Modern Machine-Shop Practice Part 137

[Ill.u.s.tration: Fig. 1971.]

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.

[Ill.u.s.tration: Fig. 1972.]

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.

[Ill.u.s.tration: Fig. 1973.]

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 pa.s.sing the lowest point of cutter revolution.

[Ill.u.s.tration: Fig. 1974.]

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 pa.s.sing their lowest point of revolution--or in other words, after pa.s.sing 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 pa.s.sing 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.

[Ill.u.s.tration: Fig. 1975.]

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.

[Ill.u.s.tration: Fig. 1976.]

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.

[Ill.u.s.tration: Fig. 1977.]

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.

[Ill.u.s.tration: Fig. 1978.]

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 circ.u.mference 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 circ.u.mference 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.

[Ill.u.s.tration: Fig. 1979.]

[Ill.u.s.tration: Fig. 1980.]

[Ill.u.s.tration: Fig. 1981.]

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:--

[Ill.u.s.tration: Fig. 1982.]

[Ill.u.s.tration: Fig. 1983.]

[Ill.u.s.tration: Fig. 1984.]

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 pa.s.sing 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 pa.s.s, 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.

[Ill.u.s.tration: Fig. 1985.]

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 pa.s.ses 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 pa.s.s through the chuck, as denoted by the dotted lines, when at the bottom of its path of revolution.

[Ill.u.s.tration: Fig. 1986.]

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.

[Ill.u.s.tration: Fig. 1987.]

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.

[Ill.u.s.tration: Fig. 1988.]

[Ill.u.s.tration: Fig. 1989.]

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 pa.s.s, 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.