The.bobbirf
thread
tension
Is
regulated
by
the
small
screw
S (Fig. 9),
as
follows:
I
Turn
It
right
for
tighter
tension,
or
left
for
looser
tension.
The tension Is correct If you have to overcome a noticeable resistance when pulling
the
thread.
Regulating
the
Needle
Thread
Tension
the
Pfaff 3334-958/01
has
two needle thread tensions. The top tension Sp 1 (Fig. 11)
controls
the
thread
tension
during
the
thread
trimming
action
and
should
be
set
so
that
the
thread
Is
trimmed
easily. If
the
tension
Is
set
too
light,
the
knives
will
draw
an
exces
sive
amount
of
thread
through
the
tension
and
the
needle
thread
will
not
be
cut.
If,
on
the
other
hand,
the
tension
Is
too
tight,
the
thread
will
be
trimmed
too
early
and
too
close
to
the
needle
eye.
The
lower, or main,
tension
Sp2
(Fig. 11)
controls
the
needle
thread
during
the
stitch
se'tting phase. Begin by setting top tension
Spl
and
then
adjust
main tension
Sp2
so
that
the
thread
will
be
drawn
Into
the
material
properly.
In
order
to
prevent
that
the
fabric will
be
contracted
by
the
first long
stitches
of
the
tack,
the
lower
tension
will
not
bnecome
operative until the machine
has
made three or four stitches. (This naturally
applies only to tacks
made
on flimsy material and Involving extremely long Initial stitch
es). The main
tension
Is
released
again
before
the
last
tying stitch Is
made
so
that
a
sufficient
amount
of
thread
can
be
pulled
through
the
tension
mechanism
before
the
threads
are
trimmed.
9.
Starting
the
Machine
Don't start the machine
unless
you have thoroughly familiarized yourself with Its
mode
of
operation.
When you
depress
the right treadle, the brake Is released and the belt shifted from the
loose
to
the
driving pulley. This action
starts
the
machine.
To acquaint yourself with the operation of the machine, turn the driving pulley by hand
and study the Individual
phases
of operation. Then thread the machine and try out the
stitching on a
piece
of material.
Whife the machine Is In operation, a locking mechanism prevents the work clamp from
being raised and the knives from cutting the thread. Conversely, the machine cannot
be
started
while
the
work,
clamp
Is
raised.
14
10.
Stopping
the
Machine
Upon completion of the sewing action, tripping lever F 1 (Fig. 5) Is
depressed
by the
stop
tripping
segment
on the rim of feed cam W and
stop
motion lever R pulled against
the stop cam by a tension spring. At the
same
time, the belt shifter shifts the belt from
the
driving to the loose pulley to slow down the machine before It stops. Then the stop
link
snaps
Into the
stop
cam which
stops
the machine. The momentum of the machine
Is
absorbed
by
double
buffer springs. After
the
machine
has
stopped,
the
locking me
chanism Is released
so
that, by depressing the left treadle,
the
clamp
can
be raised and
th^e
threads
cut.
If any trouble
should
occur
while
the
madilne
Is In operation,
stop
the
machine by
pressing
down
hand
stop
lever
A (Fig. 5).
11.
Trimming
the
Threads
The
thread
trimming
knives
are
located
on
the
underside
of
the
needle
plate.
Shortly
before
the
last
stitdi
Is
made,
main
tension
Sp
2 Is
released.
At
the
same
time,
knife
cam
u (Fig. 6)
operates
the
vertical knife
bar
tripping lever
and
knife
bar
M
and
thereby
causes
the
knives
to
be
moved
to
the
operative
position.
As
the
knives
swing
forward,
the
needle
descends
between
them
for
the
last
stitch
and
the
upper
and
lower
threads
are
caught
by
the
knife
tips.
The
knives
pull
an
adequate
amount
of
thread
through
the
upper
and
lower
tensions
to
provide
sufficient
thread
for
the
first
stitches
of
the
follow
ing tack. On completion of
the
last
stitch,
the
machine
stops
automatically. At this
stage,
the
take-up
lever
has
almost
reached
the
highest
point of Its
stroke
and
the
thread
nipper
Is
closed
and
traps
the
needle
thread.
When
the
left
treadle
Is,
depressed,
the
roller
at
the
top
end
of
the
knife
bar
tripping
lever
drops
Into a
recess
In
the
control
slot
of
the
knife
cam.
This
motion Is
transmitted
to
the
knife
bar
and
the knives which swing forward.' As a result, the
threads
slide
down
from
the
knife
tips
onto
the
cutting
edges
and
are
trimmed.
The
thread
wiper
pulls
the
needle
thread
end
up
through
the
needle
hole.
15
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