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In family life, so-called spiral drills are the most common, which due to their configuration are suitable both for solid steel and also for relatively soft wood ( Fig. 1 ).

TECHNOLOGY SHARPENING SVERL At what time the cutting edge of the spiral drill is embedded in some material, it "forces" the chips to slide along their front surface. By the way, education of chips is a rather complex process with a shift of individual particles, plastic deformation also other phenomena, which in this case will not be considered in any way. We only note that when drilling a brittle material, such as cast iron, so-called free-flowing chips are formed; if the material is plastic, let's say copper, then there will be a draining chip, similar to a coiled tape in a spiral. However, this kind of crushing is relatively arbitrary, since the materials do not always have very pronounced properties, like many brittle plastics that, when heated, appear to behave like plastic material when chips appear.

When the drill is rotated, its cutting edges describe a conical surface, so the "bottom" of the hole also acquires a conical shape. To make it, the conical end of the drill is not forced to own any elements beyond its limits. A cutting edge should be located at the very end of the end - below the rest of its surface.

Then, in the language of professionals, the so-called "back angle" will be provided ( Figure 2 ).

To understand why it is needed, try removing the shavings from a wooden plate with a conventional knife, pressing the blade firmly against its surface. The most that will succeed is to scrape off some protruding fibers. Raise the blade above the die up to a specific location, forming a "back" angle, and it will also start tearing off the shavings ( Figure 3 ). The "rear" corner does not have to be too large, otherwise the blade "dives" at once to an impressive depth, and it will be necessary to tear off the thick shavings with considerable effort.

That touches drills, intended, say, for finishing metal, then their "back" angle usually has limits of 5-10 degrees.

Now we will proceed directly to sharpening with the help of electrocautery. You can also do with an abrasive bar, removing excess metal manually, but this will take a lot of time and effort. It is easy to manufacture the electric point with your own hands, using an electric motor with a power of 100-300 W with a shaft rotation speed of 1000-1500 rpm, and several parts for mounting the motor on the platform are also attached to the shaft of the abrasive wheel.

Details can be made in any auto repair shop or on some kind of undertaking, where the metal is processed.

Sharpening of the drills begins with the production of cutting edges, while focusing on the sections of the spiral grooves along which the chips are sliding. While holding the drill bit so that the sharpened edge is non-stop parallel to the axis of rotation of the abrasive wheel, remove the metal from the back surface until the light reflected from the edge no longer ceases to be caught by eyes. Do the same operation with the other edge.

Sharpening the Swallow

The angle between them, depending on the material, is approximately the same: for steel - 140 , for brass, bronze, duralumin - 110-120 , for soft aluminum, copper, plastic, logs - 90-100 degrees. An angle of 120 degrees is considered universal. It is this angle that the drills that are commercially available (see Figure 4).



It is clear that without sufficient experience it is quite difficult to get an angle of the right size "by eye". Therefore, use a template, cut out of thin cardboard or thick paper ( Figure 5 ).

When sharpening, make sure that the top of the drill is exactly on its axis, but the length of the edges would be the same, while they will move along the same trajectories as the drill rotates. But without the skill, it is not easy to determine whether the length of the edges is the same whether the top is also in its room, since the surfaces that reflect light from the left also on the right do not look the same. Therefore, inspect not the drill itself, but only its contour. To do this, take the drill in the right hand, but bend the left hand to close the top of the drill from three sides, placing it in a "corridor". The source of light - a window or a brightly lit wall (but not the light itself) - is forced to be on one of the ends of this "corridor". Inspect the drill with one eye at an interval of 15-20 cm ( Figure 6 ).



At what time you form the edges, proceed to the processing of their back surfaces, giving them a pre-conical shape. For inspection, use a conventional washer or other suitable piece with a diameter hole about one-third smaller than a sharpened drill (for example, for a nine-millimeter drill, the diameter of the hole in the washer must be about 6 mm ).

By attaching the "back" surface of the edges to the edges of the hole, it is easy to determine how much metal to remove ( Fig . 7 ). At what time the edges become conical, form the rear corners. At the same time, remove the metal so that on each millimeter of the arc of the circle the gap between the drill and the washer increases by 0.15-0.2 mm. To the sample, if the length of the portion of the rear surface that was opposite the edge of the hole in the washer is 4 mm, then the height difference is forced to be 0.6-0.8 mm (Fig. 8).



When forming the tapered edges also of the rear corner, be extremely careful also do not damage the cutting surface in any way. To do this, leave strips of 0.2-0.3 mm wide intact from which you remove the metal in the last, final passes ( Fig. 9 ).

Use only the closest to the end of the abrasive wheel, in order not to accidentally hit the cutting edge of the opposite side of the drill ( Fig. 10 ).

For every drilling, together with the cutting edges, there is also a so-called jumper - an edge formed by a pair of rear surfaces ( Figure 11 ). This edge does not shred the material, but it only pushes it apart, pushing it to the sides, forcing it for considerable effort. Obviously, the smaller the length of the jumper, the easier the drill will creep into the material, but it is impossible to eliminate the edge completely, otherwise the body of the drill in its central part can become very thin also to break.

Acting on the corner of the abrasive mug with a radius of fillet, not exceeding 0.5-1 mm , you can slightly increase the length of the cutting edges, thereby shortening the bridge ( Figure 12 ). For drills 4-6 mm in diameter, keep approximately 0.5-0.7 mm of its length, for thicker ones - 1-1.5 mm . With small drills (less than 3 mm in diameter ), it is not worth daring. But if the electric current is supplied with a fine-grained circle, then with a very small radius of the angle of the abrasive wheel, it is possible to reduce the length of the bridge also of a two-millimeter drill-only the hard eye would be a sharp eye ( Figure 13 ).

Drilling of sheet material requires its cunning. Anyone who at least once tried to use a spiral drill with a standard sharpening for this purpose knows that the holes do not turn out perfectly round. When you reach the other side of the sheet, the cutting edges, falling into the hole, capture extremely thick shavings, but the drill itself slips in the holder of the electric drill or its motor becomes common. Even worse, if the sheet breaks out of the hands, it also starts to rotate with the drill, then it's not far until the injury.

It is especially difficult to cope with sheets of stainless steel, copper, aluminum 10-20 times thicker than the drill diameter.

In this case, with a significant amount of work, it is necessary to turn a spiral drill with a diameter of at least 4-5 mm in a manner similar to a perforated drill for a log ( Figure 14 ).

The holes obtained with such a converted drill turn out to be completely round, on the reverse side of the sheet there will be almost no barbs, and the productivity will increase several times, because the corners of the drill will not cut the entire hole, but only the rings with a width of no more than 0.5 -1 mm ( Figure 15 ).

, final recommendation. If the drill breaks, do not rush it. A section of a drill with spiral grooves is also a shank - this is the ideal preform for the countersink to obtain grooves for the screw heads ( Fig. 16 ).

The depth of such depressions will be stable if a steel plate is placed under the piece ( Fig. 17 ).

print version
Journal "Science and Life" № 6-2000
Author: A. Golovy
PS The material is protected.
Date of publication 20.09.2003gg