In Fig. 1025, for example, is a top view of the holder with a single-pointed threading tool t in place. w represents a piece of work supposed to be in the lathe, and g a tool-setting gauge; and it is obvious that, if the holder is not moved, the tool t may be removed, ground up, and replaced with the assurance that it will stand in the exact same position as before, producing the exact same effect upon the work, providing that the height is maintained equal, and the tool is not altered in shape by the grinding. To maintain the height equal, all that is necessary is to have the upper face (h, Fig. 1024) of the holder horizontally level and in line with the line of centres of the lathe, and to set the top face of the tool level with that of the holder. In sharpening the tool the top face only is ground; hence the angles are not altered.
Fig. 1026 represents the holder with a tool in position to true up a lathe centre, the angle of the tool holder to the line of centres being the same as in Fig. 1025; and Fig. 1027 represents various forms of tools for curves. All these serve to illustrate the advantages of such a tool holder.
If, for example, a piece of work requires the use of two or more such tools, and the holder is once set, the tools may be removed and interchanged with a certainty that each one put into place will stand at the exact angle and position required, not only with relation to the work, but also in relation to the other tools that have preceded it. Each hollow or round will not only be correct in its sweep, but will also stand correct in relation to the other sweeps and curves, no matter how often the tools may be changed. Inasmuch as the tool is ground at the top only for the purpose of resharpening, it maintains a correct shape until worn out.
The pin shown at f in Fig. 1024 is fast in the holder, and fits loosely in clamp c to prevent it from swinging around on b when b is loosened.
When the tool requires to preserve its exact shape it may also be made circular with the required form for the cutting edge formed round the perimeter. Thus Figs. 1028 and 1029, which are extracted from The American Machinist, represent tool holders with circular cutting tools.
The holder a fits the lathe tool post, carrying the cutting tool b, which is bolted to the holder and has at f a piece cut out to form the cutting edge.
To facilitate the grinding, holes are drilled at intervals through b. A plan view of this tool and holder is shown at c, the shape of the cutting edge being shown at d. The cutting edge is shown in the side view to be level with the centre of the tool holder height, but it may be raised to the level of the top of the tool steel by raising the hole to receive the bolt that fastens the cutter, as is shown at e; or the cutter may be mounted on top of the holder as shown at h, having a stem passing down through the holder, and capable of being secured by the taper pin i. A plan view of this arrangement is shown at j.
Another form of circular cutter is shown in Fig. 1030. It consists of a disk or cutter secured to a holder fitted to the tool post, the cutter edge being formed by a gap in the disk, as shown in the figure, which represents a cutter for a simple bead or round corner. The front end of the holder has a face a, whose height is level with the line of lathe centre when the holder is set level in the tool post. Hence the top face of the cutting edge may be known to be set level with the line of centres when it is fair with the face a of the holder. The bottom clearance is given by the circular shape of the cutter, while side clearance may be given by inclining the face b of the holder (against which the face of the cutter is bolted) to the necessary angle from a vertical line. The face c is ground up to resharpen the cutting edge, and may be reground until the circumference of the wheel is used up.
Figs. 1031, 1032, 1033, and 1034 represent lathe tool holders by Messrs. Bental Brothers, of Fullbridge Works, Maldon, England. The holder consists of a bar a, having at the front end a hub h, containing a bush in two halves, through which the tool t passes; this tool consisting of a piece of V-shaped steel. A set screw on top of the hub clamps the two half-bushes together, and these, as their faces do not meet, grip the tool.
The advantage possessed by this form of holder is that the top face of the tool may be given any desired degree of side rake or angle required by the nature of the work by simply revolving the bushes in the hub of the holder. Thus, in Fig. 1034 the top face of the tool stands level, as would be required for brass work; in Fig. 1032 the tool is canted over, giving its top face angle a rake in the direction necessary when cutting wrought iron and feeding toward the dead centre; and in Fig. 1033 the tool is in position for carrying a cut on wrought iron, the feed being toward the live centre of the lathe. This capacity to govern the angle of the top face of the tool is a great advantage, and one not possessed by ordinary tool holders, especially since it does not sensibly alter the height of the tool point with relation to the work. Again, the V-shape of the tool steel causes the bushes to grip and support the tool sideways, and, by reducing the area of tool surface requiring to be ground, facilitates the tool grinding to that extent. Altogether, this is an exceedingly handy device. It is obvious, however, that it cannot be moved from side to side of the tool rest unless a right and left-hand tool holder be used; that is to say, there must be two holders having the hub on the opposite side of the body a.
Figs. 1035, 1036, 1037, and 1038 represent tool holders in which the tools consist of short pieces of steel held end-wise and at a given angle, so that the amount of clearance is constant. The holders Figs. 1035 and 1036 are split, and the tool is secured by the screw shown. Fig. 1037 represents a tool holder in which the tool is held by a clamp, whose stem passes through the body of the holder so as to bring the fastening nut out at the end, where it is more convenient to get at than are the screw heads in Figs. 1035 and 1036. It is obvious, however, that such a holder is weak and unsuitable for any tools save those used for very light duty indeed, while all this class of holders is open to the objection that the side of the holder prevents the tool from passing up into a corner, hence the cut cannot be carried up to a shoulder on the work. This may, however, be accomplished by bending the end of the holder round; but in this case two holders, a right and a left, will be necessary.
Fig. 1038 represents a form of tool holder of this kind in which the tool may be set for height by a set screw beneath it.
Fig. 1039 represents a tool holder and work-steadying device combined. The holder is held in the lathe tool rest in the usual manner, and affords slideway to a slide operated by the handle shown at the right-hand end.
The tool is carried at the other end of this slide, there being shown in the figure a cutting-off tool in position. At the end of the holder is a hub and three adjusting screws whose ends steady the work, and which are locked in their adjusted position by the chuck nuts shown.
The Power Required to Drive Cutting Tools.—From experiments made by Dr. Hartig, he concluded that by multiplying the weight of the metal cuttings removed per hour by certain decimal figures (or constants) the horse-power required to cut off that quantity of metal might be obtained. These decimal constants are as follows:
| Lbs. of metal | cut off per hour, | cast iron | × | .0314 | = | horse‑power required to drive the lathe. |
| „ | „ | wrought iron | × | .0327 | = | „ |
| „ | „ | steel | × | .4470 | = | „ |
For Planing Tools.
| Lbs. of | steel | cut off per hour | × | .1120 | = | horse-power required | to drive planer. |
| „ | wrought iron | „ | × | .0520 | = | „ | „ |
| „ | gun metal | „ | × | .0127 | = | „ | „ |
For drilling in the lathe, the twist drill is employed not only on account of its capacity to drill true, straight, and smooth holes, but also because its flutes afford free egress to the cuttings and obviate the necessity of frequently withdrawing the drill to clear the hole of the cuttings.
In the smaller sizes of twist drill, the stem or shank is made parallel, as in Fig. 1040, while in the larger sizes it is made taper, as in Fig. 1041, for reasons which will appear hereafter.
The taper shanks of twist drills are given a standard degree of taper of 5⁄8 inch per foot of length, which is termed the Morse taper. A former standard, termed the American standard, is still used to a limited extent, its degree of taper being 9⁄16 inch per foot.
Parallel shanked twist drills are driven by chucks, while taper, shanked ones, are driven by sockets, such as in Fig. 1042, from c to d, fitting into the lathe centre hole, while the bore at the other end is the Morse standard taper, to receive the drills e e, which have a projection such as shown at a, which by fitting into a slot that meets the end of the taper holes in the socket, lock the drill and prevent its revolving in the socket, while affording a means of forcing the drill out by inserting a key k, as shown in the figure.[14]
Each socket takes a certain number of different sized drills, the shanks of the smaller drills being in some cases longer than the drill body.
| Number | 1 | socket receives | drills from | 1⁄8 | to | 19⁄32 | inch | inclusive. | ||
| „ | 2 | „ | „ | 5⁄8 | „ | 29⁄32 | „ | „ | ||
| „ | 3 | „ | „ | 15⁄16 | „ | 1 | 1⁄4 | „ | „ | |
| „ | 4 | „ | „ | 1 | 9⁄32 | „ | 2 | „ | „ | |
| „ | 5 | „ | „ | 2 | 1⁄32 | „ | 2 | 1⁄2 | „ | „ |
These sockets are manufactured ready to receive the drills, but are left unturned at the shank end so that they may be fitted to the particular lathe or machine in which they are to be used, no standard size or degree of taper having as yet been adopted.
A twist drill possesses three cutting edges marked a, b, c respectively in Fig. 1043, and of these c is the least effective, because it cannot be made as keen as is desirable for rapid and clean cutting, and therefore necessitates that the drill be given an unusually fine rate of feed as compared with other cutting tools.
The land of the drill—or, in other words, the circumference between the flutes—is backed off to give clearance, as is shown in Fig. 1044, a true circle being marked with a dotted line, and the drill being of full diameter from a to b only. The object of this clearance is to prevent the drill from seizing or grinding against the walls of the hole, as it would otherwise be apt to do when the outer corner wore off, as is likely to be the case.
Twist drills having three and more flutes have been devised and made, but the increased cost and the weakness induced by the extra flutes have been found to more than counterbalance the gain due to an increase in the number of cutting edges, Further, the increase in the number of flutes renders the grinding of the drill a more delicate and complicated operation.
The keenness and durability of the cutting edge of a twist drill are governed by the amount of clearance given by the grinding to the cutting edge, by the angle of one cutting edge to the other, and by the degree of twist of the flute. Beginning with the angle of the front face, we shall find that it varies at every point in the diameter of the drill, being greatest at the outer corner and least at the centre of the drill, whatever degree of spirality the groove or flute may possess. In Fig. 1045, for example, we may consider the angle at the corner c and at the point f in the length of the cutting edge. The angle or front rake of the corner c is obviously that of the outer edge of the spiral c d, while that of the point f is denoted by the line f f, more nearly parallel to the drill axis, and it is seen that the front rake increases in proportion as the corner c is approached, and diminishes as the drill centre or point is approached.
It follows, then, that if the angle of the bottom face of the drill be the same from the centre to the corner of the drill, and we consider the cutting edge simply as a wedge and independent of its angle presentation to the work, we find that it has a varying degree of acuteness at every point in its length. This may be seen from Fig. 1046, in which the end face is ground at a constant angle from end to end to the centre line of the drill, and it is seen that the angle a represents the wedge at point c and the angle b the wedge at the point f in the length of the cutting edge, and it follows that the wedge becomes less acute as the centre of the drill is approached from the point c. If, then, we give to the end face a degree of clearance best suited for the corner c, it will be an improper one for the cutting edge near the drill point; or if we adopt an angle suitable for the point, it will be an improper one for the corner c.
This corner performs the most cutting duty, because its path of revolution is the longest, or rather of the greatest circumference, and it operates at the highest rate of cutting speed for the same reason, hence it naturally wears and gets dull the quickest.
As this wear proceeds the circumferential surface near this corner grinds against the walls of the hole, causing the drill to heat and finally to cease cutting altogether.
For these reasons it is desirable that the angle of the end face, or the angle of clearance, be made that most suitable to obtain endurance at this corner. It may be pointed out, however, that the angle of one cutting edge to the other, or, what is the same thing, its angle to the centre line of the drill, influences the keenness of this corner. In Fig. 1045, for example, each edge is at an angle of 60° to the drill axis, this being the angle given to drills by the manufacturers as most suitable for general use. In Fig. 1047, the angle is 45°, and it will be clearly seen that the corner c is much less acute; an angle of 45° is suitable for brass work or for any work in which the holes have been cored out and the drill is to be used to enlarge them.
Referring again to the angle of clearance of the end faces, it can be shown that in the usual manner of grinding twist drills the conditions compel the amount of clearance to be made suitable for the point of the drill, and therefore unsuitable for the corner c, giving to it too much clearance in order to obtain sufficient clearance for the remainder of the cutting edge. Suppose, for example, that we have in Fig. 1048 a spiral representing the path of corner c during one revolution, the rate of feed being shown magnified by the distance p, and the spiral will represent the inclination of that part of the bottom of the hole that is cut by corner c, and the angle of the end face of the drill to the drill axis will be angle r. The actual clearance will be represented by the angle between the end face s of the drill and the spiral beneath it, as denoted by t. But if we take the path of the point f, Fig. 1045, during the same revolution, which is represented by the spiral in Fig. 1049, we find that, in order to clear the end of the hole, it must have more angle to the centre line of the drill, as is clearly shown, in order to have the clearance necessary to enable the point f to cut, because of the increased spiral. It follows that, if the same degree of clearance is given throughout the full length of the cutting edge, it must be made suitable for the point of the drill, and will therefore be excessive for the corner c.
This fault is inseparable from the method of grinding drills in ordinary drill-grinding machines, which is shown in Fig. 1050, the line a a representing the axis of the motion given to the drill in these machines. It is obvious that the line a a being parallel to the face of the emery-wheel, the angle of clearance is made equal throughout the whole length of the cutting edge. This is, perhaps, made more clear in Fig. 1051, in which we have supposed the drill to take a full revolution upon the axis a a, and as a result it would be ground to the cylinder represented by the dotted lines. We may, however, place the axis on which the drill is moved to grind it at an angle to the emery-wheel face, as at b, Fig. 1052, and by this means we shall obtain two important results: (1) The angle of b may be made such that the clearance will be the same to the actual surface it cuts at every point in the length of the cutting edge, making every point in that length equally keen and equally strong, the clearance being such as it is determined is the most desirable. (2) The clearance may be made to increase as the heels of each end face are approached from the cutting edge. This is an advantage, inasmuch as it affords freer access to the oil or other lubricating or cooling material. If we were to prolong the point of the drill sufficiently, and give it a complete revolution on the axis b, we should grind it to a cone, as shown by the dotted lines in Fig. 1052.
In Fig. 1053 we have a top, and in Fig. 1054 a sectional, view of a conical recess cut by a drill, with a cylinder r lying in the same. p represents in both views the outer arc or circle which would be described by the outer corner, Fig. 1045, of the drill, and q the path or arc described or moved through by the point at f, Fig. 1045, of the drill. At v and w are sectional views of the cylinder r, showing that the clearance is greater at v than at w. The cylinder obviously represents the end of a drill as usually ground. In Figs. 1055 and 1056 we have two views of a cone lying in a recess cut by a drill, the arcs and circles p and q corresponding to those shown in Fig. 1055, and it is seen that in this case the amount of clearance between v and p and between w and q are equal, v representing a cross-section of the cone at its largest end, and w a cross-section at the point where the cone meets the circle q. It follows, therefore, that drills ground upon this principle may be given an equal degree of clearance throughout the full length of each cutting edge, or may have the clearance increase or diminished towards the point at will, according to the angle of the line b in Fig. 1052.
In order that the greatest possible amount of duty may be obtained from a twist drill, it is essential that it be ground perfectly true, so that the point of the drill shall be central to the drill and in line with the axis on which it revolves. The cutting edges must be of exactly equal length and at an equal degree of angle from the drill axis. To obtain truth in these respects it is necessary to grind the drill in a grinding machine, as the eye will not form a sufficiently accurate guide if a maximum of duty is to be obtained. The cutting speeds and rates of feed recommended by the Morse Twist Drill and Machine Company are given in the following table.
The following table shows the revolutions per minute for drills from 1⁄16 in. to 2 in. diameter, as usually applied:—
| Diameter of Drills. |
Speed for Steel. |
Speed for Iron. |
Speed for Brass. |
Diameter of Drills. |
Speed for Steel. |
Speed for Iron. |
Speed for Brass. |
||
| inch. | inch. | ||||||||
| 1⁄16 | 940 | 1280 | 1560 | 1 | 1⁄16 | 54 | 75 | 95 | |
| 1⁄8 | 460 | 660 | 785 | 1 | 1⁄8 | 52 | 70 | 90 | |
| 3⁄16 | 310 | 420 | 540 | 1 | 3⁄16 | 49 | 66 | 85 | |
| 1⁄4 | 230 | 320 | 400 | 1 | 1⁄4 | 46 | 62 | 80 | |
| 5⁄16 | 190 | 260 | 320 | 1 | 5⁄16 | 44 | 60 | 75 | |
| 3⁄8 | 150 | 220 | 260 | 1 | 3⁄8 | 42 | 58 | 72 | |
| 7⁄16 | 130 | 185 | 230 | 1 | 7⁄16 | 40 | 56 | 69 | |
| 1⁄2 | 115 | 160 | 200 | 1 | 1⁄2 | 39 | 54 | 66 | |
| 9⁄16 | 100 | 140 | 180 | 1 | 9⁄16 | 37 | 51 | 63 | |
| 5⁄8 | 95 | 130 | 160 | 1 | 5⁄8 | 36 | 49 | 60 | |
| 11⁄16 | 85 | 115 | 145 | 1 | 11⁄16 | 34 | 47 | 58 | |
| 3⁄4 | 75 | 105 | 130 | 1 | 3⁄4 | 33 | 45 | 56 | |
| 13⁄16 | 70 | 100 | 120 | 1 | 13⁄16 | 32 | 43 | 54 | |
| 7⁄8 | 65 | 90 | 115 | 1 | 7⁄8 | 31 | 41 | 52 | |
| 15⁄16 | 62 | 85 | 110 | 1 | 15⁄16 | 30 | 40 | 51 | |
| 1 | 58 | 80 | 100 | 2 | 29 | 39 | 49 | ||
To drill one inch in soft cast iron will usually require: For 1⁄4 in. drill, 125 revolutions; for 1⁄2 in. drill, 120 revolutions; for 3⁄4 in. drill, 100 revolutions; for 1 in. drill, 95 revolutions.
The rates of feed for twist drills are thus given by the same Company:—
| Diameter of drill. |
Revolutions per inch depth of hole. |
|||||||
| 1⁄16 | inch | 125 | ||||||
| 1⁄4 | „ | „ | ||||||
| 3⁄8 | „ | 120 to | 140 | |||||
| 1⁄2 | „ | „ | „ | |||||
| 3⁄4 | „ | 1 inch | feed per | minute | ||||
| 1 | „ | „ | „ | „ | ||||
| 1 | 1⁄2 | „ | „ | „ | „ | |||
Taking an inch drill as an example, we find from this table that the rate of feed is for iron 1⁄100th inch per drill revolution, and as the drill has two cutting edges it is obvious that the rate of feed for each edge is 1⁄200th inch per revolution. But it can be shown that this will only be the case when the drill is ground perfectly true; or, in other words, when the drill is so ground that each edge will take a separate cut, or so that one edge only will cut, and that in either case the rate of feed will be diminished one-half.
In Fig. 1057, for example, is shown a twist drill in which one cutting edge (e) is ground longer than the other, and the effect this would produce is as follows. First, suppose the drill to be fed automatically, the rate of feed being 1⁄100th inch, and the whole of this feed would fall on cutting edge e, and, being double what it should be, would in the first place cause the corner c to dull very rapidly, and in the second place be liable to cause the drill to break when c became dull.
In the second place the drill would make a hole of larger diameter than itself, because the point of the drill will naturally be forced by the feed to be the axis or centre of cutting edge revolution, which would therefore be on the line b b. This would cause the diameter of hole drilled to be determined by the radius of the cutting edge e rather than by the diameter of the drill. Again, the side of the drill in line with corner c would bind against the side of the hole, tending to grind away the clearance at the corner c, which, it has been shown, it is of the utmost importance to keep sharp. But assuming 1⁄200th inch to be the proper feed for each cutting edge, and the most it can carry without involving excessive grinding, then the duty of the drill can only be one-half what it would be were both cutting edges in action.