Modern Machine-Shop Practice Part 191

FORGING.--The operation of forging consists in beating or compressing metal into shape, and may be divided into five cla.s.ses, viz., hand-forging, drop-forging, machine-forging, forging under trip or steam hammers, and hydraulic forging. In purely hand forging much work is shaped entirely by hand tools, but in large shops much work is roughed out under trip or steam hammers, and finished by hand, while some work is finished under these hammers. In drop forging the work is pressed into shape by dead blows, which compress it into shape in dies or moulds. In machine forging the work is either formed by successive quick blows rather than by a few heavy ones, or in some machines it is compressed by rolling. In hydraulic forging the metal is treated as a plastic material, and is forced into shape by means of great and continuous pressure.

In all forging the nature or quality of the iron is of primary importance; hence the following (which is taken from _The English Mechanic_), upon testing iron, may not be out of place.

"The English Admiralty and Lloyds' surveyor's tests for iron and steel are as follows:--

"Two strips are to be taken from each thickness of plate used for the internal parts of a boiler. One-half of these strips are to be bent cold over a bar, the diameter of which is equal to twice the thickness of the plate. The other half of the strips are to be heated to a cherry-red and cooled in water, and, when cold, bent over a bar with a diameter equal to three times the thickness of the plate--the angle to which they bend without fracture to be noted by the surveyor. Lloyds' Circular on steel tests states that strips cut from the plate or beam are to be heated to a low cherry-red, and cooled in water at 82 Fahr. The pieces thus treated must stand bending double to a curve equal to not more than three times the thickness of the plate tested. This is severe treatment, and a plate containing a high enough percentage of carbon to cause any tempering is very unlikely to successfully stand the ordeal. Lloyds'

test is a copy of the Admiralty test, and in the Admiralty Circular it is stated that the strips are to be one and a half inches wide, cut in a planing machine with the sharp edges taken off. One and a half inches will generally be found a convenient width for the samples, and the length may be from six to ten inches, according to the thickness of the plate. If possible, the strips, and indeed all specimens for any kind of experimenting, should be planed from the plates, instead of being sheared or punched off. When, however, it is necessary to shear or punch, the piece should be cut large and dressed down to the desired size, so as to remove the injured edges. Strips with rounded edges will bend further without breaking than similar strips with sharp edges, the round edges preventing the appearance of the small initial cracks which generally exhibit themselves when bars with sharp edges are bent cold through any considerable angle. In a h.o.m.ogeneous material like steel these initial cracks are apt to extend and cause sudden fracture, hence the advantage of slightly rounding the corners of bending specimens.

[Ill.u.s.tration: Fig. 2824.]

"In heating the sample for tempering it is better to use a plate or bar furnace than a smith's fire, and care should be taken to prevent unequal heating or burning. Any number of pieces may be placed together in a suitable furnace, and when at a proper heat plunged into a vessel containing water at the required temperature. When quite cold the specimens may be bent at the steam-hammer, or otherwise, and the results noted. The operation of bending may be performed in many different ways; perhaps the best plan, in the absence of any special apparatus for the purpose, is to employ the ordinary smithy steam-hammer. About half the length of the specimen is placed upon the anvil and the hammer-head pressed firmly down upon it, as in Fig. 2824. The exposed half may then be bent down by repeated blows from a fore-hammer, and if this is done with an ordinary amount of care it is quite possible to avoid producing a sharp corner.

[Ill.u.s.tration: Fig. 2825.]

"An improvement upon this is to place a cress on the anvil, as shown at Fig. 2825. The sample is laid upon the cress, and a round bar of a diameter to produce the required curve is pressed down upon it by the hammer-head.

[Ill.u.s.tration: Fig. 2826.]

"The further bending of the pieces thus treated is accomplished by placing them endwise upon the anvil-block, as shown in Fig. 2826. If the hammer is heavy enough to do it, the samples should be closed down by simple pressure, without any striking.

[Ill.u.s.tration: Fig. 2827.]

"Fig. 2827 is a sketch of a simple contrivance, by means of which a common punching machine may be converted temporarily into an efficient test-bending apparatus. The punch and bolster are removed, and the stepped cast-iron block A fixed in place of the bolster. When a sample is placed endwise upon one of the lower steps of the block A the descending stroke of the machine will bend the specimen sufficiently to allow of its being advanced to the next higher step, while the machine is at the top of its stroke. The next descent will effect still further bending, and so on till the desired curvature is attained. It would seem an easy matter, and well worth attention, to design some form of machine specially for making bending experiments; but with the exception of a small hydraulic machine, the use of which has, I believe, been abandoned on account of its slowness, nothing of the kind has come under the writer's notice.

"The shape of a sample after it has been bent to pa.s.s Lloyds' or the Admiralty test is that of a simple bend, the sides being brought parallel. While being bent the external surface becomes greatly elongated, especially at and about the point of the convex side, where the extension is as much even as fifty per cent. This extreme elongation corresponds to the breaking elongation of a tensile sample, and can only take place with a very ductile material. While the stretching is going on at the external surface, the interior surface of the bend is being compressed, and the two strains extend into pieces till they meet in a neutral line, which will be nearer to the concave than to the convex curve with a soft specimen. When a sample breaks, the difference between the portions of the fracture which have been subject to tensile and compressive strains can easily be seen.

[Ill.u.s.tration: Fig. 2828.]

"Fig. 2828 shows a piece of plate folded close together; and this can generally be done with mild steel plates, when the thickness does not exceed half an inch.

"Common iron plates will not, of course, stand anything like the foregoing treatment. Lloyds' test for iron mast-plates 1/2 inch thick, requires the plates to bend cold through an angle of 30 with the grain, and 8 across the grain; the plates to be bent over a slab, the corner of which should be rounded with a radius of 1/2 inch.

[Ill.u.s.tration: Fig. 2829.]

"When the sample of metal to be tested is of considerable thickness, as in the case of bars, it is often turned down in a lathe to the shape shown in Fig. 2829, so as to reduce its strength within the capacity of the machine. The part to be tested has usually a length between the shoulders of 8, 10, or 12 inches, and must be made exactly parallel with a cross-sectional area apportioned to the power of the machine and the strength of the material to be tested. When it is desired to investigate the elastic properties of materials, it is desirable to have the specimens of as great a length as the testing apparatus will accommodate.

[Ill.u.s.tration: Fig. 2830.]

[Ill.u.s.tration: Fig. 2831.]

"Many of the early experiments on the tensile strength of wrought iron were made with very short specimens, such as in Fig. 2830, which is a sketch of that used formerly in the royal a.r.s.enal at Woolwich. This had no parallel length for extension at all, its smallest diameter occurring at one only point. Mr. Kirkaldy, to whom is due in a great measure the honour of having raised 'testing' to an exact science, discovered that this form of specimen gave incorrect results. He found that experiments with such specimens, more especially when the metals were ductile, gave higher breaking strains than were obtained with specimens of equal cross-sectional area having the smallest diameter parallel for some inches of length. This was due to the form of the specimen resisting to some extent the 'flow' or alteration of shape which occurs in soft ductile materials previous to fracture. He accordingly commenced to use a specimen of the form shown in Fig. 2831, with a parallel portion for extension of several inches in length, and specimens like that in Fig.

2830 became a thing of the past.

"The specimens shown in the figures admit of being secured in the testing machine in many different ways. But whatever description of holder be employed, two absolute requirements must be kept in view. The holders must be stronger than the sample, and they must transmit the stress in a direction parallel to the axis of the sample without any bending or twisting tendency.

[Ill.u.s.tration: Fig. 2832.]

"Fig. 2832 gives two views of a very effective method of holding round specimens, used by Mr. Kirkaldy in his earlier experiments carried out for Messrs. Napier & Sons, of Glasgow. The enlarged ends of the samples are clasped in split sockets provided with eye-holes for attaching them to the shackles of the testing machine, the halves of the sockets being held together during the experiment by small bolts pa.s.sing through the projecting lugs.

[Ill.u.s.tration: Fig. 2833.]

"Fig. 2833 explains the plan adopted for testing the strength of bolts and nuts in the same series of experiments.

[Ill.u.s.tration: Fig. 2834.]

"A good holder for lathe-turned samples is shown in Fig. 2834. Close fitting socket-pieces _b_ _b_ embrace each end of the specimen, and also the turned collar at the extremity of the shackle _a_. The halves of the socket are held together by a collar _c_, the interior of which and exterior of the socket rings are turned to an equal taper, so that the socket-pieces are held quite firmly when the collar _c_ is simply slipped over them by hand. When the experiment is over, a few taps with the hammer will remove the collar _c_.

[Ill.u.s.tration: Fig. 2835.]

"Samples of plates for tensile testing are usually shaped like Fig.

2835. The parallel portion B is generally 8, 10, or 12 inches long, as in the case of the turned specimens. Two minor points in the preparation of specimens may be here alluded to. In the first place the holes _a_ _a_ must be made large enough to obviate any danger of the pins which are placed in these holes to secure the specimen being sheared in two before the specimen breaks. In the second place, enough material must be left around these pin or bolt holes to prevent the probability of the metal tearing away between the hole and the edge of the plate. The pin holes must be placed exactly in a line with the axis of the specimen, and the part B must be quite parallel in width, so that the strength (and the elongation during the testing) may be, as nearly as possible, equal throughout the length of B. The shoulders, as _c_, should be easy curves, so that sharp corners may be avoided. When a number of such specimens are required at the same time, the strips of plate may be clamped together and planed or slotted to the desired width as one piece, but the tool marks should be afterwards removed by careful draw-filing.

"When the plates are thin, small side pieces are riveted on the sides of the ends to be clamped, as shown in Fig. 2836. These stiffen those ends and afford a larger bearing for the securing pins. The connection with the shackles is made by means of steel pins pa.s.sing through the end holes, and when specimens like 2835 are properly prepared, the direction of the stress on them must be in a line with their axis. Fig. 2837 shows another form of plate specimen in which the holes are dispensed with, the ends being held in the machine by friction clips, as shown. These specimens are more easily prepared, and from the absence of holes may be made of a very narrow strip of plate.

[Ill.u.s.tration: Fig. 2836.]

[Ill.u.s.tration: Fig. 2837.]

"In Fig. 2837 the jaws or forked arms of the shackle are closed to form a rectangular ring, as shown in section in the figure. Two of the interior faces are tapered inwards to the same angle as the back of the wedges or clips _a a'_, which are perfectly smooth and free to slide upon the inclined or tapered surfaces of the shackles. The faces of the wedges, however, which come in contact with and grip the specimen to be tested, as _b_, are fluted or grooved, so that the friction of the edges against the specimen is much greater than against the inside surfaces of the shackles. The result of the arrangement is, that when the shackles are pulled, the wedges _a a'_ are tightened against the specimen with a degree of force proportionate to the load on the specimen, which is prevented from slipping through the clips by the 'bite' of their fluted faces. The grooves on the faces of the clips need not be deep--a depth of a little more than 1/16, with about the same distance apart, answering well for ordinary loads. With deep grooves and a wider pitch apart, the danger of the specimen breaking in the clips is increased.

The inclination of the backs of the wedges _a a'_ to the faces may be at an angle of 5 or 6 degrees. When the taper is too small, the removal of the halves of the specimen after breaking is sometimes difficult, while on the other hand, when too great, the specimen is apt to slip between the wedges while being tested. The wedges exert a very considerable outward pressure, and the jaws of the shackles must be made strong enough to resist any strain likely, under extreme conditions, to fall on them, otherwise they will speedily become unfit for use. In securing a specimen care must be taken that its axis is in the direct line of strain, and the opposite clips should be driven in equally so that the stress may act fairly upon it. Parallel planed strips of metal, without any enlargement at the ends, may be tested in these friction clips, though, of course, there is a chance of the specimen breaking within them. Turned specimens may also be held by such clips; as also may rough, unturned round and square bars, an advantage when it is desired to immediately ascertain approximately the strength of metal samples."

Open fires for hand forging purposes are mainly of two cla.s.ses, those having a side and those with a bottom or vertical blast.

[Ill.u.s.tration: Fig. 2838.]

Fig. 2838 represents a side draft forge. F is the fireplace, usually from 3 to 5 feet long, T is the tuyere through which the blast enters the fire, B being the blast pipe. To prevent T from being burned away it is hollow as at S, and two pipes P and P' connect to the water-tank W, thus maintaining a circulation of water through S; V is simply a valve or damper to shut off the supply of air from the tuyere; D is the opening to the chimney C.

The side blast, though not so much used as in former years, is still preferred by many skilful mechanics, on the ground that it will give a cleaner fire with less trouble. The method of accomplishing this is to dig out a hole in the fire bed and fill it in with c.o.ked coal, which will form a drain through which the slag or clinker may sink, instead of remaining in the active fire and obstructing the blast.

In cases where the fire requires to be built farther out from the chimney wall than the location of the tuyere permits, it may be built out as follows:--

[Ill.u.s.tration: Fig. 2839.]

[Ill.u.s.tration: Fig. 2840.]

A bar B, Fig. 2839, is placed in the tuyere hole and supported at the other end at P. The coal is well wetted and packed around and above the bar, which is then pulled out endwise, leaving a blast hole through the coal, as is shown in the end view Fig. 2840.

[Ill.u.s.tration: Fig. 2841.]

Fig. 2841 represents a patent tuyere of vertical or bottom draft, in which the blast pa.s.ses through pipe A and circulates around B, finding egress at C into the fire. C is hollow and receives water from the tank F by the pipe D. The steam generated in the nozzle C is conveyed to the tanks by the pipe E.

Figs. 2842 and 2843 represent a blacksmith's forge, for work up to and about 4 inches in diameter. It consists of a wind-box A, supported on brickwork which forms an ash-pit G beneath it. To this box is bolted the wind-pipe B, and at its bottom is the slide E. In an orifice at the top of A is a triangular and oval breaker D, connected to a rod operated by the handle C. This rod is protected from the filling which is placed between the brickwork and the sh.e.l.l F of the forge by being encased in an iron pipe I. The blast pa.s.ses up around the triangular oval piece D.

The operation is as follows: when D is rotated, it breaks up the fire and the dirt falls down into the wind-box, cleaning the fire while the heat is on. At any time after a heat the slide E may be pulled out, letting the slag and dirt fall into the ash-pit beneath. It is a great advantage to be able to clean the fire while a heat is on without disturbing the heat.

[Ill.u.s.tration: Fig. 2842.]

[Ill.u.s.tration: Fig. 2843.]

Blacksmiths' anvils are either of wrought iron steel faced, or of cast iron steel faced, the faces being hardened. It is sometimes fastened to the block by spikes driven in around the edges. A better plan, however, is to make the block the same size as the anvil, and secure the latter by two bands of iron and straps, as shown in Fig. 2844, because in this way the block will not come in the way of arms or projecting pieces that hang below the anvil. The square hole is for receiving the stems of swages, fullers, &c., and for placing work over to punch holes through it, and the round is used for punching small holes.