The electric drill is a portable drilling machine, powered by its own electric motor carried in the case. Apart from the motor, the essential components are the chuck, which holds the drill, and a simple gear train for gearing the speed of the motor down to a suitable speed for the drill.
The domestic electric drill usually takes the shape of a pistol, the pistol grip being a good example of ergonomic design, that is, correctly designed for easy usage. The power cord enters the case at the base of the grip, and the on-off switch is the trigger. Often an interlock is provided so that the trigger need not be held down continuously while using the drill. On larger, heavier models there are other handles on the case as well as the grip so that both hands can safely be used to bring pressure on the work. The other handle may be a simple bar extending from the case at the top, opposite the grip; it may be a stirrup at the back end of the case, opposite the chuck; or its position may be adjustable.
The chuck
The chuck is a three-jawed, self-centering device that protrudes from the transmission end of the case and holds the drill bit. Turning the outer sleeve of the chuck in a counter-clockwise direction opens the jaws; the other direction closes it, final tightening being achieved with the use of a key supplied with the drill.
The motor
Housed in the case is an electric motor of the series, or universal, type. The advantages of this type of motor are that it is suitable for use with either direct or alternating current and that it produces a high torque at low speeds: as one pushes harder while drilling, thus increasing the load, the speed decreases but the torque increases. The gear train reduces the speed to about 2,500 rpm. A disadvantage of this type of motor is that it provides interference with near-by radio and TV reception, but it can be overcome by fitting various chokes and capacitors to the motor circuitry.
Drill attachments
Drill bits are normally of the twist-drill type, having two helical grooves running from the twin cutting edges for about three-quarters of the length, the remainder being plain shank for inserting into the chuck. The capacity of electric drills ranges from ¼ in. (6.35 mm) to ½ in. (12.7 mm). Sizes larger than ½ in. are usually difficult to handle in a portable power tool.
An electric drill with a given capacity is designed to handle a drilling job of that size in drilling metal; a drill bit of a larger size, with a cut-down shank to fit the chuck, may be used with caution to drill wood or plastic. Caution is always necessary when drilling metal. Certain types of steel may need a very hard cutting edge, a specially ground angle on the cutting edge, lubrication while drilling, or all three. When drilling a hole all the way through a piece of metal, the pressure brought to bear must be carefully applied as the bit goes through, otherwise the emerging bit may "grab" the rough edge of the hole, giving the tool a severe wrench that can cause loss of control or snap off the bit itself.
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ELECTRIC DRILL |
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This Kango hammer drill has been designed to make light work of concrete, brick, or masonry. The gearing has been structured to allow the motor to be placed near the handle, which improves the balance of the drill. An important consideration for industrial drill users is the "down time" suffered when a drill is out of action. A partial solution is to make the drill in modules—mechanism, motor, and control handle. (1) Drawback drill holder: operator serviceable (2) External labyrinth dustproof seal (3) Patented hammer stop for instant drilling change (4) All-plastic body with no exposed metal: prevents electric shocks (5) Switch dust seal: prolongs switch life (6) Variable-speed switch for precise starting of holes (7) Control handle: contoured for comfort (8) Brush stops: automatic power disconnection at service intervals (9) Safety clutch: protects operator and machine (10) Specialized gearing: gives better balance (11) Adjustable front handle: rotates through full 360 degrees (12) Sealed anvil: protects mechanism from dust
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Two-speed and percussion electric drills are also available. The first provides a lower alternative speed of about 900 rpm; the second gives a percussive effect combined with low speed and is designed for use on concrete and masonry. Because of the abrasive nature of concrete and masonry, drill bits with specially hardened cutting edges must be used—these are normally sold as masonry bits.
The versatility of the electric drill is extended by the availability of attachments designed to make use of the rotary motion, such as rotary files, sanding disks, hole saws, grindstones, and so on. An electric drill is often designed to be fitted to a bench- or table-mounted machine to turn it into a drill-press or a lathe attachment or for certain applications such as use with a jig.
