The removal of the stub of a stud or bolt broken off short in the hole, leaving nothing to get hold of, is not such a hopeles job as it looks. Put a center-punch mark or "center-pop " exactly in the center of the broken bolt and drill a hole clear through it with a drill1/8 to 1/4 in. smaller in diameter than the bolt, accord ing to the size of the bolt, and drill straight, leaving a thin shell of the broken stud. The heat and vibration of the drilling frequently has the effect of loosening the stud, which is, of course, an advantage in the final operation. Then take a piece of square steel measuring diagonally, from corner to corner, one-sixteenth inch larger than the driled hole. . Taper it slightly so that the small end will just enter the hole, taking care that the corners are straight and sharp. Drive it lightly into the shell of the broken stud, just enough to make it grip, and with a wrench unscrew the shell. If it is found that the steel drift revolves without turning the stud, drive it a trifle harder; but remember that if driven too hard it will simply expand the shell and make it more difficult to remove.

A good way to loosen a stud that promises to be difficult to start is heat to a bright red a piece of iron that will just enter the hole drilled and leave it there until it cools to almost black. Then, with a squirt-can or in any other convenient way, apply cold water to the shell and immediately drive in your drift and apply the wrench. The purpose is to loosen up the rust by the alternate and unequal expansion and contraction of the stud and surrounding metal.

If all such efforts to dislonge the stump prove ineffectual, there is a last resort which seldom fails, but which should not be attempted except in case of necessity. With a narrow cape chisel or triangular file, cut a groove the full length of the hole drilled in the stub, cutting the groove just to the bottom of the thread. This will cut the shell so that the cape chisel, applied at the top near the groove, will close it into the hole, loosening it so that it will easily come out.

To place a hole with extreme accuracy, make the center-pop with a sharp-pointed punch, called a " prick-punch, " and describe a circle with your dividers just a trifle larger in diameter than the hole is to be. If the hole is to be small a plainly marked circle will be sufficient, but if the hole is over 1/4 in. make four center-pops with the prick-punch exactly on the circle at approximately equal distances apart. Start the drill, and before it has cut in far enough to bury the beveled part, stop drilling, and examine the work. If the edge of the started hole is exactly concentric with the circle, or equi-distant from each of the center-pops made on the circle, the hole is started true. If, however, the preliminary cut is off center, make a center-pop in the hole a trifle further from the exact center of the drill point than the distance by which the hole is out of true. Make the center-pop quite deep, so that the point of the drill will be drawn out of its original course enough to true the hole. Drill a trifle deeper and examine the cut, if necessary, repeating the operation. A little preliminary experimenting will make this quite clear. The process is called "drawing" a hole. When large drills are used, say from 5/8 in. up, instead of making a center-pop, a groove is cut down the side of the cut with a round-nosed chisel, as a center-pop would not be sufficient to draw so large a drill.

When drilling steel or wrought iron, the drill should be kept well oiled or it will soon overheat and lose its temper and, besides, will cut less freely. Cast iron and brass, however, must be drilled dry, and for the latter the speed of the drill should be as high as possible for small holes.

Grind twist and fluted drills so that the cutting edge is left projecting a little. If this is not done the drill will simply glide around on a beveled end, taking at most only a very slight scrape instead of a full cut. If a drill seems sharp but refuses to cut it will generally be found that the troubie is lack of "backing off, " as it is called. When grinding be careful to keep the point exactly in the center of the drill, for any little variation in its position will cause the hole drilled to be larger than the diameter of the drill. In fact, it is a common practice, where a hole of slightly larger size than the drill is wanted, to grind the tool a little off of center. This is a makeshift, of course, and is to be recommended only when there is no other way out of the difficulty.

A drill for enlarging a hole already drilled should have a small flat ground on the lip just behind the cutting edge. For ordinary drilling this would reduce the depth of cut and make the drill work very slowly and heat up quickly; but in enlarging a hole the drill has not the steadying influence of the point to guide it, and, as ordinarily ground would, as soon as it touched the metal, dig in viciously and stick. If the driving-power were greater than the resisting force, something would, of course, give way, and the something is likely to be the drill. Fluted drills are better adapted to enlarging holes than twist drills, the latter having more tendency to draw in than the former. Never trust to a drill for enlarging if you want the hole to be smooth and true, for the drill, having no support from the center, will " wabble" and make the hole irregular. A little caliper work on any enlarged hole will show this very readily. If there is no reamer at hand to ream a hole to the desired size, the hole can be plugged, the plug center-popped and a new hole of the correct diameter drilled through the plug.

To drill a hole in any metal that is too hard to be touched by a drill of ordinary hardness, try hardening in mercury or strong brine. Drilling very hard metal with a glasslike tool is, however, a very unsatisfactory business, and should be avoided, if possible. The proper hardening of tools is an important matter, and though long experience is necessary for the production of the best results, ordinary tempering may be done by the amateur with a little practice. Take a flat drill, for instance. Heat the drill to a bright red for about half an inch from the point and dip it vertically into the water, point first, completely submerging it. Do this quickly and then bring it up so that only the cutting edge and a little more of the drill remain in water, and hold it there for a few seconds. Take it out of the water, and, as quickly as possible, polish off one side of the point, brush your finger quickly over t and watch the colors appear on the clean place. If you want the drill very hard, wait until the cutting edge assumes a light straw color, and plunge it entirely under water, holding it there until it is comparatively cool. A dark blue color will give a cutting edge too soft for ordinary work, and between these two colors can be found any desired temper. This method will leave the shank of the drill soft, so that it will not snap under strain, as it would if it were made as hard as the edge. Another way to bring the drill to the correct temper is to harden it outright at once, and reheat until the proper colors appear; but this is not usually as satisfactory as the first method, which is the one generally used in shop practice. The point is hardened outright and the shank left soft and so hot that it transmits enough heat to the point to reduce the hardness to the proper degree. If the shank is too hot the colors will flash along the point so quickly that the proper one cannot be caught and the line between the hard and the soft part is apt to be too sharply defined. If too cold, there is not heat enough to do the work, and the drill will have to be re-heated to temper it.