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Vol. I.

FULL-PAGE PLATES.

MODERN

MACHINE SHOP PRACTICE.

Chapter XXII.—MILLING MACHINERY AND MILLING TOOLS.

The Milling Machine.—The advantages of the milling machine lie first in its capacity to produce work as true and uniform as the wear of cutting edges will permit (which is of especial value in work having other than one continuous plane surface); second, in the number of cutting edges its tools will utilize in one tool or cutter; and third, in its adaptability to a very wide range of work, and in the fact that when the work and the cutters are once set the operator may turn out the best quality of work without requiring to be a skilled machinist.

The extended use of the milling machine, which is an especial feature of modern machine shop practice, is due, in a very large degree, to the solid emery wheel, which provides a simple method of sharpening the cutters without requiring them to be annealed and rehardened, it being found that annealing and rehardening reduces the cutting qualifications of the steel, and also impairs the truth of the cutting edges by reason of the warping or distortion that accompanies the hardening process. Rotary cutters are somewhat costly to make, but this is more than compensated for in the uniformity of their action, since in the case of the cutter the expense is merely that involved in forming the cutting edges with exactitude to shape; once shaped the cutter will produce a great quantity of work uniform in shape, whereas in the absence of such cutters each piece of work would require, to bring it to precise form, as much precision and skill as is required in shaping the cutter.

If a piece of work is shaped in a planing machine, the different steps, curves, or members must be cut or acted upon by the tool separately, and the dimensions must be measured individually, giving increased liability to error of measurement, and requiring a fine adjustment of the cutting tool for each step or member. Furthermore, neither a planing machine or any other machine tool can have in simultaneous cutting operation so great a length of cutting edge as is possible with a rotary cutter.

Again, in the planing machine each cut requires to be set individually, and cannot be so accurately gauged for its depth, whereas with a rotary cutter an error in this respect is impossible, because the diameters of the various steps on the cutter determine the depth of the respective cuts or steps in the work.

In a milling machine the cut is carried continuously from its commencement to its end, whereas in a shaping or planing machine the tool does not usually cut during the back or return stroke. In either of these machines, therefore, the operator’s skill is required as much in measuring the work, setting the tools feeds, &c., as in shaping the tools, whereas in the milling machine all the skill required lies in the chucking and adjustment of the work to the cutter, rather than in operating the machine, which may therefore be operated by comparatively unskilled labor.

The multiplicity of cutting edges on a rotary cutter so increases its durability, and the intervals at which it must be sharpened are so prolonged, that, with the aid of the present improved cutter grinding machines, one tool maker can make and keep in order the cutters for many machines.

The speed at which milling cutters are run varies very widely in the practice in different workshops. Thus upon cast iron, cutting speeds of 15 circumferential feet per minute will be employed upon the same class of work that in another shop would be done at a cutting speed of as high as fifty feet per minute. With the quick speeds, however, lighter feeds are employed. As the teeth of milling cutters are in cutting action throughout but a small portion of a revolution, they have ample time to cool, and may be freely supplied with oil, which enables them to be used at a higher rate of cutting speed than would otherwise be the case. Yet another element of importance in this connection is that when the cut is once started on a plain cutter, the cutting edges do not meet the surface skin of the metal, this skin always being hard and destructive to the cutting edges.

The simplest form in which the milling machine appears is termed the hand milling machine, and an example of this is shown in Fig. 1878. This machine consists of a head carrying a live spindle which drives the cutting tools, which latter are called cutters or mills. The front of the head is provided with a vertical slideway for the knee or bracket that carries an upper compound slide upon which the work-holding devices or chucks are held. The work is fed to the revolving cutter by the two levers shown, the end one of which is for the vertical and the other for the horizontal motion, which is in a direction at a right angle to the live spindle axis.

In other forms of the hand milling machine the live spindle is capable of end motion by a lever.

In Fig. 1878a is shown Messrs. Brown and Sharpe’s plain milling machine, or in other words a milling machine having but one feed motion, and therefore suitable for such work only as may be performed by feeding the work in a straight line under the cutter, the line of feed motion being at a right angle to the axis of the cutter spindle.

Machines of this class are capable of taking heavy cuts because the construction admits of great rigidity of the parts, there being but one slideway, and therefore but one place in the machine in which the rigidity is impaired by the necessity for a sliding surface.

Fig. 1879 represents Pratt & Whitney’s power milling machine. The cone and live spindle are here carried in boxes carried in vertical slideways in the headstock, so as to be adjustable in height from the work table, and is provided with a footstock for supporting the outer end of the live spindle, which is necessary in all heavy milling. The carriage is adjustable along the bed, being operated by a screw whose operating hand wheel is shown at the left-hand end of the bed.

The automatic feed is obtained as follows: The large gear on the right of the main driving cone operates a pinion driving a small four-step cone connected by belt to the cone below, which, through the medium of a pair of spur-gears, drives the feed rod, on which is seen a long worm engaging a worm-wheel which drives the feed screw. A suitable stop motion is provided.

What is termed a universal milling machine is one possessing the capacity to cut spiral grooves on either taper or parallel work, and is capable of cutting the teeth of spur and bevel-gears or similar work other than that which can be held in an ordinary vice. These features may be given to a machine by devices forming virtually an integral part of the machine, or by providing the machine with separate devices which are attachable to the work table.

In Fig. 1880 is represented a small size universal milling machine, in which a is the frame that affords journal bearing to the live spindle, in the coned mouth a of which the mandrel carrying the rotary cutter is fitted, means being afforded for taking up the wear of the live spindle journal and bearings. b is the cone pulley for driving a. Upon the front face of a is a vertical slide upon which may be traversed the knee or table c, which by being raised, regulates the depth to which the cutters enter the work. To operate c the vertical screw b is provided, it being operated (by bevel-gears) from a horizontal shaft whose handle end is shown at c.

The nut for elevating screw b is formed by a projecting lug from or on the main frame a. To enable c to be raised to a definite height so that the cutters shall enter successive pieces of work to an equal depth, a stop motion is provided in the rod d, which passes through a plain hole in the lug on a that forms a nut for b. Rod d is threaded and is provided with a nut and chuck nut whose location on the length of the rod determines the height to which c can be raised, which ceases when the faces of the nuts meet the face of the projecting lug.

The upper surface of c is provided with a slide on which is a slider d, which, by means of a feed screw whose handle end is shown at e, may be traversed in a line parallel to the axial line of the live spindle or arbor, as it is more often termed, this motion being employed to set the width of the work in the necessary position with relation to the rotary cutters. To d is attached e, which is pivoted at its centre so as to be capable of swinging horizontally, means being provided to fasten it to d in its adjusted position. This is necessary to enable the line of traverse of the work to be at other than a right angle to the axial line of the cutter spindle when such is desired, as in the case of cutting spirals; e serves as a guide to the carriage f, the latter being operated endwise by means of a screw whose handle is shown at e′′, the nut being attached to e, handle e′′ being to traverse e by hand. To feed f automatically gear-wheel f is attached to the other end of the same screw, this automatic feed being actuated as follows:—

At the rear end of the live spindle is a three-stepped cone pulley attached by belt to cone pulley g, which connects by rod to and drives gear f. The construction of the rod is so designed as to transmit the rotary motion from g to f without requiring any adjustment of parts when c is raised or lowered or f traversed back or forth, which is accomplished as follows:—

At g g are two universal joints attached respectively to g and f, and to two shafts which are telescoped one within the other. The inner rod is splined to receive a feather in the outer. The rotary motion is communicated from g to the universal joint, through that joint to the outer or enveloping shaft which drives the inner shaft, the latter driving a universal joint which drives f, the inner shaft passing freely within the outer or sliding out from it (while the rotary motion is continuing) to suit the varying distance from and position of f with relation to g. This automatic feed motion may be adjusted to cease at any point in the traverse of e by a stop and lever provided for the purpose, so that if an attendant operates more than one machine, or if the feed require to be carried a definite distance, it will stop automatically when that point has been reached.

The carriage f may carry various chucks or attachments to suit the nature of the work. As shown in the cut it carries a tailblock i and head j, both fitting into a way provided in f so that they will be in line one with the other at whatever part in the length of f they may be set or fixed. Both i and j carry centres between which the work may be held, as in the case of lathe work. Part j is pivoted to j so that it may be set at an angle if required, thus setting the centre, which fits in the hole at h, above the level of that in i, as may be necessary in milling taper work, the raising of j answering to the setting over of the tailstock of a lathe for taper turning.

To enable the accurate milling of a polygon, the spindle h may be rotated through any given portion of a circle by means of the index wheel at i, it being obvious that if a piece of work be traversed beneath the cutter, and h be rotated a certain portion of a circle after each traverse, the work will be cut to a polygon having a number of sides answering to the portion of a circle through which h is rotated after each traverse. Means are also provided to rotate h while f is traversing beneath the cutter; hence when these two feed motions act simultaneously the path of the work beneath the cutter is a spiral, and the action of the cutter in the work is therefore spiral; hence spiral grooves may be cut or spiral projections left on the work, as may be determined by the shape of the cutters. k is a chuck that may be connected to h to drive the work, and h a work-holding vice, that may be used instead upon f in place of heads i j.

The countershaft shown at the foot of the machine has two loose pulleys and a tight one between them, this being necessary because, in cutting spiral work, the work must rotate while on the back traverse as well as on the forward one, hence a crossed as well as an open belt is necessary.

Fig. 1881 represents a large Brown & Sharp universal milling machine, in which the cone spindle is provided with back gear, and a supporting arm is also provided for the outer end of the cutter arbor. The feed motions for this machine correspond to those already described for the smaller one, Fig. 1880, the construction of the important parts being shown in the following figures.

The construction of the bearings for the cutter driving spindle of the machine is as in Figs. 1882 and 1883. a is the spindle having a double cone to fit corresponding cones in the sleeve b, the fit of one to the other being adjusted by means of the nut c, which is threaded upon a. The mouth of a is coned to receive the arbors or mandrels for driving the mills or cutters. At the back bearing, Fig. 1883, the journal a′, and bore of the sleeve b′, is parallel, this sleeve being split at the top so that when it is (by means of nut d) drawn within the head e its coned exterior will cause it to close to a proper fit upon a′, by which means the wear of the parts may be taken up as they become perceptible.

The head j, Fig. 1880, is used (in connection with the foot block i) to suspend or hold work by or between centres, its centre fitting into the spindle at h, which is capable of being revolved continuously (to enable the cutting of spirals), by means of change gears, and intermittently through a given part of a circle by means of the index wheel i. The block j carrying the spindle is also capable of elevation for conical or taper work, two examples of such uses being shown in Figs. 1884 and 1885, in which c is the cutter and w the work.

Fig. 1886 is a sectional view in a vertical plane through the centre of the head, and showing the construction of the spindle and the means of elevating the block j; h is the spindle having journal bearing in j, and secured from end motion by the cone at a and the nut b; its bore is coned at the front end to receive the arbor c carrying the centre d, upon which is the piece e for driving the work dog, which is secured within e by the set-screw f. Fast upon spindle h is a worm-wheel f made in two halves, which are secured together by the screws g. At g is the worm-wheel (for driving f) fast upon the shaft h′.

It is obvious that the block j may be raised at its centre end upon h as a centre of motion, the worm f simply moving around upon g. At v is a bolt to lock j to j, and thus secure it in its adjusted position. w w are lugs or blocks fitting into the slot in the work table, and serving to secure the head, being in line with the foot block (shown at 1 in Fig. 1880). A sleeve z is used to cover the thread and protect it when a chuck is not used.

Fig. 1887 is an end view partly in section to show the construction of the worm shaft and the index plate. h is a sleeve upon which j pivots, and h′ the worm shaft, which may be revolved by hand by the lever l, or automatically by means of the bevel-gear k, which connects with the train of change gears; these change gears being thrown out of operation when gear k (and therefore h) is not required to revolve automatically nor continuously. l is an arm for carrying the index pin l for the index plate i. The pin l is adjustable for radius from the centre of h (so as to come opposite to the necessary circle of holes on the plate i), the arm l being slotted to permit of this adjustment, and being secured in its adjusted position by the nut on the end of h′. Pin l is pushed into the index holes by means of the spiral spring coiled around l at m, which permits l to be withdrawn from i under an end pressure, but pushes it into i when that pressure is released. To indicate the amount of rotation of i, without counting the number of holes, a sector n n′ is employed, it having two arms adjustable for their widths apart so as to embrace any given number of holes on the required circle. At r′ is a pin which is pulled forward and into holes provided in the plate i to prevent its turning when using the lever l. n and n′ are held to the face of i by the friction of the spring q. A face view of index plate i is shown in Fig. 1888, the lever l, Fig. 1887, being removed to expose n and n′.

The surface of the plate is provided with rings of holes marked respectively 20, 19, 18, &c., the holes in each ring or circle being equidistantly spaced.

The sector arms n and n′ may be opened apart or closed together so as to embrace any required number of holes in either of the circles. As shown in the cut they embrace one quarter of the circle of 20, there being five divisions between the holes s and t. The screw w secures them in their adjustment apart. Suppose that pin l (Fig. 1887), is in s, and arm n′ is moved up against it, the arm n leaves t open, and indicates that t is the next hole for pin l, which is withdrawn from s, and lever l (Fig. 1887) is moved around until the pin will enter t, and the sector is then moved into the position shown in Fig. 1888a, indicating that hole u is the next one for the pin. This obviates the necessity of counting the holes, and prevents liability to error in the counting. Three of these index plates are provided, each having different numbers of holes in the circles, and in the following tables are given those specially prepared for use in cutting the teeth of gear-wheels: