Modern Machine-Shop Practice Part 120

[Ill.u.s.tration: Fig. 1716.]

Fig. 1716 represents the flat drill, which has three cutting edges, A, B, and C. The only advantages possessed by the flat drill are that it will stand rougher usage than the twist drill, and may be fed faster, while it can be more easily made. Furthermore, when the work is unusually hard the flat drill can be conveniently shaped and tempered to suit the conditions.

The drill is flattened out and tapered thinnest at the point C. The side edges that form the diameter of the drill are for rough work given clearance, but for finer work are made nearly cylindrical, as in the figure.

The flattening serves two purposes: first, it reduces the point of the drill down to its proper thinness, enabling it to enter the metal of the work easily, and secondly, it enables the cuttings to pa.s.s upward and find egress at the top of the hole being drilled.

The cutting edges are formed by grinding the end facets at an angle as shown, and this angle varies from 5 for drilling hard metal, such as steel, to 20 for soft metal, such as bra.s.s or copper.

[Ill.u.s.tration: Fig. 1717.]

The angle of one cutting edge to the other varies from 45 for steel to about 35 or 40 for soft metals. The object of these two variations of angles is that in hard metal the strain and abrasion is greatest and the cutting edge is stronger with the lesser degree of angle, while in drilling the softer metals the strain being less the cutting edge need not be so strong and the angles may be made more acute, which enables the drill to enter the metal more easily. The most imperfect cutting edge in a drill is that running diagonally across the point, as denoted by A in Fig. 1717, because it is less acute than the other cutting edges, but this becomes more acute and, therefore, more effective, as the angles of the facets forming it are increased as denoted by the dotted lines in the figure. It is obvious, however, that the more acute these angles the weaker the cutting edge, hence an angle of about 5 is that usually employed.

It is an advantage to make the cutting edge at A, Fig. 1717, as short as possible, which may be done by keeping the drill point thin; but if too thin it will be apt either to break or to operate in jumps (especially upon bra.s.s), drilling a hole that is a polygon instead of a true circle.

The cutting edges should not only stand at an equal degree of angle to the axial line of the drill, but should be of equal lengths, so that the point of the drill will be in line with the axial line of the drill. If the drill runs true the point will then be in the axial line of rotation, and the diameter of hole drilled will be equal to the diameter of the drill.

If, however, one cutting edge is longer than the other the hole drilled will be larger than is due to the diameter of the drill.

[Ill.u.s.tration: Fig. 1718.]

Suppose, for example, the drill to be ground as in Fig. 1718, the cutting edge F being the longest and at the least angle, then the point G of the drill, when clear of the work, will naturally revolve in a circle around the axial line H of the drill's rotation. But when the drilling begins, the point of the drill meets the metal first and naturally endeavours to become the centre of rotation, drilling a straight conical recess, the work moving around with the point of the drill. If the work is prevented from moving, either the drill will spring or bend, the point of the drill remaining (at first) the centre of rotation at that end of the drill, or else the recess cut by the drill will be as in the figure, and the hole will be larger in diameter than the drill.

[Ill.u.s.tration: Fig. 1719.]

[Ill.u.s.tration: Fig. 1720.]

If, however, the drill is ground as shown in Fig. 1719, the edge E being nearest to a right angle to the axial line H of the drill, the drilling will be performed as shown in the figure, the edge E cutting the cone L, the edge F serving simply to enlarge the hole drilled by E. Here, again, if the work is held so that it cannot move, the point of the drill will revolve in a circle, and in either case, so soon as the point of the drill emerges the diameter of the hole drilled will decrease, the finished hole being conical as shown in Fig. 1720 at A.

It may be remarked that the eye of the workman is (for rough work, such as tapping or clearing holes) sufficient guide to enable the grinding of the drill true enough to partly avoid the conditions shown in these two figures (in which the errors are magnified for clearness of ill.u.s.tration), because when the want of truth is less in amount than the thickness of the drill point, the centre of motion of the drill point when the drill has entered the work to its full diameter becomes neither at the point of the drill nor in the centre of its diameter, but intermediate between the two.

[Ill.u.s.tration: Fig. 1721.]

[Ill.u.s.tration: Fig. 1722.]

Thus, in Fig. 1721, A is the centre of the diameter of the drill, but the cutting edge C being shorter than D throws the point of the drill towards E, hence the extra pressure of D on the incline of the recess it cuts, over the like pressure exerted by C tends to throw the centre of rotation towards E, the natural endeavor of the drill point to press into the centre of the recess acting in the same direction. This is in part resisted by the strength of the drill, hence the centre of rotation is intermediate as at B in figure. The dotted circle is drawn from the axial line of the drill as a centre, while the full circle is drawn from B as a centre. The result of this would be that the point of the drill would perform more duty than is due to its thickness, and the recess cut would have a flat place at the bottom, as shown in Fig. 1722 at O. This, from the want of keenness of the cutting edge running diagonally across the drill point, would cause the drill to cut badly and require more power to drive and feed.

[Ill.u.s.tration: Fig. 1723.]

The edges at the flat end of the drill, as at A, A in Fig. 1723, should have a little clearance back from the cutting edge though they may be left the full circle as, at A, A, but in any event they should not have clearance sufficient to form them as at B, B, Fig. 1723, because in that case the side edges C, C would cut the sides of the hole. In large drills, especially, it is necessary that the edges have but little clearance, and that the form of the clearance be as shown in Fig. 1044, with reference to twist drills. When no edge clearance whatever is given the edges act to a certain extent as guides to the drill, but if the drill is not ground quite true this induces a great deal of friction between the edges of the drill and the side of the hole.

In any case of improper grinding the power required to drive the drill will be increased, because of the improper friction induced between the sides of the drill and the walls of the hole.

For use on steel, wrought iron, and cast iron the lip drill shown in Fig. 1724 is a very efficient tool. It is similar to the flat drill but has its cutting edge bent forward. It possesses the keenness of the twist drill and the strength of the flat drill, but as in the case of all drills whose diameters are restored by forging and hand grinding, it is suitable for the rougher cla.s.ses of work only, and requires great care in order to have it run true and keep both cutting edges in action.

It is sometimes attempted to give a greater cutting angle to a flat drill by grinding a recess in the front face, as at A in Fig. 1725, but this is a poor expedient.

Fig. 1726 represents what is known as the t.i.t drill. It is employed to flatten the bottoms of holes, and has a t.i.t T which serves to steady it.

The edges A, B of this drill may be turned true and left without clearance, which will also serve to steady the drill. The t.i.t T should be tapered towards the point, as shown, which will enable it to feed more easily and cut more freely. The speed of the drill must be as slow again as for the ordinary flat drill, and not more than one-third as fast as the twist drill.

To enable a drill to start a hole in the intended location the centre-punch recess in the centre of that location should be large enough in diameter at the top to admit the point of the drill, that is to say, the recess should not be less in diameter at the top than the thickness of the drill point.

[Ill.u.s.tration: Fig. 1724.]

[Ill.u.s.tration: Fig. 1725.]

[Ill.u.s.tration: Fig. 1726.]

[Ill.u.s.tration: Fig. 1727.]

If the drill does not enter true the alteration is effected as shown in Fig. 1727, in which A represents the work, B a circle of the size of the hole to be drilled, and C the recess cut by the drill, while D is a recess cut with a round-nosed chisel, which recess will cause the drill to run over in that direction.

[Ill.u.s.tration: Fig. 1728.]

It is a good plan when the hole requires to be very correctly located to strike two circles, as shown in Fig. 1728, and to define them with centre-punch marks so that the cuttings and oil shall not erase them, as is apt to be the case with lines only. The outer circle is of the size of hole to be drilled, the inner one serves merely as a guide to true the drilling by.

If the work is to be clamped to the work table an alteration in the location of the recess cut by the drill point may be made by moving the work. In this case the point of the drill may be fed up so as to enter into and press against the centre-punch mark made in the centre of the location of the hole to be drilled, which, if the drill runs true will set the work true enough to clamp it by. The alteration to the recess cut by the drill when first starting to bring the hole in its true position should be made as soon as a want of truth is discernible, because the shallower the recess the more easily the alteration may be made.

Sometimes a small hole is drilled as true to location as may be, and tested, any error discovered being corrected by a file; a larger drill is then used and the location again tested, and so on; in this way great precision of location may be obtained.

The more acute angle the cutting edges form one to the other, or in other words, the longer the cutting edges are in a drill of a given diameter, the more readily the drill will move over if one side of the recess be cut out as in Fig. 1727, and from some experiments made by Messrs. William Sellers and Co., it was determined that if the angle of one cutting edge to the other was more than 104 the drill would cease to move over.

In drilling wrought iron or the commoner qualities of steel the drill should be liberally supplied with either water or oil, but soapy water is better than pure. This keeps the drill cool and keeps the cutting edge clean, whereas otherwise the cuttings under a coa.r.s.e feed are apt to stick fast to the drill point if the speed of the drill is great.

Furthermore, under excessive duty the drill is apt to become heated and softened.

For cast steel oil is preferable, or if the steel be very hard it will cut best dry under a slow speed and heavy pressure.

For bra.s.s and cast iron the drill should run dry, otherwise the cuttings clog and jam in the hole. When the drill squeaks either the cutting edge is dulled and the drill requires regrinding, or else the cuttings have jammed in the hole, and either defect should be remedied at once.

As soon as the point of the drill emerges through the work the feed should be lessened, otherwise the drill is apt to force through the weakened metal and become locked, which will very often either break or twist the drill. This may be accomplished when there is any end play to the drilling machine spindle by operating the feed motion in a direction to relieve the feed as soon as the point of the drill has emerged through the bottom of the hole, thus permitting the weight of the spindle to feed the drill. In a drilling machine, however, in which the weight of the spindle is counterbalanced, the feed may be simply reduced while the drill is pa.s.sing through the bottom of the hole.

Drills for work of ordinary hardness are tempered to an orange purple, but if the metal to be cut is very hard a straw color is preferable, or the drill may be left as hard as it leaves the water; that is to say hardened, but not tempered. In these cases the speed of the drill must be reduced.

To a.s.sist a drill in taking hold of hard metal it is an excellent plan to jag the surface of the metal with a chisel which will often start the drill to its cut when all other means have failed. It is obvious from previous remarks that the harder the drill the less the angle of the end facets.

In cases of extreme hardness two drills may with advantage be used intermittently upon the same hole; one of these should have its cutting edges ground at a more acute angle one to the other than is the case with the other drill, thus the cutting edge will be lessened in length while the drill will retain the strength due to its diameter, so that a maximum of pressure may be placed upon it. When one drill has cut deep enough to bring its full length of cutting edge into action, it may be removed and the other drill employed, and so on.

The drill (for hard steel) should be kept dry until it has begun to cut, when a very little oil may be employed, but for chilled cast iron it should be kept dry.

Small work to be drilled while resting upon a horizontal table may generally be held by hand, and need not therefore be secured in a chuck or to the table, because the pressure of the drill forces the work surface to the table, creating sufficient friction to hold the work from rotating with the drill. For large holes, however, the work may be secured in chucks or by bolts and plates as described for lathe and planer work, or held in a vice.

The following table for the sizes of tapping holes is that issued by the Morse Twist Drill and Machine Co. In reply to a communication upon the subject that company states. "If in our estimate the necessary diameter of a tap drill to give a full thread comes nearest to a 1/64 inch measurement, we give the size of the drill in 64ths of an inch. If nearest to a 32nd size of drill we give the drill size in 32nds of an inch."

In the following table are given the sizes of tapping drills, to give full threads, the diameters being practically but not decimally correct:--

-------+-----------------+----------------------- Dia- | Number | Drill for meter | threads | [V]-thread.

of tap.| to inch. | -------+-----------------+----------------------- 1/4 |16 18 20| 5/32 5/32 11/64 9/32 |16 18 20| 3/16 13/64 13/64 5/16 |16 18 --| 7/32 15/64 -- 11/32|16 18 --| 1/4 17/64 -- 3/8 |14 16 18| 1/4 9/32 9/32 13/32|14 16 18| 19/64 21/64 21/64 7/16 |14 16 --| 21/64 11/32 -- 15/32|14 16 --| 23/64 3/8 -- 1/2 |12 13 14| 3/8 25/64 25/64 17/32|12 13 14| 13/32 27/64 27/64 9/16 |12 14 --| 7/16 29/64 -- 19/32|12 14 --| 15/32 31/64 -- 5/8 |10 11 12| 15/32 1/2 1/2 21/32|10 11 12| 1/2 17/32 17/32 11/16|11 12 --| 9/16 9/16 -- 23/32|11 12 --| 19/32 19/32 -- 3/4 |10 11 12| 19/32 5/8 5/8 25/32|10 11 12| 5/8 21/32 21/32 13/16|10 -- --| 21/32 -- -- 27/32|10 -- --| 11/16 -- -- 7/8 | 9 10 --| 45/64 23/32 -- 29/32| 9 10 --| 47/64 3/4 -- 15/16| 9 -- --| 49/64 -- -- 21/32| 9 -- --| 51/64 -- -- 1 | 8 -- --| 13/16 -- -- 1-1/32 | 8 -- --| 53/64 -- -- 1-1/16 | 8 -- --| 55/64 -- -- 1-3/32 | 8 -- --| 57/64 -- -- 1-1/8 | 7 8 --| 29/32 15/16 -- 1-5/32 | 7 8 --| 15/16 31/32 -- 1-3/16 | 7 8 --| 31/32 1 -- 1-7/32 | 7 8 --|1 1-1/32 -- 1-1/4 | 7 -- --|1-1/32 -- -- 1-9/32 | 7 -- --|1-1/16 -- -- 1-5/16 | 7 -- --|1-3/32 -- -- 1-11/32| 7 -- --|1-1/8 -- -- 1-3/8 | 6 -- --|1-1/8 -- -- 1-13/32| 6 -- --|1-5/32 -- -- 1-7/16 | 6 -- --|1-5/32 -- -- 1-15/32| 6 -- --|1-3/16 -- -- 1-1/2 | 6 -- --|1-15/64 -- -- 1-17/32| 6 -- --|1-9/32 -- -- 1-9/16 | 6 -- --|1-9/32 -- -- 1-19/32| 6 -- --|1-5/16 -- -- 1-5/8 | 5 5-1/2 --|1-9/32 1-5/16 -- 1-21/32| 5 5-1/2 --|1-5/16 1-11/32 -- 1-11/16| 5 5-1/2 --|1-11/32 1-3/8 -- 1-23/32| 5 5-1/2 --|1-3/8 1-13/32 -- 1-3/4 | 5 -- --|1-13/32 -- -- 1-25/32| 5 -- --|1-7/16 -- -- 1-13/16| 5 -- --|1-15/32 -- -- 1-27/32| 5 -- --|1-1/2 -- -- 1-7/8 | 4-1/2 5 --|1-17/32 1-17/32 -- 1-29/32| 4-1/2 5 --|1-9/16 1-9/16 -- 1-15/16| 4-1/2 5 --|1-19/32 1-19/32 -- 1-31/32| 4-1/2 5 --|1-5/8 1-5/8 -- 2 | 4-1/2 -- --|1-21/32 -- -- -------+-----------------+-----------------------

-------+---------------------+-------------------- Dia- | Drill for U.S.S. | Drill for meter | thread. | Whitworth of tap.| | thread.