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The Ordinary Drilling Machine is used for little else but small or occasional jobs. Its table can be raised or lowered, and can generally be turned round a vertical axis. The feeding is done by hand. In shipyards, where these machines are used for countersinking holes, the feeding arrangement is a balance lever.
Radial Drilling Machines are made in various forms. Sometimes they are fixed against walls, and are driven by belting from above, or they stand alone and are driven from underground. In that case they are generally made so that their arm can sweep through 360°. In some cases the table is adjustable, in others the arm can be raised or
lowered. Arrangements for doing this by power are a great convenience, and all moving parts should be balanced. It is well to make the balance weights extra heavy, so as to reduce the slack of the drill to a minimum.
Multiple Drilling Machines are passing out of favour in boiler shops, because their chief occupation, drilling shell plates before bending, is gone. They are generally arranged to slide along a frame, and each spindle can be shifted and worked independently of the others, or they can be set to the proper pitch and moved along together. In some machines the spindles
can also be moved at right angles to the frame, so that a double row of holes can be drilled without resetting the plate. In these machines the various spindles cannot of course be brought close together, which causes a slight waste of time at the two ends of a seam, but is otherwise an advantage. In another type of machine the spindles are placed very close together. Figs. 207, 208 show two arrangements for adjusting the pitch in these cases. A turn of the screw S (fig. 207) will separate the nuts N , N2, N4, N, from the centre one N3, and they are then clamped. Or a turn of the spindle C (fig. 208) will turn the hollow right- and left-handed screws 21, 22, 23, which connect the four nuts ni, N2, N3, N4, and these are then clamped. The drill spindles are attached to these nuts and move with them. Should one of the drills break, or be put out of use, all the others would of course have to be stopped for a time.
Shell Plate Drilling Machines usually consist of two vertical columns, of which one or both are movable. One, two, or even three drill spindles are attached to a slide, which can be moved up or down these columns into any required position. If shells are to be drilled they are placed on a turntable between the two columns (sometimes there are three or four). Much time is wasted over the longitudinal seams, for usually only one can be attacked at a time, and in this respect it is a great advantage if each column can be moved both in a radial and in a circumferential direction, even if only through a limited range. Then the angular adjustment need not be done by the turntable.
Another shell-drilling machine is shown in fig. 209. The boiler shell rests on four wheels, W, W. A slide rest S is movable along the bed plate, and carries an arm A, which can be set to any angle by means of the screw z. It carries the drilling spindle D, which can be
moved up or down. It is clear that the drills—for there are usually two--can, within certain limits, be set to any required angle or position. When all the holes in a given span are drilled, the boiler is turned round the necessary angle and a fresh start made.
Boiler Back End Drilling Machines usually consist of two columns, both movable along a strong bed plate; the drill spindles can, as in one of the previous cases, be raised or lowered to any convenient height, so that every part of the back end plate of a boiler could be drilled if placed in the proper position. These machines are sometimes fitted with the necessary gear for tapping holes and screwing in the screw stays, and in some cases circular saws can be attached, with which to cut off the projecting ends.
A very convenient machine for drilling furnaces in place is shown (fig. 210). A frame with three or four set screws is firmly screwed into the mouth of the furnace; a light drilling machine is attached to the centre of this frame in such a manner that it can be clamped in any radial position. The driving wheel projects beyond the mouth of the furnace, and is driven by a belt or gut rope. Of late, electric motors have been applied to this purpose. À somewhat similar machine, but fixed, is sometimes used for drilling the furnace fronts out of place after the furnaces have been fitted and the holes marked off.
Any one of the more powerful drilling machines can be used for boring the tube plate holes. A small hole is first drilled or punched in the centre. The guiding spindle S (fig. 211) of a special tool holder, which is secured to the drilling machine, is inserted into one of them, and the cutting of the circumference of the large hole is then done by the parting tool T while it revolves. P is the tube plate which is to be bored. Unless the spindle S is a very good fit in its guide hole, the tube holes are very apt to be irregular both as regards shape and size, and as the setting of the tool T is a somewhat tedious T S job, it is not to be wondered at that every time it has to be done the diameter of the hole changes. This can easily be demonstrated by measuring several holes, which
FIG. 211 will be found to differ sometimes by as much as le in. even in the same plate. No wonder, then, that tubes which are somewhat less in diameter than the smallest holes should split while being expanded into the larger ones. In some machines S is replaced by a sliding centre which rests in a large centre punch mark and guides the cutter. In some works the tube holes are rimered out after boring, which is an excellent practice. Less commendable is the system of tapping the stay tube holes before the plates are riveted up; some of the threads are sure to be nearly cut away.
Boring Tools.—Other contrivances related to drilling machines are furnace and manhole boring tools. If intended only for the former of these purposes, they are practically nothing but very powerful drilling machines; only, instead of a drill, an arm with a slide rest is attached to the vertical spindle. A parting tool is secured to this rest, as shown in fig. 212, and the furnace front plate bolted in the desired position on the bed plate and bored out. Most of these machines are now arranged in such a manner that the vertical spindle can be made to travel horizontally backwards and forwards, whereby the hole cut into the plate will be an elliptic one, as required for manholes.
As regards the drills themselves something may be learnt from W. S. Hall's paper (* M. E.,' 1878, p.
Fig. 212 565), as well from W. F. Smith's (ibid. 1883, p. 56), who discusses the cutting angles of tools and drills and speeds.
Time required for Drilling.–To drill a hole 1 in. deep per minute, and to spend about half a minute for setting, seems to be a fair allow
a ance for holes of from & in. to 1} in. diameter. W. F. Smith gives a circumferential drill speed of 20 ft. per minute with too in. feed, while
J. H. Wicksteed, in the discussion which followed, recommends 40 ft. and do in., so that as regards speed of feed the result is the same in both cases. For 1-in. holes this means a drilling speed of about 1 in. per minute, which is a very high value.
Hand drilling is much slower, being at the rate of about 5 to 6 ins. depth of 1-in. holes per hour, and about 3 minutes for setting. Smaller holes can be drilled more quickly, and, generally speaking, the labour is proportional to the weight of metal removed. The limiting conditions for automatic feed are the strength of the drill and the clearance for the borings. In both respects the twist drill is superior to the ordinary one, but very few boiler shops have retained it in use.
It is stated that after a time these drills wear away near their ends, and grow taper, and get jammed in the holes and break. Another and probably more powerful reason is that sufficient care is not taken to run them perfectly true, nor to guide them as required, nor to sharpen them correctly; and if all or nearly all the work is thrown on one cutting edge, it cannot be expected that the result will be a satisfactory one. The irregular action which takes place with ordinary drills if one cutting edge is longer than the other is sometimes made use of to produce taper holes. The deeper the drill penetrates when in this condition, the larger grows the diameter of the hole. This is of course impossible with twist drills, as they are guided by the hole they make.
Irregularly-shaped Holes.-Even ordinary drills are very liable to produce irregularly-shaped holes. The action which takes place is easily understood by examining fig. 213, in which a two-cornered spindle moves freely, but without slack, in a three-cornered hole. A three-cornered spindle would do the same in a four-cornered hole (fig. 214), and this principle has been made use of for producing square holes.
As regards the general practice, it will be found that in nearly all works the shell plates are drilled after they have been bolted together with the flanged end plates fitted into them, the holes being drilled
through shell plates and flanges in one operation. A few years ago the circumferential shell seams were often drilled before the plates were bent, and considerable ingenuity was displayed in devising means for ensuring that the holes in the two strakes should cor
respond. But this practice has died Fig. 213 Fig. 214
out, except in small works, and vertical
multiple drilling machines are now used chiefly for the holes in the seams of the end plates and of the combustion chambers. If, as is usual, several plates are being drilled at a time, they should be screwed together by properly fitted bolts, and not clamped only; otherwise they slide on each other, and the holes of the lower plates will be out of place. Twist drills should be used for this work, because ordinary ones do not drill in a straight line.
Marking off Holes.-Should it be necessary to drill shell plates before bending, it is very important to make accurate templets both