Drilling MCQ Quiz in తెలుగు - Objective Question with Answer for Drilling - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Concept:

Drilling:

• Drilling is used for producing holes in the components by using a rotating multipoint cutting tool.
• During producing a hole, it is always recommended to use the smallest size of the drill bit first and enlarge the hole slowly by using different sizes of drill bit until the required size of the hole is produced.
• Drilling time is given by:

 \(​​{\bf{T}} = \frac{{\bf{L}}}{{{\bf{f}} \times {\bf{N}}}}\)

where, Tool travel (L) = CAP + AP + OR + t, 

Compulsory approach (CAP) = Depth of cut in drilling operation(d)

∴ CAP = d = D/2

T = Drilling time in minutes, L = Tool travel = approach (AP) + over run (OR) + thickness of workpiece(t), f = Feed rate, N = Speed of rotation (rpm)

Concept:

For drilling a through-hole of diameter D and length L_w, the machining time T_c is evaluated from –

where

s_o = feed (mm/rev)

N = rpm of tool (rev/min)

L_c = L_w + A + O + C

A = approach, O = over-travel and C = compulsory approach

\({\rm{C}} = \frac{{\rm{D}}}{2}\cot {\rm{\rho \;\;where\;\rho }} = {\rm{half\;of\;drill\;point\;angle}}.\)

[Note: If angle is not given then neglect compulsory approach]

Calculation:

Given:

D = 8 mm, L_w = 24 mm, N = 360 rpm ⇒ 6 rps and s_o = 0.5 mm/rev

L_c = L_w + A + O + C

⇒ L_c = L_w (∵ A = C = 0 and ρ is not given)

\(\therefore {{\rm{T}}_{\rm{c}}} = \frac{{{{\rm{L}}_{\rm{c}}}}}{{{{\rm{s}}_{\rm{o}}}{\rm{N}}}} \Rightarrow \frac{{{{\rm{L}}_{\rm{w}}}}}{{{{\rm{s}}_{\rm{o}}}{\rm{N}}}}\)

\(T_c = \frac{{24}}{{0.5 \times 6}}\)

T_c = 8 s

Concept:

Flutes are the spiral grooves which run to the length of the twist drill; 

The flutes help:

• to form the cutting edges
• to curl the chips and allow these to come out
• the coolant to flow to the cutting edge

Flutes are not used for cutting purposes.

Explanation:

• Generally, drills are manufactured to standard sizes in the metric system.
• These drills are available in specified steps.
• The drills, which are not covered under the above category, are manufactured in,
  1. Letter drills
  2. Number drills

Number drills:

• The number drill series consists of drills numbered from 1 to 80.
• The No.1 drill is the largest, with 5.791 mm diameter, and the No.80 drill is the smallest, with 0.35 mm diameter.
• There is no uniform variation in the drill diameters from number to number.
• To find the correct diameter of a number drill, refer to a drill Size Chart or a Hand-book.
• Number drill series are also known as Wire gauge series​

Additional Information

Letter drills:

• The letter drill series consists of drill sizes from ‘A’ to ‘Z’. The letter ‘A’ drill is the smallest with a 5.944 mm diameter, and the letter ‘Z’ is the largest, with a 10.49 mm diameter.
• In the number drill and the letter drill series, the correct diameter of the drill is gauged with the help of the respective drill gauges.
• A drill gauge is a rectangular or square-shaped metal piece containing a number of different diameter holes.
• The size of the hole is stamped against each hole.

Explanation:

The lips, also known as cutting edges, are the edges formed by the intersection of flanks and faces. They are two in number with identical length and angle.

The various parts of a drill can be identified from the figure:

Point 

• The cone-shaped end which does the cutting is called the point; It consists of a dead centre, lips or cutting edges, and a heel. 

Shank 

• This is the driving end of the drill which is fitted on the machine; Shanks are of two types: Taper shank used for larger diameter drills, and straight shank used for smaller diameter drills

Tang

• This is a part of the taper shank drill which fits into the slot of the drilling machine spindle

Body

• The portion between the point and the shank is called the body of a drill; The parts of the body are a flute, land/margin, body clearance and web

Flutes

Flutes are the spiral grooves which run to the length of the drill; The flutes help:

• to form the cutting edges
• to curl the chips and allow these to come out
• the coolant to flow to the cutting edge

Land/margin:

• The land/margin is the narrow strip which extends to the entire length of the flutes; The diameter of the drill is measured across the land/ margin

Body clearance

• Body clearance is the part of the body which is reduced in diameter to cut down the friction between the drill and the drilled hole

Web

• The web is the metal column which separates the flutes; It gradually increases in thickness towards the shank

Important Points

Explanation:

Drilling:

• Drilling is a cutting process in which a hole is originated using 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.
• Smaller the rake angle. greater will be the torque required to drive the drill at a given feed.

Point Angle:

• It is known as the 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 angles 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 that is ground to the cutting edges in order 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 due to the fact that 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.

Explanation:

Drilling angle used for various materials

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.

Tool geometry:

Following are the important angles in the twist drill.

Chisel edge angle is the angle between the chisel edge and cutting lip.

Chisel edge angle for hard material is 55° and for soft material is 51°.

Lip angle

The standard point angle of 118° is most common.

• The point angle is the angle between the cutting edges (lips).

• The point angle varies according to the hardness of the material to be drilled

Helix angle

Rake angle (helix angle) is the flute angle. The helix angle is the angle between the leading edge of the land and the axis of the drill. It is also known as the spiral angle. The standard helix angle is 24°.

Relief angle

• The relief angle given behind the cutting lips is called the lip clearance angle.
• It is provided to prevent the friction behind the cutting edges and helps in the penetration of the cutting edges into the work-piece.

Lip Relief Angle 

• The axial relief angle at the outer corner of the lip.
• It is measured by projection into a plane tangent to the periphery a the outer corner of the lip.
• Lip relief angle is usually measured across the margin of the twist drill.

Explanation:

Reaming removes a small amount of material from the surface of holes. It is done for two purposes:

1. To bring holes to a more exact size

2. To improve the finish of an existing hole

Important Point:

Boring: Boring is an operation to enlarging of an existing hole, which may have been made by a drill or maybe the result of a core in a casting.

Drilling: Drilling is a cutting process that uses a drill bit to cut or enlarge a hole of circular cross-section in solid materials. The drill bit is a rotary cutting tool.

Reaming: Reaming is a sizing operation that removes a small amount of metal from a hole already drilled. It is done for two purposes: to bring holes to a more exact size and to improve the finish of an existing hole.

Mistake Points

Boring is used to enlarge the diameter. Reaming operation is meant to slightly increase the size and to provide a better tolerance and surface finish of an initially drilled hole. 

Concept:

Heat absorbed by aluminum block:

δQ = mcΔT

Calculation:

Given:

Power of the machine, P = 10 kW

Now,

As 50% of power is used in heating

∴ Heat generated = 0.5 P = 5 kW = 5 kJ/s

Heat generated in 2.5 min (i.e 150 s),

∴δQ = 5 × 150 = 750 kJ

Heat absorbed by aluminum block:

δQ = mcΔT

750 = 8 × 0.91 × ΔT

Concept:

Two types of co-ordinate systems are used to define and control the position of the tool in relation to the workpiece.
The co-ordinate systems used are:
(1) Absolute co-ordinate system
(2) Incremental co-ordinate system

Absolute System:
In the absolute system (G90), all dimensions or positions are given from a zero or reference point.
Incremental System:
In the incremental system (G91), dimensions or positions are given from a previous known point.

Calculation:

In the incremental system. Point P will become the new origin for point Q.

x-coordinate of point Q = 4

y-coordinate of point Q = 7

Q (x,y) = Q (4,7)