Machines and tools employed in the working of sheet metals

The scarcity of the literature on Presses and Press Tools is in itself sufficient justification for the publication of these articles, which appeared originally, at intervals, in the columns of The Practical Engineer, and are now published in book form at the request of many readers.

The production of tools for the working of sheet metals is a distinctly separate branch from that of engine-fitting and general machine work. It is therefore difficult for an engineer to thoroughly grasp the work of the press tool maker, unless he has had an opportunity of closely watching the tool work in progress and the subsequent operations performed by the tools in the production of numerous articles made from sheet metals.

This difficulty is largely due to the many technical points of detail connected with the processes through which an article may have to pass before reaching its final stage, which will become obvious upon reading Chapter XI. Drawing and Re-drawing and Chapter XIX. on Tool Setting.

The subject embracing, as it does, many industries, including a vast number of separate processes, no systematic treatment has been attempted. As far as possible each section has been taken separately, and where necessary to a complete and thorough understanding of a point under consideration, either a little recapitulation has been deemed advisable, or reference made to the chapter or section where the particular point has been previously mentioned.

The original articles were the result of many years' work, and study of a business which plays an important part in many British industries, and it is hoped that these collective notes in book form will be found useful to engineers, mechanics, manufacturers, and to young technical students, assisting them to overcome some of the difficulties which they may meet with in technical schools and workshops. A few remarks in reference to the machine tools to be found in every machine shop have been included, to enable the younger readers to better understand the matter contained in those chapters which refer to the manufacture of press tools.

At present very little seems to be known by mechanics in the workshops concerning the work done in either a Fly Press or Drop Stamp. It is therefore hoped that the special chapter devoted to the following subjects work done in Copying Press, Fly Press compared with the Screw Press, work done by the Stamp Hammer, the reasoning regarding the Hammer Blow will be readily followed by the student and practical mechanic, who may not have sufficient scientific and mathematical knowledge to follow the reasoning of the more advanced text-books on the subject.

The illustrations used are of a special character, and do not represent every possible type of Power Press. Those selected may be regarded as being good practical examples of modern machines and calculated to be useful to the greatest number of those who are engaged in sheet-metal working industries.

Calculations and definitions have been introduced to enable practical mechanics to readily grasp the point under consideration with tin least possible calculation, and at the same time more accurately and quickly than by "rule-of-thumb" methods.

The durability of dies and punches depends to some extent upon the nature of the work upon which they are used. Further, the work to be done by these tools governs the point as to the amount of accuracy and finish that shall be given to them. It is, therefore, necessary that care and judgment be exercised upon this point, for when the metal after being cut into blanks has to be stamped, or raised and clipped as often is the case, the cutting tools need not be so accurate as they would necessarily have to be if several pairs of tools were producing similar blanks, which may have to undergo many processes before finally coming together and be interchangeable when being assembled. The cutting- out tools for such articles as cycle chain links, quick firing shells, small arms ammunition, type writing machines, cycle and motor car parts, electrical apparatus, jewelry, locks, musical instruments, stencils, watches, or any similar delicate work, must necessarily be made very accurate and be of first-class finish; whereas in the case of such articles as stamped hollow-ware, coal hods, elevator buckets, trunk trimmings, kitchen utensils, agricultural implement parts, hinges, lanterns, shovels, etc., a small fraction of an inch larger or smaller in the size of the cutting-out tools, or a slight variation in their shape would l>e of small consequence.

It is interesting to trace the process of making a die and punch to a standard pattern, such for instance as the two ordinary shapes, a pedal plate blank, fig. 83, and a spanner blank, fig. 84, the die D having been planed or milled both on the top and bottom and the necessary angle planed upon its sides to fit the bolster. The top of the die should now be cleaned oft with a smooth file and emery cloth, after which the top must be smeared with sulphate of copper, commonly known in the workshop as "blue stone." This is done to enable a fine line to be seen, for without the blue stone, it would be of little use marking out a shape upon the bright face of the steel die with a marking scriber, because the lines could only be traced by the eye with great difficulty. In some instances the standard blank would but used as a template to mark out the shape upon the die, but this is not considered a good method unless a perfectly accurate blank is used as the template. Assuming that the blank is one that has been cut by a pair of ordinary press tools, probably it would not be accurate, therefore the better plan would be for the tool maker to carefully set out and mark the shape accurately upon the face of the die. This would be done with square, compasses, and scriber, in a similar manner that a detail drawing would be made upon a piece of drawing paper. From this it follows that a knowledge of geometrical drawing is very useful to the tool maker. Having carefully indicated the outline of the figure by dots from a small centre punch, as much of the metal as possible should be removed by drilling various sized holes straight through the die; the size of the drills used will depend upon the size and shape of the hole required in the die in other words the size and shape of the required blank. After removing as much metal as is possible by drilling, the die may be turned over and larger drills passed up the back to a certain depth, thereby forming a clearance; this clearance will be seen in the section at h.

If the holes have been carefully set out to follow the outline of the shape a little chipping with a thin flat chisel on top and bottom of the die will remove the bulk of the metal in one piece or lump. The back or clearance side of the die may now be cut away by the aid of larger chisels, after which the die is turned over in the vice so that the top, showing the outline of the figure, faces the operator, who will carefully chip and file out the shape of the blink by using the various small chisels and files.


CHAPTER IX - DIE MAKING BY DRIFT

As illustrating how any number of dies may be readily made to one standard size and shape, take the case of a cutting-out die for cycle-chain links. It is possible to make dies or beds of this kind by the hundreds, so that they shall not vary more than one thousandth part of an inch. Figs. 90 to 93 show a good method of making these and similar tools. A standard bed and punch would first be made by the method described by figs. 83 and 84, for making standard dies and punches. The bed would be called the standard bed, and must necessarily be made very carefully ; in fact, both bed and punch must be made absolutely accurate, the punch being sized so that it can just be pressed into its bed or die, being what is known to a tool maker as a tight fit.

Now, referring to fig. 90, the die is seen in plan at A, and the inverted plan B shows the bottom of the die ; between A and B the die is seen in section. Supposing this standard die to have been finished, hardened, and tempered, the next step is to prepare an accurate drift, with which it may be finished, or sized and corrected, so that all subsequent dies will be alike in shape and size. This drift will deal with the bar H H of the die, fig. 91. The method of dealing with the round holes or large ends of this die may for the present be left out of the question, as that portion of the work requires separate attention. To make the drift, procure a piece of tool steel about 3 in. long, and of such sectional area that will allow it to be shaped up for its whole length, the same shape as the hole in the standard die, figs. 90 and 91. Having first filed up the ends and covered them with sulphate of copper, mark out the shape on each end, and indicate by small centre dots, plane or shape it from cud to end by means of the shaping machine, afterwards carefully file it with suitable files until it is possible to pass the drift straight through the die with the assistance of light blows from a hand hammer. It is not an easy matter to make this drift, and it requires the greatest possible care and judgment to be exercised during each step in the process, and the micrometer gauge should be used throughout, for if the drift was carelessly driven through the standard die a tight driving fit it would be liable to burst the die, in addition to the drift being so ripped and knocked about as to make it absolutely useless.

The standard die, fig. 90, is used more as a guide to shape, and to enable the extreme ends of the drift to be fitted with the die, rather than to be used for forming the general shape of the drift for its entire length. This must be done by careful filing, when the drift has been made perfectly parallel and of uniform shape.


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Machines and tools employed in the working of sheet metals