Test: Drilling - Civil Engineering (CE) MCQ

Drilling machine:

• The drilling machine is one of the most important machine tools in a workshop and is second to lathe. It was primarily designed to originate a hole. Drilling machines are made in many different types and sizes, each designed to handle a class of work or specific job to the best advantage.

The different types of drilling machines are:

• Portable drilling machine
• Sensitive drilling machine
• Upright drilling machine
• Radial drilling machine
• Gang drilling machine
• Multiple spindle drilling machine
• Automatic drilling machine
• Deep hole drilling machine

Sensitive Drilling Machine:

• The sensitive drilling machine is a small machine designed for drilling a small hole at high speed in light jobs, the base of the machine may be mounted on a bench or on the floor.
• The maximum size of drill that can be held in a sensitive drilling machine is 12.5 mm.
• It consists of a vertical column, a horizontal table, a head supporting the motor and driving mechanism, and a vertical spindle for driving and rotating the drill.
• There is no arrangement for any automatic feed of the drill spindle.
• The drill is fed into the work by purely hand control. High-speed and hand feed are necessary for drilling small holes.
• High speeds are necessary to attain the required cutting speed by a small-diameter drill.
• Hand feed permits the operator to feel or sense the progress of the drill into the work so that the drill becomes worn out or jams on any account, the pressure on the drill may be released immediately to prevent it from breaking.

Machining time for drilling:

D = diameter, N = speed in rpm of the drill, L = Length of axial travel in mm, f = feed

• Drilling is the production of cylindrical holes of definite diameters in workpieces by using a multi-point cutting tool called a drill.
• Various cutting tools are available for drilling, but the most common is the twist drill.
• Almost all drilling operation is done using a twist drill. 
• It is called a twist drill as it has two or more spiral or helical flutes formed along its length. 
• The two basic types of twist drills are, parallel shank and taper shank twist drills are available below 13 mm size.
• The various parts of a drill can be identified from the figure:

Designation of drill:

• In the Indian standard system, twist drills are designated by the series to which they belong, the diameter, the I.S number, and the material of the drill.
• Unless mentioned in the designation, it should presume that the drill type is N and the point angle 118º.
• The drills are made in three types, namely,
  • Normal (N)
  • Hard (H)
  • Soft (S).
• These designations are based on the material to be cut and the design requirements of the drill.
•  The maximum size of a parallel shank twist drill is 13 mm.
• Thus a parallel shank twist drill of long series, 13 mm dia, conforming to I.S standard, made of carbon steel, of type S and point angle 80º is designated as:
• Parallel shank dwell twist drill (Long) 13.00- IS : 599-CS-S80

Types of Twist Drill:
Following are the types of twist drills:

• Short Series or Jobbers Parallel Shank Twist Drill
• Sub Series Parallel Shank Twist Drill
• Long Series Parallel Shank Twist Drill
• Taper Shank Twist Drill
• Taper shank Core Drill (Three or Four Fluted)
• Oil Tube Drill
• Centre Drills

Drilling machine:

• The drilling machine is one of the most important machine tools in a workshop and is second to lathe. It was primarily designed to originate a hole. Drilling machines are made in many different types and sizes, each designed to handle a class of work or specific job to the best advantage.

The different types of drilling machines are:

• Portable drilling machine
• Sensitive drilling machine
• Upright drilling machine
• Radial drilling machine
• Gang drilling machine
• Multiple spindle drilling machine
• Automatic drilling machine
• Deep hole drilling machine

Sensitive Drilling Machine:

• The sensitive drilling machine is a small machine designed for drilling a small hole at high speed in light jobs, the base of the machine may be mounted on a bench or on the floor.
• The maximum size of drill that can be held in a sensitive drilling machine is 12.5 mm.
• It consists of a vertical column, a horizontal table, a head supporting the motor and driving mechanism, and a vertical spindle for driving and rotating the drill.
• There is no arrangement for any automatic feed of the drill spindle.
• The drill is fed into the work by purely hand control. High-speed and hand feed are necessary for drilling small holes.
• High speeds are necessary to attain the required cutting speed by a small-diameter drill.
• Hand feed permits the operator to feel or sense the progress of the drill into the work so that the drill becomes worn out or jams on any account, the pressure on the drill may be released immediately to prevent it from breaking.

The helix angle is the angle between the leading edge of the land and the axis of the drill. Sometimes it is also called a spiral angle.

• The helix results in a positive cutting rake. This angle is equivalent to the back rake angle of a single-point cutting tool.
• The usual range of helix angle used in the drill is 20° to 35°.
• Large helix angle 45° to 60° suitable for deep holes and softer work materials.
• The small helix angle of less than 45° is suitable for harder and stronger materials.
• Zero helix angles are used in spade drills for high production drilling, micro‐drilling, and hard work materials.

Important Points

• An increase in helix angle is given for more quick removal of chips but a decrease in helix angle will give greater strength of cutting edges.
• The larger the value of helix angle lesser will be the power required in drilling.

Drilling:

• Drilling is a cutting process in which a hole is originated by means of a multi-point, fluted, end-cutting tool.
• As the drill is rotated and advanced into the workpiece, the material is removed in the form of chips that move along the fluted shank of the twist drill.

Important angles of a drill:
Chisel edge angle:

• The obtuse angle included between the chisel edge and the lip as viewed from the end of the drill.
• The usual value of this angle varies from 120º to 135º.

Rake Angle or Helix Angle: 

• It is the angle formed between the plane containing the drill axis and the lead edge of the land.
• It can be zero, positive or negative.
• Higher values are suitable for softer materials and lower values for harder materials.
• If the flute is straight, parallel to the drill axis then there would be no rake. if the flute is right handed then it is a positive rake, and if it is left-handed then the rake is negative.
• The usual value of rake angle is 30º although it may vary up to 45º for different materials.
• The smaller the rake angle, the greater will be the torque required to drive the drill at a given feed.

Point Angle:

• It is known as cutting angle.
• It is the angle included between the two opposite lips of a drill measured in a plane containing the axis of the drill and both lips.
• Smaller point angles are used for ductile materials and larger point angle for brittle materials.
• The point angle of a twist drill for general purpose work is 118°

Clearance angle:

• The angle formed between the flank and a plane normal to the drill axis, measured at the periphery of the drill.
• The angle is measured on the back side of the cutting edge.
• Lip clearance is the relief ground to the cutting edges to allow the drill to enter the metal without interference.
• The lip clearance angle should increase towards the center of the drill than at the circumference.
• This is because different points on the drill cutting edge follow different helical paths.
• Any point on the cutting edge at the circumference moves through a smaller helical angle than a point on the cutting edge near the center.
• This happens to be such due to the lead of the helix being the same in each case and hence the clearance angle given to the drill cutting edge should increase towards the center.
• The clearance angle is 12º in most cases. The clearance angle should be minimum to add rigidity and strength to the cutting edge.

Centre Lathe → Knurling
Milling → Slotting
Grinding → Dressing
Drilling → Counter-boring
Knurling
Knurling is the operation of producing a straight-lined, diamond-shaped pattern or cross lined pattern on a cylindrical external surface by pressing a tool called knurling tool. Knurling is not a cutting operation but it is a forming operation.
A lathe is used for many operations such as turning, threading, facing, grooving, Knurling, Chamfering, centre drilling

Counter - boring
Counter - boring is an operation of enlarging a hole to a given depth, to house heads of socket heads or cap screws with the help of a counterbore tool.

Dressing:
When the sharpness of grinding wheel becomes dull because of glazing and loading, dulled grains and chips are removed (crushed or fallen) with a proper dressing tool to make sharp cutting edges.
The dressing is the operation of cleaning and restoring the sharpness of the wheel face that has become dull or has lost some of its cutting ability because of loading and glazing.
Slot Milling:
Slot milling is an operation of producing slots like T - slots, plain slots, dovetail slots etc.

Drilling time can be calculated by;

where T = Machining time in min, L = (Approach Length + Thickness of plate) in mm, f = Feed (mm/rev), N = Speed in rpm (revolution per min)
Approach Length = D/tanθ
Where θ = Half drill bit angle, D = diameter of the hole
∵ The angle of a drill bit is not given in the question hence we will take approach length zero.
∴ L = Thickness of the plate
Calculation:
Given:
Thickness of plate (L) = 30 mm, Feed (f) = 1 mm/rev, Speed (N) = 60 rpm, Hole diameter (d) = 25 mm

T = 0.5 min
T = 0.5 × 60 sec
T = 30 sec
Hence drilling time will be 30 sec.
Important Points
If in the ques it is given that "Neglect approach and over travel" OR "Drill bit angle is not given" then effective length equal thickness of the workpiece.

Helix angle

• It is the angle formed by the leading edge of the land with a plane having the axis of the drill.
• Its usual value is 30°.
• The low-helix drill was developed primarily to drill brass and other thin materials like plastics. Because of its design, the low-helix drill can remove the larger volume of chips formed by high rates of penetration.
• High helix-drills are designed for drilling deep holes in aluminium, copper, die-cast material, and other metals where the chips have a tendency to jam in a hole. The angles vary between (35° - 45°).

Advantages of radial drilling machine

• Drill head can be moved up and down.
• Drill head can be moved along the radial arm
• Radial drilling machines are used for heavy job which cannot moved around easily
• Radial drilling machines used for drilling number of holes in the job
• Radial drilling machine is powerful machine used for drilling large diameter hole.