Modern Machine-Shop Practice Part 136

[Ill.u.s.tration: Fig. 1951.]

Suppose, for example, that steps, such as shown in Fig. 1950, were required to be cut in a piece of bra.s.s 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 in Fig. 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.

[Ill.u.s.tration: 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 s.p.a.ced 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 s.p.a.cing may be coa.r.s.er, as follows:--

Diameter of cutter 6 inches, number of teeth in cutter 40 " " 7 " " " 45 " " 8 " " " 50

[Ill.u.s.tration: 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 in Fig.

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 s.p.a.cing 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 as follows:--

Between each pair of face teeth is a slit sleeve, which meets them and has a taper base, through which pa.s.ses 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 circ.u.mferential 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 in figure represents 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.

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 is 1/2 inch per foot of length. The keyway is semicircular, as shown at G in Fig. 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.

Fig. 1955 (from _The American Machinist_) represents an arbor, having a cone at A, so that the cutter bore being coned to correspond, the cutter will run true, notwithstanding that it may not fit the stem B. It is obvious, however, that the nut and washer must be made quite true or the cutter will be thrown out of line with the arbor axis and therefore out of true, and also that such an arbor is not suitable for cutters of a less width of face than the length of the cone A.

[Ill.u.s.tration: Fig. 1954.]

[Ill.u.s.tration: Fig. 1955.]

[Ill.u.s.tration: Fig. 1956.]

Shank cutters that have parallel shanks as in Fig. 1928 should have their sockets eased away on the upper half of the bore as denoted by the dotted arc D in Fig. 1956, which will enable the cutter shanks to be made the full size of the socket bore proper, and thus run true while enabling their easy insertion and extraction from the socket. Or the same thing may be accomplished by leaving the socket bore a true circle fitting the cutter shanks in tight, and then easing away that half of the circ.u.mference that is above the centre line C in the figure. It is preferable, however, to ease away the bore of the socket, which entails less work than easing away the shanks of all the cutters that fit to the one socket. When the cutter is held in a socket of this kind it allows it to be set further in or out, to suit the convenience of the work in hand, which cannot be done when the cutter has a taper shank fitting into the coned bore of the machine spindles.

[Ill.u.s.tration: Fig. 1957.]

It is obvious that when the cutter requires to pa.s.s within the work, or cut its way, as in the case of milling out grooves, a nut cannot be used; hence, inch cutters are driven by a key as usual, but secured by a screw, as in Fig. 1957, which is from the pen of John J. Grant, in _The American Machinist_.

[Ill.u.s.tration: Fig. 1958.]

[Ill.u.s.tration: Fig. 1959.]

In many cases it becomes a question whether it is better to do a piece of work with plain mills, with an end mill, or with face mills, a common hexagon nut forming an example. Thus, in Fig. 1958, we have a nut being operated upon by a plain mill; in Fig. 1959 by an end mill, and in Fig.

1960 by a pair of twin face mills.

In the case of the plain mill, it is obvious that only one side of the nut is operated upon at a time, and as the whole of the pressure of the cut falls on one side of the work it acts to spring or bend the mandrel or arbor used to hold the nut, and this spring is sufficient, if several nuts are milled at once on the same arbor, to make the arbor bend and cause the nuts in the middle to be thicker than those at the ends of the arbor. In the case of hand-forged nuts in which there may be more metal to take off some nuts or some sides of nuts than off others, the extra spring due to an increased depth of cut will make a sensible difference to the size the work is milled to. In the case of the end mill the pressure of the cut falls in line with the arbor axis and downwards; hence the arbor spring is less and does not affect the depth of the cut.

In the case of the face mills the pressure of the cut falls on both sides of the work, and the spring is mainly endways of the nut arbor; hence, it does not affect the depth of the cut nor the truth of the work. Furthermore, in both the end and the face mills, the work will be true notwithstanding that the cutter may not be quite true, because each point of the work surface is pa.s.sed over by every tooth in the cutter, so that the work will be true whether the cutter runs true or not; whereas in the plain mill or cutter each tooth does its individual and independent proportion of finishing. This is shown in Figs. 1961 and 1962. In Fig. 1961 we have the plain mill, and it is obvious that the tooth does the finishing on the vertical line B, that being the lowest point in its revolution. After a tooth has pa.s.sed that point the work in feeding moves forward a certain distance before the next tooth comes into action; hence to whatever amount a tooth is too high it leaves its mark on the work in the form of a depression, or _vice versa_, a low tooth will leave a projection.

[Ill.u.s.tration: Fig. 1960.]

[Ill.u.s.tration: Fig. 1961.]

[Ill.u.s.tration: Fig. 1962.]

In Fig. 1962 we have a piece of work being operated on by a face mill, and it is obvious that while the teeth perform cutting duty throughout the distance A, yet after the work has fed past the line A it is met by the cutter teeth during the whole time that the work is feeding a distance equal to A on the other side; hence the prolonged action of the teeth insures truth in the work. On the other hand, however, it is clear that the work requires to feed this extra distance before it is finished.

[Ill.u.s.tration: Fig. 1963.]

Suppose, however, that the cutter being dead true the cutting action ceases on the centre line, and therefore exists through the distance A only, and if we take a plain cutter of the same diameter as in Fig. 1963 we see that its period of feed only extends through the length B, and it becomes apparent that to perform an equal amount of work the face cutter is longer under feed, and therefore does less work in a given time than the plain cutter, the difference equalling twice that between A and B in the two figures, because it occurs at the beginning and at the end of the cut.

There is, however, another question to be considered, inasmuch as that the face cutter must necessarily be of larger diameter than the plain one, because the work must necessarily pa.s.s beneath the washer (C, Fig.

1915), that is between the two cutters; hence the cutter is more expensive to make.

[Ill.u.s.tration: Fig. 1964.]

We may in very short work overcome this objection by feeding the work, as at K in Fig. 1964, the face L to be milled requiring to feed the length of the teeth instead of the distance H in the figure. In the end mill the amount of feed also is greater for a given length of finished surface than it is in the plain cutter, as will be readily understood from what has already been said with reference to face mills.

Face milling possesses the following points of advantage and disadvantage, in addition to those already enumerated: If the work is sprung by the pressure of the holding devices it is in a line with the plane of motion of the teeth, hence the truth of the work is not impaired. On the other hand, the teeth meet the scale or skin of the work at each cut, whereas in a cylindrical cutter this only occurs when the cutter first meets the work surface.

The strain of the cut has more tendency to lift the work table than in the case of a cylindrical cutter. The work must be held by end pressure; hence the chuck or holding jaws must be narrower than the work, rendering necessary more work-holding devices. Since, however, both sides of the work are simultaneously operated on, there is no liability of error in parallelism from errors in the second chucking, as is the case with plain cutters.

[Ill.u.s.tration: Fig. 1965.]

[Ill.u.s.tration: Fig. 1966.]

To cut [V]-shaped grooves in cylindrical work, when it is required that one face or side of the groove shall be a radial line from the centre of the work, two methods may be employed. First we may form the cutter, as in Fig. 1965, the side B of the cutter being straight and the point of the cutter being set over the centre of the work. The objection to this is that the finished groove will have a projection or burr on the radial side of the groove, as shown at D in the figure, entailing the extra labor of filing or grinding, to remove it; furthermore, that face will have fine scored marks upon it, as denoted by the arcs at C, these scores showing very plainly if the cutter has any high teeth upon it, and more especially in the case of cutting spirals, as will appear presently. The reason of this is that the side B of the cutter being straight or flat the whole of the teeth that are within the groove have contact with the side C of the groove, that is to say, all the teeth included in the angle E in the figure, because the teeth on the side A tend, from the pressure of the cut to force the cutter over towards the side C of the groove. The second method referred to, which is that commonly adopted for cutting the flutes of tapes, reamers, milling cutters, &c., is to form the cutter on the general principle ill.u.s.trated in Fig. 1966, and set it to one side of the centre of the work so that one of its faces forms a radial line, as shown in the figure, the distance to which it is set to one side depending upon the angle of its cutting edge to the face of the cutter.

[Ill.u.s.tration: Fig. 1967.]

Fig. 1967 represents a common form of cutter of this cla.s.s that is used for cutting spiral grooves on milling cutters up to 3 inches in diameter, which contain eight teeth per inch of diameter. The angle of the teeth on B is 12 to the side face A of the cutter, and the angle of the teeth at C is 40 to the face D.

The effect produced by making face B at an angle instead of leaving it straight, or in other words, instead of cutting the teeth on the face A, may be shown as follows:--

[Ill.u.s.tration: Fig. 1968.]

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 circ.u.mferential 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 pa.s.sing 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 pa.s.ses the line G, which pa.s.ses 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 pa.s.sing 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 pa.s.s the line G.

[Ill.u.s.tration: Fig. 1969.]

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 pa.s.sing 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 circ.u.mference 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.

[Ill.u.s.tration: Fig. 1970.]