Grinding - IIT Bombay

Transcription

Grinding and Finishing
Illegitimi non carborundum
ver. 1
ME 338: Manufacturing Processes II
Instructor: Ramesh Singh; Notes: Profs.
Singh/Melkote/Colton
1
Overview
• Processes
• Analysis
2
3
Horizontal Grinding
4
5
Horizontal grinding
Vertical grinding
6
Centered grinding
Centerless grinding
7
Creep Feed Grinding
• Full depth and stock is removed with
one or two passes at low work speed
• Very high forces are generated
• High rigidity and power
Advantages
• Increased accuracy
• Efficiency
• Improved surface finish
• Burr reduction
• Reduced stress and fatigue
8
9
10
11
12
Grinding Wheels
13
Grinding Wheels
14
Grinding Wheel Information
15
Correctly Mounted Wheel
16
Grinding Wheel Surface
17
Grind Wheel Dressing
18
Grinding Wheel Dressing
19
Grinding Chips
20
Chip formation geometry
w
l
t
D
q
t
d
V
l
v
21
Chip geometry
• As with rolling contact length, the chip length,
l
– D = wheel diameter, d = depth of cut
l  Dd
• Material removal rate, MRR
– v = workpiece velocity, d = depth of cut, b = width
of cut
MRR  v  d  b
23
Material removal rate
• The chips have a triangular cross-section,
and ratio (r) of chip thickness (t) to chip width
w
(w)
w
r   10 to 20
t
t
l
• So, the average volume per chip
1 1
1
Volchip  w  t  l  wtl
2 2
4
24
Chips
• The number of chips removed per unit time
(n), where c = number of cutting edges
(grains) per unit area (typ. 0.1 to 10 per mm2,
and V = peripheral wheel velocity
n V bc
25
Combining
MRR  v  d  b  n Volchip
1
v  d  b  Vbc  wtl
4
w  r t
l  Dd
1
vd b V cb r t t  Dd
4
26
Chip thickness
4v  d
t 
V cr Dd
2
or
4v
t
V cr
d
D
27
Specific grinding energy, u
• Consist of chip formation, plowing, and sliding
u  uchip  u plowing  usliding
28
Total grinding force
• Get force from power
Power  u  MRR
Fgrinding V  u  v  d  b
v  d b
Fgrinding  u 
V
29
• From empirical results, as t decreases, the
friction component of u increases
1
u
t
or
1
u  K1 
t
substituting
1 v  d b
Fgrinding  K1  
t V
30
Substituting for t
v  d b
Fgrinding  K1 

V
4v
d

V cr D
1
rearranging
d cr v
Fgrinding  K1  b 
 Dd
4V
31
ME 6222: Manufacturing Processes and
Systems Prof. J.S. Colton © GIT 2006
Force on a grain
• The force per grain can be calculated
Fgrain  u  Area
1
Fgrain  u  wt
2
w  rt
and
1
u  K1 
t
1 1
Fgrain  K1    r  t  t
t 2
32
Force on a grain
substituting for t, and rearranging
K1
4v
Fgrain   r 
2
V cr
d
D
vr d
Fgrain  K1 
V c D
33
ME 6222: Manufacturing Processes and
Systems Prof. J.S. Colton © GIT 2006
Grinding temperature
• Temperature rise goes with energy delivered
per unit area
Energy input
T  K2 
area
u bl  d
1
T  K2 
 K2  K1   d
bl
t
T  K1  K2  d 
1
4v d
Vcr D
34
Grinding temperature
• Rearranging
∆𝑇 =
3
𝐾1 𝐾2 𝑑 4
𝑉𝑐𝑟
𝐷
4𝑣
• Temperatures can be up to 1600oC, but for a
short time.
35
Grinding – Ex. 1-1
• You are grinding a steel, which has a specific
grinding energy (u) of 35 W-s/mm3.
• The grinding wheel rotates at 3600 rpm, has a
diameter (D) of 150 mm, thickness (b) of 25 mm, and
(c) 5 grains per mm2. The motor has a power of 2
kW.
• The work piece moves (v) at 1.5 m/min. The chip
thickness ratio (r) is 10.
• Determine the grinding force and force per grain.
• Determine the temperature (K2 is 0.2oK-mm/N).
Room temperature is 20oC.
36
• First we need to calculate the depth of cut.
We can do this from the power.
Power  u  MRR  u  v  d  b
2
W s
m
min
6 mm
2000 W  35
1.5
 d  25 mm 10

3
2
min
60 sec
mm
m
d  91.4 106 m
37
• Now for the total grinding force
v  d b
Fgrinding  u 
V
mm
3
1500

91
.
4

10
mm  25 mm
W s
min
Fgrinding  35

m
mm3 3600 rev 150 mm 
min
rev 1000 mm
Fgrinding  70.7 N
38
• Next, the force per grain
1
Fgrain  u  wt
2
and
w  rt
1
Fgrain  u  r  t  t
2
we need t
t
4v
V cr
mm
d
91.4 103 mm
min

mm grains
D
150 mm
3600  150
5

10
min
mm2
4  1500
t  1.32 103 mm
39
• Substituting


1
1
3 2
Fgrain  u  r  t  t  35  10  1.32 10
2
2
4
Fgrain  3.05 10 J / mm
40
• For the temperature, we need K1 and K2. K2 is
given, so we need to calculate K1.
1
1 1
1
Fgrain  u   r  t  t  K1    r  t  t  K1   r  t
2
t 2
2
1
3.0510 N  K1  10 1.32106 m
2
3 N
K1  46.2 10
m
1
41
• substituting
1
T  K2  K1   d
t
T  0.2
K m
N
1
6
 46.2 

91
.
4

10
m  640K

6
N
m 1.32 10 m
T  Tinitial  T  20  640  660C
42
Honing and Superfinishing
Honing tool used to improve
the surface finish of bored or
ground holes.
Schematic
illustrations of the
superfinishing
process for a
cylindrical part.
(a) Cylindrical
mircohoning, (b)
Centerless
microhoning.
Lapping
(a) Schematic illustration of the lapping process. (b)
Production lapping on flat surfaces. (c) Production lapping
on cylindrical surfaces.
Polishing Using Magnetic
Fields
Schematic illustration of polishing of balls and rollers
using magnetic fields. (a) Magnetic float polishing of
ceramic balls. (b) Magnetic-field-assisted polishing of
rollers. Source: R. Komanduri, M. Doc, and M. Fox.
Abrasive-Flow Machining
Schematic illustration of
abrasive flow machining to
deburr a turbine impeller.
The arrows indicate
movement of the abrasive
media. Note the special
fixture, which is usually
different for each part
design. Source: Extrude
Hone Corp.
Robotic Deburring
A deburring operation on a robot-held die-cast
part for an outboard motor housing, using a
grinding wheel. Abrasive belts or flexible
abrasive radial-wheel brushes can also be used
for such operations. Source: Courtesy of
Acme Manufacturing Company and
Manufacturing Engineering Magazine,
Society of Manufacturing Engineers.
Conformal Hydrodynamic Nanopolishing:
Case-study
Applications
Challenges
• Most processes can polish only flat
surfaces; concave/profiled surfaces
• Defence & Nuclear
are difficult to superfinish
• Concave surface on hard brittle
• Electronics
materials, such as single crystal
• Industrial
sapphire can not be finished via form
Existing Methods
grinding due to process-induced
cracks
• Diamond Turning
• Diamond turning center can be used
• Precision Grinding
for non ferrous materials but it is a
• Lapping
super-precision machine-tool (The
equipment cost is ~ 20 crores besides
• Ion Beam Polishing
• Spot Hydrodynamic Polishing the expensive operational cost)
• Optics
1
Conformal Hydrodynamic
Nanopolishing Process and Machine
Process Description
•
•
•
•
Improvement over existing
spot hydrodynamic polishing
methods
Superfinish hard and brittle
concave surfaces, specially,
sapphire and hardened steels
Mitigates
existing surface
microcracks
Polishing action due to
elastohydrodynamic film in
the slurry
submerged
rotating conformal contact
(silicone ball in the cavitynd
being
polished)
12st Generation Conformal Hydrodynamic Nanopolishing Machin
Delivered to Precison Engineering Division BARC
Generation
(designed and fabricated in the Machine Tools Lab
at IIT Bombay)
2
Novelty and Technology Breakthrough
• Existing hydrodynamic polishing employs spot polishing unable to
polish small cavities (< 6mm in dia)
– More than 3 degrees of freedom required to polish the entire cavity
effectively
– Small actuation system and polishing tool is required
• Conformal contact has a rotating soft tool which conforms to the shape
of the cavity
• Axis of this tool is inclined at 45˚ and the workpiece is rotated inside a
slurry filled tank which ensures non-zero relative motion between tool
and workpiece at every location in the cavity
• The entire cavity can be polished at once which will be much cheaper
and faster than programming the tool path
• Alternative to expensive Diamond Turning
• Can finish wide range of materials ceramics (sapphire, glass) and
hardened steels
3
Technology Outcomes
•
•
•
•
•
Superfinished surfaces in
steels (< 3nm 3D surface
roughness)
Crack-free surface of < 100
nm 3D surface roughness in
single
crystal
sapphire
cavity
Process
knowhow
and
machine
transferred
to
Precision
Engineering
Division, BARC for strategic
applications in a nuclear
device
The superfinished obtained
at a fraction of cost of
Diamond turning
It can also be used by gem
polishers
which
could
reduce the health hazard by
reducing the dust inhalation
and automation is possible
Nanometric finish on hardened steel
Crack-free superfinished surface on single
crystal sapphire cavity
4
Economics of Grinding and Finishing
Operations
Increase in the cost of
machining and finishing a
part as a function of the
surface finish required.
This is the main reason
that the surface finish
specified on parts should
not be any finer than
necessary for the part to
function properly.
Summary
• Overview of processes
• Analysis of process
• Example problem
54

Grinding - IIT Bombay

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