Fig. 73.

In another case where the punching was only partially performed, as shown in fig. 74, the thickness of the blank was easily seen to be less than the penetration of the punch without having recourse to measurement. A further experiment may be mentioned, in which a series of lead discs were placed upon each other and then punched. After l>eing cut in two the appearance of the block and blank produced was as shown in fig. 75. The blank consists of an equal number of discs, but their thickness has been changed, and it will be seen from the figure that the depth of each disc differs, the thickness of each gradually increasing from the top to the bottom of the blank. Of the metal forced to flow laterally the greater part has been obtained from the upper discs. From these experiments the student will easily infer that under the pressure of the punch, the metal becomes plastic and flows, till the remaining metal is so thinned that its resistance to shearing is less than the load on the punch. Further details upon the"Flow of Solids

Fig. 74.

Fig. 75.

"can be found in the Proceedings of the Institution of Mechanical Engineers of 1876 and 1878; and also in la article on"Elasticity"in the"Encylopaedia Brittanica"by Lord Kelvin.

Obviously the best way to fix the punch is that shown in fig. 72, berause the whole load comes upon the shoulder which can be turned true, and thus there cannot be any tendency to force the punch out of line. Probably the reason that screwed punches have been so much used in the past is due to the facility with which a punch can be secured tightly into the ram, by means of a spanner, as contrasted with the trouble given by a punch with a shank that is not parallel, although it may be called so.

Parallel punches may be made from round bar without the aid of the smith, and if suitable jigs and gauges are available may be completed ready for use without the tool maker going near any of the presses in which they will be used.

In order to prevent the punch from being drawn out of the ram on the up-stroke, a flat or hole should be provided for the set screw to bed against, and so hold the punch firmly. This precaution is very necessary in the case of large cutting-out tools, also in cupping and drawing presses, in each of which operations there is often a very heavy drag put on the punch.

Screwed shanks cannot be used when the shape of the punch is irregular, square, rectangular, &C., as in such cases the punch must always have some particular position with respect to the ram, and the slightest deviation from this position will cause the punch to foul the die. With a screwed shank it would be very difficult to maintain this accuracy; but by means of a parallel shank, with a flat planed upon it, a set screw will secure it in the same position accurately any number of times. When extreme accuracy is essential some arrangement of dowel pin can be easily devised upon the face of the ram to suit the special case, and for this purpose a jig should be made, so that the tools can be accurately made in the tool room to suit one or more presses in the shop, as may be required. One such arrangement is shown in fig. 76, where the punch P is secured to the ram by means of the parallel shank S, which is held by means of a set screw E pressing on the flat F. The punch, however, has a small dowel pin D fixed near its outer edge, and this entering the hole in the ram forces the punch to take up its proper position automatically, without any effort on the part of the workman fixing it. The dowel pin should have its upper end slightly tapered so that it may readily enter the hole in the ram, and should be made large enough to prevent its being damaged with ordinary shop usage.

Fig. 76.

Fig. 77.

In an American cutting-out press the author noticed that the punch was fixed to the ram by means of a key similar to a method commonly used for attaching dies, but it does not seem to offer any advantages over the parallel shank type, except in some kinds of forcing work. For small cutting-out work, such as this press was intended for, it would be comparatively expensive to make, as well as more difficult to handle.

Fig. 78.

A good method of holding the bed is shown in fig. 77. The bolster B is fixed to the press by means of two set screws S, and the bed D is secured in its place by means of the key K, its position being adjusted by means of thin packing strips at P. This packing strip is also useful in case the key is not tight enough to hold the bed D securely.

Another method of holding the bed is shown in fig. 78, where, instead of taper grooves, the bolster is planed with vertical edges and two set screws S are used to fix the bed in its place. This method is not so good as that shown in fig. 77, because there is nothing to ensure the block being tightened hard against the bolster B; indeed, it may be easily raised up slightly by the turning of the set screws, whereas the pressure of the key K and the taper grooves forces the bed down in a very positive fashion ensuring a solid fixing. The bolster is attached to the press frame by means of the dog plate E and a set screw in the way familiar to turners. This method of fixing gives more latitude in adjusting the position of the bed upon the press.