Modern Machine-Shop Practice Part 134

The machine has longitudinal feed only, but where it is desired an automatic feed motion can be applied to the elevating screw in the head, giving feed in a vertical direction.

The table is arranged to be run back rapidly by power, by a device which is not seen in the engraving. As the table weighs one ton, the relief to the operator by this improvement is obvious.

All the operations of the machine are intended to be conducted from the front side, without any change in the position of the operator. The feed can be thrown out by hand at any moment by means of a rod which connects with the latch shown in the front of the cut, and the power quick-return applied; or the table can be run back by hand, and the feed thrown in by a foot lever, which lifts the drop box shown in front of cut. Adjustable dogs automatically drop the feed motion at any point.

The machine is ma.s.sive in all its parts, and is intended for heavy milling of any description, but more particularly for shafting, railroad, or engineering shops, being specially adapted for key-seating long and heavy shafting, finis.h.i.+ng guide bars, connecting rods, &c.

Its weight is 7,500 pounds. The work table is 7 feet long by 20 inches wide; length of longitudinal feed, 84 inches; distance between uprights, 24 inches. The cast-steel spindle is 4 inches in diameter, and the mill arbor 2-1/2 inches diameter. Arm for outer centre support 5 inches diameter at its smallest part.

[Ill.u.s.tration: Fig. 1909.]

[Ill.u.s.tration: _VOL. II._ =EXAMPLES OF MILLING MACHINES.= _PLATE III._

Fig. 1907.

Fig. 1908.]

MILLING CUTTERS OR MILLS.--The simplest form of milling cutter is that shown in Fig. 1909, the teeth being equidistantly arranged upon the circ.u.mference only. Its size is usually designated by its length, which is termed the face. Thus a cutter having its teeth parallel to its axis and an inch long would be said to have 1 inch face. Cutters of more than about half an inch face usually, however, have their teeth cut spirally, as in Fig. 1910; the degree of spiral is one turn in a length of 3 feet for cutters between 2-1/4 and 4 inches in diameter. For cutters of less than 2-1/4 the degree in the spiral is increased; thus for an inch cutter, the degree is one turn in 15 inches, while for 6 inches one turn in about 60 inches is used.

[Ill.u.s.tration: Fig. 1910.]

In the following table is given the sizes of cutters as made by one company, the bores being 1 inch.

+---------+-----------------------++---------+-----------------------+ | Width of| || Width of| | | face. | Diameter of cutter. || face. | Diameter of cutter. | +---------+-------+-------+-------++---------+-------+-------+-------+ | inch. | inch. | inch. | inch. || inch. | inch. | inch. | inch. | | 1/8 | 2-1/2 | 3 | 4 || 15/16 | 2-1/2 | 3 | 4 | | 3/16 | 2-1/2 | 3 | 4 || 1 | 2-1/2 | 3 | 4 | | 1/4 | 2-1/2 | 3 | 4 || 1-1/8 | 2-1/2 | 3 | 4 | | 5/16 | 2-1/2 | 3 | 4 || 1-1/4 | 2-1/2 | 3 | 4 | | 3/8 | 2-1/2 | 3 | 4 || 1-1/2 | 2-1/2 | 3 | 4 | | 7/16 | 2-1/2 | 3 | 4 || 1-3/4 | 2-1/2 | 3 | 4 | | 1/2 | 2-1/2 | 3 | 4 || 2 | 2-1/2 | 3 | 4 | | 9/16 | 2-1/2 | 3 | 4 || 2-1/4 | 2-1/2 | 3 | 4 | | 5/8 | 2-1/2 | 3 | 4 || 2-1/2 | 2-1/2 | 3 | 4 | | 11/16 | 2-1/2 | 3 | 4 || 3 | 2-1/2 | 3 | 4 | | 3/4 | 2-1/2 | 3 | 4 || 3-1/2 | 2-1/2 | 3 | 4 | | 13/16 | 2-1/2 | 3 | 4 || 4 | 2-1/2 | 3 | 4 | | 7/8 | 2-1/2 | 3 | 4 || | | | | +---------+-------+-------+-------++---------+-------+-------+-------+

The keyways are semicircular, the key being composed of a piece of No.

25 Stubbs steel wire.

The following is a table of the sizes of milling cutters made by another company.

+----------+------------+---------++----------+------------+---------+ | | | || | | | | Width of | Diameter | Size of || Width of | Diameter | Size of | | face. | of cutter. | hole. || face. | of cutter. | hole. | +----------+------------+---------++----------+------------+---------+ | inch. | inch. | inch. || inch. | inch. | inch. | | 1/8 | 2-1/4 | 1 || 3/4 | 2-7/8 | 1 | | 3/16 | 2-1/4 | 1 || 7/8 | 2-7/8 | 1 | | 1/4 | 2-1/2 | 1 || 1 | 2-1/2 | 1 | | 5/16 | 2-1/2 | 1 || 1-1/4 | 2-1/2 | 1 | | 3/8 | 2-5/8 | 1 || 1-1/2 | 2-1/2 | 1 | | 7/16 | 2-5/8 | 1 || 1-3/4 | 2-1/2 | 1 | | 1/2 | 2-3/4 | 1 || 2 | 2-1/2 | 1 | | 9/16 | 2-3/4 | 1 || 2-1/2 | 2-1/2 | 1 | | 5/8 | 2-3/4 | 1 || 3 | 2-1/2 | 1 | | 11/16 | 2-7/8 | 1 || | | | +----------+------------+---------++----------+------------+---------+

Cutters of 1 inch face and over have teeth of a spiral form.

The object of providing spiral teeth is to maintain a uniformity of cutting duty at each instant of time.

Suppose, for example, that the teeth are parallel to the cutter axis, when the cutter first meets the work the tooth will take its cut along its full length at the same instant, causing in wide cuts a jump to the work because of the spring of the various parts of the work-holding devices, and of the cutter driving spindle; furthermore as the cutter revolves the number of teeth in action upon the work varies. Thus in Fig. 1912 it is seen that one tooth only is in action, but when the cutter has revolved a little more there will be two teeth in action, as shown in Fig. 1913. This variation causes a corresponding variation of spring or give to the machine, producing a surface very slightly marked by undulations. But if the teeth are cut spiral the cut begins at one end of the tooth and proceeds gradually along it, thus avoiding violent shock, and after the cut is fairly started across the work the length of cutting edge in action is maintained uniform, producing smoother work, especially in the case of wide surfaces and deep cuts.

[Ill.u.s.tration: Fig. 1911.]

[Ill.u.s.tration: Fig. 1912.]

[Ill.u.s.tration: Fig. 1913.]

When the cutter is required to cut on the sides of the work as well as on its upper face it is termed a face cutter, and its side faces are provided with teeth, as shown in Fig. 1914; and when these cutters are arranged in pairs as in Fig. 1915, so as to cut in the side faces only of the work D, they are termed twin or straddle mills, both being of the same diameter.

[Ill.u.s.tration: Fig. 1914.]

In mills or cutters used in this way the cutting duty is excessive on the outer corners of the teeth, which, therefore, rapidly dull; hence it is usual to provide teeth on both sides of the cutter, as in Fig. 1916, so that after having been used in the position shown in the engraving until the teeth are dull the positions of cutters may be changed, bringing the unused cutting edges into use.

[Ill.u.s.tration: Fig. 1915.]

Twin or heading cutters are right and left hand, a right-hand one being that in which the teeth at the top of the wheel revolves towards the right, while a left-hand one revolves (at the top) towards the left.

[Ill.u.s.tration: Fig. 1916.]

[Ill.u.s.tration: Fig. 1917.]

If the machine is belted so that it can be revolved in either direction, both sides of the cutter may be utilised by taking the cutters off the arbor, turning them around and then replacing them in their original positions on the same. Thus in Fig. 1917 we have at A a left-hand cutter that if reversed upon its arbor would be a right-hand one as at B, and it is obvious that the direction of revolution must be in each case as denoted by the arrows F G, which are in opposite directions. In this case the direction of work feed must be reversed, the work for A feeding in the direction of C, and that for B in the direction of D. It is to be observed, however, that the cutter could not be reversed if it was driven by an arbor that screwed upon the driving spindle of the milling machine. For if the machine has a right-hand thread then the cutter must revolve in the direction of G, and the work feed must be in that of C; whereas if the machine spindle drives its chucks, arbors, &c., by a left-hand thread, then the direction of cutter revolution must be as at F, and that of work feed as at D. But if the cutters are upon an arbor that is driven by a conical seat in the machine spindle, or by any other means enabling the arbor to revolve in either direction without becoming released from that spindle, then the cutter may be simply turned around and the feed direction reversed, as already explained. The reason for reversing the direction of feed when the direction of cutter revolution is reversed is as follows:--

[Ill.u.s.tration: Fig. 1918.]

In Fig. 1918 A and B represent two pieces of work of which B is to be fed in the direction of arrow C, so that the pressure of the cut tends to force the work back from under the cutter, whereas in the case of the work A, feeding in the direction of D, the teeth act to pull the work beneath the cutter, which causes tooth breakage.

Suppose, for example, that in Fig. 1919 P is a piece of work fastened to the table T, feeding in the direction of A, the cutter W revolving in the direction of arrow B, N representing the feed nut operated by the feed screw S. Now while the table is being pulled in the direction of A, the sides C of the feed screw thread will bear against the sides of the thread in the nut, and whatever amount of looseness there may be between the threads of the screw and nut will in this case be on the sides D of the threads. So soon, therefore, as the wheel meets the work P, it will suddenly pull the work forward to the amount of the play or looseness on the sides D of the threads, and this in addition to the feed given by the rotating screw S, would cause the wheel to lock upon the work surface.

[Ill.u.s.tration: Fig. 1919.]

In all milling operations, therefore, the work is fed against the cutter as at B, in Fig. 1918, unless, in the case of twin mills, it is fed (as at E and F in the same figure) in the middle of the cutters, in which case it is preferable to present it as at F, so that the pressure of the cut will tend to hold the work down to the table, and the table down upon its guideways. This position of the work presents some advantages for small work which will be explained hereafter.

[Ill.u.s.tration: Fig. 1920.]

Fig. 1920 represents angular cutters, the teeth being at an angle to the cutter axis. These cutters are made right and left as at A and B in Fig.

1921, the teeth of A being cut in the opposite direction to those at B, so as to be able to cut equal angles on the work when these angles lie in opposite directions, as C and D in the figure. Furthermore these cutters are sometimes screwed to their arbors, and can therefore be revolved in one direction only, which prevents their being turned around end for end, even though the machine be so belted as to be capable of revolving its spindle in either direction.

[Ill.u.s.tration: Fig. 1921.]

The angular cutters shown in Fig. 1921 have their teeth arranged for a Brainard milling machine, in which the live spindle has a right-hand thread for driving the chucks, arbors, &c.; hence the direction of cutter revolution, and the arrangement of the teeth are as in the figure.

[Ill.u.s.tration: Fig. 1922.]

[Ill.u.s.tration: Fig. 1923.]

In Fig. 1922 are segments of two wheels, A and B (corresponding to A and B in Fig. 1921), but with their teeth arranged for a Brown and Sharpe milling machine, in which the machine spindle has a left-hand thread; hence the direction of cutter revolution is reversed, as denoted by the arrows in the two figures.

[Ill.u.s.tration: Fig. 1924.]

Fig. 1923 represents a round edge cutter; and it is obvious that the curvature or roundness of the cutting edges may be made to suit the nature of the work, whether the same be of regular or irregular form. In cutters of this description it would be a difficult matter to resharpen the teeth by grinding their backs, hence they are ground on the front faces; and to maintain the form or profile of the cutting edges, notwithstanding the grinding, we have a patent form of cutter, an example of which is shown in the gear tooth cutter in Fig. 1924. The backs of the teeth are of the same form throughout their entire length, so that grinding away the front face to sharpen the cutting edge does not alter the contour or shape of the cutting edge. This is of especial advantage in cutters for gear teeth, and those for irregular forms, Figs. 1925, 1926, and 1927 forming prominent examples.

[Ill.u.s.tration: Fig. 1925.]

End mills or shank cutters are formed as in Fig. 1928, the shank sometimes being made parallel with a flat place at A, to receive the set-screw pressure, and at others taper, the degree of taper being 1/2 inch per foot. The hole at the end facilitates both the cutting of the tooth in the making and also the grinding. Shank cutters may be used to cut their way into the work, with the teeth on the end face, and then carry it along, bringing the circ.u.mferential teeth into operation; or the end teeth may be used to carry the cut after the manner of a face cutter.

[Ill.u.s.tration: Fig. 1926.]

[Ill.u.s.tration: Fig. 1927.]

Shank cutters are rarely made above an inch in diameter, and are largely used for cutting grooves or recesses, and sometimes to dress out slots or grooves that have been cast in the work, as in the case of the steam and exhaust ports of steam engine cylinders. In work of this kind the direction of the feed is of great importance and must be varied according to the depth of cut taken on the respective sides of the cutter. Suppose, for example, that the conditions are such as ill.u.s.trated in Fig. 1929, the cut being deepest on the side A of the slot, and the cutter must be entered at the end of the slot and fed in the direction of D, so that the pressure of the cut may tend to push the cutter back, it being obvious that on the side B the cutter has a tendency to walk or move forward too rapidly to its cut, and if the cut was heaviest on that side it would do, this increasing the cut rapidly and causing tooth breakage.