Analysis of form threads using fluteless taps in cast magnesium alloy (AM60)

Abstract

Threads are used in the most assemblies of industrial products. Commonly, mechanical components need to have threaded parts allowing fast and accurate assemblies and disassemblies. Internal tapping is one of the most demanded machining operations, and threads obtained by forming have been a good alternative. This work investigates the effect of the factors the hole diameter, the forming speed and types of tool on the responses: torque, hardness, fill rate, and thrust force of the form tapping process. The experiments were carried out with three diameters, three forming speeds, and two coatings. The material used was the AM60 alloy due to its high ductility and wide application as head engine. The results revealed torque is more affected by the hole diameter than thrust force, and little hardening occurred using high forming speed with a small diameter. The fill rate of the thread profile was not significantly affected by the intermediate and large diameters. Finally, it can be stated that the recommended hole diameter provided by the tool's supplier can be modified to achieve more accurate thread profiles.

Full text

Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760
Contents
lists
available
at
SciVerse
ScienceDirect
Journal
of
Materials
Processing
Technology
jou
rnal
h
om
epa
g
e:
www.elsevier.com/locate/jmatprotec
Analysis
of
form
threads
using
fluteless
taps
in
cast
magnesium
alloy
(AM60)
Alessandra
Olinda
de
Carvalho, Lincoln
Cardoso
Brandão∗, Túlio
Hallak
Panzera, Carlos
Henrique
Lauro
Sustainable
Manufacturing
Group,
Department
of
Mechanical
Engineering,
Federal
University
of
São
João
del
Rei,
Minas
Gerais,
Brazil
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
12
October
2011
Received
in
revised
form
13
February
2012
Accepted
14
March
2012
Available online 30 March 2012
Keywords:
Form
tapping
Torque
Thrust
force
AM60
alloy
Screw
threads
a
b
s
t
r
a
c
t
Threads
are
used
in
the
most
assemblies
of
industrial
products.
Commonly,
mechanical
components
need
to
have
threaded
parts
allowing
fast
and
accurate
assemblies
and
disassemblies.
Internal
tapping
is
one
of
the
most
demanded
machining
operations,
and
threads
obtained
by
forming
have
been
a
good
alternative.
This
work
investigates
the
effect
of
the
factors
the
hole
diameter,
the
forming
speed
and
types
of
tool
on
the
responses:
torque,
hardness,
fill
rate,
and
thrust
force
of
the
form
tapping
process.
The
experiments
were
carried
out
with
three
diameters,
three
forming
speeds,
and
two
coatings.
The
material
used
was
the
AM60
alloy
due
to
its
high
ductility
and
wide
application
as
head
engine.
The
results
revealed
torque
is
more
affected
by
the
hole
diameter
than
thrust
force,
and
little
hardening
occurred
using
high
forming
speed
with
a
small
diameter.
The
fill
rate
of
the
thread
profile
was
not
significantly
affected
by
the
intermediate
and
large
diameters.
Finally,
it
can
be
stated
that
the
recommended
hole
diameter
provided
by
the
tool’s
supplier
can
be
modified
to
achieve
more
accurate
thread
profiles.
© 2012 Elsevier B.V. All rights reserved.
1.
Introduction
The
assemblies
provided
by
screws
have
been
widely
used
in
every
industrial
sector.
Generally,
tapping
is
the
last
stage
of
manufacturing
process,
consequently
good
accuracy
and
surface
finishing
must
be
reached
for
a
perfect
assembly
with
not
air-gaps.
The
Al–Si
alloys
are
the
most
widely
material
used
in
the
auto-
motive
industry,
mainly
for
engine
head
and
gearbox
(Bhowmicka
et
al.,
2010).
Several
tap
geometries
with
different
coatings
can
be
used
for
machining
of
metals,
as
well
as
non-ferrous
and
non-
metallic
materials.
The
most
industrial
processes
produce
internal
threads
using
cutting
taps.
According
to
Fromentin
et
al.
(2010)
the
form
tapping
has
increased
in
industrial
applications
in
the
last
years
due
to
the
environmental
requirements,
such
as
the
waste
generation
by
the
chip
formation.
Ivanov
and
Kirov
(1997)
stated
the
efficiency
of
fluteless
taps
on
the
shop
floor
is
not
satisfactory
because
of
basic
issues
which
are
associated
with
the
physical
phe-
nomenon
of
the
cutting
taps.
Despite
the
tapping
process
has
been
widely
applied
in
industry,
there
is
a
relevant
gap
between
the
researches
and
the
practical
cases.
Although
the
form
tapping
process
has
existed
for
decades,
very
few
studies
have
been
published
due
to
the
poor
interest
by
the
industry
(Gontarz
et
al.,
2004).
Screws
are
usually
manufactured
by
forming
process
due
to
the
amount
of
parts
produced
by
hours
and
∗Corresponding
author.
E-mail
addresses:
alessandraolinda@yahoo.com.br
(A.O.
de
Carvalho),
lincoln@ufsj.edu.br
(L.C.
Brandão),
panzera@ufsj.edu.br
(T.H.
Panzera),
caiquelauro@gmail.com
(C.H.
Lauro).
the
strength
necessary
to
join
different
parts.
Generally,
forming
threads
support
higher
mechanical
efforts
than
cutting
threads.
In
particular,
the
automotive
industry
has
great
interest
in
this
process
due
to
the
possibility
of
eliminating
steps
to
remove
chips
from
internal
threads
(Stephan
et
al.,
2011).
In
addition,
the
great
advantage
of
form
tapping
is
linked
to
the
characteristics
of
the
screw
thread.
The
thread
is
formed
by
the
plastic
deformation
of
the
worked
material,
providing
a
perfect
screw
thread
with
no
waste
material.
In
the
automotive
industry,
for
example,
the
engine
heads
are
manufactured
with
non-ferrous
materials
which
have
a
superior
capacity
to
deform
and
main-
tain
an
acceptable
mechanical
strength.
Baldo
et
al.
(2010)
have
investigated
the
form
tapping
process
in
Aluminum
alloy
7055
determining
the
tensile
strength
of
internal
threads.
The
results
demonstrated
that
the
formed
threads
exhibited
the
same
mechan-
ical
strength
of
machined
threads
considering
similar
parameters
of
production,
pitch
and
hole
diameter.
Magnesium
and
aluminum
alloys
are
non-ferrous
materials
applied
frequently
in
the
automotive
industry
due
to
their
low
weight,
ease
of
work
and
the
possibility
of
recycling
(Chowdhary
et
al.,
2002).
The
aluminum
alloys
have
great
potential
in
the
Brazil-
ian
industry
corresponding
to
95
wt%
of
recycling.
According
to
Agapiou
(1994),
smaller
hole
diameter
provides
higher
fill
screw
thread,
however,
the
process
needs
high
level
of
torque,
and
the
operation
must
be
monitored
to
avoid
tool
breakage.
The
threads
produced
by
fluteless
taps
have
some
peculiarities,
such
as
the
appearance
of
a
split
crest
on
the
top
of
the
thread.
According
to
Fromentin
et
al.
(2002)
the
rate
of
crest
formation
depends
directly
on
the
initial
hole
diameter
because
smaller
diam-
eters
provide
a
smaller
split
crest
on
the
top
of
the
screw
thread
0924-0136/$
–
see
front
matter ©
2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2012.03.018

1754 A.O.
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Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760
Fig.
1.
Work
piece
dimensions
and
experimental
setup.
after
form
tapping.
Small
diameters,
however,
increase
the
torque
values.
Based
on
this
statement,
form
tapping
is
suitable
for
high
ductile
and
metallic
materials,
mainly
for
internal
threads,
which
are
used
to
bear
high
load
and
perfect
fixture.
The
experimental
and
numerical
researches
have
been
carried
out
to
determine
the
load
level
which
provides
the
best
surface
finishing
in
thread
forming
process
(Mathurin
et
al.,
2009).
The
finite
element
technique
has
been
used
to
preview
the
load
in
the
machining
process;
however
the
results
must
be
compared
with
the
experimental
data.
Finally,
the
selection
of
the
initial
diameter
must
be
set
for
each
material
as
a
function
of
the
torque,
the
split
crest
formation,
and
the
surface
hardness
values.
A
design
of
experiment
(DOE)
was
conducted
to
identify
the
effect
of
the
factors,
tool
coating,
hole
diameter
and
forming
speed
on
the
responses
torque,
thrust
force,
hardness,
and
fill
rate
for
form
tapping
process.
The
optical
and
scanning
electron
microscopes
were
used
to
analyze
the
topogra-
phy
of
the
forming
thread’s
profiles.
2.
Methodology
A
machining
center,
ROMI
Discovery
560,
with
10,000
rpm
and
15
kW
of
main
power
was
used
to
perform
the
experimental
tests.
A
rigid
tapping
system
consisted
of
a
BT-40
tool
holder
and
mechani-
cal
collet.
The
axial
and
rotational
movement
synchronization
was
based
on
the
SINUMERIC
840D
CNC
software.
A
start
distance
of
10
mm
was
set
from
the
work
piece
to
ensure
the
full
acceler-
ation
of
the
tool
holder.
All
experimental
tests
were
carried
out
under
emulsion
coolant
with
6%
of
concentration
at
a
flow
rate
of
20
l/min.
Specimens
of
magnesium
cast
alloy,
AM60
grade,
with
dimensions
69
mm
of
diameter
and
30
mm
of
height
were
manu-
factured
as
illustrated
in
Fig.
1.
A
relief
hole
with
14
mm
of
diameter
and
15
mm
of
height
was
set
at
the
bottom
to
allow
the
output
of
Fig.
3.
Assembly
of
the
experimental
test
setup.
Table
1
Factors
and
levels
of
the
experiment.
Input
factors
Levels
−1
0
+1
Diameter
(mm)
9.1
9.3
9.5
Forming
speed
(m/min)
60
80
100
Tool
Coated
Uncoated
the
tap.
The
diameter
indicated
as
“A”
in
Fig.
1
corresponds
to
the
diameter
variation
for
the
initial
holes
used
in
the
tests,
and
the
holes
with
8
mm
of
diameter
were
used
to
assemble
the
work
piece
on
the
dynamometer.
In
order
to
run
the
experiments,
nine
form
tapping
setup
were
randomly
machined
for
each
work
piece.
The
hardness
was
measured
in
three
random
regions
of
the
work
piece,
ranging
from
65
to
74
HB.
The
taps,
model
M10
6HX
Druck-S,
manufactured
by
Emuge-Franken
were
used
in
the
experiment.
The
taps
are
manufactured
according
to
the
DIN-13-1
and
ISO-68
stan-
dards
for
metric
threads.
Uncoated
and
TiN
coated
HSSE
taps
were
used
with
polygonal
geometry
and
a
lead
taper
of
2–3.5
threads,
according
to
Fig.
2(a)
and
(b)
(DIN
2175,
2008).
However,
the
coated
taps
provided
five
slots
for
lubrication,
as
shown
in
Fig.
2(b).
The
assembly
of
the
work
pieces
on
the
dynamometer
was
per-
formed
to
avoid
interference
between
the
fixture
and
intermediate
devices.
The
work
pieces
were
fixed
directly
upon
the
dynamome-
ter
as
shown
in
Fig.
3.
Tests
of
microhardness
Vickers
were
carried
out
using
a
MVK-G1
MitutoyoTM hardness
tester
with
50
grf
of
load
and
20
s
of
pre
load
time.
The
distance
between
the
micro
indentations
of
the
thread’s
border
were
set
based
on
three
times
of
the
indentation
size
avoiding
the
displacement
of
material
at
the
border
and
consequently,
influencing
the
results.
Table
1
shows
the
evaluated
factors
and
levels
investigated
in
the
experiment.
The
zero
level
for
diameter
(9.3
±
0.2
mm)
and
forming
speed
(80
m/min)
were
set
based
on
the
results
of
Baldo
et
al.
(2010)
which
investigated
aluminum
alloy.
These
levels
were
also
checked
on
Emuge-Franken
(2010)
for
non-ferrous
materi-
als.
The
diameter
dimension
was
verified
at
both
sides
of
the
hole
Fig.
2.
Design
of
the
taps
(Emuge-Franken,
2010).

A.O.
de
Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760 1755
Fig.
4.
Typical
variation
of
thrust
force
(setup:
diameter
9.3
mm,
forming
speed
of
60
m/min
and
uncoated
tap).
using
an
internal
three-point
digital
micrometer
with
10
␮m
of
accuracy.
The
fill
rate
measurements
of
the
thread
profile
were
carried
out
using
an
optical
microscope
MitutoyoTM model
TM-
500
at
40×
of
magnification.
The
fill
rate
was
analyzed
based
on
a
line
drawn
at
the
border
of
thread
to
ensure
the
area
used
for
the
calculus.
The
Moticam
software
automatically
calculated
the
area
of
the
generated
profile,
comparing
the
results
with
the
theoreti-
cal
value
of
thread
profile.
The
topographic
analysis
of
the
threads
was
performed
using
a
scanning
electron
microscope
Hitachi
model
TM3000.
Preliminary
tests
demonstrated
that
the
forming
speed
of
100
m/min
is
the
upper
limit
for
this
alloy,
which
avoids
spindle
lock
and
ensure
an
acceptable
process
finishing.
A
full
factorial
design
(3221),
running
the
total
of
18
experimental
conditions,
was
performed
to
identify
the
main
and
interaction
effects
of
input
fac-
tors
(see
Table
1)
on
the
responses,
such
as
fill
rate,
torque,
thrust
force,
and
hardness
of
work
pieces.
Each
experimental
condition
was
repeated
three
times
and
two
replicates
were
set
in
the
exper-
iment.
The
statistical
software
Minitab
14
was
used
to
analyze
the
data
using
DOE
and
ANOVA
tools.
3.
Results
and
discussion
3.1.
Torque
and
thrust
force
Fig.
4(a)
and
(b)
shows
typical
graphs
for
thrust
force
during
the
form
tapping
process.
The
process
behavior
can
be
divided
in
three
steps;
the
first
step
corresponds
to
the
production
of
the
thread
in
0.35
s,
the
second
step
represents
the
stop
of
the
tap
at
the
end
of
the
hole
in
0.25
s,
and
the
last
step
demonstrates
the
return
of
the
tool.
The
total
time
of
tapping
is
nearly
2
s.
The
first
step
of
the
form
tapping
process
is
highlighted
in
Fig.
4(b).
This
curve
can
be
divided
into
two
subregions;
the
first
one
represents
a
quick
increase
of
the
force
due
to
the
chamfered
length
of
the
tap,
followed
by
a
constant
thrust
force
during
the
action
of
the
cylindrical
length.
The
increase
of
thrust
force
in
the
end
of
the
process
can
be
attributed
not
only
to
the
reduction
of
space
for
the
material’s
strain,
but
also
an
elastic
recovery
of
the
material
which
tends
to
lock
the
tap
into
the
hole,
consequently
increases
the
tap
return
force.
The
same
behavior
was
observed
in
all
experimental
conditions,
just
varying
the
peak
level
of
the
thrust
force.
The
second
step
features
a
constant
force
which
quickly
drops
to
the
minimum
thrust
force
value,
as
shown
in
Fig.
4(a).
It
can
be
observed
that
the
minimum
thrust
force
during
the
return
oper-
ation
is
higher
than
the
thrust
force
during
the
tapping
process,
considering
the
modulus
of
the
values.
The
form
tapping
process
is
suitable
for
ductile
materials,
such
as
the
AM60
magnesium
alloy,
which
provided
low
efforts
and
high
plastic
strains.
Thrust
forces
during
the
return
process
were
observed
on
both
coated
and
uncoated
tools.
Similar
to
the
tapping
process,
the
return
also
shows
a
constant
region
at
the
cylindrical
part
and
a
slope
that
matches
the
output
of
the
chamfered
length
of
the
tap.
The
return
speed
was
set
at
20
m/min,
which
represents
1/3
of
the
minimum
forming
speed
value
during
tapping.
Fig.
5(a)
shows
the
graph
of
torque
during
the
form
tapping.
It
was
verified
the
torque
is
similar
to
the
force
graph
behavior
exhibiting
three
different
steps.
The
first
step
corresponds
to
the
increase
of
torque
due
to
the
chamfered
and
cylindrical
lengths
of
Fig.
5.
Typical
variation
in
torque
(setup:
diameter
9.5
mm,
forming
speed
100
m/min,
and
uncoated
tap).

1756 A.O.
de
Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760
Table
2
Analysis
of
variance.
ANOVA
Experimental
factors
P-value
≤0.05
Fill
rate
Thrust
force
Torque
Main
factors
Forming
speed
0.000
0.081
0.299
Hole
diameter
0.000
0.000
0.000
Type
of
tool
0.151
0.505
0.004
Interaction
of
factors Forming
speed
×Hole
diameter
0.451 0.357 0.428
Forming
speed
×Type
of
tool
0.138
0.297
0.209
Hole
diameter
×
Type
of
tool
0.014
0.006
0.114
Forming
speed
×
Hole
diameter
×
Type
of
tool
0.080
0.628
0.592
R2(adjunct)
98.74%
79.76%
93.15%
the
tap.
As
observed
in
Fig.
4(b)
the
second
step
represents
the
stop
of
tap
and
the
third
the
returning.
The
main
difference
between
the
graphs
is
the
entrance/return
relation
of
the
tap.
Unlike
the
thrust
force,
the
return
value
is
proportional
to
the
tapping
process.
The
return
of
the
tap
also
shows
a
constant
torque
and
a
quick
decrease
which
corresponds
to
the
chamfered
region
of
the
tap.
Fig.
5
shows
the
results
for
the
uncoated
tap
with
100
m/min
of
forming
speed
and
initial
hole
of
9.5
mm,
however
the
same
behavior
was
revealed
for
all
experi-
mental
conditions,
as
exhibited
for
the
thrust
force
values.
The
rotation
of
the
tool
minimizes
the
torque
force
due
to
the
presence
of
rolling
friction
at
tool/work
piece
interface,
which
pro-
vides
low
torque
values
during
the
tap
return.
In
addition,
more
thrust
force
is
needed
for
the
tap
displacement
to
unlock
it
from
the
hole.
According
to
Fromentin
(2004)
and
Stéphan
et
al.
(2012)
the
thrust
force
is
high
for
the
rigid
tapping
systems
and
low
for
the
self-reversible
systems.
Generally,
it
occurs
due
to
the
com-
pensation
used
in
self-reversible
systems
in
relation
to
the
axial
and
rotational
movements.
3.2.
Design
of
experiment
The
results
from
the
analysis
of
variance
(ANOVA)
are
given
in
Table
2.
If
the
P-value
is
less
than
or
equal
to
0.05
the
effect
is
con-
sidered
significant.
An
˛-level
of
0.05
for
a
level
of
significance
with
a
95%
probability
indicates
the
effect
being
significant.
All
P-values
less
than
or
equal
to
0.05
are
underlined
in
Table
2.
The
signifi-
cant
effects
are
shown
on
main
effect
plot
or
interaction
plot.
These
graphics
are
not
a
typical
‘scatter’
plot
of
data,
but
illustrate
the
sta-
tistical
analysis
and
provide
the
variation
on
the
significant
effects.
The
value
of
‘R2adjust’
shown
in
the
ANOVA
(Table
2)
indicates
the
adjustment
of
the
models
is
satisfactory.
Larger
values
of
adjusted
R2suggest
models
of
greater
predictive
ability
(Wu
and
Hamada,
2000).
3.2.1.
Thrust
force
The
thrust
force
data
for
all
experiments
varied
from
580.57
to
1541.96
N.
Two
main
factors
(forming
speed
and
hole
diameter)
and
an
interaction
effect
(diameter
×tool)
exhibited
P-values
(see
Table
2)
lower
than
0.05,
which
significantly
affect
the
thrust
force.
Fig.
6(a)
shows
the
thrust
force
is
much
affected
when
the
form-
ing
speed
decreases
from
80
m/min
to
100
m/min,
which
can
be
attributed
to
the
reduction
of
friction
by
the
increase
of
tempera-
ture
which
is
responsible
to
the
rearrangement
of
atoms
making
it
more
ductile.
For
this
reason,
high
speed
forming
is
desired
in
the
form
tapping
process.
Fig.
6(b)
shows
the
interaction
effect
plot
of
type
of
tool
and
hole
diameter.
As
expected
the
thrust
force
increases
with
the
reduction
of
the
hole
diameter,
however
it
is
possible
to
observe
the
coated
tool
was
able
to
decrease
the
thrust
force
only
for
the
smaller
hole
diameter
(9.1
mm).
In
opposite,
the
uncoated
tool
provided
lower
levels
of
force
when
the
hole
diameters
of
9.3
and
9.5
mm
were
used.
According
to
Nedic
and
Lakic-Globocki
(2005)
the
coated
tool
(TiN)
exhibits
superior
friction
which
can
contribute
to
the
force
increasing.
The
thrust
force
is
reduced
as
the
increase
of
hole
diam-
eter
considering
there
is
less
bulk
material
to
be
deformed
(Fig.
7).
The
coated
tool
has
five
slots
used
for
lubrication
during
the
process,
however,
for
small
diameters
(i.e.
9.1
mm)
the
slots
are
not
sufficient
to
lubricate
during
the
tapping
process,
consequently,
increasing
the
thrust
force.
3.2.2.
Torque
The
torque
value
varied
from
6.9
Nm
to
15.39
Nm.
The
main
fac-
tor
hole
diameter
and
the
interaction
hole
diameter
and
type
of
tool
significantly
affected
the
toque
response,
exhibiting
P-values
lower
than
0.05
(see
Table
2).
Fig.
8(a)
shows
the
main
effect
plot
of
the
hole
diameter
on
the
torque.
The
increase
of
the
hole
diameter
provided
the
reduction
of
the
torque
as
expected.
This
reduction
is
more
evident
(15.77%)
when
the
hole
diameter
is
changed
from
9.1
to
9.3
mm.
Fig.
6.
Main
effect
plot
of
forming
speed
(a)
and
diameter
(b)
for
thrust
force.

A.O.
de
Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760 1757
Fig.
7.
Interaction
plot
of
hole
diameter
and
type
of
tool
for
thrust
force.
Fig.
8(b)
shows
the
interaction
effect
plot
of
hole
diameter
and
type
of
tool
for
torque.
A
relevant
contrast
is
observed
between
the
hole
diameter
levels
and
type
of
tool.
The
coated
tool
used
at
the
low
level
(9.1
mm)
of
diameter
was
able
to
reduce
the
torque
in
comparison
to
9.3
and
9.5
mm
levels,
however
the
lowest
torque
was
achieve
when
the
hole
diameter
was
set
as
9.5
mm
and
the
uncoated
tool
was
used.
In
contrast,
a
significant
increase
of
torque
was
observed
when
the
diameter
of
9.1
mm
and
uncoated
tool
were
used,
which
can
be
attributed
to
the
large
amount
of
forming
mate-
rial
responsible
to
generate
the
thread
profile.
3.2.3.
Fill
rate
The
fill
rate
data
varied
from
0.3221
to
0.7948
mm2.
The
main
factors
hole
diameter
and
type
of
tool
significantly
affected
the
fill
rate
response,
showing
P-values
lower
than
0.05
(see
Table
2).
Fig.
9(a)
shows
the
main
effect
plot
of
the
factor
hole
diameter.
The
increase
of
the
hole
diameter
provided
the
decrease
of
the
fill
rate
as
expected.
A
relevant
percent
reduction
of
28%
was
observed
between
the
levels
9.3
and
9.8
mm.
Similarly
to
the
thrust
force
and
torque,
the
large
diameter
provided
the
lowest
effort
during
the
form
tapping,
consequently
achieving
a
low
fill
rate.
Fig.
9(b)
shows
the
main
effect
plot
of
type
of
tool
for
the
fill
rate
response.
The
uncoated
tool
provided
a
percent
reduction
of
8.77%
on
the
fill
rate.
This
behavior
is
also
related
to
the
amount
of
material
which
must
be
deformed
during
the
tapping
operation.
3.3.
Microstructural
and
topographic
analysis
of
the
profile
Fig.
10
shows
the
optical
microscopic
image
of
the
cross
sec-
tion
of
thread
profile
manufactured
with
uncoated
tool,
diameter
of
9.5
mm
and
forming
speed
of
80
m/min
at
50×
of
magnification.
The
fill
rate
of
the
thread
profile
is
not
perfect
and
a
split
crest
is
observed
due
to
the
small
amount
of
bulk
material.
The
closing
of
the
split
crest
occurs
when
small
diameters
are
used
as
shown
in
Fig.
11
when
the
hole
diameter
is
9.1
mm.
The
form
tapping
process
provides
a
plastic
deformation
of
the
mate-
rial
which
is
responsible
to
move
up
to
the
crest
of
the
thread.
The
deformation
of
the
material
can
enhance
the
surface
hardness,
Fig.
8.
Main
effect
plot
of
diameter
(a)
and
interaction
plot
of
hole
diameter
and
type
of
tool
(b)
for
torque.
Fig.
9.
(a)
Main
effect
plot
of
hole
diameter
for
the
fill
rate.
(b)
Main
effect
plot
of
type
of
tool
for
fill
rate.

1758 A.O.
de
Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760
Fig.
10.
Optical
image
of
the
thread
profile
(forming
speed
80/m/min
and
diameter
of
9.5
mm)
at
50×
of
magnification.
Fig.
11.
Optical
image
of
the
thread
profile
(forming
speed
80/m/min
and
diameter
of
9.5
mm)
at
50×
of
magnification.
which
is
revealed
in
Figs.
10
and
11
by
the
flattening
the
grain
size
at
the
borders.
It
is
observed
in
Fig.
11
that
the
bent
mass
of
the
material
is
concentrated
in
the
right
side,
which
corresponds
to
the
feed
rate
direction
of
the
tool.
The
best
closure
of
the
split
crest
in
the
experi-
ment
was
achieved
using
an
uncoated
tool
with
9.1
mm
of
diameter
and
100
m/min
of
speed
as
seen
in
Fig.
11.
Fig.
12
shows
a
scanning
electron
microscope
image
of
the
topography
of
the
thread
profile
for
the
diameter
of
9.3
mm
and
forming
speed
of
80
m/min.
Fig.
12(a)
reveals
the
presence
of
split
crest
on
both
sides
of
the
thread
profile
(see
detail
(a)),
due
to
the
plowing
effect
resulting
in
a
saw
format.
It
is
noted
the
saw
for-
mat
has
a
trend
to
bend
in
the
direction
of
the
feed
rate
(see
detail
(b)).
This
phenomenon
has
occurred
only
when
the
coated
tool
was
used,
due
to
the
high
friction
coefficient
of
TiN
coating.
The
high
friction
coefficient
leads
to
increase
the
adhesion
in
the
tool/material
interface,
raising
the
strain
of
the
material
and
plucking
a
small
bulk
during
the
displacement
of
the
tool.
In
the
opposite
situation,
as
shown
in
Fig.
12(b)
the
saw
format
is
not
formed
when
the
uncoated
tool
is
used.
Thus,
a
clear
split
crest
shows
some
errors
and
an
incomplete
thread
profile
is
achieved.
Fig.
13
shows
the
experimental
data
under
the
following
setup
condition:
diameter
of
9.5
mm
and
speed
of
60
m/min.
In
the
same
way
that
occurred
in
Fig.
12,
the
thread
profile
is
incomplete
and
exhibits
some
errors.
However,
the
profile
did
not
demonstrate
the
saw
format
in
both
type
of
tool
(coated
and
uncoated),
the
thread
profile
is
more
homogeneous
throughout
the
analyzed
profile.
Comparing
the
uncoated
and
coated
tools
based
on
a
visual
analysis
of
the
surface
roughness
of
the
thread,
the
uncoated
tool
provided
a
better
surface
roughness
than
the
coated
one.
Based
on
the
SEM
image,
a
small
amount
of
material
in
chip
format
was
observed,
which
demonstrates
that
the
removal
of
material
occurred
even
when
a
small
amount
of
material
is
deformed,
as
shown
in
Fig.
13(b).
Fig.
14
shows
the
SEM
images
of
the
thread
profile
obtained
under
the
following
setup
condition:
diameter
of
9.1
mm,
speed
of
100
m/min,
coated
and
uncoated
tools.
The
fill
rate
was
more
efficient
when
a
diameter
of
9.1
mm
was
set,
due
to
the
excess
of
material.
In
this
case,
the
bulk
of
material
was
superior
to
that
one
recommended
by
the
tool
supplier.
In
this
way,
the
use
of
these
input
parameters
provides
not
only
the
perfect
closing
of
the
split
crest
for
both
tools,
but
also
the
adhesion
of
small
chips
on
the
thread
profile.
Fig.
14
shows
that
the
bulk
of
material
attached
to
the
thread
profile
occurs
more
intense
for
coated
than
uncoated
tool.
Therefore,
it
can
be
stated
that
small
amount
of
material
merges
on
thread
profile
due
to
the
high
friction
coefficient
and
the
lifting
of
the
material
after
the
tool
removal,
leading
to
a
poor
surface
roughness.
The
same
condition
was
observed
in
the
experiments
using
uncoated
tool
as
shown
in
Fig.
14(b).
It
can
be
observed
in
Fig.
14(a),
that
the
complete
closing
of
split
crest
is
random
and
it
does
not
occur
throughout
the
thread
profile,
generating
small
slots
on
the
profile.
Fig.
12.
SEM
images
of
the
thread
for
9.3
mm,
cutting
speed
of
80
m/min
for
(a)
coated
tool
and
(b)
uncoated
tool
(30×of
magnification).

A.O.
de
Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760 1759
Fig.
13.
SEM
images
of
the
thread
for
9.5
mm,
cutting
speed
of
80
m/min
(a)
coated
tool
and
(b)
uncoated
tool
(30×
of
magnification).
Fig.
14.
SEM
images
of
thread
for
9.1
mm
forming
speed
of
100
m/min
– (a)
coated
tool
and
(b)
uncoated
tool
(30×of
magnification).
3.4.
Analysis
of
microhardness
The
tests
of
microhardness
were
carried
out
to
confirm
the
evidence
of
hardening
due
to
the
material’s
deformation.
Five
mea-
surements
were
performed
on
the
top,
bottom,
and
both
sides
of
the
thread
profile.
A
central
measurement
(number
6)
was
carried
out
at
1
mm
underneath
the
bottom
of
the
thread
in
order
to
verify
the
primary
microhardness
of
the
work
piece.
The
indentations
were
performed
in
those
threads
manufac-
tured
with
the
following
setup:
high
and
low
levels
of
speed
for
the
diameter
of
9.1
mm;
low
level
of
speed
for
the
diameter
of
9.5
mm
and
both
for
coated
and
uncoated
tools.
The
microhardness
mea-
surements
of
the
experimental
condition,
diameter
of
9.3
mm
and
the
speed
of
80
m/min
was
compared
with
the
results
obtained
for
the
standard
setup
recommended
the
supplier
(Fig.
15).
Fig.
16
confirms
the
form
tapping
process
provided
a
hardening
of
the
work
pieces
in
the
region
of
thread.
All
indentations
were
superior
to
the
primary
microhardness
of
work
piece,
indepen-
dently
of
the
input
parameter.
The
highest
values
of
microhardness
were
measured
at
the
top
of
thread
profile
(ID-3)
for
all
experi-
ments.
The
microhardness
of
side
of
the
thread
profile
(ID-2
and
ID-4)
exhibited
superior
values
than
those
measured
at
the
bottom.
These
results
imply
the
deformation
of
material
is
more
evident
in
the
side
than
in
the
bottom
of
the
thread
profile.
In
the
same
way,
the
values
of
microhardness
at
the
bottom
of
the
thread
profile
(ID-1
and
ID-5)
were
higher
than
the
primary
microhardness,
demonstrating
there
was
a
material’s
hardening
in
all
sides
of
the
thread
profile.
The
microhardness
data
for
all
indentations
varied
from
56.5
mHV
to
123.0
mHV.
The
mean
value
of
microhardness
based
on
the
ID-6
indentation
was
55.6
mHV.
The
ID-3
and
ID-4
indentations
showed
the
highest
values
of
microhardness
which
can
be
attributed
to
the
feed
rate
direction,
in
other
words,
the
tap
displaces
from
ID-4
to
ID-2
indentation,
moving
the
material
in
the
direction
of
the
crest.
The
ID-3
indenta-
tion
corresponds
to
the
top
of
thread
profile
where
an
accumulation
Fig.
15.
Scheme
of
measurements
of
hardness
carried
out
in
the
thread
profile.

1760 A.O.
de
Carvalho
et
al.
/
Journal
of
Materials
Processing
Technology
212 (2012) 1753–
1760
Fig.
16.
Results
of
microhardness.
of
deformed
material
occurs.
The
highest
values
of
microhardness
are
related
to
the
direction
of
the
tap
displacement.
4.
Conclusions
Based
on
the
results,
the
following
points
are
summarized:
•The
torque
changes
from
high
to
low
diameters
with
a
decrease
of
23.68%
in
the
mean
values.
The
torque
for
hole
diameter
of
9.3
mm
is
nearly
constant
(increase
of
1.65%)
for
different
coatings.
The
torque
increases
17.03%
for
uncoated
tools
and
decreases
13.67%
for
uncoated
tools.
•The
thrust
force
is
constant
from
60
m/min
to
80
m/min
forming
speed
values
and
decreases
from
80
m/min
to
high
100
m/min
forming
speed
values.
•The
thrust
force
exhibits
a
percent
increase
nearly
47.84%
when
reducing
the
hole
diameter
from
9.5
to
9.1
mm.
•The
thrust
force
was
higher
for
the
coated
than
uncoated
tools
for
the
hole
diameters
of
9.3
and
9.5
mm.
However,
for
the
hole
diam-
eter
of
9.1
mm
the
thrust
force
was
superior
for
uncoated
tools.
The
thrust
force
during
the
tap
return
was
higher
than
torque
during
the
tap
return
for
all
experiments.
•The
fill
rate
varied
from
0.3221
to
0.7948
mm2.
The
main
fac-
tors
that
affected
the
fill
rate
were
the
hole
diameter
and
type
of
tool.
Small
hole
diameters
provide
a
better
closing
of
the
split
crest.
•The
best
thread
profile
was
achieved
when
uncoated
tools
and
forming
speed
of
100
m/min
were
set.
•The
microhardness
tests
revealed
a
small
hardening
of
the
thread
profile
mainly
in
the
sides
and
the
top
of
the
thread.
Acknowledgments
The
authors
would
like
to
thank
Emuge-Franken
and
TRW
Automotive
Systems
for
the
tooling
support
and
the
AM60
alloy,
respectively.
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