defects is accentuated by the adjoining parts being tough and ductile, and such a plate might be compared to one made up of glass ribs and sheets of lead. There is naturally much difficulty in reproducing these conditions experimentally, but the danger exists all the same, and should be guarded against by careful annealing.

Cases have occurred where unflanged plates whose corners had been drawn out and then laid aside cracked overnight. It is difficult to imagine that the quality had nothing to do with this, though the tests were good, but stresses had evidently been set up in a similar manner to those explained on p. 257.

Hydraulic Flanging. Formerly presses similar to that shown at fig. 223, p. 244, were used for flanging, but the necessity of requiring

numerous moulds and various other inconveniences have led to the very extensive adoption of Tweddell's flanging press (fig. 261). It consists of three hydraulic cylinders, A, B, C, to whose rams various moulds or head-pieces can be keyed, a, b, c. The whole is supported on a strong bed-plate, B, to which the mould block M and the angle frame N (for guiding b) can be bolted. The press is shown in the act of flanging a boiler end plate, P, which is firmly held down by a, while the moulding iron b is forced down the side and bends the plate. After each stroke a is lifted so as to allow the plate P to be moved a little, and then b descends again. When the whole length of one heat has been dealt with in this way, the flange will be very irregular and frilled, as shown in fig. 262. These irregularities are removed by forcing the ram c against the circumference of the flange. The head

piece a should be of ample size, so as not to injure the plate, but not too large, otherwise the plate remains perfectly flat after flanging, in which condition it is more liable to crack than if somewhat warped or buckled. The plate should be hottest at the edge, for if that part is left dark red, the thickness of the plate will be reduced at the bend, and the puckers cannot be easily removed. Fig. 263 shows an end plate partly flanged. One part, a, is bent down, the other part, b, is still straight, and the metal between a and b must have stretched

FIG. 262

FIG. 263

a

FIG. 264

FIG. 265

considerably. Besides this the circumference of the plate at b is greater than that of the flange a, so that when finished there must be a considerable compression stress in the flange. It has the effect of slightly bending the plate edgeways, as can be noticed by the curving of any straight line which has been scribed on the plates before flanging.

Short lengths, varying from 4 to 6 feet of the circumference (and even 8 feet of thick plates), are heated at one time and the flanging completed before the next length is taken. The heating takes about twenty minutes, and the flanging is at the rate of about 1 foot in

FIG. 266

2 minutes. The extreme ends of the flanges are never of the shape finally required, and have to be dealt with subsequently by hand.

Fig. 264 shows how the corners of the plates are cut previous to flanging. The outside plate (see fig. 265) is drawn out at the corner, and when subsequently flanged has a very irregular appearance, as shown in fig. 280, p. 269. Fig. 266 shows how the corners rise up. This is due to the above-mentioned compression of the material. Before allowing the flange to cool, the corners of the inside flange have to be knocked in with a few blows of a hammer, and the corners of the outside flange knocked out; otherwise these

parts will have to be re-heated before fitting together, which would be a great waste of time.

Flanging with a Steam Hammer is done when no other means are available. The plate is placed at an angle on rollers (fig. 267). H is the moulded hammer-head; M is the lower mould, keyed to the anvil block. A tie rod or plate is also bolted to it for holding the pivot of the plate

The

P, which has been firmly bolted to the centre. flanges occasionally get torn by this rough treatment, and must then be welded.

Hand Flanging. It goes without saying that up to certain thicknesses flanging can be done by hand. It is usual to dispense with pivots and stops, which are necessary with machine flanging, and to be guided as regards shape by deep centre punch marks on the plate. For end plates it is always better to use a pivot; the circumference can then be made more truly circular.

P

FIG. 267

The size to which plates have to be sheared for flanging depends, of course, on the depth of the flange, but also on the skill of the operator. With some a very much greater margin must be left for irregular work than with others. The following is a customary rule, and produces flanges with an average of 1 in. for waste:

To the external size of the flanged plate add the depth of the flange, measured inside.

For end plates of boilers this is the usual custom. The furnace holes are, however, cut 2 ins. smaller in diameter than would be required by this rule, while for machine-pressed dome ends the extra margin for the flanges is reduced to about 75% of the above, because they draw out considerably.

Tube plates and combustion chamber back plates have to be ordered with a margin which is 1 in. in excess of the depth of the flange.

An extra allowance of 1 in. has to be added at the corners of plates to be flanged by a steam hammer. (See fig. 264.)

Flanging Furnace Holes.-Another operation which is carried out under the press is the flanging of the holes in the furnace front plates. Having been bored out to the correct diameter, as indicated above, the circumference of each hole is heated and flanged separately. This is also done under a Tweddell's flanging press. A strong cast-iron ring mould is bolted to the bed plate, and the two plungers a and b (fig. 261) are secured to a strong cast-iron die. The cocks and valves

of the press are then altered, so that the plungers work in unison, and when the furnace front plate has been placed in its proper position on the ring mould, the die is forced through it, producing the desired flange. The power required seems to be at the rate of about 5 tons per foot of circumference with a 3-in. plate and with a sufficient depth for one row of rivets, and double this power for inch plates or for treble-riveted flanges. After being heated it takes about 15 minutes to carry out the flanging.

The furnace front plate should be firmly held in its proper position, because a tendency exists for the die to drag down only one side of the flange, and in doing this the plate gets moved. If the ring mould is strong enough, the plunger c might secure the plate. The die should also be as taper as the length of the stroke will allow; less force is then required, and the tendency to shift the plate is reduced to a minimum.

Irregular Shapes of Furnace Holes.-Simple as the operation appears, it will be found that the holes produced in this way do

not always turn out perfectly circular, unless certain precautions have been taken. One of these is not to heat the plate further than is absolutely necessary, so as not to warm, and thereby weaken, those flanges which have already been finished. Thus, if the finished flange at a (fig. 268) were to be heated while the circumference of the hole O is over the furnace, a slight contraction would take place at this point during flanging, and the diameter D would be reduced. Similarly, a contraction would take place at b if the outside flange were heated at this point. This contraction is the necessary consequence of the stretching of every part of the circumference of O, just as the closing-up action, while flanging the outer circumference of end plates, tends to elongate the adjoining parts.

These deformations can be partly prevented by placing a stout iron ring (fig. 269), cut as shown, inside the finished flange and tightly wedging it into position, but only after the adjoining hole has been heated, and just before it is placed under the press. In some works all three or four furnace holes are flanged in one operation, and

sometimes also the outside flange is done at the same time. This of course requires a very powerful press, similar to the one shown at fig. 223, p. 244. The great inconvenience of this method is the large number of moulds and dies required; but the results are highly satisfactory, the only serious trouble being the drawing away of the

FIG. 269

FIG. 270

metal from the weaker or lower flange towards the stronger one (see fig. 270), which very often leads to the outer flange being higher and the inner one being lower than either should be. Fig. 271 is a section through a furnace-hole flange, and shows the deformation to be expected.

The actual flanging operation is easier with iron than with steel, because this metal is softer, and because it can be worked at a higher

FIG. 271

temperature, and fewer heats are necessary. On the other hand, only the very best qualities of iron can pass through this operation without showing cracks or other defects, and there is also much waste of thickness, due to the burning of the surface. That steel possesses great advantages is proved by the fact that hardly any other material is now used for this purpose.

Annealing. It might have been better to postpone the necessary remarks about annealing until all flanging operations have been dis