IES 1996

DrillingBy S K MondalDrillingDrilling is a operation that cuts cylindrical holes.

2Video3TYPES OF DRILL PRESSESVertical or pillar typeRadial Arm typeGang drillMulti Spindle drillNumerical Control drill

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Video5Drilling Operations

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Chip formation of a drillDrillThe 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.8Drill

9IES - 2004Consider the following statements:The helical flute in a twist drill provides the necessaryClearance angle for the cutting edgeRake angle for the cutting edgeSpace for the chip to come out during drillingGuidance 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

Ans. (b)10IES - 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

Ans. (c)

11DrillAxial 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.12DrillDrill sizes are typically measured across the drill points with a micrometerMost widely used material is High Speed SteelThe 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.Deep holes more than three times the diameter of the hole are difficult to be produced by conventional drilling. This is because of the large volume of chips generated. The work materials that produce continuous chips will further compound this problem. Special deep hole drilling methods are to be used which are expensive.

13GATE- 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 operationAns. (a) Rake angle of a drill varies along the cutting edge ( or lips ), being negative close to the point and equal to the helix angle out at the lip.14IES - 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.Ans. (a)15Point Angle (2)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 118It 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 plastics16Helix Angle ()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 materialsSpade drills Spade drills are used to make holes with smaller diameter using low cutting speeds and highfeed rates. These have long supporting bar with the cutting blade attached at the end. These are lessexpensive since the support structure can be made more rigid using ordinary steel with no spiral flutes. Spadedrills are also used to machine small conical shapes for subsequent drilling or making a bevel (similar tocountersinking) on the existing holes to facilitate the subsequent tapping and assembling operations.

17IES - 1992A drill for drilling deep holes in aluminum should have(a)High helix angle(b)Taper shank(c)Small point angle (d)No lipAns. (a)18GATE- 1997Helix angle of fast helix drill is normally(a)35o (b)60o (c)90o (d)5oAns. (a) drill very rapidly use helix angle above 300 .The helix angle of most drill = 240

19IES - 1992Low helix angle drills are preferred for drilling holes in(a)Plastics(b)Copper(c)Cast steel(d)Carbon steel

Ans. (d)

20IFS-2011Discuss deep-hole drilling keeping in mind speed and feed, mentioning the technique of applying coolant.[5-marks]

Cutting Speed in DrillingThe cutting speed in drilling is the surface speed of the twist drill.

where, V= cutting speed (surface), mlminD = diameter of the twist drill, mmN = rotational speed of the drill, rev/min

22Drilling TimeTime for drilling the hole

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MRR in Drilling

24ExampleA 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.25

Some Formulae for Drilling

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How drill tool manufactured.28GATE- 2002The 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

Ans. (b)

Time for drilling the hole

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GATE- 2004Through 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

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GATE- 2012In 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) 30.1Ans. (a)31

IES - 2002The 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

Ans. (a)32IES - 1994The 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

Ans. (b)33IES - 2009What 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

Ans. (a)

34IES - 2002A 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

Ans. (d)35GATE - 2014A 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) 1.75Ans. (b)

36GATE-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 _______Ans. 68 to 7037IAS - 1999To 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

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GATE 2007 (PI) Linked S-1Blind 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) 0.86Ans. (c) 39

GATE 2007 (PI) Linked S-2Blind 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. Taylors tool life equation is of the form VT0.3 = C, where V = cutting speed in m/minute and T = tool life in minutes. Taylors constant C will be(a) 15(b) 72(c) 93(d) 490Ans. (b)40

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.

Ans. (d)

41IES - 1999Match List-I (Drill bits) with List-II (Applications) and select the correct answer using the codes given below the Lists:List-IList-IIA. Core drill 1.To enlarge a hole to a certain depth so as toaccommodate the bolt head of a screwB.Reamer 2.To drill and enlarge an already existing hole in acastingC.Counter bore drill 3.To drill a hole before making internal threadD.Tap drill4.To improve the surface finish and dimensionalaccuracy of the already drilled holeCode:ABCDABCD(a)1324(b)2314(c)2413(d)3241Ans. (c)42