Drilling

By S K Mondal

DrillingDrilling is a operation that cuts cylindrical

holes.

Video

TYPES OF DRILL PRESSES

Vertical or pillar type

Radial Arm type

Gang drill

Multi Spindle drill

Numerical Control drill

Video

Drilling Operations

Chip formation of a drill

DrillThe 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.

Drill

IES - 2004Consider the following statements:The helical flute in a twist drill provides the necessary1. Clearance angle for the cutting edge2. Rake angle for the cutting edge3. Space for the chip to come out during drilling4. Guidance for the drill to enter into the

workpieceWhich of the statements given above are correct?(a) 1 and 2 (b) 2 and 3(c) 3 and 4 (d) 1 and 4

IES - 2003The purpose of helical grooves in a twist drill is to1. Improve the stiffness2. Save a tool material3. Provide space for chip removal4. Provide rake angle for the cutting edgeSelect 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

DrillAxial 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.

DrillDrill 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- 1996The 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 - 1997The 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 ironIt is 125° for hardened steel It is 130° to 140° for brass and bronzeIt 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 rakeThis angle is equivalent to back rake angle of a

single point cutting tool.Usual – 20° to 35° – most commonLarge helix : 45° to 60° suitable for deep holes

and softer work materialsSmall helix : for harder / stronger materialsZero helix : spade drills for high production

drilling micro-drilling and hard work materials

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

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

(d) 5o

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

Cutting Speed in DrillingThe cutting speed in drilling is the surface

speed of the twist drill.

/ min1000

DNV m

Drilling TimeTime for drilling the hole

, minL

TfN

MRR in Drilling

23, / min

4

DMRR 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

( )2 tan

( ) sin2

( )2sin

2 / tan( ) tan

sin

DConeheight h

fUncut chip thickness t

DWidthof cut b

r DOrthogonal rakeangle

Video

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

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 - 1994The 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

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