What is drilling machine

A drilling machine is one of the most important machine tools in a workshop is a material removing or cutting process in which the tool uses a drill bit to cut into solid materials a hole of circular cross-section.

While performing drilling operation rotating edge of a cutting tool known as the twist drill which exerts a large force on the work clamped on a table.

The Egyptians drilled holes about 3000 years ago by bow drills in 1200 B.C.

There are different types of drilling machine used to perform the drilling operation which is following below. 

Types of  Drilling Machine :

  1. Portable drilling machine
  2. Sensitive drilling machine
  3. Upright drilling machine
  4. Radial drilling machine
  5. Gang drilling machine
  6. Multiple spindle drilling machine
  7. Automatic drilling machine
  8. Deep hole drilling machine

Drilling machine Operations :

  1. Drilling
  2. Reaming
  3. Boring
  4. Counterboring
  5. Countersinking
  6. Spot facing
  7. Tapping
  8. Lapping
  9. Grinding
  10. Trepanning

Types of the drill commonly used are : 

  • Flat or spade drill
  • Straight fluted drill
  • Two-lip twist drill
Parallel shank (short series or "Jobbers" twist drill)
Parallel shank (stub series) twist drill
Parallel shank (long series) twist drill
Taper shank twist drill

  • Taper shank core drill
  • Oil tube drill
  • Centre drill

Drill Size :

Drills from 0.2 to 100 mm are commonly produced in the metric system.

In the British system, drills are manufactured in three different sizes.

1. Number Sizes :

The drill size range from No. 1 to No. 80. 

Number 80 is the smallest having diameter equal to 0.0135 inches and the number 1 is the largest diameter equal to 0.228 inches.

2. Letter sizes :

The drill size range from A to Z.

A being the smallest having diameter equal to 0.234 inches and Z being the largest having diameter equal to 0.413 inches.

3. Fractional sizes :

The drill sizes range from 1/64 inch to 5 inches.

Designation of Drill :

Twist drills are designated in the Indian standard system by the series they belong to, the diameter, the I.S. Number and drilling material.

The drill is made in three types, namely, normal (N), hard (H), soft (S).

Thus a long-series 10 mm dia parallel shank twist drill conforming to I.S. Standard, made from carbon steel, type S and point angle 80 degrees are designated as follows. 

Parallel shank twist drill (Long) 10.00 - IS:599-CS-S-80

Drill Material :

For one piece of construction, high-speed steel or carbon steel are used. 

For two-piece of construction and cutting portion, high-speed steel is used.

For two-piece of construction and shank portion carbon steel with a minimum tensile strength of 70 kg per sq mm.

High-speed drills are more widely used due to its greater cutting efficiency.

Cutting Speed :

The cutting speed in a drilling operation is the peripheral speed of a point on the surface of the drill in contact with the work.

It is usually expressed in meters per minute.

 v = Π d n /1000 m per min

Where d is the diameter of the drill in mm and n is the r.p.m of the drill spindle.

Cutting speed of drill depends on several factors which are following below. 

  • The kind of material is being drilled. Softer material the higher the speed.
  • The cutting tool material. 

For example - Drill of high-speed steel can be operated at about twice the speed of drill of high carbon steel.

  • The efficient use of cutting fluid.
  • The quality of the surface finish desired.
  • The method of holding the work.
  • The size, type, and rigidity of the machine.

Feed :

The feed of a drill is the distance the drill moves into the work at each spindle revolution.

The feed per minute may be defined as the axial distance moved by the drill into the work per minute.

Sm = Sr × n

Sm = Feed per minute in mm.
Sr = Feed per revolution in mm.
n = r.p.m of the drill.

The amount of feed is depending upon :

  • Material being cut 
  • The rigidity of the job and machine
  • Depth of hole
  • Type of finish desired
  • Power available
  • Range of feed available

Depth of cut :

Depth of cut in drilling is equal to one half the diameter of the drill.

Thus if d be the diameter of the drill the depth of cut (t) is expressed as :

t = d / 2 mm

What is torque

The net force acting on a body accelerates it and takes something to give the body an angular acceleration. It needs a force, but it needs to be applied in a way that creates a twisting or turning action. Torque, τ is the rotational version of force and results from the application of one or more forces and is specified relative to a chosen rotation axis or pivot.

Torque is a measure of how much force an object act causes the object to rotate.

Torque is dependent upon :
  • The distance from the rotation axis to the force application point (Refer to the first figure).
  • The magnitude of the force, F.
  • The orientation of the force relative to the displacement from the axis to force application point (Refer to the second figure).

The torque that a force produces is defined by 
τ = R x F 

τ = R x F sinθ

In other words, torque is the cross product between the vector of distance (the distance from the pivot to the point where force is applied) and the vector of force, 'θ' being the angle between r and F.

Example :
Let’s say we’re using a 0.5m long wrench to tighten a wheel nut, and we need to lean on the far end of the wrench with a force of 50 Newtons to do it up tightly. Simply multiplying the two numbers give us the required torque figure in Newton meters.

Torque τ = 50 (N) x 0.5 (m) x = 25 Nm
The SI units of torque are Newton-meter (N.m).
Rotational Equilibrium is analogous to transnational equilibrium, where the sum of the forces is equal to zero. 

There may be more than one force that acts on an object, and each of these forces may act on an object at a different point. Then every force is going to cause torque. The net torque is the sum of the torques in each case.

The sum of the torques is equal to zero in rotational equilibrium. In other words, the object does not have a net torque.

∑τ = 0