Ch 5 drilling

Drilling
 Drilling is a operation that cuts cylindrical holes.

In a machining operation , if the generatrix
and directrix both are straight lines, the
surface obtained is
a)Cylindrical
b)Helical
c)Plane
d)Surface of revolution
GATE-2015

TYPES OF DRILL PRESSES
 Vertical or pillar type
 Radial Arm type
 Gang drill
 Multi Spindle drill
 Numerical Control drill

Drill
 The twist drill does most of the cutting with the tip of
the bit.
•There are flutes
to carry the chips
up from the
cutting edges to
the top of the
hole where they
are cast off.

IES - 2004
Consider the following statements:
The helical flute in a twist drill provides the necessary
1. Clearance angle for the cutting edge
2. Rake angle for the cutting edge
3. Space for the chip to come out during drilling
4. Guidance for the drill to enter into the workpiece
Which of the statements given above are correct?
(a) 1 and 2 (b) 2 and 3
(c) 3 and 4 (d) 1 and 4

IES - 2003
The purpose of helical grooves in a twist drill is to
1. Improve the stiffness
2. Save a tool material
3. Provide space for chip removal
4. Provide rake angle for the cutting edge
Select the correct answer using the codes given below:
Codes:
(a) 1 and 2 (b) 2 and 3
(c) 3 and 4 (d) 1 and 4

Drill
 Axial rake angle is the angle between the face and the line
parallel to the drill axis. At the periphery of the drill, it is
equivalent to the helix angle.
 The lip clearance angle is the angle formed by the portion of
the flank adjacent to the land and a plane at right angles to
the drill axis measured at the periphery of the drill.
 Lead of the helix is the distance measured parallel to the drill
axis, between corresponding point on the leading edge of the
land in one complete revolution.

Drill
 Drill sizes are typically measured across the drill points with
a micrometer
 Most widely used material is High Speed Steel
 The drill blanks are made by forging and then are twisted to
provide the torsional rigidity. Then the flutes are machined
and hardened before the final grinding of the geometry.
 Deep hole drilling requires special precautions to take care of
the removal of large volume of chips.

GATE- 1996
The rake angle in a drill
(a) Increases from centre to periphery
(b) decreases from centre to periphery
(c) Remains constant
(d) Is irrelevant to the drilling operation

IES - 1997
The rake angle in a twist drill
(a) Varies from minimum near the dead centre to a
maximum value at the periphery
(b) Is maximum at the dead centre and zero at the
periphery
(c) Is constant at every point of the cutting edge
(d) Is a function of the size of the chisel edge.

Point Angle (2β)
 The point angle is selected to suit the hardness and brittleness of
the material being drilled.
 Harder materials have higher point angles, soft materials have
lower point angles.
 An increase in the drill point angle leads to an increase in the
thrust force and a decrease in the torque due to increase of the
orthogonal rake angle.
 This angle (half) refers to side cutting edge angle of a single point
tool.
 Standard Point Angle is 118°
 It is 116° to 118° for medium hard steel and cast iron
 It is 125° for hardened steel
 It is 130° to 140° for brass and bronze
 It is only 60° for wood and plastics

Helix Angle (ψ)
 Helix angle is the angle between the leading edge of the
land and the axis of the drill. Sometimes it is also called
as spiral angle.
 The helix results in a positive cutting rake
 This angle is equivalent to back rake angle of a single
point cutting tool.
 Usual – 20° to 35° – most common
 Large helix : 45° to 60° suitable for deep holes and softer
work materials
 Small helix : for harder / stronger materials
 Zero helix : spade drills for high production drilling
micro-drilling and hard work materials

IES - 1992
A drill for drilling deep holes in aluminum should
have
(a) High helix angle (b) Taper shank
(c) Small point angle (d) No lip

GATE- 1997
Helix angle of fast helix drill is normally
(a) 35o
(b) 60o
(c) 90o
(d) 5o

IES - 1992
Low helix angle drills are preferred for drilling
holes in
(a) Plastics (b) Copper
(c) Cast steel (d) Carbon steel

IFS-2011
Discuss deep-hole drilling keeping in mind speed and
feed, mentioning the technique of applying coolant.
[5-marks]

Cutting Speed in Drilling
 The cutting speed in drilling is the surface speed of the
twist drill.
/ min
1000
DN
V m



Drilling Time
 Time for drilling the hole
, min
L
T
fN


MRR in Drilling
2
3
, / min
4
D
MRR fN mm
 
  
 

Example
A hole with 40-mm diameter and 50-mm depth is to
be drilled in mild steel component. The cutting
speed can be taken as 65 m/min and the feed rate as
0.25 mm/rev. Calculate the machining time and the
material removal rate.

Some Formulae for Drilling
 1
( )
2tan
( ) sin
2
( )
2sin
2 / tan
( ) tan
sin
D
Coneheight h
f
Uncut chipthickness t
D
Widthof cut b
r D
Orthogonal rakeangle










 
  
 

GATE- 2002
The time taken to drill a hole through a 25 mm thick
plate with the drill rotating at 300 r.p.m. and
moving at a feed rate of 0.25 mm/revolution is
(a) 10 sec (b) 20 sec
(c) 60 sec (d) 100 sec

GATE- 2004
Through holes of 10 mm diameter are to be drilled
in a steel plate of 20 mm thickness. Drill spindle
speed is 300 rpm, feed 0.2 mm/ rev and drill point
angle is 120°. Assuming drill over travel of 2 mm, the
time for producing a hole will be
(a) 4 seconds (b) 25 seconds
(c) 100 seconds (d) 110 seconds

GATE- 2012
In a single pass drilling operation, a through hole of
15 mm diameter is to be drilled in a steel plate of 50
mm thickness. Drill spindle speed is 500 rpm, feed
is 0.2 mm/rev and drill point angle is 118°. Assuming
2 mm clearance at approach and exit, the total drill
time (in seconds) is
(a) 35.1 (b) 32.4
(c) 31.2 (d) 30.1

IES - 2002
The arm of a radial drilling machine is being raised
at a speed of 3.9 m/min by single start square
threads of 6 mm pitch and 30 mm diameter. The
speed of the screw
(a) Is 650 rpm
(b) Is 180 rpm
(c) Is 130 rpm
(d) Cannot be determined as the data is insufficient

IES - 1994
The ratio between two consecutive spindle speeds
for a six-speed drilling machine using drills of
diameter 6.25 to 25 mm size and at a cutting velocity
of 18 m/min is
(a) 1.02 (b) 1.32
(c) 1.62 (d) 1.82

IES - 2009
What is the drilling time for producing a hole in an
MS sheet of 25 mm thickness using an HSS drill of
20 mm diameter? The cutting speed and feed for
drill are 20 m/min and 0.25 mm/revolution
respectively, Neglect time taken for setting up,
approaching and travelling of tools.
(a) 0.314 min (b) 0.236 min
(c) 0.438 min (d) 0.443 min

IES - 2002
A 31.8 mm H.S.S. drill is used to drill a hole in a cast
iron block 100 mm thick at a cutting speed 20
m/min and feed 0.3 mm/rev. If the over travel of
drill is 4 mm and approach 9 mm, the time required
to drill the hole is
(a) 1 min 40 s (b) 1 min 44 s
(c) 1 min 49 s (d) 1 min 53 s

GATE - 2014
A hole of 20 mm diameter is to be drilled in a steel block
of 40 mm thickness. The drilling is performed at
rotational speed of 400 rpm and feed rate of 0.1 mm/rev.
The required approach and over run of the drill together
is equal to the radius of drill. The drilling time (in
minute) is
(a) 1.00 (b) 1.25 (c) 1.50 (d) 1.75

GATE-2014 (PI)
An HSS drill of 20 mm diameter with 5 mm cone
height is used to drill a through hole in a steel work-
piece of 50 mm thickness. Cutting speed of 10
m/min and feed rate of 0.3 mm/rev are used. The
drilling time, in seconds, neglecting the approach
and over travel, is _______

IAS - 1999
To drill a 10 mm diameter hole through a 20 mm
thick M.S. plate with a drill bit running at 300 rpm
and a feed of 0.25 mm per revolution, time taken
will be
(a) 8 s (b) 16 s
(c) 24 s (d) 32 s

GATE – 2007 (PI) Linked S-1
Blind holes 10 mm diameter, 50 mm deep are
being drilled in steel block. Drilling spindle
speed is 600 rpm, feed 0.2 mm/rev, point angle of
drill is 120o.
Machining time (in minutes) per hole will be
(a) 0.08 (b) 0.31 (c) 0.44 (d) 0.86

GATE – 2007 (PI) Linked S-2
Blind holes 10 mm diameter, 50 mm deep are
being drilled in steel block. Drilling spindle
speed is 600 rpm, feed 0.2 mm/rev, point angle of
drill is 120o.
During the above operation, the drill wears out
after producing 200 holes. Taylor’s tool life
equation is of the form VT0.3 = C, where V =
cutting speed in m/minute and T = tool life in
minutes. Taylor’s constant C will be
(a) 15 (b) 72 (c) 93 (d) 490

IAS - 1994
The time (in minutes) for drilling a hole is given by
where 'h' is the
(a) Length of the drill
(b) Drill diameter
(c) Flute length of the drill
(d) Cone height of the drill.
RPMFeed
hholetheofDepth
t




IES - 1999
Match List-I (Drill bits) with List-II (Applications) and select the correct
answer using the codes given below the Lists:
List-I List-II
A. Core drill 1. To enlarge a hole to a certain depth so as to
accommodate the bolt head of a screw
B. Reamer 2. To drill and enlarge an already existing hole in a
casting
C. Counter bore drill 3. To drill a hole before making internal
thread
D. Tap drill 4. To improve the surface finish and dimensional
accuracy of the already drilled hole
Code:A B C D A B C D
(a) 1 3 2 4 (b) 2 3 1 4
(c) 2 4 1 3 (d) 3 2 4 1

Ch 5 drilling

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