The term punching is generally understood to include all operations of cutting out blanks from sheet metal The shape cut away is known as the blank, whilst the unavoidable amount of metal sheet left afterwards is only fit for scrap. Some ingenuity is required to so arrange the sequence of successive punchings that the amount of scrap shall be a minimum. _

Shearing is the operation of cutting up sheet metal, flat bars, etc, into lengths, strips, etc. The action of a punch is that of shearing, and a punch may be regarded as an endless shearing tool.

The load that comes upon a punch is wholly compressive, and very heavy. The limiting thickness of metal that can be punched without heating the punch, being dependent upon the relative resistances of the die to compression, and of the plate to shearing.

Fig. 65.

Let fe = crushing stress in pounds per square inch of punch; d - diameter of the punch in inches; D = diameter of the hole punched in the plate in inches; fc = shearing stress in pounds per square inch at which the plate yields to the punch; t = thickness of plate in inches.

Then at the instant of rupture

The load on the punch = maximum resistance offered by the plate to punching

π.d2/4 x f c = π. D.t.fs d2 . fe = 4.D.t f8 if the difference between d and D is so small that it may be neglected, then putting d = D we have d/2=4.f/fe;

Taking the case of best Yorkshire iron plates, then fs is, according to Professor Goodman, about 19 tons as the upper limit, and for tempered cast steel fc may be taken at about 85 tons, the exact figure, however, depends upon the amount of hardening and the quality of the steel, then using these values the ratio

That is, the theoretical limit of thickness is, in this case, equal to the diameter x by 1.118, which agrees with the practical rule which puts the maximum thickness as being equal to the diameter of the hole punched. The limiting diameter of any hole in a plate t inches thick of any material is:

In this connection the following figures will be useful in connection with the above formula.

From experiments that have been made, it is found that the area surrounding the hole in punched boiler plates is injured by the severe stresses caused by the pressure of the die, and the extent of this injury is given by Professor Goodman as about 1/10th of an inch deep round the hole. For this reason, where plates have been punched or cut by shears it is customary to plane the edges of such plates and ream out the punched holes. The pressure on the plates makes them brittle, and if the plates be bent, cracks develop in the crushed parts.

Fig . 66.

To reduce the magnitude of the total load, it is common practice to give a slope to one of the cutters in a shearing machine, as shown in fig. 66, the amount of this dip being from 5 deg. to 15deg. This dip has also the effect of extending the time during which the cutting is going on over a longer interval, with the result of reducing the stresses generally on the machine. The angle of the cutting edges, and the position of the plate to be operated upon, is shown clearly in the figure.

Fig. 67.

The details of punching tools, it may be noted first that whilst cutting-out punches are given shear for the purpose first referred to in shearing cutters, yet generally the ends of the punches are faced off flat-that is, at right angles to the line of motion, thus there is no shear, and the cutting angle is 90 deg. An example is shown in fig. 67, where the cutting edge D is equal to 90 deg., whilst in fig. 68 is a similar punch, but the central part of the face is cut away, making the cutting edge D less than 90 deg. This is of some advantage when the punch becomes worn, and requires upsetting to enable the original diameter to be regained.

Fig. 68.

For tools that are to cut blanks from thin sheets of brass and tin, particularly those of large area, it is usual to harden and temper the lower die to a straw colour, and leave the 6wp punch comparatively soft; i.e., it would be hardened, but 1et down in tempering to a blue colour. When such a punch has worn, the punch can be hammered up around its cutting edge, and then forced into its own die, to bring it up to correct size again. This is necessary when the metal sheet is thin, say less than 1/32 in., then the cutting tools to do good work require to be a good fit, but with thicker sheets there must be some clearance varying from 1/100 in. to about 1/16 in. according to the nature of the work. The result of any freedom when working on thin plates is to leave a ragged edge or fraze on the blanks. If the blanks have to be cut from steel plates, the punch must be hardened and tempered the same as the die if uniformity of size is necessary, but this hardened punch must have some freedom in the lower die, or the friction will increase the rate of wear, and entail extra work upon the tool-maker in grinding, etc. The amount of clear ance required depends upon the size of the punch, the thickness of the plate, and the nature of the work to be done; in some cases 1/100 in.will be sufficient, whereas,if it is boiler plate, or girder work, the clearance may be as much as 1/16 in. The effect of clearance upon the size of the blanks is to render them taper, their top side will be the same diameter as the punch, and the lower face will correspond to the diameter of the hole in the die.

Pouches may be made solid, in which case they require some forging, or they may be made up in the tool room without the aid of the smith. Each plan has its advantages, the built-up tool requiring to be well made or it will lack rigidity, and fail to do good work in consequence. In punches of small diameter they are drawn down from a larger bar of tool steel, the upper end of a suitable size to fit the ram; the lower end the diameter of the hole or blank required. Sometimes the punch is left perfectly parallel, but in many cases there is a slight taper which may tend to slightly increase the diameter of the punch as it wears back or may reduce its diameter.