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Simplifying the Drilling of Marked Off Holes By H. Moore a mix up of counterbored holes when there are LTHOUGH the standard procedure for equality is overcome. Needless to say, however, many so closely approximated that a glance at marking off the positions of drilled holes is the drill must still be fed in carefully, and it may the marked off circles would hardly reveal the well known there are some aspects that require further attention from the chisel, but such difference. This hint cannot be credited to fore deserve special attention. Chief among these, preliminary gashes are a great help. sight because it was from an actual occurrence perhaps, is the general disinclination to attempt Worst of all blow holes is the one that occurs of this error that the idea originated. But by any alteration of the standard rules. Surely it in the exact centre of the hole, FIG. 4. It is of stamping the particulars on the face of the work should be the aim of every good marker off to little use attempting to correct this with the aid the driller is spared the trouble of searching the simplify the work of the driller and not merely of a centre punch alone. The better way is to drawing and the afterwards unwanted informa to indicate where the hole is to be drilled. Should chisel the blow hole away as evenly as possible; tion is removed by the counterbore itself. he not study each job apart, note its peculiarities, concentric with the centre. On a valuable piece FIG . 11 shows how to deal with a row of small and seek to offset these with any reasonable inno of work it might pay to end-mill the blow hole holes. Circles around these are impossible and vations which promise an easier drilled hole? away, but for all ordinary purposes a chisel, skil generally a centre punch mark at the intersection There is no end to the assisting ways open to him. fully handled, will remove the defect. of each cross line is considered sufficient. But it It may be an extra circle here, a line there, a few A rather different use for a chisel is shown in centre pops elsewhere or, perhaps, a radical is not easy to tell when a small hole is out when TIG. 5. In this case it is used as a marking tool in departure from all accepted rules. The present only single intersecting lines are used. The better place of a centre punch. The reason for this is way is to use the box system in which double writer, whose experience includes both drilling that many surfaces are porous and centre pops intersecting lines are used, the boxes thus formed and marking off, has often adopted such expedi on such material are not easily distinguishable being of the same size as the drilled holes. The ents in his work and proposes, for the first time, from the tiny porous holes. Narrow chisel marks, marker off in this case—holding his punch firm to describe a few of his own origination. on the other hand, stand out clearly and a circle and straight—taps it until the mark completely of these is not likely to cause any confusion. Perhaps the best example of how a few extra fills the box. He can easily see when it is truly circles are useful is shown in FIG. 1. Here, besides For very accurate work only light punch marks inside the box and since the punch mark is the the usual hole-diameter circle and the slightly are permissible and if only the usual four of these same size as the drill it is impossible for the latter greater identifying circle, there are four others are made on the circle it is not unlikely that inside. The purpose of these is to show immedi trouble will be encountered in detecting them ately any tendency of the drill to run off. When when the actual drilling is being done. To ensure this is not done it is difficult to detect any error easier detection, place three or more pops at until the drill is well spotted, and the larger the each side, FIG. 6. The chances of mistaking some spot the harder it is to pull the drill back. With thing else for a pop mark is thereby greatly a number of circles any error can immediately be reduced, if not eliminated altogether. detected and corrected before it has gone too far. One of the peculiar conditions attending the Next is the blow hole nuisance which is seldom drilling of cylindrical pieces is that the drill spot absent from drilling. Take the case of a couple spreads to the full diameter on the top before it of blow holes X at one side of the centre punch reaches the line on the sides, FIG. 7. For this indentation marking the centre of the hole, reason the drill may appear to be central when it FIG. 2. If these are not attended to the drill will is not. What is needed is a guiding line that will almost certainly run over on that side. But if a show the progress of the drilling sideways before couple of chisel marks are cut approximately as it is too late to correct an error. This is served to deep and in the same positions on the opposite the best extent by placing one leg of the dividers to stray from centre. The marker off, in effect, side, the resistance to the drill will be equalized on the circle and scribing a radius at each side. starts the drill since the box is the same size and and the work of the driller facilitated. This oval shape closely approximates the shape the punch mark fills it. A similar method of equalizing resistance is of the spot made by a drill on a round surface. For centre drilling eccentric shafts the same shown in FIG. 3, which indicates a boss with a Drilling out a square hole that will leave as idea can be used. In FIG. 12 one end of what is to slightly inclined surface. Normally the drill little as possible for finishing either by punching be an eccentric shaft is shown. A double circle would run down the lower side but if the high or machining is a job that requires careful has been scribed to give the right amount of side is relieved by a few chisel gashes the in marking off. One of the best practical ways is eccentricity, a straight line scribed from the shown in FIG. 8. With the square marked off, centre and—with the shaft turned slightly— four small holes are set out in the corners. These another line to form a complete box the same are drilled first and the main central hole—the size as the diameter of the centre drill. The other diameter of which is often better when slightly end of the shaft is, of course, treated in the same greater than the width of the square—last. way at the same setting. If the boxes thus formed A glance at the illustration will show how little are filled with the centre punch indentations the metal remains to be removed before finishing the resulting centres will be exactly in line without square. It may be added that this method has undue care on the part of the driller. been used for blind square holes with pro This concludes the number of selected ex nounced success—a common square drift easily amples but by no means concludes the number of removing all that was left. ways in which a marker off can show profitable The removal of an irregular-shaped piece of consideration for the man who drills the holes. sheet metal is often effected by the trepanning But, perhaps, enough has been said to indicate method. A row of holes following the line of the the necessity for studying the other fellows' diffi cut-out allows the piece to be broken out and the culties and attempting to assist them. After all, jagged edges afterwards smoothed down to the the marker off is usually a highly skilled man and line. It is this part of the work that is dealt with it should be his function to apply that extra skill in FIG. 9. If only one line is made the finishing to overcome the underskill of the man who might operation is apt to stray and there is no way of be detailed to drill to his marks. telling how far or by how much. An identification line similar to the one used with a circle is a diffi cult proposition except in one way, and that is by using a flat scriber, about 1/16 wide. This is run round the template used to mark the shape of the hole and centre pops indented at each side of the resulting thick line. In this way an outer line is obtained that is an exact duplicate of the inner—and true—line. FIG. 10 illustrates a useful dodge for avoiding 366 Aircraft Engineering
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Nov 1, 1947
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