1. What is the drilling process?

  • Drilling is an operation of making a circular hole by removing a volume of metal from the job by cutting a tool called a drill.
  • In the olden days, a flat drill was used for drilling a hole, but these days, a twist drill is universally used.
  • A twist drill is the cutting tool and it is used in conjunction with a drilling machine.
  • Besides drilling round holes, many other operations can also be performed on the drilling machine such as counterboring, countersinking, honing, reaming, lapping, sanding, etc.
Drilling process
Fig.1 Drilling of a hole

2. What are the drills?

  • Drills typically have high length-to-diameter ratios hence, they are capable of producing relatively deep holes.
  • The chips that are produced within the hole move in a direction opposite to the forward movement of the drill.
  • Thus, chip disposal and ensuring cutting-fluid effectiveness can present significant difficulties in drilling.
  • Drills generally leave a burr on the bottom surface upon breakthrough, necessitating deburring operations.
  • The diameter of a hole produced by drilling is slightly larger than the drill diameter (oversize), as one can note by observing that a drill can easily be removed from the hole it has just produced.
  • The amount of oversize depends on the quality of the drill and of the equipment used, as well as on the machining practices employed.
  • For better surface finish and dimensional accuracy, drilled holes may be subjected to subsequent operations, such as reaming and honing.

3. What are the different types of drills?

The different types of drill are as follows:

A. Step drill

B. Core drill

C. Counterboring and countersinking drills

D. Center drill

E. Spot drill

F. Spade drills

G. Solid carbide and carbide tipped drills

4. Which is the most widely used drill?

  • A twist drill is the most widely used drill.
  • Twist drills usually have a tapered shank, at the end which is fitted into the drilling machine having a tapered sleeve of matching taper.
  • When the tapered sleeve rotates, the twist drill also rotates along with it due to the friction between two tapered surfaces.
  • Sometimes the shank is machined parallel, then a special collet chuck is fitted in the drilling machine, in which the drill is held.
  • The drill has two lips at the other end where the cutting takes place when the drill rotates.
  • The angle between the two cutting lips is usually 118°. The chips formed at the cutting edges are automatically guided upwards through the helical grooves cut into the body of the drill; these grooves are called flutes.
  • For rotating the drill and to overcome the resistance in cutting, a torque is needed to rotate the drill. An axial force is also needed which keeps pushing the drill deeper and deeper into the hole being drilled.
  • Twist drills are made of solid high speed steel, hardened and ground to shape.
  • Drills with tungsten carbide inserts are also available.

5. Describe the geometry of the Twist drill?

Fig.2 Twist Drill Geometry
  • The drill axis is the longitudinal centerline.
  • Drill point is the sharpened end of the drill body consisting of all that part which is shaped to produce lips, faces and chisel edge.
  • Lip or cutting edge is the edge formed by the intersection of the flank and face
  • Lip length is the minimum distance between the outer corner and the chisel-edge corner of the lip.
  • Face is that portion of the flute surface adjacent to the lip on which the chip impinges as it is cut from the work.
  • Chisel edge is the edge formed by the intersection of the flanks.
  • Flank is that surface on a drill point which extends behind the lip to the following flute.
  • Flutes are the grooves in the body of the drill, which provide lips, allow the removal of chips, and permit cutting fluid to reach the lips.
  • Flute length is the axial length from the extreme end of the point to the termination of the flutes at the shank end of the body.
  • Body is that portion of the drill nomenclature, which extends from the extreme cutting end to the beginning of the shank.
  • Shank is that portion of the drill by which it is held and driven.
  • Heel is the edge formed by the intersection of the flute surface and the body clearance. Body clearance is that portion of the body surface reduced in diameter to provide diametric clearance.
  • Core or web is the central portion of the drill situated between the roots of the flutes and extending from the pointed end towards the shank; the pointed end of the core forms the chisel edge.
  • Lands are the cylindrically ground surfaces on the leading edges of the drill flutes. The width of the land is measured at right angles to the flute.
  • Recess is the portion of the drill body between the flutes and the shank provided so as to facilitate the grinding of the body. Parallel shank drills of small diameter are not usually provided with a recess.
  • Outer corner is the corner formed by the intersection of the lip and the leading edge of the land.
  • Chisel edge comer is the corner formed by the intersection of a lip and the chisel edge.
  • Drill diameter is the measurement across the cylindrical lands at the outer corners of the drill.
  • Lead of helix is the distance measured parallel to the drill axis between corresponding points on the leading edge of a flute in one complete turn of the flute.
  • Helix angle is the angle between the leading edge of the land and the drill axis.
  • Rake angle is the angle between the face and a line parallel to the drill axis. It is bigger at the face edges and decreases towards the center of the drill to nearly 0°. The result is that the formation of chips grows more unfavorable towards the center.
  • Lip clearance angle is the angle formed by the flank and a plane at right angles to the drill axis; the angle is normally measured at the periphery of the drill. To make sure that the main cutting edges can enter into the material, the clearance faces slope backward in a curve. The clearance angle is measured at the face edge, must amount to 5° up to 8°.

6. What are the different types of drilling machines?

Drilling machines are of the following types:
(1)Portable drilling machine
(2) Sensitive drilling machine

Fig.3 Sensitive drilling machine

(3) Upright drilling machine
(a) Round column section
(b) Box column section machine
(4) Radial drilling machine

Fig.4 Radial drilling machine

(5) Gang drilling machine
(6) Multiple spindle drilling machine
(7) Automatic drilling machine
(8) Deep hole drilling machine
(a) Vertical
(b) Horizontal

7. What are the different operations performed on drilling machine?

Fig.5 Common operations associated with drilling

A. Core drilling: Holes made in castings by use of cores, are rough and require a special kind of drill, called core drill to clean up the holes. This operation is called core drilling.
B. Step drilling: More than one diameter can be ground on the drill body which saves an extra operation.
C. Counter boring: Often a flat surface is needed around a hole to provide a good seating area for the washer and nuts/head of a bolt. The counterboring tool has a pilot, which ensures that the counterbore is concentric with the hole.
D. Countersinking: Countersinking provides a tapered entrance to the hole. A special countersinking tool with a pilot is used as shown.
E. Reaming: Reaming is an operation of sizing and improving the geometry and finish of a previously drilled hole. Hand, machine, and shell reamers are used for this purpose. Machine reamers are used with a drilling machine. To work efficiently, proper stock allowance is very important. Reamers cannot remove much material, but at the same time, enough material should be available all around. For
holes up to 12.5 mm in diameter, about 0.38–0.4 mm of material is left as a reaming allowance. A reamer follows the original hole and cannot shift its center.
F. Tapping: Tapping is also done on a drilling machine with a special flexible adapter for holding machine taps. Tapping means cutting internal threads in a hole. A machine tap set consists of two taps rough and finish. Both taps should be used in the same order. During tapping, the spindle r.p.m. is drastically reduced and a good lubricant used.