Drills and Drill Bits

The following are the commonly used portable drills these days.

Traditional Hand Drill: This portable drill is also known as egg-beater drill. It can only handles holes up to ¼” in diameter.

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Breast Drill: In this, the breastplate is attached at the upper end of the drill in order to permit the use of body weight to increase the cutting power. These drills are used to drill small holes horizontally.

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Pneumatic Power Drill or Air Drill: It is driven by compressed air stored in small cylinders. They are safer to run and are preferably used around places with flammable materials.

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Drill bits are usually made from high carbon steel and are used for drilling holes. They are available in various forms to meet different requirements. High speed or alloy drills contain 14% tungsten and can be used at higher cutting rates. Thus these drills can also withstand high temperature. The following are the main types of drill bits in use.

Twist Drill Bit: It is a cylindrical tool that is rotated to cut and enlarge holes in materials. These are the most common type of drill bit in which the shank provides the drive. It has spiral flutes running the length of the body and a conical point with cutting edges. The flute clears out the swarf and paves way to allow coolant.

To provide extra strength, the web of the drill increases in thickness from the tip to the shank. The diameter of a twist drill can be expressed in fractions, letters or numbers. On large drills, the web may be thinned by local grinding to reduce the size of the non-cutting edge. The cutting angle is 59^o from the centre line with the edges being of equal angle and length. It may be altered to suit specific drilling operations. Normally, the angle between the web and the cutting edge is 130^o and the clearance angle is 12^o.

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There are various convenient twist drill grinders available to sharpen twist drills. Instead, the following steps can be carried out for sharpening drills.

1. Adjust the grind tool rest to a convenient position.
2. Hold the drill between the thumb and index finger of right/left hand and grasp the body of the drill near the shank with the other hand.
3. Place the hand on the tool rest with the centreline of the drill making 59^o angle with the cutting face of the grinding wheel.
4. Slightly lower the shank end of the drill.
5. Slowly place the cutting edge of the drill against the grinding wheel and twist the drill in a clockwise direction.
6. Maintain the same pressure against the grinding surface until the heel of the drill is reached.
7. Check the results of grinding with a gauge to ensure the lips are of same length and at 59^o angle.

Flat Drill Bit: It is used to finish the bottom of a blind hole.

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The cutting speed depends on drill size, type and the material being drilled. The larger the drill or the harder the material, the slower the rpm. Most drill bits are placed in a self-centring chuck on the drilling machine. A Morse Taper is present to provide stronger grip that is required with larger drills. It engages directly with the tapered adapter of the machine. A slot is provided in the adapter to remove the drill by inserting a wedge on top of the drill tang and tapping the wedge with a hammer in order to free the drill.

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Lubricating the drill prevents excessive heat by reducing friction and helps preserves the temper of the machine. Cutting oil, soapy water, turpentine and paraffin are the lubricants used depending on the type of material.