Modern Machine-Shop Practice Part 135

[Ill.u.s.tration: Fig. 1928.]

[Ill.u.s.tration: Fig. 1929.]

This tendency, however, is resisted by the pressure on the side A of the slot, which acts, as already stated, to push the cutter back. In starting the cutter therefore, it is necessary to do so at that end of the slot that will cause the deepest cut to act in the direction to r.e.t.a.r.d the feed. Suppose, for example, that the heaviest or deepest cut, instead of being on the side A of the slot, as in Fig. 1929, was on the side B, and in that case it would be necessary to start the cut from the other end of the slot as in Fig. 1930, the arrow C denoting the direction of feed.

[Ill.u.s.tration: Fig. 1930.]

[Ill.u.s.tration: Fig. 1931.]

Similarly when a groove has been roughed out from the solid, and it is determined to take a finishing cut, the direction of the feed for the latter is of importance. Suppose, for example, a [T]-groove is to be cut, and that a slot is first cut with a shank cutter as in Fig. 1931, leaving a light finishing cut of, say, 3/64 inch to finish the neck to the dotted lines A B, and entering to within 1/16 inch of the full depth as denoted by line C. The enlarged part of the groove may then be cut out, leaving about 3/64 inch at the top and bottom, D and E, the cutter having a plain shank (as in Fig. 1933), whose diameter should just clear the narrow part of the groove already roughed out. The work will then be ready for the finishing cutter, formed as in Fig. 1932, whose teeth (on both the shank and the enlarged end) should have a diameter of 3/32 less than that of the finished slot. In taking the finishing cut this cutter must be set first to cut the sides B E to finished size, the direction of the feed being such that the pressure of the cut acts to push the cutter back as already explained, and when the cut is finished on this side the finishing cut may be put on the side A D, without traversing the cutter back, or in other words the feed must be carried in the opposite direction, so that the cutter will run under the cut and be pushed back by it, so as to prevent it from running forward as explained with reference to figure.

[Ill.u.s.tration: Fig. 1932.]

For ordinary work not requiring great truth, however, the first cutter, Fig. 1931, may be made of the finished diameter, and be followed by a cutter such as in Fig. 1933, also of the finished diameter.

[Ill.u.s.tration: Fig. 1933.[31]]

[31] Figs. 1928, 1931, 1932, 1933, are from an article by John J.

Grant, in _The American Machinist_.

When a shank-cutter is required to enter solid metal endways, as in the case of cutting grooves around the circ.u.mferential surface of a cylinder, it is necessary to drill a hole to admit the cutter, leaving a light finishing cut for the diameter of the cutter, and sufficient in the depth to let the end face of the cutter remove or square up the cone seat left by the drill. Shank cutters may obviously be made taper, or to any other required angle or curvature, Figs. 1934 and 1935 being examples which can be used in situations where other cutters could not, as for example on the arms or spokes of wheels.

[Ill.u.s.tration: Fig. 1934.[32]]

[32] Figs. 1934, 1935, 1936, are from articles by John J. Grant, in _The American Machinist_.

Fig. 1936, from _The American Machinist_, represents an example of the employment of shank cutters, the work being a handle for a lathe cross-feed screw, and it is obvious that the double cornering cutter may be used upon both edges, and the cut being carried around the hub by the parallel part of the cutter; the whole of the work on the handle including the boring, if the hole is cast in, may be done by the shank cutter, the handle end being finished and the boring done first, the hub being finished on an arbor.

[Ill.u.s.tration: Fig. 1935.]

Shank mills may obviously be made of various shapes; thus in Fig. 1937 is shown two applications of an end or shank mill, one for cutting a dovetailed groove and the other an angular one. In the case of the dovetail groove the cutter will work equally well, whether it be used on straight or spiral grooves; but this is not the case with angular grooves for reasons which are explained with reference to angular cutters and spiral groove cutting.

[Ill.u.s.tration: Fig. 1936.]

Shank cutters are provided with finer teeth than ordinary cutters, the following being the numbers of teeth commonly employed for the respective diameters:--

Diameter of cutter 1/8 or 3/16 inch, number of teeth 6 " " 1/4 " " " 7 " " 3/8 " " " 8 " " 1/2 " " " 8 " " 5/8 " " " 10 " " 3/4 " " " 10 " " 7/8 " " " 12 " " 1 " " " 14

The front faces of the teeth are radial as in other cutters, the angle of the back of the tooth being 40 for the smaller, 50 for the medium, and 60 for an inch cutter.

[Ill.u.s.tration: Fig. 1937.]

Fly cutters are single-toothed cutters, or rather tools, that are largely used by watchmakers for cutting their fine pitches of gear wheels.

Fig. 1938 represents a fly cutter in place in its holder or arbor, its front face D being in line with the axis C of the arbor.

[Ill.u.s.tration: Fig. 1938.]

Let it be required to make a fly cutter for a very fine pitch of gear tooth, such as used for watches, and a template, shown greatly magnified at T in Fig. 1939, is made to fill a s.p.a.ce and one half of each of the neighboring teeth. From this template a cutting tool is made, being carefully brought to shape with an oil-stone slip and a magnifying gla.s.s. This tool is used for the production of fly cutters, and may be employed by either of the following methods:--

[Ill.u.s.tration: Fig. 1939.]

The piece of steel to form the cutter is fastened in an arbor back from the centre, as at D in Fig. 1940, and is then cut to shape by the tool before referred to. It is then set for use in the milling machine, or in such other machine as it may be used in, in the position shown in Fig.

1938, its front face D being in line with the axis C of the arbor. The change of position has the effect of giving the tool clearance, thus enabling it to cut while being of the same shape throughout its whole thickness; face D may be ground to resharpen the cutter without altering the shape it will produce. It is this capacity to preserve its shape that makes the fly cutter so useful as a milling machine tool, since it obviates the necessity of making the more expensive milling cutters, which, unless made on the principle of the Brown and Sharpe cutters, do not preserve their shapes.

[Ill.u.s.tration: Fig. 1940.]

It is to be observed, however, that a fly cutter made as above does not produce work to exactly correspond to the template it was made from, because moving it from the position it was made in (Fig. 1938) to the position it is used in (Fig. 1940) causes it to cut slightly shallower, but does not affect its width.

[Ill.u.s.tration: Fig. 1941.]

Another method of cutting up a fly cutter by the tool made to the template is shown in Fig. 1941. The blank cutter is placed at an angle to an arbor axis, and is cut to shape by the tool. For use it is placed in line with the arbor axis as in Fig. 1942, the change of position here again giving clearance as shown by the dotted arcs, the inside ones showing the arc the cutter revolved on when it was in the arbor in Fig.

1938. Here again, however, the change of position causes the fly cutter to produce a shape slightly different from the template to which the first tool was made, hence the best method is as follows:--

[Ill.u.s.tration: Fig. 1942.]

The blank is let into an arbor of small diameter, as in Fig. 1943, its face D being in line with the arbor axis. It is then cut up with the tool made from the template. For use it is set in a larger arbor, as in Fig. 1944, the difference in its path of revolution giving it the necessary clearance. Thus, in the figure the inner dotted arcs show the path of revolution of the cutter when it was in the small arbor, and the outer arc of the path in the large arbor. The front face can be ground without altering the shapes; the cutter will produce this front face, being kept in line with the arbor axis by grinding the body of the steel as much as the front face is ground when it is resharpened. Curves or irregular shapes may be readily produced and preserved by fly cutters.

[Ill.u.s.tration: Fig. 1943.]

[Ill.u.s.tration: Fig. 1944.]

It is obvious, however, that when the tool made to the original template is worn out, another must be made, and to avoid this trouble and preserve the original shape beyond possible error, we have recourse to the following additional method:--

[Ill.u.s.tration: Fig. 1945.]

With the tool made from the template we may cut up a wheel, such as in Fig. 1945, and this wheel we may use as a turning tool to cut up fly cutters, the principle of the wheel cutter having been shown in connection with lathe tools. It may here be pointed out, however, that if a wheel or circular cutter, as it is termed, is to be used, we may make the template, and the master tool we make from it, for one side of a tooth only, and use the master tool to cut up one side only of the corner of the circular cutter, as shown in Fig. 1945.

[Ill.u.s.tration: Fig. 1946.]

[Ill.u.s.tration: Fig. 1947.]

The method of using the circular cutter is ill.u.s.trated in Fig. 1946, in which H is a holder, whose end face P is level with the axis of the cutter, which is held to the holder by a screw. The side face of the holder is out of the vertical so as to give the cutter side clearance. A second holder has its side face inclined in the opposite direction, thus enabling the one edge of the circular cutter to be used as a right or as a left-hand tool and insuring uniformity, because the same edge of the circular cutter is used in both cases, so that if used for say a tool for a gear tooth, both sides of the tool will be cut from the same side of the circular cutter.

It is obvious that instead of having one continuous cutter, the necessary breadth of cutter face may be obtained by means of two or more cutters placed side by side. Thus to mill a piece of work two inches wide we may use two cutters of an inch face each (both of course being of equal diameter), or we may use one cutter, of 1-1/4 inch and another of 3/4 inch face. It is preferable, however, to use two cutters of an inch face each, and to set one beam left-hand and the other right-hand spiral teeth, because spiral teeth have considerable tendency to draw the machine spindle endways in its bearings, because the teeth correspond to a certain extent to a screw, and the work to a nut. A cutter with a left-hand spiral exerts end pressure tending to draw the driving spindle out from its bearings, while a right-hand one tends to push it within them; hence by making the two cutters of equal length and of the same degree of spiral, the effect of one cutter offsets that of the other. Furthermore, it is found that the tendency to chatter which increases with an increase in the width of the work, is diminished by using right and left spiral cutters side by side.

[Ill.u.s.tration: Fig. 1948.]

In order that the cutting edges of cutters placed side by side in this way may be practically continuous so as not to leave a line on the finished work, the teeth may be made to overlap in two ways as in Fig.

1948, both representing magnified portions of cutters. In the method shown on the left of the figure the usefulness of either cutter to be used singly is not impaired, all that is necessary to insure the overlapping being to cut the keyways in different positions with relation to the teeth; whereas on the left of the figure neither cutter would be efficient if used singly, except upon work as narrow as the narrowest part of the cutter. On the other hand, however, it affords excellent facilities for grinding, since the two cutters may be ground together, thus ensuring that they be of equal diameters except in so far as may be influenced by the wear of the emery wheel, which is, however, almost inappreciable even in cutters of considerable width of face. In the method shown on the left there is the further advantage that as the teeth are not in line the cutting action is more continuous and less intermittent, the arrangement having in a modified degree the same advantage as the spiral cutter.

[Ill.u.s.tration: Fig. 1949.]

In both methods some lat.i.tude is given to adjust the total width of face by placing paper washers between the cutters. If the plan on the right is employed the projections may occupy one-fourth of the circ.u.mference, there being two projections and two depressions on one end of the cutter. When cutters of different diameters and shapes are put together side by side on the same arbor the operation is termed gang milling.

[Ill.u.s.tration: Fig. 1950.]

Thus, in Fig. 1949 is shown a sectional view of a gang of three mills or cutters, A, B, and C, of which A and C are recessed to admit of the ends of B pa.s.sing within them. The heavy black line representing a paper washer inserted to adjust the distance apart of A and C, it being obvious that this gives a means of letting them together after their side teeth at D and E have been ground. As shown in the figure, A has teeth on one only of its sides, while C has them on both sides as well as in its circ.u.mference, while all three are of different widths of face. This would capacitate A only for the inside cutter, as in the figure, while B would be serviceable only when there was a cutter on each side of it; or if used singly, only when its face overlapped the width of the work on each side. But C, being cut on each side, could be used singly for grooving or recessing, or for plain milling, or in the position of B or A in the figure; hence it is preferable in gang milling for general purposes to provide teeth on both sides as well as on the circ.u.mference of the mill or cutter. But if a gang of mills are to be made for some special purpose, and used for no other, the teeth may be provided on the sides or not, as the circ.u.mstances may require.