Fig. 1911Fig. 1911.
Fig. 1911.
Fig. 1912Fig. 1912.
Fig. 1912.
Fig. 1913Fig. 1913.
Fig. 1913.
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 inFig. 1912it 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 inFig. 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.
Fig. 1914Fig. 1914.
Fig. 1914.
Fig. 1915Fig. 1915.
Fig. 1915.
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 inFig. 1914; and when these cutters are arranged in pairs as inFig. 1915, so as to cut in the side faces only of the workd, they are termed twin or straddle mills, both being of the same diameter.
Fig. 1916Fig. 1916.
Fig. 1916.
In mills or cutters used in this way the cutting duty is excessiveon the outer corners of the teeth, which, therefore, rapidly dull; hence it is usual to provide teeth on both sides of the cutter, as inFig. 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.
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.
Fig. 1917Fig. 1917.
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 inFig. 1917we have ataa left-hand cutter that if reversed upon its arbor would be a right-hand one as atb, and it is obvious that the direction of revolution must be in each case as denoted by the arrowsfg, which are in opposite directions. In this case the direction of work feed must be reversed, the work forafeeding in the direction ofc, and that forbin the direction ofd. 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 ofg, and the work feed must be in that ofc; whereas if the machine spindle drives its chucks, arbors, &c., by a left-hand thread, then the direction of cutter revolution must be as atf, and that of work feed as atd. 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 asfollows:—
Fig. 1918Fig. 1918.
Fig. 1918.
InFig. 1918aandbrepresent two pieces of work of whichbis to be fed in the direction of arrowc, so that the pressure of the cut tends to force the work back from under the cutter, whereas in the case of the worka, feeding in the direction ofd, the teeth act to pull the work beneath the cutter, which causes tooth breakage.
Fig. 1919Fig. 1919.
Fig. 1919.
Suppose, for example, that inFig. 1919pis a piece of work fastened to the tablet, feeding in the direction ofa, the cutterwrevolving in the direction of arrowb,nrepresenting the feed nut operated by the feed screws. Now while the table is being pulled in the direction ofa, the sidescof 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 sidesdof the threads. So soon, therefore, as the wheel meets the workp, it will suddenly pull the work forward to the amount of the play or looseness on the sidesdof the threads, and this in addition to the feed given by the rotating screws, would cause the wheel to lock upon the work surface.
In all milling operations, therefore, the work is fed against the cutter as atb, inFig. 1918, unless, in the case of twin mills, it is fed (as ateandfin the same figure) in the middle of the cutters, in which case it is preferable to present it as atf, 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.
Fig. 1920Fig. 1920.
Fig. 1920.
Fig. 1921Fig. 1921.
Fig. 1921.
Fig. 1920represents angular cutters, the teeth being at an angle to the cutter axis. These cutters are made right and left as ataandbinFig. 1921, the teeth ofabeing cut in the opposite direction to those atb, so as to be able to cut equal angles on the work when these angles lie in opposite directions, ascanddin 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.
The angular cutters shown inFig. 1921have 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.
Fig. 1922Fig. 1922.
Fig. 1922.
InFig. 1922are segments of two wheels,aandb(corresponding toaandbinFig. 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.
Fig. 1923Fig. 1923.
Fig. 1923.
Fig. 1924Fig. 1924.
Fig. 1924.
Fig. 1925Fig. 1925.
Fig. 1925.
Fig. 1926Fig. 1926.
Fig. 1926.
Fig. 1927Fig. 1927.
Fig. 1927.
Fig. 1923represents 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 inFig. 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, and1927forming prominent examples.
Fig. 1928Fig. 1928.
Fig. 1928.
End mills or shank cutters are formed as inFig. 1928, the shank sometimes being made parallel with a flat place ata, to receive the set-screw pressure, and at others taper, the degree of taper being1⁄2inch per foot. The hole at the end facilitates boththe 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 circumferential teeth into operation; or the end teeth may be used to carry the cut after the manner of a face cutter.
Fig. 1929Fig. 1929.
Fig. 1929.
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 illustrated inFig. 1929, the cut being deepest on the sideaof the slot, and the cutter must be entered at the end of the slot and fed in the direction ofd, so that the pressure of the cut may tend to push the cutter back, it being obvious that on the sidebthe 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.
Fig. 1930Fig. 1930.
Fig. 1930.
This tendency, however, is resisted by the pressure on the sideaof 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 retard the feed. Suppose, for example, that the heaviest or deepest cut, instead of being on the sideaof the slot, as inFig. 1929, was on the sideb, and in that case it would be necessary to start the cut from the other end of the slot as inFig. 1930, the arrowcdenoting the direction of feed.
Fig. 1931Fig. 1931.
Fig. 1931.
Fig. 1932Fig. 1932.
Fig. 1932.
Fig. 1933Fig. 1933.[31]
Fig. 1933.[31]
[31]Figs. 1928,1931,1932,1933, are from an article by John J. Grant, inThe American Machinist.
[31]Figs. 1928,1931,1932,1933, are from an article by John J. Grant, inThe American Machinist.
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, aT-groove is to be cut, and that a slot is first cut with a shank cutter as inFig. 1931, leaving a light finishing cut of, say,3⁄64inch to finish the neck to the dotted linesab, and entering to within1⁄16inch of the full depth as denoted by linec. The enlarged part of the groove may then be cut out, leaving about3⁄64inch at the top and bottom,dande, the cutter having a plain shank (as inFig. 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 inFig. 1932, whose teeth (on both the shank and the enlarged end) should have a diameter of3⁄32less than that of the finished slot. In taking the finishing cut this cutter must be set first to cut the sidesbeto 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 sidea 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.
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 inFig. 1933, also of the finished diameter.
Fig. 1934Fig. 1934.[32]
Fig. 1934.[32]
Fig. 1935Fig. 1935.
Fig. 1935.
[32]Figs. 1934,1935,1936, are from articles by John J. Grant, inThe American Machinist.
[32]Figs. 1934,1935,1936, are from articles by John J. Grant, inThe American Machinist.
When a shank-cutter is required to enter solid metal endways, as in the case of cutting grooves around the circumferential 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 mayobviously be made taper, or to any other required angle or curvature,Figs. 1934and1935being examples which can be used in situations where other cutters could not, as for example on the arms or spokes of wheels.
Fig. 1936Fig. 1936.
Fig. 1936.
Fig. 1936, fromThe 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.
Fig. 1937Fig. 1937.
Fig. 1937.
Shank mills may obviously be made of various shapes; thus inFig. 1937is 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.
Shank cutters are provided with finer teeth than ordinary cutters, the following being the numbers of teeth commonly employed for the respectivediameters:—
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.
Fly cutters are single-toothed cutters, or rather tools, that are largely used by watchmakers for cutting their fine pitches of gear wheels.
Fig. 1938Fig. 1938.
Fig. 1938.
Fig. 1938represents a fly cutter in place in its holder or arbor, its front facedbeing in line with the axiscof the arbor.
Fig. 1939Fig. 1939.
Fig. 1939.
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 attinFig. 1939, is made to fill a space 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 glass. This tool is used for the production of fly cutters, and may be employed by either of the followingmethods:—
Fig. 1940Fig. 1940.
Fig. 1940.
The piece of steel to form the cutter is fastened in an arbor back from the centre, as atdinFig. 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 inFig. 1938, its front facedbeing in line with the axiscof 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; facedmay 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 expensivemilling cutters, which, unless made on the principle of the Brown and Sharpe cutters, do not preserve their shapes.
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.
Fig. 1941Fig. 1941.
Fig. 1941.
Fig. 1942Fig. 1942.
Fig. 1942.
Another method of cutting up a fly cutter by the tool made to the template is shown inFig. 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 inFig. 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 inFig. 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 asfollows:—
Fig. 1943Fig. 1943.
Fig. 1943.
Fig. 1944Fig. 1944.
Fig. 1944.
The blank is let into an arbor of small diameter, as inFig. 1943, its facedbeing 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 inFig. 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.
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 additionalmethod:—
Fig. 1945Fig. 1945.
Fig. 1945.
With the tool made from the template we may cut up a wheel, such as inFig. 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 inFig. 1945.
Fig. 1946Fig. 1946.
Fig. 1946.
Fig. 1947Fig. 1947.
Fig. 1947.
The method of using the circular cutter is illustrated inFig. 1946, in whichhis a holder, whose end facepis 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. Asecond holderhas 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 cutteris 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 11⁄4inch and another of3⁄4inch 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.
Fig. 1948Fig. 1948.
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 inFig. 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.
In both methods some latitude 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 circumference, 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.
Fig. 1949Fig. 1949.
Fig. 1949.
Thus, inFig. 1949is shown a sectional view of a gang of three mills or cutters,a,b, andc, of whichaandcare recessed to admit of the ends ofbpassing within them. The heavy black line representing a paper washer inserted to adjust the distance apart ofaandc, it being obvious that this gives a means of letting them together after their side teeth atdandehave been ground. As shown in the figure,ahas teeth on one only of its sides, whilechas them on both sides as well as in its circumference, while all three are of different widths of face. This would capacitateaonly for the inside cutter, as in the figure, whilebwould 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. Butc, being cut on each side, could be used singly for grooving or recessing, or for plain milling, or in the position ofborain the figure; hence it is preferable in gang milling for general purposes to provide teeth on both sides as well as on the circumference 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 circumstances may require.
Fig. 1950Fig. 1950.
Fig. 1950.
Fig. 1951Fig. 1951.
Fig. 1951.
Suppose, for example, that steps, such as shown inFig. 1950, were required to be cut in a piece of brass work, and that, the work requiring to be very true, a set of roughing and one of finishing cutters be used, then the latter may be put together as inFig. 1951, there being eight separate cutters, and their ends being slightly recessed but without teeth. Such cutters would wear a long time and may be readily sharpened, and there being no side teeth, the widths of the cutters, individually and collectively, would not be altered by the grinding; hence no readjustment with washers would be necessary. The tooth corners must, however, be kept sharp, for in proportion as they get dull or blunt, the sides of the cutter wedge in the work, causing friction and extra power to drive them as well as producing inferior work.
Fig. 1952Fig. 1952.
Fig. 1952.
Fig. 1952, which is from an article by John J. Grant, represents a gang of cutters arranged to mill out the jaws and the top faces of a head for a lathe; and it is obvious that a number of such heads may be set in line and all milled exactly alike.
The Number of Teeth in Mills or Cutters.—The teeth of cutters must obviously be spaced wide enough apart to admit of the emery wheel grinding one tooth without touching the next one, and the front faces of the teeth are always made in the plane of a line radiating from the axis of the cutter.
In cutters up to 3 inches in diameter, it is good practice to provide 8 teeth per inch of diameter, while in cutters above that diameter the spacing may be coarser, asfollows:—
Fig. 1953Fig. 1953.
Fig. 1953.
Milling Cutters with Inserted Teeth.—When it is required to use milling cutters of a greater diameter than about 8 inches, it is preferable to insert the teeth in a disk or head, so as to avoid the expense of making solid cutters and the difficulty of hardening them, not merely because of the risk of breakage in hardening them, but also on account of the difficulty in obtaining the uniform degree of hardness or temper. The requirements for the heads for inserted teeth are, that the teeth shall be locked firmly in position without lost motion, and be easily set to gauge, ease of insertion and of removal being of secondary consideration, as such teeth should be ground in their places in the head, and are therefore rarely removed. The manner in which these requirements are attained in the Brainard heads are, as shown inFig. 1953. A disk of wrought iron of suitable thickness and diameter is turned and squared, then a circle of index holes corresponding to the number of teeth required is drilled in its face; this circle of holes is used to insure the accurate spacing of the dovetail seats for the teeth, and to attain accuracy in grinding the teeth. All the teeth are a driving fit, and being milled are, of course, interchangeable. In order to obtain a larger number of teeth in a given size of head than could be got into the face, only one-half of the teeth are dovetailed into the periphery of the head and the other half into its face, but yet all the teeth are effective for face cutting, the construction being asfollows:—
Between each pair of face teeth is a slit sleeve, which meets them and has a taper base, through which passes a taper bolt having a nut on the back face of the head. Tightening this nut expands the sleeve, thus locking the pair of teeth in their dovetail grooves. The circumferential teeth are each counter-based to receive a screw tapped in the head, and are firmly locked thereby. This affords a simple and reliable means of inserting and adjusting other teeth with the certainty that they will be true with those already in use.
The large size of some of these heads makes it convenient and desirable to grind them in their places on the machine, and for this purpose a special grinder is made by the same company. This grinder sets upon the machine table and has a point or pin for the index holes or the cutter head; by this means the grinding may be made as accurate as in small milling cutters.
The head shown infigurerepresents one that has been in use ten years, its cutters having been renewed but once; it is 28 inches in diameter, contains 84 teeth, and weighs 400 lbs.
Fig. 1954Fig. 1954.
Fig. 1954.
Arbors for milling cutters may be driven in two ways. In the first the shank is made taper to fit the taper bore of the live spindle. The standard taper is1⁄2inch per foot of length. The keyway is semicircular, as shown atginFig. 1954, the key consisting of a piece of No. 25 Stubbs steel wire, which being of uniform diameter enables a number of keys of different lengths to be easily obtained or made, and the nut is usually cylindrical, having two flat sides,a.