Shearing And Punching

Shearing is explained in the early part of this chapter, and the action is the same in punching, as both edges cut in each case, and both act on the two sides of the metal. This can be seen by referring to Fig. 1, No. 6. It will be noticed, however, that the punch itself has parallel sides while the die has a very slight clearance to prevent the punch binding. The use of the punch in smiths' shops, however, is quite different, as when a piece of iron is brought to a bright red heat and then a hole is punched in it by driving a punch which tapers a little outwardly towards the top, through the metal with the aid of a hammer, the hot metal is first of all pressed from underneath the face of the punch and then the punch is usually driven half-way through from each side so forcing the metal outwardly and enlarging the hole. This action makes a swelling on the bar where the hole has been punched, so retaining practically all of the material and hardly weakening the bar to any extent. In the action of punching as illustrated in Fig. 1, No. 6, the bar is weakened according to the amount of material removed, but it is quicker and a machine does the work.

Drilling And Turning

Drilling, turning, and boring are three different operations. In drilling the work is stationary and the drill revolves, in turning the work revolves, and in boring the work is fixed to the saddle of a lathe which travels in a given direction, while the boring tool is fixed in a boring bar which is placed between the centres of a lathe and revolves.

This is the general practice, but of course there are many variations; the action of boring is similar to drilling, though one could not drill a hole with a boring tool. Drilling differs in principle from almost every other operation in metal cutting, as it cannot be guided in any given direction and is supported by the bearing of the cutting edges of the drill against the material. The ordinary flat or diamond-pointed drill (Ch. VI, f. 6) is a most useful tool, because it can be easily made and tempered, and will withstand very rough usage, but it does not cut in the true sense but scrapes, and requires great pressure to force it into the metal, and cuts very slowly.

Fig. 2.-Position when filing.

If the lip A (Fig. 1, No. 6) is longer than lip C the diameter of the hole will be twice the radius of A, as this drill is guided solely by its point, and the hole so drilled will bear no relation to the diameter B. Another disadvantage of this form of drill is that if the metal is spongy or has blow holes in it where a hole is being drilled it will run into the softest part, or into the space away from the harder metal, so giving an uneven or slanting hole.

For rapid and good work twist drills, brought to perfection by Mr. Morse of America, are universally adopted. This drill is illustrated in Ch. XVI, f. 22. To obtain the best results with this drill it should be ground on an emery wheel with an automatic twist drill grinding attachment, for unless the lips are equal a larger hole is made than the diameter of the drill, as illustrated at B in Fig. 1, No. 7. If the angles of both the lips (which should be 1200, that is 6o° each side of the centre of the drill) are not the same, one lip does all the work, consequently it soon dulls and requires regrinding. Another important point is that if the cutting edge or lip C of the drill is not the highest point or has not the proper amount ofclearance,it will refuse to cut, or if it is too high it gets drawn into the metal and is then broken off.

When the point of the drill protrudes through the stock being drilled, but is not quite through, extra care must be taken or the drill will catch in the fash of the hole and get broken. The spiral grooves of the twist drill provide the

"rake", and this causes the chips to curl out of the way instead of being pulverized as is the case with the flat drills and less power is required. Some twist drills have oil holes through the centre so that the cutting edges can be lubricated. The sense of feeling is or can be utilized more in drilling holes than in any other operation, and so the operator can tell whether the drill is doing the work as it should, so avoiding accidents.