Chapter XXII: Milling Machinery and Milling Tools (3)

Suppose that in Fig. 1968 we have a sectional view taken through the middle of the thickness of a cutter for a rectangular groove, the circumferential surface being at a right angle to the side faces, and it is evident that the teeth, at every point in their length across the cutter, except at the extreme corner that meets the side faces as C, will have contact with the seat of the groove while passing through the angle F only (which is only one half of the angle E in Fig. 1965); or in other words, each tooth will have contact with the seat of the groove as soon as it passes the line G, which passes through the axis of the cutter; whereas, when the teeth are parallel with the side of the cutter, as was shown in Fig. 1965, the teeth continue to have contact with the side walls of the groove after passing the line G.

By forming the cutter as in Fig. 1967, therefore, we confine the action to the angle F, Fig. 1968, the teeth having contact with the walls of the groove as soon as they pass the line G.

In cutting spiral grooves this is of increased importance, for the following reasons: In Fig. 1969 we have a cutter shown in section, and lying in a spiral groove. Now suppose a tooth to be in action at the bottom of the groove, and therefore on the line G G, and during the time that it moves from that line until it has moved above the level of the top of the groove, the work will have performed some part of a revolution in the direction of the arrow, and has therefore moved over towards that side of the cutter; hence, if that side of the cutter had teeth lying parallel, as shown at B in Fig. 1965, the walls of the groove would be scored as at C in that figure, whereas by placing the teeth at an angle to the side face, they recede from the walls after passing line G, and therefore produce smoother work.

A cutter of this kind must, for cutting the teeth of cutters, be accurately set to the work, and the depth of cut must be accurate in order to cut the grooves so that one face shall stand on a radial line, and the top of the teeth shall not be cut to a feather edge. If the teeth were brought up to a sharp edge the width of the groove at the top would be obtained with sufficient accuracy by dividing the circumference of the work by the number of flutes or teeth the work is to contain, but it is usual to enter the cutter sufficiently deep into the work to bring the teeth tops up to not quite a sharp edge. The method of setting the cutter is to mark on the end of the work a central line R, Fig. 1970, and make the distance E in same figure equal to about one tenth the diameter of the work.

Obviously the cutter is set on opposite sides of the work centre, according to which side of the groove is to have the radial face. Thus for example, in Fig. 1970, the cutter is set to the left of line R, the radial face of the groove being on the left, while in Fig. 1971 the cutter is set on the right of line R, because the radial face is on the right hand side of it, the work consisting (in these examples) in cutting up a right and a left-hand mill or cutter.

The acting cutter J may in both cases be used to cut either a right or a left-hand flute, according to the direction in which the work W is revolved, as it is fed beneath the cutter J.

In Fig. 1972 we have an example of cutting straight grooves or teeth, with an angular cutter having one side straight, and it is seen that we may use the operating or producing cutter in two ways: first, so that the feed is horizontal, as at A, or vertical, as at B; the first produces a right-hand, and the second a left-hand cutter, as is clearly seen in the plan, or top view. The feeds must, however, be as denoted by the respective arrows being carried upwards for B, so that the cutter may run under the cut and avoid cutter breakage.

The number of grooves or flutes producible by an angular cutter depends upon the depth of the groove and the width of land or tooth between the grooves. Thus Fig. 1973 represents a cutter producing in one case four and in the other eight flutes with the same form of cutter, the left being for taps, and the right for reamers.

For cutting the teeth of cutters or mills above 3 inches in diameter, the angles of the acting or producing cutter are changed from the 12° and 40° shown in Fig. 1967, to 12° as before on one side, and a greater number on the other; thus in the practice of one company it is changed to 12° and 48°, the 12° giving the radial face as before, and the 48° giving a stronger and less deep tooth, the deep tooth in the small cutters being necessary to facilitate the grinding of the teeth to sharpen them.

In cutting angular grooves in which the angle is greater on one side than on the other of the groove, the direction of cutter revolution and the end of the work at which the groove is started; or in other words, the direction of the feed, is of importance, and it can be shown that the feed should preferably be so arranged that the side of the groove having the least angle to the side of the cutter should be the one to move away from the cutter after passing the lowest point of cutter revolution.

In Fig. 1974, for example, we have at R a cylinder with a right-hand groove in it, whose side C, representing the face of a tooth, is supposed to be a radial line from the cylinder axis, the side B representing the back of a cutter tooth, being at an angle of 40°.

Now if the work revolves in the direction of arrow A, and the cut be started at end G (as it must to cut a right-hand groove with the work revolving as at A), then the side C of the groove will move over towards and upon the side of the cutter for the reasons explained with reference to Fig. 1969, and the teeth on this side being at the least angle to the side of the cutter, do not clear the cut so well, the teeth doing some cutting after passing their lowest point of revolution--or in other words, after passing the line G in Fig. 1968. The effect of this is to cause the cutter to drag, as it is termed, producing a less smooth surface on that side (C) of the groove or tooth.

We may, however, for a right-hand groove revolve cylinder R, as denoted by arrow E, and start the cut at end D. The result of this is that the side C of the groove, as the roller revolves, moves away from the side of the cutter, whose teeth therefore do no cutting after passing their lowest point of revolution (G, Fig. 1968), and the dragging action is therefore avoided, and the cut smoother on this which is the most important side of the tooth, since it is the one possessing the cutting edge. When "dragging" takes place the burr that was shown in Fig. 1965 at D, is formed, and must, as stated with reference to that figure, be removed either by filing or grinding.

Obviously if the direction of cutter revolution and of feed is arranged to cause side C to move away from the side of the cutter, then side B will move over towards the other side of the cutter; but on account of the cutter teeth on this side being at a greater angle to the side of the cutter, they clear better, as was explained with reference to Fig. 1968, and the dragging effect caused by the revolving of the work is therefore reduced.

We have now to examine the case of a left-hand groove, and in Fig. 1975 we have such a groove in a cylinder L. Let it be supposed that the direction of its revolution is as denoted by arrow F, and if the cutter is started at H (as it must be to cut a left-hand groove if the work revolves as at F), then the side C moves over towards the cutter, and the dragging or crowding action occurs on that side; whereas if the direction of revolution is as at K, and the cutter starts at N and feeds to H, then side B of the groove moves towards the cutter; hence face C of the groove is cut the smoothest. Obviously then the direction of cutter and work revolution and of feed, in cutting angular grooves in which one angle of the cutter is at a greater degree of angle than the other to the side of the cutter, should be so arranged that the work revolves towards that side of the cutter on which its teeth have the greater angle, whether the spiral be a right-hand or a left-hand one. In cutting grooves not truly circular the same principle should be observed.

In Fig. 1976, for example, it is better if the side B is the one that moves towards the cutter, the direction of revolution being as denoted by the arrow, whether the groove be a right-hand or left-hand (supposing, of course, that the cutter starts from end E of the work).

Obviously, also, the greater the degree of spiral the more important this is, because the work revolves faster in proportion to the rate of feed, and therefore moves over towards the outer faster.

In cutting spirals it is necessary first to put on such change gears as are required to revolve the work at the required speed for the given spiral, and to then set the work at such an angle that the cutter will be parallel to the groove it cuts, for if this latter is not the case the groove will not be of the same shape as the cutter that produces it.

In Fig. 1977 we have a spiral so set, the centre of the cutter and of the groove being in the line O O, and the work axis (which is also the line in which the work feeds beneath the cutter) being on the line C C. The degrees of angle between the centre of the cutter, or line O O, and the axis of the work, or line C C, are the number of degrees it is necessary to set the work over to bring the cutter and the groove parallel, this number being shown to be 20 in the example.

To find this angle for any given case we have two elements: first, the pitch of the spiral, or in other words, the length or distance in which it makes one complete turn or revolution; and second, the circumference of the work; for in a spiral of a given pitch the angle is greater in proportion as the diameter is increased as may be seen in Fig. 1978, in which the pitch of the spirals is that in Fig. 1977, while the angle is obviously different.

To find the required angle for any given case we may adopt either of two plans, of which the first is to divide the circumference of the work in inches by the number of inches which the spiral takes to make one turn. This gives us the tangent of angle of the spiral.

The second method of setting the work to cut a given spiral is to chuck the work and put on the necessary change gears. The cutter is then set to just touch the work and the machine is started, letting the work traverse beneath the cutter just as though the work was set at the required angle to the cutter:

When the cutter has arrived at the end of the work it will have marked on it a line, as in Fig. 1979, this line representing the spiral it will cut with those change gears, and all that remains to do is to swing the work over so that this line is parallel with the face of the cutter, as shown in Fig. 1980. If the diameter of the cutter is small we may obviously secure greater accuracy by placing a straight-edge upon the side of the cutter so as to have a greater length to sight by the eye in bringing the line fair with the cutter. This being done it remains to merely set the cutter in its required position with reference to the work diameter.

If an error be made in setting the angle of the work to the cutter the form of groove cut will not correspond to that of the cutter. This is shown in Fig. 1981, in which the cutter being at an angle to the groove the latter is wider than the cutter thickness, and it is obvious that by this means different shapes of grooves may be produced by the same cutter. In proportion, however, as the cutter is placed out of true the cutting duty falls on the cutting edges on one side only of the cutter, which is the leading side C in the figure, while the duty on the other side, B, is correspondingly diminished.

The simplest method of holding work to be operated upon in the milling machine is either between the centres or in the vice that is provided with the machine. The principles involved in holding work in the vise so as to keep it true and avoid springing it for milling machine work, are the same as those already described with reference to shaping machine vises.

In milling tapers the work, if held in centres, should be so held that its axial line is in line with the axes of both centres, for the following reasons:--

In Figs. 1982 and 1983 we have a piece of work in which the axes of the centres and of the work are not in line, and it is clear that the horn _d_ of the dog D will, in passing from the highest to the lowest point in its revolution, move nearer to the axis of the work. Suppose, then, that the driver E is moved a certain portion of a revolution with tail _d_ at its highest point, and is then moved through the same portion of a revolution with _d_ at its lowest point in its path of revolution, and being at a greater distance or leverage when at the top than when at the bottom it will revolve the work less. Or if the tail _d_ of the dog is taper in thickness, then in moving endways in the driver E (as it does when the work is revolved) it will revolve the work upon the centres. Suppose, then, that the piece of work in the figures required to be milled square in cross-section, and the sides would not be milled to a right angle one to another. This is avoided by the construction of the Brainard back centre, shown in Fig. 1984, in which T represents the surface of the work table and H the back centre. The block B is fitted within head H, and has two slots A A, through which the bolts S S pass, these bolts securing B in its adjusted position in H. The centre slide C operates in B; hence B, and therefore C, may be set in line with the work axis.

For heads in which the back centre cannot thus be set in line, the form of dog shown in Fig. 1985 (which is from _The American Machinist_) may be employed to accommodate the movement of the tail or horns through the driver. Its horn or tail B is made parallel so as to lie flat against the face of the slot in the driver. The other end of tail B is pivoted into a stud whose other end is cylindrical, and passes into a hub provided in one jaw of the dog, the set-screw A being loosened to permit this sliding motion. This locks the horn in the clamp and permits the dog to adjust itself to accommodate the motion endwise that occurs when it is revolved. The amount of this motion obviously depends upon the degree of taper, it being obvious (referring to Fig. 1982) that horn _d_ would pass through the chuck, as denoted by the dotted lines, when at the bottom of its path of revolution.

It is obvious that when the head or universal head of the machine is elevated so that it stands vertical, it may have a chuck screwed on and thus possess the capacity of the swiveled vise. It is preferable, however, to have a separate swiveled chuck, such as in Fig. 1986 (from _The American Machinist_), which will not stand so high up from the machine bed, and will therefore be more solid and suitable for heavy work.

Another very handy form of chuck for general work is the angle chuck shown in Fig. 1987, which is from an article by John J. Grant, in _The American Machinist_. The work-holding plate has [T]-grooves to chuck the work on and is pivoted at one end, while at the other is a segment and bolt to secure it in its adjusted angle. Two applications of the chuck are shown in the figure.

Fig. 1988 represents a top, and Fig. 1989 an end view of a chuck to hold rectangular bars that are to be cut into pieces by a gang of mills. A, A, A, are grooves through the chuck jaws through which the cutters pass, severing the bar through the dotted lines. Each piece of the bar is held by a single screw on one side and by two screws on the other, which is necessary in order to obtain equal pressure on all the screws and prevent the pieces from moving when cut through, and by moving, gripping the cutters and causing them to break.

In chucking the bar the two end screws D D must be the first to be set up to just meet the bar: next the screws B C on the other side must be set up, holding the bar firmly. The two screws between D D are then set up to just bind the bar, and then the middle four on the other side are screwed up firmly. By this method all the screws will hold firmly and the pieces cannot move.

VERTICAL MILLING, DIE SINKING, OR ROUTING MACHINE.--Fig. 1990 represents Warner & Swazey's die sinking machine. The cutter driving spindle is here driven by belt direct, imparting a smooth motion. The knee is adjustable for height on the vertical slideway on the face of the column, which is provided with a stop adjustable to determine how high the knee and work-holding devices can be raised, and, therefore, the depth to which the cutter can enter the work, and a _former_ pin is placed 6 inches behind the cutter to act as a stop against which a pattern may be moved when work is to be copied from a _former_ or pattern piece. The work-holding device consists of a compound rest and a vise capable of being swiveled to any angle or of being revolved to feed the work to the cutter, hence the work may be moved in any required direction, in either a straight line, in a circle, or in any irregular manner to suit the shape of the work.

PROFILING MACHINE.--The profiling machine is employed mainly to cut the edges of work, and to sink recesses or grooves in the upper surface of the same to correspond to a pattern. A provisional template of the form of the work is fastened on the bed of the machine, and from this is cut in the machine a thicker one termed the "former," which is then used to copy the work from.

Fig. 1991 represents Pratt & Whitney's profiling machine. On the cross slide are two separate sliding heads, each of which carries a live spindle for the cutting tool, and beside it a spindle to receive a pin, which by being kept against the pattern or _former_ causes the work to be cut to the same shape as the former.

The work is fastened to the table, which is operated upon the raised [V]s shown by the handle on the left, which operates a pinion geared to a rack on the underneath side of the table. The handle on the right operates the heads along the cross slide also by a rack and pinion motion. The gearing and racks in both cases are double, so that by two independent adjusting screws the wear of the teeth may be taken up and lost motion prevented. By means of these two handles the work may be moved about the cutter with a motion governed by the form or shape of the _former_, of which the work is thus made a perfect pattern both in size and shape. The tool used is a shank or end mill, such as was shown in Fig. 1928. In some profiling machines the spindle carrying the guide or former pin is stationary, in which case the provisional template is put beneath it and the _former_ is cut by the live spindle, and for use must be moved from the position in which it was cut and reset beneath the _former_ spindle. This machine, however, is provided with Parkhurst's improvement, in which the _former_ spindle is provided with a gear-wheel, by which it may be revolved from the live spindle, hence the provisional template may be set beneath the live spindle in which the guide pin is then placed. The cutter is then placed in the _former_ spindle, and the _former_ cut to shape from the provisional template while in the actual position it will occupy when used.

Fig. 1992 represents Brainard's machine for grinding milling cutters. It consists of a threaded column A to which is fitted the knee B, which as it fits the top of the threads on the column may be swung or revolved about the column without being altered in its height upon the same except by means of the threaded ring C. At D is a lever for clamping the knee B to the column after adjustment; W represents the emery wheel mounted on the end of the horizontal spindle having journal bearing at the top of the column. The face of the knee B has a slideway _d_ for the fixtures, &c., which hold the cutters to be ground, and at E is a lug pierced to receive an arbor whereon to place cutters to be ground, the lug being split and having a binding screw to lock the arbor firmly in place. F is a slide for receiving the grinding attachments, one of which is shown at K carrying a milling cutter in position to be ground on the face.

Fig. 1993 shows the fixture employed to grind parallel cutters, S representing a stand upon slide F (which corresponds to slide F in the general view of the machine in Fig. 1992) in which is fixed the arbor H. The cutter C is slid by hand along arbor H and beneath the emery wheel, the method of guiding the cutter to the wheel being shown in Fig. 1994, which represents a front view of the machine. At E is the lug (shown also at E in the general view) which has a hole to receive a rod P, and is split through at S, so that operating binding screw L locks rod P in E. At R is a rod secured to the rod P, and G is a gauge capable of swivelling in the end of R and of being secured in its adjusted position. The end of this gauge is adjusted to touch the front face of the tooth to be ground on the cutter C, which must be held close against the end of the gauge in order to grind the cutting edge to a straight line parallel to its axis.

A not uncommon error is to place the gauge G against the tooth in front of that which is being ground, as in Fig. 1995, the gauge being against tooth C while tooth B is the one being ground. In this case the truth of the grinding depends upon the accuracy of the tooth spacing. Suppose, for example, that teeth B and C are too widely spaced, tooth C being too far ahead, and this error of spacing would cause tooth B to be too near the centre of the emery wheel and its cutting edge to be ground too low.

The object of feeding the cutter by hand along the arbor H is twofold: first, the amount of cut must be very light and the feed very delicate, for if the grinding proceeds too fast the cutting edge will be what is termed burned, that is to say, enough heat will be generated to soften the extreme cutting edge, which may be discovered by holding the front face of the tooth to the light, when a fine blue tint will be found along the cutting edge, showing that it has been softened in the grinding, and this will cause it to dull very rapidly.

The second object is to insure parallelism in the cutter. Suppose, for example, that the cutter C was fast upon arbor H and was fed to the wheel by moving slide F, and if the arbor H stood at an angle, as in Fig. 1996, to the slide upon which F moved, the cutter would be ground taper, whereas if the cutter is fed along the arbor it will be ground parallel whether the arbor is true or not with the slideway of F, the only essential being that the arbor H be parallel and straight, which is much easier to test and to maintain than it is in the slideway (D, Fig. 1992). Here it may be noted that oil should not be applied either to arbor H or to the cutter bore or slideway D, as lubrication only increases the wear of the parts, causing the fine emery particles that inevitably fall upon them to cut more freely.

As thin cutters would not have sufficient length of bore to steady them upon the arbor and insure parallelism, the cutter sleeve shown in Fig. 1997, which is from _The American Machinist_, is employed to hold them. It is provided with a collar, is threaded at T for the nut N to hold the cutter against collar C, and is bored to fit the cutter arbor H, which corresponds to H in Fig. 1993.

This device also affords an excellent means of holding two or more thin cutters requiring to be ground of exactly equal diameters.

It follows from what has been said that taper tools, such as taper reamers, must be held with their upper face parallel to the line of their motion in being fed to the wheel, as in Fig. 1998, in which line M represents this line of motion, line N the axis of the reamer, and line O the line on which the fixture that holds the reamer must move, O being parallel to M.

Fig. 1999 represents Slate's fixture for this class of work. A is a stand that bolts upon the slideway _d_ in Fig. 1992. Upon A is fixed a rectangular bar B, upon which (a sliding fit) is the shoe C. Upon C fits the piece D, which is pivoted to shoe C by the pin at E. At the other end of D is a lug, against which abuts the end of screw G, which is threaded through the end of C, so that by operating the screw G, D may be set to any required angle upon C, and at F is a set-screw threaded through D and abutting against C, so as to lock D in its adjusted position. At P is a pointer for the graduations on C, which are marked to correspond with the graduations upon the taper turning attachment of a lathe.

The work is held between centres, the head H fitting to a slideway on the top of D, and being secured in its adjusted position by the screw I. The work should obviously be set so that its upper face lies horizontal, and is fed to the wheel by moving shoe C by hand along bar B, the long bearing keeping C steady, and the lightness of the moving parts making the feeding more sensitive than it would be were it required to move bar B.

The tooth being ground is held by hand against the gauge G in Fig. 1994, as was described with reference to that figure, and the reamer, therefore, in the case of having spiral grooves, revolves upon its centre while being fed to the emery wheel.

For tapers that are beyond the capacity of this device, and also for holding cutters to have their face teeth ground, the device shown in Fig. 2000 is employed. Upon the slide F is fixed knee K (the corresponding parts to which are seen in the general view, Fig. 1992), whose disk face at R is graduated as shown. Piece S is pivoted by a pin passing through the hub of K and having a nut T to secure it in its adjusted position. S is bored to receive the cutter arbor H, and is split through so that by means of the screw at V the arbor may be gripped and locked in S. The stud W for holding the gauge G passes into a bore in the bracket X, and is secured therein by the screw at Y, the lugs through which Y passes being split through into the bore for W. As shown in the figure, the arbor H is set for grinding the side teeth of the cutter, but it is obvious that S being pivoted to K may be swung out of the vertical and to any required angle, so as to bring the face of the tooth that is to be ground horizontally beneath the emery wheel, as shown in Fig. 2001, which represents an angular cutter in position. We have now to consider the adjustment of the cutter to the emery wheel, necessary in order that the cutting edges may be given the necessary clearance.

First, then, suppose in Fig. 2002 that the line A A represents the line of centres of the emery-wheel spindle and the cutter arbor, and if the front face B of the tooth be set coincident with this line, as in the figure, then the top of the tooth partaking of the curvature of the wheel that grinds it would have its heel C the highest; hence the edge at B could not cut.

If, however, the line A A in Fig. 2003, still representing the line of centres, we so set the gauge (G, Fig. 1994) that the heel C of the tooth comes up to line A A, then the curvature of the emery wheel would give clearance to the heel C, and therefore a cutting edge to face B of the tooth.

The amount of clearance that may be given in this way is limited by the spacing of the teeth and the diameter of the emery wheel, as is seen from Fig. 2004, it being obvious that when tooth A is being ground the emery wheel must clear the rear tooth B or it will grind its edge off, and it is obvious that the smaller the emery-wheel diameter the more the tooth to be ground may be set in advance of the line of centres of the wheel and spindle. It may be pointed out, however, that there are two methods of adjusting the cutter to the wheel.

In Fig. 2005, for example, let A A represent the line of centres of the cutter and the wheel, and line B the plane of the front face of the tooth being ground; and in Fig. 2006 let line A represent a vertical line from the axis of the wheel, and B a vertical line passing through the axis of the cutter, the tooth edge C occupying the same position in both figures. Now suppose we employ cutting edge C as a centre and swing the cutter until its axis or centre moves along the arc D to the dot E, and it is evident that during this motion the heel of the tooth will have approached the axis of the emery wheel and that more clearance will therefore have been given to the cutting edge C.

The actual curve of the top face, as C, Fig. 2007, of the tooth T will remain the same in either case, but its position with relation to the front face will be altered. As this curve is greater in proportion as the diameter of the emery wheel is diminished, and as the curvature weakens the cutting edge of the tooth, it is obviously desirable to employ a wheel of as large a diameter as possible.

To eliminate this curvature it would appear that the position of the emery wheel might be reversed, as in Fig. 2008, but as the emery wheel would wear only where in contact with the tooth, it would gradually assume the shape in Fig. 2009, there being a shoulder at S that would destroy the cutting edge of the tooth.

This may to a great extent be remedied by presenting the cutter diagonally to the wheel, as in Fig. 2010, employing a wheel so thin that the whole of its face will cross the tooth top during a revolution. Or if the side faces of the wheel be recessed, leaving only a narrow annular grinding ring at the circumference, the wheel might be mounted as in Fig. 2011, thus making the top of the tooth quite flat. It may be observed, however, that the usual plan is to revolve the wheel at a right angle to the work axis, as was shown in Fig. 1994.

In grinding cutters having their teeth a right-hand spiral, care must be taken that in grinding one tooth the emery wheel does not touch the cutting edge of the next tooth.

Thus in Fig. 2013 it is seen that the corner C of the emery wheel is closer than corner D, and being at the back of the wheel and out of sight it is apt to touch at C unless a thin emery wheel be used.

In a left-hand spiral, Fig. 2012, it is the corner D that is apt to touch the next tooth, the liability obviously being greatest in cutters of large diameter.

The emery wheel should be of a grade of not less than 60 or more than 70. If it is too coarse it leaves a rough edge, which may, however, be smoothed with an oilstone slip. If the wheel is too fine it is apt to _burn_ the cutter, or in other words, to soften the cutting edge, which may be known by a fine blue burr that may be seen on the front face of the tooth, the metal along this line being softened.

The diameter of the wheel may be larger for small cutters than for large ones, since the teeth of small cutters clear the wheel better. The larger the wheel the less the curvature on the top of the tooth.

For general work a diameter of 2-1/2 inches will serve well, the thickness being about 5/16 inch or 3/8 inch. The speed of a wheel of this diameter varies in practice from 3,000 to 4,500 revolutions per minute, but either too fast or too slow a speed will cause the wheel to burn the cutter, and the same thing will occur if the cutter is fed too fast to the wheel, or if too deep a cut is taken. The finishing cut should obviously be very small in amount, especially in cutters of large diameter, for otherwise the wear in the diameter of the wheel will sensibly affect the teeth height, those last ground being the highest.