1. Chapter 6A: Machining Process
Objectives:
Define the fundamental of cutting.
Describe the cutting tool material, cutting fluid and machining process to produce different kind of shapes

2. Machining
A material removal process in which a sharp cutting tool is used to mechanically cut away material so that the desired part geometry remains
~Most common application: to shape metal parts
~Most versatile of all manufacturing processes in its capability to produce a diversity of part geometries and geometric features with high precision and accuracy
Casting can also produce a variety of shapes, but it lacks the precision and accuracy of machining

3. Classification of Machined Parts
Figure: Machined parts are classified as: (a) rotational, or (b) nonrotational, shown here by block and flat parts.
Rotational ~cylindrical or disk‑like shape
Non-rotational (also called prismatic) ~block‑like or plate‑like

4. Machining Operations and Part Geometry
Each machining operation produces a characteristic part geometry due to two factors:
Relative motions between tool and workpart
Generating – part geometry determined by feed trajectory of cutting tool
Shape of the cutting tool
Forming – part geometry is created by the shape of the cutting tool

5. Generating Shape
Figure: Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling.

6. Forming to Create Shape
Figure : Forming to create shape: (a) form turning, (b) drilling, and (c) broaching.

7. TURNING
Turning is a machining process in which a single point tool removes material from the surface of a rotating cylindrical workpiece ; the tool is fed linearly in a direction parallel to the axis of rotation.
Turning is traditionally carried out on a machine tool called a lathe, which provides power to turn the part at a given rotational speed and to feed the tool at a specified rate and depth of cut.

8. Turning Operation

9. 1.2 Turning Process Turning is performed at various:
rotational speeds, (N) of the workpiece clamped in a spindle
depths of cut, (d)
feeds, (f) depending on the workpiece materials, cutting tool materials, surface finish, dimensional accuracy

10. Cutting Conditions in Turning
a. Relation of rotational and cutting speed :
N = rotational speed (rev/min)
v = cutting speed (m/min)
Do = original diameter of part (m)
b. The change in diameter is determined by the depth of cut :
d = depth of cut (m)
Df = final diameter of part (m)
Do = original diameter of part (m)

11. Tm = time of actual machining (min) L = length of the workpart (mm)
c. The feed in turning is generally expressed in mm/rev. This feed can be converted to a linear travel rate in mm/min by :
N = rotational speed (rev/min) f = feed (mm/rev ) fr = feed rate (mm/min)
d. The time to machine from one end of a cylindrical workpart to the other is given by :
Tm = time of actual machining (min)
L = length of the workpart (mm)

12. e. Material Removal Rate
Material removal rate (MRR) in turning is the volume of material removed per unit time with the unit of mm3/min.
The average diameter of the ring is
Since there are N revolutions per minute, the removal rate is
Since the distance traveled is l mm, the cutting time is

13. Types of Turning Operation
Instead of feeding tool parallel to axis of rotation, tool follows a contour that is other than straight, thus creating a contoured shape
Tool is fed radially inward
Figure: facing
Figure : contour turning

14. Types of Turning Operation
Figure: cutoff
Figure: chamfering
Cutting edge cuts an angle on the corner of the cylinder, forming a "chamfer"
Tool is fed radially into rotating work at some location to cut off end of part

15. Engine Lathe
The basic lathe used for turning and related operations is an engine lathe.
It is a versatile machine tool, manually operated, and widely used in low and medium production.
The term ‘engine’ dates from the time when these machines were driven by steam engines.

16. EXAMPLE
A 150-mm-long, 12.5-mm-diameter 304 stainless steel rod is being reduced in diameter to 12.0 mm by turning on a lathe. The spindle rotates at N 400 rpm, and the tool is travelling at an axial speed of 200 mm/min. Calculate the cutting speed, material-removal rate and cutting time.
SOLUTION
The cutting speed at the machined diameter is
The depth of cut is

17. The feed is
The material-removal rate is
The actual time to cut is

18. Drilling
Drilling is a machining operation used to create a round hole in a workpart.
Drilling is usually performed with a rotating cylindrical tool which has 2 cutting edges on its working end. The tool is called a drill or drill bit.
The rotating drill feeds into the stationary workpart to form a hole whose diameter is equal to the drill diameter.
Drilling is customarily performed on a drill press, although other machine tools can also perform this operation.

19. Through Holes vs. Blind Holes
Through‑holes - drill exits opposite side of work
Blind‑holes – does not exit work opposite side
Figure:Two hole types: (a) through‑hole, and (b) blind hole.

20. Cutting Conditions in Drilling
a. The rotational speed of the drill :
N = rotational speed (rev/min)
v = cutting speed (mm/min)
D = diameter of the drill (mm)
b. The feed rate :
N = rotational speed (rev/min)
f = feed (mm/rev)
fr = feed rate (mm/min)

21. Cutting Conditions in Drilling
c. Drilled holes are either through holes or blind holes. The machining time required to drill a through hole can be determined by :
Tm = drilling time (min)
t = work thickness (mm)
A = an approach allowance (mm)

22. Tm = drilling time (min) d = hole depth
d. In a blind hole, hole depth d is defined as the distance from the work surface to the ‘point’ of the hole. The drill point angle allowance does not affect the time to drill the hole. Thus the drilling time is :
Tm = drilling time (min)
d = hole depth
e. The rate of metal removal in drilling is determined as the product of the drill cross-sectional area and the feed rate :

23. Operations Related to Drilling
(a) Reaming – it is used to slightly enlarge a hole, to provide a better tolerance on its diameter, and to improve its surface finish. The tool is called a reamer, and usually has straight flutes.
(b) Tapping – this operation is performed by a tap and is used to provide internal screw threads on an existing hole.

24. Operations Related to Drilling
Counterboring – provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole. A counterbored hole is used to seat bolt heads into a hole so the heads do not protrude above the surface.
Countersinking – similar to counterboring, except that the step in the hole is cone-shaped for flat head screws and bolts.

25. Operations Related to Drilling
Centering – also called centerdrilling. This operation drills a starting hole to accurately establish its location for subsequent drilling. The tool is called a centerdrill.
Spotfacing – similar to milling. It is used to provide a flat machined surface on the workpart in a localized area.

26. Types of Drill Press Machine
Upright drill press stands on the floor
Bench drill similar but smaller and mounted on a table or bench
Figure: Upright drill press

27. EXAMPLE
A hole is being drilled in a block of magnesium alloy with a 10-mm drill bit at a feed of 0.2 mm/rev and with the spindle running at N = 800 rpm. Calculate the material-removal rate.
SOLUTION
The material-removal rate is

28. MILLING
Milling is a machining operation in which a workpart is fed past a rotating cylindrical tool with multiple cutting edges.
The axis of rotation of the cutting tool is perpendicular to the direction of feed. This orientation between the tool axis and the feed direction is one of the features that distinguishes milling from drilling.
The cutting tool in milling is called a milling cutter and the cutting edges are called teeth.
The machine tool that traditionally performs this operation is a milling machine.

29. The geometric form created by milling is a plane surface.
Milling is an interrupted cutting operation ; the teeth of the milling cutter enter and exit the work during each revolution. This interrupted cutting action subjects the teeth to a cycle of impact force and thermal shock on every rotation.
Therefore, the tool material and cutter geometry must be designed to withstand these conditions.

30. Two Forms of Milling
Figure: Two forms of milling: (a) peripheral milling, and (b) face milling.

31. Peripheral Milling vs. Face Milling
~Cutter axis parallel to surface being machined
~Cutting edges on outside periphery of cutter
Face milling
~Cutter axis perpendicular to surface being milled
~Cutting edges on both the end and outside periphery of the cutter

32. Peripheral Milling
(a) Slab milling.
(b) Slotting.

33. (c) Side milling.
(d) Straddle milling.

34. In peripheral milling, the rotation direction of the cutter distinguishes 2 forms of milling i.e. up milling and down milling. In down milling, the cutter is engaged in the work for less time per volume of material cut – higher tool life.
Two forms of milling with a 20-tooth cutter : (a) up milling, and (b) down milling.

35. Types of Face Milling Figure: (a) conventional face milling
~Cutter overhangs work on both sides
~Cutter diameter is less than work width, so a slot is cut into part
Figure: (a) conventional face milling
Figure: end milling

36. Figure: profile milling
Types of Face Milling
~Form of end milling in which the outside periphery of a flat part is cut
~Another form of end milling used to mill shallow pockets into flat parts
Figure: profile milling
Figure: pocket milling

37. Types of Face Milling
Ball‑nose cutter fed back and forth across work along a curvilinear path at close intervals to create a three dimensional surface form
Figure: surface contouring

38. Cutting Conditions in Milling
a. The spindle rotation speed :
N = spindle speed (rev/min)
v = cutting speed (mm/min)
D = outside diameter of cutter (mm)
b. The feed f in milling is usually given as a feed per cutter tooth ; called the chip load, it represents the size of the chip formed by each cutting edge. Therefore the feed rate :
N = spindle speed (rev/min)
fr = feed rate (mm/min)
nt = number of teeth on the cutter
f = chip load (mm/tooth)

39. c. Material removal rate is determined using the product of the cross-sectional area of the cut and the feed rate.
w = cutting width (mm)
d = cutting depth (mm)
fr = feed rate (mm/min)
d. The time to mill the workpiece of length L for a slab milling is given by :
T = milling time (min)
L = length of the workpart (mm)
A = approach distance (mm)

40. L = length of the workpart (mm) A = approach distance (mm)
e. The time to mill the workpiece of length L for a face milling is given by :
T = milling time (min)
L = length of the workpart (mm)
A = approach distance (mm)

41. EXAMPLE
Refer to above figure and assume that D=150mm, w=60mm, l=500mm, d=3mm, v=0.6m/min and N=100rpm. The cutter has 10 inserts, and the workpiece material is a high-strength aluminum alloy. Calculate the material-removal rate, cutting time and feed-per- tooth.
SOLUTION
The material removal rate (MRR) = (60)(3)(600) = mm3/min
Noting that 0.6 m/min = 600 mm/min
The cutting time is given by
The feed-per-tooth can be obtained from equation by
Noting that 100 rpm = 1.67 rev/s

42. Milling Machines
Milling machines can be classified as horizontal or vertical.
A horizontal milling machine has a horizontal spindle, and this design is well-suited for performing peripheral milling on workparts that are roughly cube-shaped.
A vertical milling machine has a vertical spindle, and this orientation is appropriate for face milling, end milling, surface contouring, and diesinking on relatively flat workparts.

43. Machining Centers
Highly automated machine tool can perform multiple machining operations under CNC control in one setup with minimal human attention
~Typical operations are milling and drilling
~Three, four, or five axes
Other features:
~Automatic tool‑changing
~Pallet shuttles
~Automatic workpart positioning

44. Several features make an economics of machining center :
Automatic tool-changing – to change from one machining operation to the next, the cutting tools must be changed. This is done on a machining center under NC program control by an automatic tool-changer designed to exchange cutters between the machine tool spindle and a tool storage drum. Capacities of these drums commonly range from 16 to 80 cutting tools.
Pallet shuttles – some machining centers are equipped with two or more pallet shuttles, which can be automatically transferred to the spindle to machine the workpart. With 2 shuttles, the operator can be unloading the previous part and loading the next part while the machine tool is engaged in machining the current part. This reduces nonproductive time on the machine.
Automatic workpart positioning – many machining centers have more than three axes. One of the additional axes is often designed as a rotary table to position the part at some specified angle relative to the spindle. The rotary table permits the cutter to perform machining on 4 sides of the part in a single setup.

45. Example of Machine Center
Figure: Universal machining center; highly automated, capable of multiple machining operations under computer control in one setup with minimal human attention
Figure: CNC 4‑axis turning center; capable of turning and related operations, contour turning, and automatic tool indexing, all under computer control.

46. End Chapter 6A

47. Chapter 6B: Fundamental of Machining

48. Chapters OVERVIEW OF MACHINING TECHNOLOGY CUTTING FORCES AND POWER
TEMPERATURES IN MACHINING
CUTTING TOOLS AND CUTTING FLUIDS

49. Overview Of Machining Technology
A family of shaping operations, the common feature of which is removal of material from a starting workpart so the remaining part has the desired geometry
~Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling
~Abrasive processes – material removal by hard, abrasive particles, e.g., grinding
~Nontraditional processes - various energy forms other than sharp cutting tool to remove material (waterjet)

50. Machining
(a) A cross‑sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).
Cutting action involves shear deformation of work material to form a chip
~As chip is removed, new surface is exposed

51. Types of Machining Operation
b) Drilling
a) Turning
c) Peripheral Milling
d) Face Milling

52. Theory of Chip Formation
The geometry of most practical machining operations is somewhat complex. A simplified model of machining is available which neglects many of the geometric complexities, yet describes the mechanics of the process fairly accurate. It is called the orthogonal cutting model.
Although an actual machining process is 3- dimensional, the orthogonal model has only 2 dimensions that play active roles in the analysis.

53. The Orthogonal Cutting Model
Orthogonal cutting :- as a 3-dimensional process
Orthogonal cutting :- reduces to 2-dimensions in the side view.

54. The Orthogonal Cutting Model
Orthogonal cutting uses a wedge-shaped tool in which the cutting edge is perpendicular to the direction of cutting speed.
As the tool is forced into the material, the chip is formed by shear deformation along a plane called the shear plane, which is oriented at an angle with the surface of the work.
The tool in orthogonal cutting has only 2 elements of geometry i.e. rake angle and clearance angle. The rake angle determines the direction that the chip flows as it is formed ; and the clearance angle provides a small clearance between the tool flank and the newly generated work surface.

55. Four Basic Types of Chip in Machining
(a) Discontinuous chip
when relatively brittle materials (e.g. cast irons) are machined at low cutting speeds, the chips often form into separate segments. This tends to impart an irregular texture to the machined surface.
secondly, high tool-chip friction and large feed and depth of cut also promote to the formation of this kind of chip.

56. (b) Continuous chip
when ductile materials are cut at high speeds and relatively small feed and depths, long continuous chips are formed. A good surface finish typically results when this chip type is formed.
a sharp cutting edge on the tool and low tool-chip friction also encourage the formation of continuous chips.

57. (c) Serrated chips
these chips are semicontinuous in the sense that they possess a sawtooth appearance that is produced by a cyclical chip formation of alternating high shear strain and followed by low shear strain.
this chip is most closely associated with certain difficult- to-machine metals such as titanium alloys, nickel-base superalloys, and austenitic stainless steels when they are machined at higher cutting speeds.

58. (d) Continuous chip with built-up edge
when machining ductile materials at low- to-medium cutting speeds, friction between tool and chip tends to cause portions of the work material to adhere to the rake face of the tool near the cutting edge. This formation is called a built-up- edge (BUE).
the formation of a BUE is cyclical ; it forms and grows, then becomes unstable and breaks off.
much of the detached BUE is carried away with the chip, sometimes taking portions of the tool rake face with it, which reduces the life of the cutting tool.
portions of the detached BUE that are not carried off with the chip become imbedded in the newly created work surface, causing the surface to become rough.

59. Increase the cutting speeds Decrease the depth of cut
(d) Continuous chip with built-up edge can be reduced by :
Increase the cutting speeds
Decrease the depth of cut
Increase the rake angle
Use a sharp tool
Use an effective cutting fluid /lubricant
Use a cutting tool that has lower chemical affinity for the workpiece material

60. Cutting Forces and Power
The forces applied against the chip by the tool can be separated into 2 mutually perpendicular components :
(a) Friction force (F) – friction force between the tool and chip resisting the flow of the chip along the rake face of the tool ;
(b) Normal force to friction (N) – the force that is normal to the friction force.
These 2 components can be used to define the coefficient of friction between the tool and the chip :

61. (b) The normal force to shear, Fn is normal to the shear force.
The friction force and its normal force can be added vectorially to form a resultant force, R. R is oriented at an angle b , called the friction angle. The friction angle is related to the coefficient of friction as :
In addition to the tool forces acting on the chip, there are 2 force components applied by the workpiece on the chip :
(a) The shear force, Fs is the force that causes shear deformation to occur in the shear plane.
(b) The normal force to shear, Fn is normal to the shear force.

62. As = area of the shear plane.
Based on the shear force, we can define the shear stress S which acts along the shear plane between the work and the chip :
As = area of the shear plane.
Note : the shear stress S represents the level of stress required to perform the machining operation. Therefore, this stress is equal to the shear strength of the work material under the conditions at which cutting occurs.

63. Cutting Force and Thrust Force
F, N, and Fs cannot be directly measured
Forces acting on the tool that can be measured:
~ Cutting force Fc and Thrust force Ft
Cutting force ( Fc ) is in the direction of cutting, the same direction as the cutting speed v . Thrust force (Ft ) is associated with the chip thickness before the cut to . It is perpendicular to the cutting force. R’’ is the resultant force of Fc and Ft .

64. Forces in Metal Cutting
The respective directions of these forces are known, so the force transducers in the dynamometer can be aligned accordingly.
Equations can be derived to relate the 4 force components that cannot be measured to the 2 forces that can be measured by using the force diagram (as illustrated ...)
(1)
(2)
(3)

65. Effect of Higher Shear Plane Angle
Effect of shear plane angle  : (a) higher  with a resulting lower shear plane area; (b) smaller  with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation
Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature

66. CUTTING FORCES AND POWER
The product of cutting force and speed gives the power (energy per unit time) required to perform a machining operation : (energy used every second in metal cutting)
Pc = cutting power (Nm/s or W)
Fc = cutting force (N)
v = cutting speed (m/s)

67. The gross power required to operate the machine tool is greater than the power delivered to the cutting process because of mechanical losses in the motor and drive train in the machine.
This losses can be accounted for by the mechanical efficiency of the machine tool :
Pg = gross power of the machine tool motor (W)
E = mechanical efficiency of the machine tool. Typical values of E for machine tools are around 90%.

68. It is often useful to convert power into power per unit volume rate of metal cut. This is called the unit power, Pu , defined as :
MRR = material removal rate, mm3/s. The material rate can be calculated as the product of vtow.

69. Cutting Temperature
Of the total energy consumed in machining, nearly all of it (approximately 98%) is converted into heat. This heat can cause temperatures to be very high at the tool-chip interface – over 600 oC is not unusual.
The remaining energy (about 2%) is retained as elastic energy in the chip.

70. Analytical Method
The following equation can be used to predict the rise in temperature at the tool-chip interface during machining :
T = mean temperature rise at the tool-chip interface (oC)
U = specific energy in the operation (Nm/mm3 or J/mm3)
v = cutting speed (m/s)
to = chip thickness before the cut (m)
rC = volumetric specific heat of the work material (J/mm3C)
K = thermal diffusivity of the work material (m2/s)

71. Measurement of Cutting Temperature
Experimental methods have been developed to measure temperatures in machining. The most frequently used measuring technique is the tool-chip thermocouple.
The thermocouple has been utilized by researchers to investigate the relationship between temperature and cutting conditions such as speed and feed.
Speed-temperature relationship:
T = measured tool-chip interface temperature
v = cutting speed
K and m - depend on cutting conditions (other than v) and work material.

72. Speed-temperature relationship
Experimentally measured cutting temperature plotted against speed for 3 work materials, indicating general agreement with equation above.

73. CUTTING TOOLS AND CUTTING FLUIDS
There are 3 modes of tool failure that can be used to identify some important properties required in a tool material :
1) Toughness – to avoid fracture failure, the tool material must possess high toughness. Toughness is the capacity of a material to absorb energy without failing. It is usually characterized by a combination of strength and ductility in the material.
2) Hot hardness – is the ability of a material to retain its hardness at high temperatures. This is required because of the high temperature environment in which the tool operates.

74. 3) Wear resistance – is the single most important property needed to resist abrasive wear. All cutting tool materials must be hard. However, wear resistance in metal cutting depends on more than just tool hardness, because of the other tool wear mechanism.
Other characteristics affecting wear resistance include surface finish on the tool (a smoother surface means a lower coefficient of friction), chemistry of tool and work materials, and whether a cutting fluid is used.

75. Materials of Tool High-Speed Steel and Its Predecessors
a. High-speed steel (HSS)
a highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels.
its good hot hardness permits tools made of HSS to be used at higher cutting speeds.
a wide variety of high-speed steels is available, but they can be divided into 2 basic types :
(i) tungsten-type, and (ii) molybdenum-type

76. Commercially, HSS is one of the most important cutting tool materials in use today. It is especially suited to applications involving complicated tool geometries, such as drills, taps, milling cutters, and broaches.
They can be heat-treated so that cutting edge hardness is very good (Rockwell C 65) while toughness of the internal portions of the tool is also good.
HSS cutters possess better toughness than any of the harder nonsteel tool materials used for machining, such as cemented carbides and ceramics.

77. Materials of Tool b. Cast Cobalt Alloys
consist of cobalt, around 40% to 50% ; chromium, about 25% to 35% ; and tungsten, usually 15% to 20%. These tools are made into the desired shape by casting in graphite molds and then grinding to final size and cutting edge sharpness.
high hardness is achieved as cast, an advantage over HSS, which requires heat treatment to achieve its hardness. Wear resistance of the cast cobalt is better than HSS, but not as good as cemented carbide. Toughness of cast cobalt tools is better than carbides but not as good as HSS.
the applications are generally between those of HSS and cemented carbides. They are capable of heavy roughing cuts at speeds greater than HSS and feeds greater than carbides. Work materials include both steels and nonsteels, as well as nonmetallic materials.

78. Materials of Tool c. Cemented Carbides
a class of hard tool material formulated from tungsten carbide (WC) using powder metallurgy techniques, with cobalt (Co) as the binder.
cemented carbides are divided into 2 basic types :
(i) nonsteel cutting grades, consisting of only WC-Co. Suitable for machining aluminum, brass, copper, magnesium, titanium and other nonferrous metals.
(ii) steel cutting grades, with combinations of TiC and TaC added to the WC-Co. Suitable for machining low carbon steel, stainless steel and other alloy steels.

79. The general properties of the 2 types of cemented carbides are similar :
high compressive strength but low-to-moderate tensile strength
high hardness (90 to 95 HRA)
good hot hardness
good wear resistance
high thermal conductivity
high modulus of elasticity – E values up to around 600x103 MPa
toughness lower than high-speed steel.

80. Cutting Fluids
Cutting fluids are any liquids or gases that are applied directly to the machining operation to improve cutting performance.
Cutting fluids address 2 main problems :
(a) heat generation at the shear zone and friction zone ;
(b) friction at the tool-chip and tool-work interfaces.
In addition to removing heat and reducing friction, cutting fluids wash away chips (especially in grinding and milling), reduce the temperature of the workpart for easier handling, reduce cutting forces and power requirements, improve dimensional stability of the workpart, and improve surface finish.

81. Types of cutting fluids :
(i) Coolants are cutting fluids designed to reduce the effects of heat in the machining operation.
They have a limited effect on the amount of heat energy generated in cutting ; instead, they carry away the heat that is generated, thereby reducing the temperature of tool and workpiece. This helps to prolong the life of the cutting tool.
Water has high specific heat and thermal conductivity relative to other liquids, which is why water is used as the base in coolant-type cutting fluids. These properties allow the coolant to draw heat away from the operation, thereby reducing the temperature of the cutting tool.

82. Coolant-type cutting fluids seem to be most effective at relatively high cutting speeds where heat generation and high temperatures are problems. They are most effective on tool materials that are most susceptible to temperature failures, such as HSS, and are used frequently in turning and milling operations where large amounts of heat are generated.
Coolants are usually water-based solutions or water emulsions, since water has thermal properties that are ideally suited for these cutting fluids.

83. (ii) Lubricants are usually oil-based fluids (since oil possess good lubricating qualities) formulated to reduce friction at the tool-chip and tool-work interfaces.
Lubricant cutting fluids operate by extreme pressure lubrication, a special form of lubrication that involves formation of thin solid salt layers on the hot, clean metal surfaces through chemical reaction with the lubricant.
Lubricant-type cutting fluids are most effective at lower cutting speeds. They tend to lose their effectiveness at high speeds – above about 120m/min – because the motion of the chip at these speeds prevents the cutting fluid from reaching the tool-chip interface.

84. Machining operations such as drilling and tapping usually benefit from lubrication. In these operations, built-up-edge (BUE) formation is retarded, and torque on the tool is reduced.
Although the principal purpose of a lubricant is to reduce friction, it also reduces the temperature in the operation due to (a) the specific heat and thermal conductivity of the lubricant help to remove the heat from the operation, (b) since friction is reduced, the heat generated from friction is also reduced, and (c) lower coefficient of friction means lower friction angle.

85. THANK YOU