5 October 2019

What is drilling? | Types of drills | Drilling operation formula

Drilling is the most common process of machining. The drilling process accounts for nearly 75% of all metal cutting material removed.

Drilling is an operation to create cylindrical holes by extracting the metal from solid material or by widening existing holes using multi-tooth cutting tools called drilling. There are various cutting tools available for drilling, but the most common is the twist drill.

The Egyptians drilled holes some 3000 years ago through bow drills in 1200 B.C. The bow drills are the mother of the current drilling machine for metal cutting. There are various types of drilling machine available in the market. 

Drilling characteristics:

  • The chips must exit out from the hole created by the drilling process. 
  • When chips are large or continuous, existing chips can cause problems. 
  • At the entrance and for deep holes, the drill can wander. 
  • Coolant may need to be delivered to the cutting front through the drill shaft for deep holes in large workpieces.
  • The most likely drilling on the drill press is by someone who is not a machinist in the process of powder metal cutting.
There are several apparatus needed during the drilling operation. 
  • Drilling machine
  • Center punch 
  • Hammer
  • Center drill 
  • Twist drill 
  • Coolant
  • Vernier calliper
  • Two flute drill set 
  • Center drill
  • Countersink drill 
  • Counterbore drill
  • Drill various diameter

Types of drills:

Step drill to produce a hole of two or more different diameters. 

Core drill to enlarge existing holes.

Counterboring and countersinking to produce depression on the surface to accommodate heads of screws and bolts. 

Center drill a short and stubby drill to produce holes so that workpiece can be mounted between lathe centres. 

Spot drill to start a hole. 

Spade drill to remove large and deep holes. 

Crankshaft drill for good centring. Suitable for deep holes. 

Gun drilling for deep hole making, self-centring, lubrication, and coolant passage. 

Trepanning removal of disk-shaped piece.  

Twist drills to remove the metal in large volume in a minimum period of time. 

The formula for drilling operation:

The cutting speed of the drilling operation is the peripheral speed of the cutting edge. 

Cutting speed = π D N 

Where D =  drill diameter
N = Rotational speed in rpm 

A feed is the distance the drill penetrates per revolution ( mm / rev ). 
Each cutting edge is f / 2. 

Depth of cut is taken as half the diameter of drilling. 

Depth of cut = D / 2

Drilling time is the time taken to complete the drilling operation. 

 T = L / f * N

Where, f = feed-in mm / rev

N = rotational speed ( rpm )
L = the sum of hole depth 

Material removal rate ( MMR ) is the volume of material removed by the drill per unit time.