Lecture 10 - Abrasive Machining and Finishing Operation-W15
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Transcript of Lecture 10 - Abrasive Machining and Finishing Operation-W15
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1/23/2015
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February 5th, 2015
MECH 360
Principles of Manufacturing
Lecture 10: Abrasive machining and finishing operations
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Outline
Grinding
Related abrasive and finishing operations
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Abrasive Machining
Material removal by action of hard, abrasive particles
usually in the form of a bonded wheel
Typical abrasive grains
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Abrasive Machining
Abrasive machining:
Generally used as finishing operations after part geometry
has been established by conventional machining
Grinding is most important abrasive process
Other abrasive processes: honing, lapping, superfinishing,
polishing, and buffing
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Abrasive Machining
Why Abrasive Processes are Important
Can be used on all types of materials
Some can produce extremely fine surface finishes, to
0.025 m (1 -in)
Some can hold dimensions to extremely close tolerances
Grinding
Material removal process in which abrasive particles are
contained in a bonded grinding wheel that operates at very
high surface speeds
Grinding wheel is usually disk-shaped and precisely balanced
for high rotational speeds
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Grinding
Different types of grinding operations 6
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Grinding
The Grinding Wheel
Consists of abrasive particles and bonding material
Abrasive particles accomplish cutting
Bonding material holds particles in place and establishes shape
and structure of wheel
Attritious wear -- the dulling of the cutting edges of a sharp
grain caused by rubbing
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Grinding
The Grinding Wheel
Superabrasive wheels
Conventional grinding wheels
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Grinding
Grinding Wheel Parameters Abrasive material
Grain size
Bonding material
Wheel grade
Wheel structure Abrasive Material Properties High hardness harder than conventional cutting tool material
Wear resistance
Toughness
Friability - capacity to fracture when cutting edge dulls, so a
new sharp edge is exposed
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Grinding
Traditional Abrasive Materials
Aluminum oxide (Al2O3) - most common abrasive
Used to grind steel and other high-strength alloys
Silicon carbide (SiC) - harder than Al2O3 but not as
tough
Used on aluminum, brass, stainless steel, some cast irons
and certain ceramics
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Grinding
Newer Abrasive Materials Abrasive material Knoop hardness
Aluminum oxide 2100
Silicon carbide 2500
Cubic boron nitride 5000
Diamond (synthetic) 7000
Diamond Even harder, very expensive
Occur naturally and also made synthetically
Not suitable for grinding steels
Used on hard, abrasive materials such as ceramics, cemented carbides, and glass
Cubic boron nitride (CBN) very hard, very expensive
Suitable for steels
Used for hard materials such as hardened tool steels and aerospace alloys
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Grinding
Grain size Small grit sizes produce better finishes
Larger grit sizes permit larger material removal rates
Harder work materials require smaller grain sizes to cut
effectively
Softer materials require larger grit sizes
Measurement of Grain Size
Grit size is measured using a screen mesh procedure
Smaller grit sizes indicated by larger numbers in the screen
mesh procedure and vice versa
Grain sizes in grinding wheels typically range between 8 (very
coarse) and 250 (very fine)
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Grinding
Measurement of Grain Size
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Grinding
Bonding Material Properties
Must withstand centrifugal forces and high temperatures
Must resist shattering during shock loading of wheel
Must hold abrasive grains rigidly in place for cutting yet
allow worn grains to be dislodged to expose new sharp
grains
Bonding Materials
Vitrified bond Resinoid
Thermoplastic Rubber
Metal
Reinforced one or more layers of fiberglass mats
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Grinding
Wheel Structure
Refers to the relative spacing of abrasive grains in wheel
In addition to abrasive grains and bond material, grinding
wheels contain air gaps or pores
Volumetric proportions of grains, bond material, and pores
can be expressed as:
0.1 pbg PPP
Open structure small,
large
gP
pP
Dense structure large,
small
gP
pP
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Grinding
Wheel grade
Indicates bond strength in retaining abrasive grains
during cutting
Depends on amount of bonding material in wheel structure (Pb)
Measured on a scale from soft to hard
Soft wheels lose grains readily - used for low material
removal rates and hard work materials
Hard wheels retain grains - used for high stock removal rates
and soft work materials
Standard grinding wheel marking system used to designate
abrasive type, grit size, grade, structure, and bond material
Example: A-46-H-6-V
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Grinding
Wheel grade
Grinding is a chip-removal process that uses an individual
abrasive grain as the cutting tool
Grain force is proportional to the process variables:
ForceGrain
D
d
V
v
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Grinding
Grinding process
Material Removal Rate in grinding
dwvRR M
Energy dissipated in producing a grinding chip consists
of energy required from:
1. Chip formation
2. Plowing
3. Friction
Specific Energy
Specific Energy energy per unit volume of material ground
from the workpiece surface, u, Ws/mm3
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Grinding
EXAMPLE
Forces in Surface Grinding
A surface-grinding operation is being performed on low-
carbon steel with a wheel of diameter D=250 mm that is
rotating at N=4000 rpm and a width of cut of w=25 mm.
For carbon steel wheel, the specific energy is 40 Ws/mm3.
The depth of cut is d=0.05 mm and the feed rate of the
workpiece, is 1.5 m/min. Calculate the cutting force.
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Grinding
Solution
Forces in Surface Grinding
The material-removal rate (MRR) is
The power consumed is
Noting that 1 W = 1 Nm/s = 60 Nm/min
Power=1250W = 75000 Nm/min
Since power is
N 8.23400025.075000
cc
cc
FF
DNFVFPower
Wmm
smm
sWMRRuPower 1250
min1875
60
min140
3
3
min/10*875.1min/mm 187515002505.0 363 mdwvMRR
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Grinding
Temperature rise in grinding is important as it can:
1. Adversely affect the surface properties
2. Cause residual stresses on the workpiece
3. Cause distortions due to thermal expansion and
contraction of the workpiece surface
Surface-temperature rise in grinding is
Temperature increases with increasing d, D, and V
2/1
4/34/1
v
VdDT
Temperature
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Grinding
Wheel wear
Attritious Grain Wear
Similar to flank wear in cutting tools
Cutting edges become dull and develop a wear flat
Selection of abrasive is based on the reactivity of the
grain, workpiece hardness and toughness
Grain Fracture
The grain should fracture at a moderate rate
So that new sharp cutting edges are produced
continuously during grinding
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Grinding
Grinding Ratio
Grinding ratio is defined as
Higher the force, greater the tendency for the grains to
fracture
Higher the wheel wear, lower the grinding ratio
wear wheelof Volume
removed material of VolumeG
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Grinding
Dressing, Truing, and Shaping of Grinding Wheels Dressing Resharpening the wheel
Functions:
Break off dulled grits to expose new sharp grains
Remove chips clogged in wheel
Truing Use of a diamond-pointed tool fed slowly and
precisely across wheel as it rotates
Very light depth is taken (0.025 mm or less) against the wheel
Not only sharpens wheel, but restores true disk shape and insures
straightness across outside perimeter
Shaping Use of a diamond-pointed tool fed slowly and
precisely to shape the grinding wheel into the designed form
Used for forming the desired workpiece gometry
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Grinding
Surface grinding operations and machines (a) Horizontal spindle with reciprocating
worktable, (b) horizontal spindle with rotating
worktable
(c) Vertical spindle with reciprocating
worktable, and (d) vertical spindle with rotating
worktable
Surface grinder with
horizontal spindle and
reciprocating worktable
(most common grinder
type)
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Other finishing operations
Honing Abrasive process performed by a set of bonded abrasive sticks
using a combination of rotational and oscillatory motions
Common application is to finish the bores of internal combustion engines
Grit sizes range between 30 and 600
Surface finishes of 0.12 m (5 -in) or better
Creates a characteristic cross-hatched surface that retains lubrication
(a) Honing tool used for internal bore surface,
(b) cross-hatched pattern created by the
action of honing tool
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Other finishing operations
Lapping
Uses fluid suspension of very small abrasive particles between
workpiece and lap (tool)
Lapping compound - fluid with abrasives, general appearance of a chalky
paste
Typical grit sizes between 300 to 600
Applications: optical lenses, metallic bearing surfaces, gages
The lapping process
in lens-making
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Other finishing operations
Superfinishing
Similar to honing - uses bonded abrasive stick pressed against
surface and reciprocating motion
Differences with honing:
Shorter strokes
Higher frequencies
Lower pressures between tool and surface
Smaller grit sizes
Superfinishing on an
external cylindrical
surface
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