fig. 2197A we have a scale or rule constructed upon the foregoing principles, but marked to sixteenths, and it may now be shown that the same scale may be used in finding the actual curve as well as the depth of cutter edge necessary to produce the moulding of any member of it.

Let the lower curves _s_, _t_, for example, represent the moulding to be produced, and the upper outline represent the blank piece of steel of which the cutter is to be made, the edges C, D being placed in line one with the other. We may then draw across both the moulding and the steel, lines such as E E, F F, G G, H H, I I, J J, all these lines being parallel to the edges C, D. To get the total depth of cutter edge for the moulding we measure with a common measuring rule the total height of the moulding, and supposing it to measure an inch, we set a pair of compa.s.ses to an inch on our cutter scale, and with them mark from the base _m_ of the steel, the line P giving total depth of cutter edge. We next measure with a common rule the depth of member _s_ of the moulding, and as it measures 1/2 inch we set the compa.s.ses to the 1/2 on the cutter scale, and with these compa.s.ses mark from line _m_ line B, showing the depth of member _s_. In order to find the exact curve for each member, we have first to find a number of points in the curve and then mark in the curve by hand, and it is for the purpose of finding these points that the lines E E, F F, G G, H H, I I, J J, have been drawn. These lines, it may be remembered, need not be equally s.p.a.ced, but they must be parallel, and as many of them may be used as convenient, because the greater their number the more correctly the curve can be drawn.

[Ill.u.s.tration: Fig. 2197A.]

[Ill.u.s.tration: Fig. 2198.]

[Ill.u.s.tration: Fig. 2199.]

The upper edge or base line, _m_ _m_, both of the steel and of the drawing, is that from which all measurements are to be taken in finding the points in the curve, which is done as follows: With an ordinary measuring rule we measure on the moulding and from line _m_ _m_ of the moulding as a base the length of the line F F below _m_ _m_, to the curve, which in this case measures say 5/16 inch; we then set a pair of compa.s.ses or compa.s.s calipers to the 5/16 on the cutter scale, and from base _m_ _m_ on the cutter steel, mark, on line F F, an arc, and where the arc cuts F F is one point in the curve.

Similarly we measure on the moulding, or drawing of the moulding, the length of line G G from line _m_ _m_ to the moulding curve, and find that it measures, say 7/16 inch, hence we mark from base line _m_ _m_ of the steel, on line G G, arc V, distant 7/16 according to the cutter scale. Similar measurements are taken at each vertical line of the drawing which represents the moulding, and by means of the corresponding divisions of the cutter scale, arcs are marked on the vertical lines on the cutter steel, and where the arcs cut the vertical lines are points in the curve, and through these points the curve may be drawn by hand.

We may make a cutter scale from an ordinary parallel rule, marking one end to correct inches and the other end for a cutter scale. Measurements from the moulding may then be made on one end of the rule; measurements for the cutter may be taken from the other end of the rule, and the rule may be used at the same time to draw the parallel lines E E, &c. Or, as each size of cutter head requires a different cutter scale, we may make a rule out of a piece of box or other close-grained wood, say 3/4 inch square, using one side for each size of cutter head. One end of each face of this rule may be marked in correct inches and parts of an inch (the divisions being thirty-seconds of an inch), and the other end may be marked as a cutter scale, the divisions being found as described with reference to Fig. 2197.

An instrument, patented by R. Drummond, for finding the depth of cutting edge and also for finding the curves, is shown in Figs. 2198 and 2199.

It consists essentially of a bar G bent at a right angle, thus making two arms. Upon one arm is a slide W (best seen in Fig. 2199) secured by a set-screw B, and having at A a pivot to carry a second bar H, which is slotted throughout its length to permit bar G to slide freely through it. Upon the other arm of G is a slide P secured by a set-screw C, and carrying a compa.s.s point E. The bar H carries an adjustable slide Z secured by a set-screw D and carrying the compa.s.s point F.

[Ill.u.s.tration: Fig. 2200.]

In using the instrument but three very simple operations are necessary.

First, the two slides W and P are set to the numerals on the bar, which correspond to the size of the head on the moulding machine the cutter is to be used upon; thus in Fig. 2199 they are shown set to numeral 2, as they would be for a 2-inch cutter head. The instrument is next opened, its two bars occupying the position shown in Fig. 2199, and the two compa.s.s points are set to the height of the moulding or to any desired member of it, as the case may be. The bars are then opened out into the position shown in Fig. 2200, and the compa.s.s points at once give the depth of cutter edge necessary to produce the required depth of moulding.

It will be noted that the pivot A represents the centre upon which the cutter revolves, and that while the face of the bar H corresponds to the line of moulding formation answering to line A A in Fig. 2196, the face of bar G corresponds to the face C F of the cutter in Fig. 2196; hence the instrument simply represents a skeleton head and cutter, having motions corresponding to those of an actual cutter head and cutter.

[Ill.u.s.tration: Fig. 2201.]

[Ill.u.s.tration: Fig. 2202.]

THE FILE.--The file is a piece of hardened steel having teeth produced upon its surface by means of rows of chisel cuts which run more or less diagonally across its width at an angle that is varied to suit the nature of the material the file is to be used upon. The vertical inclination of the tooth depends upon the inclination of the face of the chisel with which it is cut, the two being equal, as is shown in Fig.

2201, which is an enlarged view of a chisel and some file teeth. In order that the tops of the teeth shall be sharp, and not rounded or curved, as in Fig. 2202, it is necessary that the edge of the chisel be kept sharp, an end that is greatly aided by the improved form of chisel shown in Fig. 2203. When a file possesses curved points, or caps, as they are technically termed, a few strokes upon a narrow surface will cause them to break off, reducing the depth of the teeth and causing the cuttings to clog in them. If, however, the file is used upon a broad surface these caps will remain, obviously impairing the cutting qualifications of the file, even when new, and as they soon get dulled the file loses its grip upon the work and becomes comparatively valueless.

[Ill.u.s.tration: Fig. 2203.]

Files were, until the past few years, cut entirely by hand--file cutting by machinery having previously been a wide field of mechanical experiment and failure. Among the most prominent causes of failure was that the teeth produced by the earlier machines were cut too regular, both as to their s.p.a.cing and their height; hence the points of the rear teeth fell into the same channels as those in advance of them, thus giving the tooth points too little opportunity to grip the work. This also gives too broad a length of cutting edge and causes the file to vibrate on the forward or cutting stroke, an action that is technically known as chattering, and that obviously impairs its cutting capacity.

The greatest amount of duty is obtained from a file when the rear tooth cuts off the projection left by the preceding one, because in that case the duty of the tooth is confined to cutting off a projection that is already weakened and partly separated from the main body by having the metal cut away around its base. Workmen always practically recognise this fact, and cause the file marks to cross each other after every few strokes. In the machine-cut files made by The Nicholson File Co., the teeth are arranged to attain this object by the following means:--1. The rows of teeth are s.p.a.ced progressively wider apart from the point towards the middle of the file length by regular increments of s.p.a.cing, and progressively narrower from the middle toward the heel. 2. This general law of the s.p.a.cing is modified by introducing as the teeth are cut an element of controllable irregularity in the s.p.a.cing, which irregularity is confined within certain limits, so that neither the increment nor decrement of s.p.a.cing is entirely regular. 3. In arranging the teeth so that the successive rows shall not be exactly parallel one to the other, the angle of inclination being reversed as necessity requires. The irregularity of s.p.a.cing, while sufficient to accomplish the intended object, is not enough to practically vary the cut of the file, or, in other words, it is insufficient to vary its degree of coa.r.s.eness or fineness to any observable extent. But it enables the file to grip the work with as little pressure as possible, and enables the teeth to cut easily without producing deep file marks or furrows.

Files and rasps have three distinguis.h.i.+ng features: 1. Their length, which is always measured exclusively of their tangs. 2. Their cut, which relates not only to the character, but also to the relative degree of coa.r.s.eness of the teeth. 3. Their kind or name, which has reference to the shape or style. In general, the length of files bears no fixed proportion to either their width or thickness, even though of the same kind. The tang is the spiked-shaped portion of the file prepared for the reception of a handle, and in size and shape should always be proportioned to the size of the file and to the work to be performed.

The heel is that part of the file to which the tang is affixed.

Of the cut of files we may say that it consists of three distinct forms; viz.: "single cut," "double cut" and "rasp," which have different degrees of coa.r.s.eness, designated by terms as follows viz.:--

Single-cut. Double-cut. Rasp.

Rough Coa.r.s.e Coa.r.s.e Coa.r.s.e b.a.s.t.a.r.d b.a.s.t.a.r.d b.a.s.t.a.r.d Second-cut Second-cut Second-cut Smooth Smooth Smooth Dead-smooth

[Ill.u.s.tration: Fig. 2204.]

The terms "rough," "coa.r.s.e," "b.a.s.t.a.r.d," "second-cut," "smooth" and "dead-smooth," have reference only to the coa.r.s.eness of the teeth, while the terms "single-cut," "double-cut" and "rasp" have special reference to the character of the teeth. The single-cut files (the coa.r.s.er grades of which are sometimes called "floats") are those in which the teeth are unbroken, the blanks having had a single course of chisel-cuts across their surface, arranged parallel to each other, but with a horizontal obliquity to the central line, varying from 5 to 20 in different files, according to requirements. Its several gradations of coa.r.s.eness are designated by the terms "rough," "coa.r.s.e," "b.a.s.t.a.r.d," "second-cut"

and "smooth." The rough and coa.r.s.e are adapted to files used upon soft metals, as lead, pewter, &c., and, to some extent, upon wood. The b.a.s.t.a.r.d and second-cut are applied princ.i.p.ally upon files used to sharpen the thin edges of saw teeth, which from their nature are very destructive to the delicate points of double-cut files. The smooth is seldom applied upon other than the round files and the backs of the half-rounds. Fig. 2204 represents the cut of single-cut rough files, their lengths ranging from 16 inches down to 4 inches. Fig. 2205 shows the cut of the coa.r.s.e, b.a.s.t.a.r.d, and second-cut, whose lengths also range from 16 to 4 inches, and whose cut is also finer as the length decreases. The float is used to some extent upon bone, horn, and ivory, but princ.i.p.ally by plumbers and workers in lead, pewter, and similar soft metals. It will be seen that the teeth are nearly straight across the file and are very open, both of these features being essential requirements for files to be used on the above-named metals.

Double-cut files are those having two courses of chisel cuts crossing each other. The first course is called the over cut, and has a horizontal obliquity with the central line of the file, ranging from 35 to 55. The second course, which crosses the first, and in most double cuts is finer, is called the up-cut, and has a horizontal obliquity varying from 5 to 15. These two courses fill the surface of the file with teeth inclined toward its point, the points of which resemble somewhat, when magnified, those of the diamond-shaped cutting tools in general use. This form of cut is made in several gradations of coa.r.s.eness, which are designated by the terms "coa.r.s.e," "b.a.s.t.a.r.d,"

"second-cut," "smooth" and "dead-smooth." Fig. 2206 represents the cut of double-cut b.a.s.t.a.r.d files, from the 16 inch down to the 4 inch, and Fig. 2207 the cut of the coa.r.s.e, second-cut, and smooth. For very fine finis.h.i.+ng a still finer cut, called the dead-smooth, is made, being like the smooth, but considerably finer. It is a superior file for finis.h.i.+ng work in the lathe, or for draw-filing machine work that is to be highly finished. The double-cut is applied to most of the files used by the machinist, and, in fact, to most of the larger number in general use.

For unusually fine work, tool-makers and watch-makers use the Swiss or Groubet files--so called from their being made by M. Groubet, of Switzerland. These files are double-cut, and their degree of coa.r.s.eness is denoted by number; thus, the coa.r.s.est is a b.a.s.t.a.r.d and the finest number 8. The prominent characteristics of these files are their exceedingly even curvature and straightness, and, in the finer grades, the unusual fineness of the cut, which feels soft and velvety to the touch. They are made in sizes ranging from 2 to 10 inches, and are always double-cut.

[Ill.u.s.tration: Fig. 2205.]

[Ill.u.s.tration: Fig. 2206.]

[Ill.u.s.tration: Fig. 2207.]

Rasps differ from the single or double-cut files in that the teeth are disconnected from each other, each tooth being made by a single-pointed tool, denominated by file-makers a punch, the essential requirement being that the teeth thus formed shall be so irregularly intermingled as to produce, when put in use, the smoothest possible work consistent with the number of teeth contained in the surface of the rasp. Rasps, like files, have different degrees of coa.r.s.eness, designated as "coa.r.s.e," "b.a.s.t.a.r.d," "second-cut" and "smooth." The character and general coa.r.s.eness of these cuts, as found in the different sizes, are shown in Figs. 2208 and 2209. Generally speaking, the coa.r.s.e teeth are applied to rasps used by horsesh.o.e.rs, the b.a.s.t.a.r.d to those used by carriage makers and wheelwrights, the second-cut to shoe-rasps, and the smooth to the rasps used by cabinet-makers.

[Ill.u.s.tration: Fig. 2208.]

[Ill.u.s.tration: Fig. 2209.]

[Ill.u.s.tration: Fig. 2210.]

[Ill.u.s.tration: Fig. 2211.]

[Ill.u.s.tration: Fig. 2212.]

Figs. 2210, 2211 and 2212 are respectively coa.r.s.e, b.a.s.t.a.r.d, and finis.h.i.+ng second-cut files, the first two being for bra.s.s.

Fig. 2210 represents a file open in both its over and up-cut, which is not, therefore, expected to file fine, but fast, and is adapted for very rough work on the softer metals, as in filing off sprues from bra.s.s and bronze castings, filing the ends of rods, and work of a similar nature.

It is also, to some extent, used upon wood. The essential difference between the b.a.s.t.a.r.d file shown in Fig. 2211 and that just described is the degree of fineness of the up-cut, which is nearly straight across the tool. This form of teeth, which may be applied to any of the finer cuts, and upon any of the shapes usually made double-cut, is especially adapted to finis.h.i.+ng bra.s.s, bronze, copper and similar soft metals, and is not so well adapted to the rougher work upon these metals as the coa.r.s.e bra.s.s file previously described. Fig. 2212 is a finis.h.i.+ng file.

The first or over-cut in this case is very fine, and, contrary to the general rule, has the least obliquity, while the up-cut has an unusual obliquity, and is the coa.r.s.er of the two cuts. The advantages in this arrangement of the teeth are that the file will finish finer, and by freeing itself from the filings is less liable to clog or pin than files cut for general use. This form of cut is especially useful when a considerable quant.i.ty of finis.h.i.+ng of a light nature is required upon steel or iron. It is not recommended for bra.s.s or the softer metals, nor should it be made of a coa.r.s.er grade than the second-cut.

The names of files are sometimes derived from the purpose for which they are to be used. Thus, we have saw files, slitting files, warding files, and cotter files. The term "warding" implies that the file is suitable for use on the wards of keys, while "cotter" implies that it is suitable for filing the slots for that cla.s.s of key which the machinist terms a cotter. In other cases files are named from their sections, as in the case of "square," "round," "half-round," and "triangular," or "three-square" files, as they are often termed.

The term "flat" may be considered strictly as meaning any file of rectangular section whose width exceeds the thickness. Hence, "mill files," "hand files," and "pillar files" all come under the head of flat files, although each has its own distinguis.h.i.+ng features. The general form of the flat file is shown in Fig. 2213, while the cross-sections of various quadrangular files are shown in Figs. from 2214 to 2218. From these views it will be seen that the thicknesses gradually increase from the mill to the square file. Mill files are slightly tapered from the middle to the point both in their width and thickness. They are single-cut, and are usually either b.a.s.t.a.r.d or second-cut, although they are sometimes double-cut. Mill files of both cuts are princ.i.p.ally used for sharpening mill saws, mowing-machine knives and ploughs, and in some machine shops for rough lathe work, and, to some extent, in finis.h.i.+ng composition bra.s.swork. Mill sections are occasionally made blunt--that is to say, their sectional shape is alike from end to end--in which case they are mostly double-cut, and seldom less than 8 inches in length.

They are suitable for filing out keyways, mortises, &c., and for these purposes should have at least one safe edge. A safe edge is one having no teeth upon it, which allows the file to be used in a corner without cutting more than one of the work surfaces. When the corner requires to be very sharp it is preferable to take a file that has teeth upon its edge and grind the teeth off, so as to bring the corner of the file up sharp, which it will not be from the cutting, because the teeth do not come fully up to a sharp corner.