Drilling By S K Mondal Drilling Drilling is

Drilling By S K Mondal

Drilling �Drilling is a operation that cuts cylindrical holes.

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

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Drilling Operations

Chip formation of a 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.

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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 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 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) 35 o (b) 60 o (c) 90 o (d) 5 o

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

Cutting Speed in Drilling �The cutting speed in drilling is the surface speed of the twist drill.

Drilling Time �Time for drilling the hole

MRR in Drilling

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

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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

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.