Miller 900019 Manuale di installazione e funzionamento - Pagina 13
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a-
made.
The
amount
that the electrode is with
drawn is small and
depends
upon the
diameter;
this distance is known
as
the
arc
length.
If
in
striking
an
arc,
the electrode
freezes,
it may be
freed
by
a
quick
twist of the wrist.
Another method of
establishing
the
arc
is avail
able. It
is known
as
the
tapping
method
and
is
shown in
Figure
4.1. In this the electrode
in
the holder is
brought straight
down
on
the work
and
immediately
after
contact,
is withdrawn
to
the proper
arc
length.
Practice
striking
the
arc
using
both methods.
Generally
the
scratching
method
is
preferred
for
a-c
welding.
Determination of the
correct
arc
length
is diffi
cult since there is
no
ready
means
of
measuring
it.
As
a
preliminary guide,
use
about
1/16
arc
length
on
1/16
and 3/32
electrode;
for
1/s
and
5/32
electrodes
use
about
1/8
arc
length.
When
skill is
acquired,
the sound of the
arc
will be
a
good guide.
A short
arc
with
correct current will
give
a
sharp, crackling
sound.
Once the knack of
starting
and
holding
an
arc
has been
learned,
turn
next
to
depositing
weld
metal.
In the
beginning
it is best to
run
beads of
weld metal
on
flat
plates using
a
full electrode.
Practice
moving
from left
to
right
and from
right
to
left. The electrode should be held
more
or
less
perpendicular
to
the
work,
except
that
tilting
it
ahead,
in
the direction of travel will prove
help
ful. The
correct
position
is
shown
in
Figure
5.1.
SPATtER
A proper bead is
shown
in
Figure
6.1 while
Figure
7.1,
illustrates
a
cross-section
through
the bead
and identifies the various
terms
used in describ
ing
a
weld. To
produce
these
results it is
neces
sary
to
hold
a
short
arc,
travel
at
a
uniform
speed,
and feed the electrode downward
at
a
constant
rate
as
it
melts.
BEAD
BUILD UP
PENETRATION
GOOD
Probably
the first
attempts
in
the
practice
will
fall short of the results shown. Too
long
an
arc
will be held
or
the travel
speed
will vary from
slow
to
fast
and
the welds will look like the
one
in
Figure
8.1.
A
cross
section
through
such
a
weld
SPATTER
is
given
in
Figure
9.1. In
addition the weld will
probably
be spongy
(porous)
and of low
strength.
Continue
practicing
until
uniform
beads
as
shown
in
Figure
6.1
can
be
produced
every time.
A
good
method
of
practicing
is
to
deposit
a
series
of
beads,
one
next to
the other until the
plate
is
covered. Then
deposit
another series of beads
at
right angles
to
the
first,
thus
building
up
the
plate
to
a
greater
thickness.
TAPPING METHOD
Ii
1,~r
SPATTER
Figure
6.1
Figure
4.1
Figure
7.1
50 TO
l5~
ELECTRODE
SPATTER
Figure
8.1
BASE METAL
Figure
5.1
Examination
of
the
deposited
bead will
give
a
further check.
IOM-114A-.
Page
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