Dies and punches are of infinite variety and shape, special forms being devised daily by engineers in order to produce some new shape that may have to be manufactured. It would be absolutely useless to attempt anything here in the way of a complete exposition of the subject, and only the most commonly used forms will be dealt with, pointing out the leading features of the design, so that some idea of the conditions that have to be fulfilled will be brought before the notice of the reader.

The die is the lower tool that is fixed to the bed of the press, and has a_hole pierced through it, so that the blanks forced out by the punch can fall away; or it may be hollowed out to suit the shape required, where the work to be done is pressing, or stamping some special shape from sheet metal; or there may be combination of form, with one or more holes to be punched.

It will be obvious, therefore, at the outset that the die will have to sustain a very heavy load, during the time that the operation is being carried out, without suffering any deformation. In man}* instances the blanks formed, or the holes punched, have to be true within very small limits, and the dies must not need continuous examination to discover where the wear, etc, is such that the work produced will be rejected, but should retain its efficiency for a long time.

In order to retain its shape the effective portion of the die. must be made of the best steel, suitably hardened to meet the special requirements of the operation to be executed; and to meet the great strains imposed, its mass must be very large. But since the cost of suitable steel is much greater than that of iron, it is the practice frequently to fit the steel die proper into a suitable block of wrought iron. The weight of the iron put into the block must be proportioned to the severity of the strains that may be expected in actual work, and for which no mathematical rules are easily applicable, the designers experience alone being usually depended upon. There is a certain size at which the cost of building up the compound die will exceed the saving effected by using iron in the place of steel. In punching presses, for example, for ordinary sizes there is not any advantage gained by making the outer part of the die of iron. In some cases the cast-iron frame of the machine itself is quite sufficient, because its mass may be easily made comparatively very great.

1st.

2nd.

3rd.

4th.

Fig. 53.

The process usually followed in the manufacture of a compound die is illustrated in fig. 53, the first stage being the formation by the smith of a hollow in the wrought-iron bed; next, a piece of steel is inserted and the iron closed as tightly upon it as possible. The third stage shows the block with the iron and steel firmly welded together. The hollow in the steel centre is formed by the tool smith to reduce the work of the tool maker when shaping the die accurately to the specified dimensions and contour, as shown in the fourth stage. Fig. 54 shows three other different shapes of stamp dies. The dotted line indicates the depth of metal left by the tool smith for machining purposes in the tool room; whilst fig. 55 illustrates a drawing die in which the steel centre has a wrought-iron ring surrounding it, the hole in the steel centre being afterwards bored out accurately by the tool maker, where the work to be done is that of cutting out blanks to some special shape; then the steel is worked into the iron bed by the smith, the face and hole being shaped by the tool maker, so that the upper edge of the hole in the steel centre forms with the punch a pair of cutting edges. Fig. 56 shows two methods of making beds intended for cutting out long round-ended blanks. The bed A has an iron bottom and steel top, which, after being forged, is machined nearly all over; whereas, in the case of B, the bed is cut off a solid bar of steel by the tool maker and machined, thus avoiding the cost of forging. The hole to receive the punch is clearly shown in fig. B. The holes should increase slightly, so that there is sufficient clearance for the blanks to fall freely immediately they are formed, or otherwise difficulties may arise.

Fig. 55.

Where possible the correctness of the die is tested before hardening, but this cannot always be done, because the material will be unable to withstand the stress in its un-hardened state. When the dies are correct they are hardened and tempered to a light straw colour. This process of hardening and tempering dies so that they are able to successfully withstand the rough usage of the press is one of great importance, depending ultimately upon the care exercised by the tool maker entrusted with the work. The great point apparently is to heat the articles to be hardened as uniformly as possible to the required temperature, and then to cool them more or less quickly and evenly, so that the internal stresses shall be as equal as possible. Cracks result from failure of the material, due to the excessive internal stress; whilst fracture soon after work is commenced indicates that the stress due to the external load sufficed, when added to the internal stresses already existing, to strain the material beyond its"yield point," and failure results. At other times failure only occurs after many applications of the load, the"fatigue"reducing the ultimate strength more or less quickly according to the magnitude of the internal stresses in the material due to the hardening and tempering processes.

Fig. 56.

The practice of the careless smith is to put the tool to be hardened into the fire, so that whilst one portion is rapidly warmed up another part is out of the fire and almost cold. The tool is presently turned over to warm up the colder part, when the heated side is cooled down; finally the tool is rapidly twisted about, so that as far as the eye can judge it is properly heated. If a fire is used, then the tool should be actually covered entirely over, and the process of heating up carried out slowly and evenly throughout the whole mass, so that the temperature is even throughout. Probably the best way to ensure this regular heating is to use a gas-fired furnace or muffle, which can easily be arranged to maintain the correct temperature by adjusting the gas supply, and in this way there is no risk of burning the steel if the workman should be unable to remove it when ready for cooling. Another incidental advantage of the gas-fired furnace is its economy in working both as regards cost for labour in handling the fuel and the increased cleanliness due to the freedom from smoke and dust, which are inseparable where the ordinary smiths' fires are used.

The dies should be left as soft as the nature of the work will permit as the internal stresses are proportionately reduced, leaving a wider margin of safety in actual work, and a proportionately longer life, before repairs or renewals are required in order to preserve the necessary accuracy of form.

For the manufacture of dies and punches a careful, selection of the steel employed is of the utmost importance, for if the quality of the steel employed is not suitable, it will be impossible to make satisfactory tools, notwithstanding the utmost care exercised by all concerned during their production. The amount of hardening possible depends upon the quantity of carbon present, and for the purpose of tools that will stand a great pressure, as in the case of dies, etc., the best proportion of carbon is about 3/4 per cent.. Increasing the amount of carbon to 7/8 per cent gives an exceedingly tough steel suitable for"cold sets,"and similar tools that have to withstand very heavy blows. For chisels and similar tools, where ability to withstand heavy usage, and yet to harden sufficiently to give a cutting edge, the steel should contain about 1 per cent of carbon. Increasing the amount of carbon beyond 1 per cent yields steels having greater hardening powers, and consequently increasingly brittle, whilst the metal requires much more care to work satisfactorily.

Fig. 57.

The upper die used in stamping out work is shaped to suit the inner form of the article to be produced, and is usually termed"the force."It is made of various materials, to suit the special conditions to be satisfied. Tin. brass, iron, and steel are the materials most commonly used. The force is held in the stamp hammer in several ways. Fig. 57 shows the face of the hammer notched or grooved, into which the force is pressed. In fig. 58 the force is keyed in the, groove, whilst in figs. 59 and 60 it is secured by means of a screw S pressing on to the spindle as in an ordinary driilling machine.

The operation of making a tin force is illustrated in figs. 61 and 62. In the first of these C is the clay mould formed round the top of the tool, into which the molten tin T has been poured, thus forming a casting for the force. The head of the hammer K-the face being ready notched-.is then carefully lowered, the tin filling the interstices in the face of K, and .securing the force accurately in place.

Fig. 58.

The brass force is made by using a tin force as a pattern to make a mould from in the foundry as in the ordinary course.

Iron and steel forces are formed first by forcing the hammer face K into the rough force F, fig. 63, the iron being heated for this purpose. When this has been done the force is again re-heated, and repeatedly forced into the tool until it has been squeezed into the exact form. Where the shape of the force has been much upset by the process of forming the"jag,"time may be saved in the last operation if some of the metal at a, b, fig. 64, be roughly turned off. This is always true when the die is a deep one. The turning operations are facilitated by the use of a template. Care should be taken in these operations not to let the force get cold whilst it is being formed, or the lower die will be damaged. In fig. 65 the bar of iron B is placed above the die to protect it whilst the force is having the jag formed in its upper face.

Fig. 59.

Fig.

Fig. 63.

Fig. 64.