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(M. E.,'1878, p. 244) recommends one-sixth of the thickness of plate. This amounts to 11% where the diameters of the holes are half as large again as the thickness of the plate, and 17 % when they are equal.
Spiral and Slanting Punches have been used not only to lessen the injury to the plate, but also to reduce the pressure on the machine. W. Barr (1880, p. 92) mentions that Kennedy's spiral punch (fig. 199) requires only one-third the power of an ordinary one. Stern (M.E., 1878, p. 239) gives the following information :
L. Hill (M. E.,' 1878, p. 244) mentions slanting punches (figs. 200, 201).
It is only natural that, in comparison with ordinary punches, less. force, though possibly just as much power, is required with spiral or slanting punches, because with them only a small part of the circumference is doing work, just in the same way as less force is
required in a shearing machine if the cutting blade is set at a steep angle. When large holes have to be punched, there ought to be no hesitation in substituting one of the above punches for the ordinary flat one. Fig. 200 seems to be the best suited for hand holes, &c., not only because of its being symmetrical, but because the greater part of the edge is shaped like a good cutting tool. The reverse or V shape would in this respect be the worst possible. The spiral punch leaves
a mark in the circumference, which in large holes may be an inconvenience.
A favourite plan in some works is to punch the holes in all those plates which for some reason, such as flanging, have to be annealed, but, on account of the blindness of some of these holes when fitted together, a great deal of hand labour has to be expended on them in the way of chipping and rimering, so that if added together the expenses for such holes would be found greater than if they had been drilled; besides, they will not be satisfactory jobs.
The holes of the seams in the furnaces are also sometimes punched ; but here, particularly with the circumferential seams at the front ends, there is danger that the drawing of the furnace mouth, to meet the flanged front plate, will produce cracks.
Various seams in the flat plates of the front and back ends are sometimes punched, even when they are in. thick; but the warping of the plates and the certainty of having to chip and rimer a large number of holes ought to be a sufficient objection to this practice.
The lower edges of the back tube plates are almost invariably punched, leaving them quite ragged. In some works they are left in this condition, as it is useless to caulk this edge. For appearance sake, most boiler-makers chip it. Under any circumstances it would be best to leave a good bevel, so as to prevent steam lodging there and causing the saddle seam to heat. By using a square or rectangular punch such edges would be left in a better condition for chipping.
Punching is also resorted to for producing holes for guiding the trepanning tools with which the tube plates are bored. This is à bad practice, and leads to irregularities amounting to fin. in the various diameters of the finished holes, and even affects their roundness.
A less objectionable though still unsatisfactory use of the punch is the practice of perforating those points of the back end plates and of the combustion chamber plates where the screwed stays are to be fitted. When placed in position drills and then taps are passed through them, which remove all brittleness, even if the plates have not been annealed after punching. (For injury done by punching see p. 224.)
Illustrations of various types of shearing and punching machines will be found in the following publications :
Engineering,' vol. xxxix. p. 219, ‘Hydraulic Shearing Machine'; vol. xlii. p. 221, ' Portable Pneumatic Punching Machine'; vol. xliv. p. 16, 'Shearing Machine'; vol. 1. p. 688, Punching and Shearing Machine'; vol. 1. pp. 177-179, 243, 247, 494, 519, Punching Machines.
Planing Operations. -Illustrations of plate-edge planing machines will be found in Engineering,' vol. xxxvi. p. 384 ; vol. xlix. pp. 245, 252; vol. 1. pp. 536, 625. All the shell plates have to be planed at their edges. They are bolted down on the planing machine (fig. 202), and a tool in the slide rest R, travels along its edge, at a speed of about 10 to 18 ft. per minute, mean speed 12 ft. with a feed of 3 in. for 1-in. plates, and proportionately more or less for thicker or thinner plates, removing the superfluous material at the rate of 5 to 6 cubic
inches per minute. To this has to be added the time for setting the plate. The total time required to set and plane four shell plates measuring 20 ft. x 5} ft. x 15 in., equal to 102 ft. running, amounted to about 12 hours, from which it is clear that the proper setting of the plates, which had to be repeated eight times, takes up very much more time than that required for the removal of the material. The machine used had two sets of frames, placed at right angles (see fig. 202).
With the old-fashioned machines, in which the plate has to be reset for each edge, the time required would have been twice as long, and much more space would have been required, for the plate, which is shown by the dotted lines a, b, c, d, would have to be turned round to the position e, f, g, h.
A little saving in time is effected if the planing tool be so fitted that it can be turned round, and cut both during the forward and backward travel (see fig. 203). But when the edges have to be bevelled the tool must have two cutting faces (see fig. 204).
The amount of bevel of edges to be caulked varies from nothing to 1: 3 (see p. 298), but for the butts of shell plates it should be about 1: 30; otherwise they will not close properly (see fig. 205).
Should the planing machine be shorter than the plates, their edges will have to be planed in two operations. The usual plan is to withdraw the tool gradually when it reaches the end of the stroke, but in some works part of the material at this point is first removed by chipping
In old machines, where the holding-down frame is secured at its ends, as shown in fig. 206, only definite lengths of plates can be planed, unless a bracket is bolted to the bed.
Large plates are generally ordered with a margin of 1 inch all round, and this is occasionally exceeded by another inch when shearing.
All this material has to be removed on the planing machine, and great care has to be taken that the final dimensions are correct. They should never be marked off with anything else but a steel or iron rule, for the differences of expansion of wood, iron, and brass are very appreciable on large dimensions. I'he working drawings should always contain the widths
and lengths of the plates, though the Fig. 206
latter dimension is generally omitted.
To find the exact circumferential length of a shell plate multiply the mean diameter of the particular strake by 3:1416."
This length has to be divided by two, three, or four, according to the number of joints in the shell. The length of the adjoining strake
A rough approximation to this value is 2?; where very great accuracy is desired the fraction 15 may be used.
may be found in the same way, but should be checked by adding or subtracting 6.283 times the mean thickness of the two plates as measured at their overlapping edges. It is necessary to be accurate on this point, otherwise the plates will not butt properly. In lap-jointed boilers one width of the lap has to be added to the length of each plate.
The furnace and combustion-chamber side plates are also planed on these machines, and their dimensions are either marked on the drawings or have to be measured from them. The lengths should be found by measuring the circumferences of the flanged plates and furnaces when fitted together.
The flat edges of the front and back end plates are also planed in these machines, but only after having been flanged and fitted together.
Drilling Operations. In all first-class works every hole in a boiler is drilled, and generally drilled in place. A considerable amount of ingenuity has therefore been expended in designing machines which will do this work with speed and accuracy, and which can be adapted to various uses. Naturally a great many different patterns are in use, as will be seen from the following list :
W. S. Hall, M. E.,' 1878, p. 565. Drilling machinery. He mentions Hutchinson's, Welch's, Buckton and Wicksteed's, and Buckton's multiple drilling machines, Adamson's, Dickinson's, Jordan's, and Kennedy's drilling machines; Hall's portable, Brown's and Thorn's steam drilling machines ; McKay's equilibrium drill and other furnace and manhole boring tools, and also Shaw's flexible shaft.
W. Arrol ('M. E.,' 1887, p. 312) describes the drilling machinery used in the construction of the Forth Bridge. This paper is well worth studying, though not intended for boiler work. In Engineering' will be found sketches of the following:
Ordinary drilling machines, vol. xxxiii. p. 348; vol. xxxv. p. 99; vol. xlvi. p. 327; vol. xlviii. p. 191.
Radial drilling machines, vol. xxxiii. p. 134 ; vol. xxxviii. p. 388; vol. xxxix. pp. 57,360; vol. xl. p. 246; vol. xli. p. 29; vol. xliii. p. 269; vol. xliv. p. 42; vol. xlvi. p. 468; vol. xlviii. p. 180; vol. xlix. p. 248;
. vol. 1. p. 184.
Multiple drilling machines, vol. xxxiv. p. 373 ; vol. xliii. p. 69; vol. xliv. pp. 150, 289, 292; vol. xlv. p. 451 ; vol. xlvi. p. 90; vol. xlix. pp. 250, 251.
Shell and back end drilling machines, vol. xxxiii. p. 586 ; vol. xl. p. 424; vol. xli. p. 620; vol. xlii. p. 420; vol. xlviii. p. 501; vol. lv. p. 318; vol. lviii. p. 780; vol. lxiv. p. 751,
Portable drilling machines (for furnaces), vol. xxxii. p. 162; vol. xxxix. p. 570; vol. xlii. p. 637; (for shells) vol. xl. p. 320; (hydraulic) vol. xliii. pp. 130, 131 ; vol. xliv. p. 295; vol. xlvii. p. 644; (hand) vol. xlv. p. 185. Ring-shaped shell drilling machine (Forth Bridge), vol. xxxix. p. 57; vol. xlix. p. 248.
Combined multiple and radial drill, vol. xlii. p. 613.
p Drill grinders, vol. xl. p. 320; vol. xliii. p. 101; vol. xliv. p. 6; vol. 1. p. 674.
These numerous references make it needless to discuss the distinguishing features of the various types, and attention will only be drawn to a few points.