It is sometimes convenient to build up a punch of large diameter for cutting out blanks, and one such composite punch is shown in figs. 69 and 70. In this example the tool consists of three parts, the cutting part F formed of hardened steel, attached to the part B (which may be a casting) by means of three or more small screws. The shank S is parallel and enters the ram, whilst H enters the casting B. This tool can be made without the aid of a toolsmith, and several different sizes of cutter F can be made to suit B, the hole in F being machined, so as to fit exactly concentrically without the assistance of the screws. The casting B may be attached to the shank H by heating and shrinking in the usual way.

Fig. 69.

A very common method of fixing punches to the ram is shown in fig. 71, where the shank is screwed into the ram, flats being provided so that it may be tightened by means of a spanner. A better method of fixing is shown in fig. 72, where the shank of the punch is turned parallel and made to fit the hole in the ram, and secured in its place by means of a set-screw. This is necessary in order to prevent its being pulled out on the up stroke of the ram, when considerable force is often required to withdraw the punch from the hole it has made.

Fig. 7O.

Fro. 71.

Fig. 72.

A badly-fixed punch means bad work and many broken tools, because when the pressure comes upon the tool it will"kick"or spring aside, if it does not press evenly upon the material from which the blanks are being formed. The load that comes upon the punch is very great even when but small holes are being punched, and when this is fully realised the necessity for good workmanship will be readily admitted. Dr.Anderson-- gives some interesting figures obtained by means of a series of careful experiments in which the load upon the punch was gradually increased until the hole was punched.

In the Transactions of the Institution of Mechanical Engineers of 1858, particulars are given of some experiments made with punching presses, from which we take the following results:-

Mr. Hick, of Bolton, found the load required to force a punch 8 in. diameter through a plate 3 1/2 in. thick was 2,000 tons, or 22.74 tons per square inch of area.

The load upon the punch can be estimated roughly by multiplying together the diameter of the punch, the thickness of the plate, by the stress per square inch of area, and 3.14. Stated algebraically, if D = diameter of punch in inches, t = the thickness of plate in inches, f = the resistance offered by metal in tons per square in., and the load upon the punch in tons = L, then

L = d x t x f x 3.14.

The figures given above show that the total load upon the punch is very great, and unless it is properly secured the tool must be very severely strained. These figures also show-that the load upon a punch is approximately equal to the tensile strength of a bar, the area of whose cross-section is the same as the area of the sides of the hole. Thin plates .appeared in many instances to require a greater load than was necessary in the case of thick plates, and the reason for this probably will be found due to the skin effect of the hard exterior surface of rolled plates.

It must be remembered that the operation of punching and shearing is not strictly cutting, but rather a detruding action. The time occupied in punching is due to the elasticity of the material, the punch, and the parts of the press. When this elasticity and slackness are all taken up, then the resistance of the material is at once overcome, and the metal is detruded, or pushed off, rather than cut away. After the operation is completed the resilience of the machine in assuming its normal condition gives rise to the well-known jerk or knock which is heard immediately the blank is detached.

The behaviour of materials under pressure has been the subject of many experiments, and the plastic properties of solids are now fairly well known. If a solid substance be subjected to a stress gradually increased, careful observation will show that at first there is a period of nearly perfect elasticity, when the variations of length are exactly proportional to the stress, and if the stress be removed the body returns to its original length, If, however, the load applied be increased still further, then a second period is reached when the alteration of length is no longer proportional but rapidly increases with the augmented loads. If the load be removed after this second stage is reached the body will not regain its original length, but will remain permanently stretched. If this second period of deformation be further considered, it will be seen that it tends towards that condition in which a force sufficiently great would go on stretching the material, as occurs in the operation of drawing lead wire. This particular condition M. Tresca first called the period of fluidity. In certain materials, such as glass, this period practically disappears, but in the case of malleable metals it is very much extended. M. Tresca found from his experiments that large plastic deformation is not associated with any sensible change of density. The following examples will serve to illustrate the flow of metal that takes place under the action of the punch, and are taken from the results of the experiments already referred to. Suppose a block to be supported on the die of a machine, and to be perforated by a punch. If the blank produced is carefully measured and compared with the thickness of the block, in general it will be found to be less, that is, h is less than H (see fig. 73). In one case H was 10 centimetres, the hole punched was 2 centimetres diameter, and the thickness of the blank was only 3 centimetres. The density of the blank was determined precisely, and found to be the same as that of the original block, consequently during the act of punching 70 per cent of the metal must have flowed laterally into the block.