JUKI MF-7900 Series Quick start guide

70002000
No. E406-01
ENGINEER’S MANUAL
MF-7900 Series
High-speed, Cylinder-bed Top and Bottom Coverstitch Machine
MF-7900D Series
High-speed, Semi-dry-head, Cylinder-bed Top and Bottom Coverstitch Machine

PREFACE
This Engineer’s Manual is written for the technical personnel who are responsible for the service and
maintenance of the machine.
The Instruction Manual for these machines intended for the maintenance personnel and operators at
an apparel factory contains operating instruction in detail. And this manual describes “Standard Adjust-
ment”, “Adjustment Procedures”, “Results of Improper Adjustment”, and other important information
which are not covered in the Instruction Manual.
When carrying out the maintenance work on the sewing machine, be sure to refer also to the Instruction
Manual and the Parts List.
In addition, for the motor for the sewing machine with thread trimmer, refer to the separate Instruction
Manual or This manual gives the “Standard Adjustment” on the former page under which the most basic
adjustment value is described, and on the latter page “Results of Improper Adjustment” under which
stitching errors and troubles arising from mechanical failures and “How to adjust” are described.

i
For the sewing machine, automatic machine and ancillary devices (hereinafter collectively referred to as
"machine"), it is inevitable to conduct sewing work near moving parts of the machine. This means that there
is always a possibility of unintentionally coming in contact with the moving parts. Operators who actually
operate the machine and maintenance personnel who are involved in maintenance and repair of the machine
are strongly recommended to carefully read to fully understand the following Safety precautions of this
engineer's manual before using/maintaining the machine. The content of the Safety precautions of this
engineer's manual includes items which are not contained in the specications of your product.
The risk indications are classied into the following three different categories to help understand the meaning
of the labels of this engineer's manual and the product. Be sure to fully understand the following description
and strictly observe the instructions.
(I) Explanation of risk levels
(II) Explanation of pictorial warning indications and warning labels
Warning label
❶
❷
❸
❶
• There is the possibility that slight to serious injury or death may be caused.
• There is the possibility that injury may be caused by touching moving part.
❷• To perform sewing work with safety guard.
• To perform sewing work with safety cover.
• To perform sewing work with safety protection device.
❸
• Be sure to turn the power OFF before carrying out "machine-head threading,"
"needle changing," "bobbin changing" or "oiling and cleaning.
❸
❷
❶
Electrical-shock
danger label
DANGER :
This indication is given where there is an immediate danger of death or serous injury if the person in charge or any third
party mishandles the machine or does not avoid the dangerous situation when operating or maintaining the machine.
WARNING :
This indication is given where there is a potentiality for death or serious injury if the person in charge or any third party
mishandles the machine or does not avoid the dangerous situation when operating or maintaining the machine.
CAUTION :
This indication is given where there is a danger of medium to minor injury if the person in charge or any third party
mishandles the machine or does not avoid the dangerous situation when operating or maintaining the machine.
Items requiring special attention
Pictorial warning indication
There is a risk of injury if contact-
ing a moving section.
Pictorial warning
indication
Be aware that holding the sewing
machine during operation can
hurt your hands.
There is a risk of electrical shock if
contacting a high-voltage section.
There is a risk of entanglement
in the belt resulting in injury.
There is a risk of a burn if contact-
ing a high-temperature section.
There is a risk of injury if you
touch the button carrier.
Be aware that eye deciency can
be caused by looking directly at
the laser beam.
Indication label
The correct direction is indicat-
ed.
There is a risk of contact between
your head and the sewing ma-
chine.
Connection of a earth cable is
indicated.
TO ENSURE SAFE USE OF YOUR SEWING MACHINE
Adjustment :
It means replacement of parts,
disassembly, and repair assembly.

ii
Basic precaution
1. Be sure to read the engineer's manual and other explanatory documents supplied with accessories of
the machine before using the machine. Carefully keep the engineer's manual and the explanatory docu-
ments at hand for quick reference.
2. The content of this section includes items which are not contained in the specications of your product.
3. Be sure to wear safety goggles to protect against accident caused by needle breakage.
4. Those who use a heart pacer have to use the machine after consultation with a medical specialist.
5. Turn OFF the power before starting the work in order to protect against accident that can result in per-
sonal injury.
If it is inevitable to carry out work with the power ON, utmost care should be taken to prevent from de-
pressing the foot pedal or pressing the start switch by mistake.
6. Make sure, after the completion of replacement work, that neither connectors nor cables are damaged,
slipped off or loosened in order to protect against accident that can result in personal injury.
Safety devices and warning labels
1. Be sure to operate the machine after verifying that safety device(s) is correctly installed in place and
works normally in order to prevent accident caused by lack of the device(s).
2. If any of the safety devices is removed, be sure to replace it and verify that it works normally in order to
prevent accident that can result in personal injury or death.
3. Be sure to keep the warning labels adhered on the machine clearly visible in order to prevent accident
that can result in personal injury or death. If any of the labels has stained or come unstuck, be sure to
change it with a new one.
Application and modication
1. Never use the machine for any application other than its intended one and in any manner other than that
prescribed in the engineer's manual in order to prevent accident that can result in personal injury or death.
JUKI assumes no responsibility for damages or personal injury or death resulting from the use of the
machine for any application other than the intended one.
2. Never modify and alter the machine in order to prevent accident that can result in personal injury or death.
JUKI assumes no responsibility for damages or personal injury or death resulting from the machine
which has been modied or altered.
Education and training
1. In order to prevent accident resulting from unfamiliarity with the machine, the machine has to be used
only by the operator who has been trained/educated by the employer with respect to the machine oper-
ation and how to operate the machine with safety to acquire adequate knowledge and operation skill. To
ensure the above, the employer has to establish an education/training plan for the operators and edu-
cate/train them beforehand.
Items for which the power to the machine has to be turned off
Turning the power off: Turning the power switch off, then removing the power plug from the outlet. This
applies to the following.
1. Be sure to immediately turn the power off if any abnormality or failure is found or in the case of power
failure in order to protect against accident that can result in personal injury or death.
2. To protect against accident resulting from abrupt start of the machine, be sure to carry out the follow-
ing operations after turning the power off. For the machine incorporating a clutch motor, in particular,
be sure to carry out the following operations after turning the power off and verifying that the machine
stops completely.
2-1. For example, threading the parts such as the needle, looper, spreader etc. which have to be thread-
ed, or changing the bobbin.
2-2. For example, changing or adjusting all component parts of the machine.
2-3. For example, when inspecting, repairing or cleaning the machine or leaving the machine.
3. Be sure to remove the power plug by holding the plug section instead of the cord section in order to pre-
vent electrical-shock, earth-leakage or re accident.
4. Be sure to turn the power off whenever the machine is left unattended between works.
5. Be sure to turn the power off in the case of power failure in order to prevent accident resulting of break-
age of electrical components.
PRECAUTIONS TO BE TAKEN IN VARIOUS OPERATION STAGES
Accident means "to cause
personal injury or death or
damage to property."
CAUTION
DANGER
SAFETY PRECAUTIONS
1. When it is necessary to open the control box containing electrical parts, be sure to turn the power off and
wait for ve minutes or more before opening the cover in order to prevent accident leading to electrical
shock.

iii
Transportation
1. Be sure to lift and move the machine in a safe manner taking the machine weight in consideration. Refer
to the text of the engineer's manual for the mass of the machine.
2. Be sure to take sufcient safety measures to prevent falling or dropping before lifting or moving the
machine in order to protect against accident that can result in personal injury or death.
3. Once the machine has been unpacked, never re-pack it for transportation to protect the machine against
breakage resulting from unexpected accident or dropping.
Unpacking
1. Be sure to unpack the machine in the prescribed order in order to prevent accident that can result in
personal injury or death. In the case the machine is crated, in particular, be sure to carefully check nails.
The nails have to be removed.
2. Be sure to check the machine for the position of its center of gravity and take it out from the package
carefully in order to prevent accident that can result in personal injury or death.
Installation
(I) Table and table stand
1. Be sure to use JUKI genuine table and table stand in order to prevent accident that can result in personal
injury or death. If it is inevitable to use a table and table stand which are not JUKI genuine ones, select the
table and table stand which are able to support the machine weight and reaction force during operation.
2. If casters are tted to the table stand, be sure to use the casters with a locking mechanism and lock
them to secure the machine during the operation, maintenance, inspection and repair in order to prevent
accident that can result in personal injury or death.
(II) Cable and wiring
1. Be sure to prevent an extra force from being applied to the cable during the use in order to prevent electri-
cal-shock, earth-leakage or re accident. In addition, if it is necessary to cable near the operating section
such as the V-belt, be sure to provide a space of 30 mm or more between the operating section and the cable.
2. Be sure to avoid starburst connection in order to prevent electrical-shock, earth-leakage or re accident.
3. Be sure to securely connect the connectors in order to prevent electrical-shock, earth-leakage or re
accident. In addition, be sure to remove the connector while holding its connector section.
(III) Grounding
1. Be sure to have an electrical expert install an appropriate power plug in order to prevent accident caused
by earth-leakage or dielectric strength voltage fault. In addition, be sure to connect the power plug to the
grounded outlet without exceptions.
2. Be sure to ground the earth cable in order to prevent accident caused by earth leakage.
(IV) Motor
1. Be sure to use the specied rated motor (JUKI genuine product) in order to prevent accident caused by
burnout.
2. If a commercially available clutch motor is used with the machine, be sure to select one with an entan-
glement preventive pulley cover in order to protect against being entangled by the V-belt.
Before operation
1. Be sure to make sure that the connectors and cables are free from damage, dropout and looseness be-
fore turning the power on in order to prevent accident resulting in personal injury or death.
2. Never put your hand into the moving sections of the machine in order to prevent accident that can result
in personal injury or death.
In addition, check to be sure that the direction of rotation of the pulley agrees with the arrow shown on
pulley.
3. If the table stand with casters is used, be sure to secure the table stand by locking the casters or with
adjusters, if provided, in order to protect against accident caused by abrupt start of the machine.
During operation
1. Be sure not to put your ngers, hair or clothing close to the moving sections such as the handwheel,
hand pulley and motor or place something near those sections while the machine is in operation in order
to prevent accident caused by entanglement that can result in personal injury or death.
2. Be sure not to place your ngers near the surround area of the needle or inside the thread take-up lever
cover when turning the power on or while the machine is in operation in order to prevent accident that
can result in personal injury or death.
3. The machine runs at a high speed. Never bring your hands near the moving sections such as looper,
spreader, needle bar, hook and cloth trimming knife during operation in order to protect your hands
against injury. In addition, be sure to turn the power off and check to be sure that the machine complete-
ly stops before changing the thread.
4. Be careful not to allow your ngers or any other parts of your body to be caught between the machine
and table when removing the machine from or replacing it on the table in order to prevent accident that
can result in personal injury or death.
5. Be sure to turn the power off and check to be sure that the machine and motor completely stop before re-
moving the belt cover and V-belt in order to prevent accident caused by abrupt start of the machine or motor.
6. If a servomotor is used with the machine, the motor does not produce noise while the machine is at rest.
Be sure not to forget to turn the power off in order to prevent accident caused by abrupt start of the motor.
7. Never use the machine with the cooling opening of the motor power box shielded in order to prevent re
accident by overheat.

iv
Lubrication
1. Be sure to use JUKI genuine oil and JUKI genuine grease to the parts to be lubricated.
2. If the oil adheres on your eye or body, be sure to immediately wash it off in order to prevent inamma-
tion or irritation.
3. If the oil is swallowed unintentionally, be sure to immediately consult a medical doctor in order to pre-
vent diarrhea or vomiting.
Maintenance
1. In prevention of accident caused by unfamiliarity with the machine, repair and adjustment has to be car-
ried out by a service technician who is thoroughly familiar with the machine within the scope dened in
the engineer's manual. Be sure to use JUKI genuine parts when replacing any of the machine parts. JUKI
assumes no responsibility for any accident caused by improper repair or adjustment or the use of any
part other than JUKI genuine one.
2. In prevention of accident caused by unfamiliarity with the machine or electrical-shock accident, be sure
to ask an electrical technician of your company or JUKI or distributor in your area for repair and mainte-
nance (including wiring) of electrical components.
3. When carrying out repair or maintenance of the machine which uses air-driven parts such as an air cyl-
inder, be sure to remove the air supply pipe to expel air remaining in the machine beforehand, in order to
prevent accident caused by abrupt start of the air-driven parts.
4. Be sure to check that screws and nuts are free from looseness after completion of repair, adjustment
and part replacement.
5. Be sure to periodically clean up the machine during its duration of use. Be sure to turn the power off
and verify that the machine and motor stop completely before cleaning the machine in order to prevent
accident caused by abrupt start of the machine or motor.
6. Be sure to turn the power off and verify that the machine and motor stop completely before carrying out
maintenance, inspection or repair of the machine. (For the machine with a clutch motor, the motor will
keep running for a while by inertia even after turning the power off. So, be careful.)
7. If the machine cannot be normally operated after repair or adjustment, immediately stop operation and
contact JUKI or the distributor in your area for repair in order to prevent accident that can result in per-
sonal injury or death.
8. If the fuse has blown, be sure to turn the power off and eliminate the cause of blowing of the fuse and
replace the blown fuse with a new one in order to prevent accident that can result in personal injury or
death.
9. Be sure to periodically clean up the air vent of the fan and inspect the area around the wiring in order to
prevent re accident of the motor.
10. In order to prevent accidents resulting in personal injury or death, adjustments have to be carried out
within the instructions described in this Engineer's Manual and in the Instruction Manual by the person-
nel who have been well trained by the employer with respect to the latest knowledge and safety for the
machine.
11. Make sure that water is contained in cylinders and pipes, then remove water from them in order to pre-
vent accidents due to a malfunction of the machine.
Operating environment
1. Be sure to use the machine under the environment which is not affected by strong noise source (electro-
magnetic waves) such as a high-frequency welder in order to prevent accident caused by malfunction of
the machine.
2. Never operate the machine in any place where the voltage uctuates by more than "rated voltage ±10 %"
in order to prevent accident caused by malfunction of the machine.
3. Be sure to verify that the air-driven device such as an air cylinder operates at the specied air pressure
before using it in order to prevent accident caused by malfunction of the machine.
4. To use the machine with safety, be sure to use it under the environment which satises the following
conditions:
Ambient temperature during operation 5°C to 35°C
Relative humidity during operation 35 % to 85 %
5. Dew condensation can occur if bringing the machine suddenly from a cold environment to a warm one.
So, be sure to turn the power on after having waited for a sufcient period of time until there is no sign
of water droplet in order to prevent accident caused by breakage or malfunction of the electrical compo-
nents.
6. Be sure to stop operation when lightning ashes for the sake of safety and remove the power plug in
order to prevent accident caused by breakage or malfunction of the electrical components.
7. Depending on the radio wave signal condition, the machine may generate noise in the TV or radio. If this
occurs, use the TV or radio with kept well away from the machine.
8. In order to ensure the work environment, local laws and regulations in the country where the sewing
machine is installed shall be followed.
In the case the noise control is necessary, an ear protector or other protective gear should be worn ac-
cording to the applicable laws and regulations.
9. Disposal of products and packages and treatment of used lubricating oil should be carried out properly
according to the relevant laws of the country in which the sewing machine is used.

v
Transportation
1. Be sure to lift this machine with four or more workers and use a carriage for moving it in order to pre-
vent personal injury.
2. Be sure to take sufcient safety measures to prevent falling or dropping before lifting or moving the
machine in order to protect against accident that can result in personal injury or death.
3. Installation is described in the Instruction Manual. Be sure to fully understand the description before
putting the machine into operation.
Replacement of parts
1. In prevention of accident that can result in personal injury, be sure to carry out disassembly/assembly
work within the specied range given in this Engineer's Manual and in the Instruction Manual.
2. Turn OFF the power before starting the work in order to protect against accident that can result in per-
sonal injury.
If it is inevitable to carry out work with the power ON, utmost care should be taken to prevent from de-
pressing the foot pedal or pressing the start switch by mistake.
3. Be sure to carry out replacement work after having installed the machine in a stable state in order to
protect against accident that can result in personal injury.
In addition, be sure to select appropriate tools.
4. In prevention of accident that can result in personal injury, make sure, after the completion of adjust-
ment work, that neither screws nor nuts are loosened or come in contact with other parts.
5. Be sure to use JUKI genuine parts when replacing any of the machine parts.
JUKI assumes no responsibility for any accident caused by any part other than JUKI genuine one.
In addition, in the event you cannot replace parts within the specied range, immediately stop the re-
placement work and ask JUKI or distributor in your area for replacement of the parts.
Adjustment
1. Be sure to adjust according to the instructions given in this Engineer's Manual and in the Instruction
Manual in order to protect against accident that can result in personal injury.
2. Be sure to carry out replacement work after having installed the machine in a stable state in order to
protect against accident that can result in personal injury.
In addition, be sure to select appropriate tools.
3. In prevention of accident that can result in personal injury, make sure, after the completion of adjust-
ment work, that neither screws nor nuts are loosened or come in contact with other parts.
4. Turn OFF the power before starting the work in order to protect against accident that can result in per-
sonal injury.
If it is inevitable to carry out work with the power ON, utmost care should be taken to prevent from de-
pressing the foot pedal or pressing the start switch by mistake.
5. In prevention of accident that can result in personal injury or entanglement accident, be sure to ensure
safety at the time of test run. In addition, be sure to take care not to allow hair or cloths to come in con-
tact with the machine belt.
Disassembly/assembly
1. In prevention of accident that can result in personal injury, be sure to carry out disassembly/assembly
work within the specied range given in this Engineer's Manual and in the Instruction Manual.
2. Be sure to carry out replacement work after having installed the machine in a stable state in order to
protect against accident that can result in personal injury.
In addition, be sure to select appropriate tools.
3. In prevention of accident that can result in personal injury, make sure, after the completion of adjust-
ment work, that neither screws nor nuts are loosened or come in contact with other parts.
4. In prevention of accident that can result in personal injury, be sure to tighten screws and nuts in assem-
bly work with a specied torque, if specied, or with an appropriate torque, if not specied.
After the completion of assembly work, be sure to check that screws and nuts are not loosened before
starting test run.
5. In prevention of accident that can result in personal injury, be sure to check whether the direction of
rotation is correct at the time of test run.
6. In prevention of accident that can result in personal injury or entanglement accident, be sure to ensure
safety at the time of test run. In addition, be sure to take care not to allow hair or cloths to come in con-
tact with the machine belt.
PRECAUTIONS TO BE TAKEN IN VARIOUS OPERATION STAGES
Mechanical components
CAUTION

vi
Transportation
1. Be sure to lift this machine with four or more workers and use a carriage for moving it in order to pre-
vent personal injury.
2. Be sure to take sufcient safety measures to prevent falling or dropping before lifting or moving the
machine in order to protect against accident that can result in personal injury or death.
3. Installation is described in the Instruction Manual. Be sure to fully understand the description before
putting the machine into operation.
Replacement of parts
1. In prevention of accident caused by unfamiliarity with the machine or electrical-shock accident, be sure
to ask an electrical technician of your company or JUKI or distributor in your area for replacement of
electrical components.
2. When it is necessary to open the control box containing electrical parts, be sure to turn the power off
and wait for ve minutes or more before opening the cover in order to prevent accident caused by un-
familiarity with the machine or electrical-shock accident. In addition, do not carry out replacement work
with wet hands in order to prevent electrical-shock accident.
3. Be sure to replace parts according to the instructions given in this Engineer's Manual and in the Instruc-
tion Manual in order to protect against accident that can result in personal injury.
4. Be sure to carry out replacement work after having installed the machine in a stable state in order to
protect against accident that can result in personal injury.
In addition, be sure to select appropriate tools.
5. Make sure, after the completion of replacement work, that there is no loose soldering, no contact with
other parts, inadequate contact between connectors and receptacles, and loose screws/nuts in order to
protect against accident that can result in personal injury.
6. Make sure, after the completion of replacement work, that neither connectors nor cables are damaged,
slipped off or loosened in order to protect against accident that can result in personal injury.
It should be remembered that some parts have been factory-insulated with tubes or tapes, or oated
above the PWB for safety's sake. In addition, internal wiring has been factory-routed or -clamped in such
a way that it does not come close to high-voltage parts. Be sure to re-place those parts as they are at the
time of delivery.
7. Be sure to use JUKI genuine parts when replacing any of the machine parts.
JUKI assumes no responsibility for any accident caused by any part other than JUKI genuine one.
In addition, in the event you cannot replace parts within the specied range, immediately stop the re-
placement work and ask JUKI or distributor in your area for replacement of the parts.
8. If the fuse has blown, be sure to turn the power off and eliminate the cause of blowing of the fuse and
replace the blown fuse with a new one in order to prevent accident that can result in personal injury or
death.
Adjustment
1. In prevention of accident caused by unfamiliarity with the machine or electrical-shock accident, be sure
to ask an electrical technician of your company or JUKI or distributor in your area for adjustment of
electrical components.
2. When it is necessary to open the control box containing electrical parts, be sure to turn the power off
and wait for ve minutes or more before opening the cover in order to prevent accident caused by un-
familiarity with the machine or electrical-shock accident. In addition, do not carry out adjustment work
with wet hands in order to prevent electrical-shock accident.
3. In prevention of accident that can result in personal injury, adjust adjustment variable resistor or the like
installed on PWB within the specied range given in this Engineer's Manual and in the Instruction Manu-
al.
4. Be sure to carry out replacement work after having installed the machine in a stable state in order to
protect against accident that can result in personal injury.
In addition, be sure to select appropriate tools.
5. In prevention of accident that can result in personal injury, make sure, after the completion of adjust-
ment work, that neither screws nor nuts are loosened or come in contact with other parts.
6. Make sure, after the completion of replacement work, that neither connectors nor cables are damaged,
slipped off or loosened in order to protect against accident that can result in personal injury.
7. In prevention of accident that can result in personal injury or entanglement accident, be sure to ensure
safety at the time of test run. In addition, be sure to take care not to allow hair or cloths to come in con-
tact with the machine belt.
Disassembly/assembly
1. In prevention of accident that can result in personal injury, be sure to carry out disassembly/assembly
work within the specied range given in this Engineer's Manual and in the Instruction Manual.
PRECAUTIONS TO BE TAKEN IN VARIOUS OPERATION STAGES
Electrical components
CAUTION

vii
2. In prevention of accident caused by unfamiliarity with the machine or electrical-shock accident, be sure
to ask an electrical technician of your company or JUKI or distributor in your area for disassembly/as-
sembly of electrical components.
3. When it is necessary to open the control box containing electrical parts, be sure to turn the power off
and wait for ve minutes or more before opening the cover in order to prevent accident caused by unfa-
miliarity with the machine or electrical-shock accident. In addition, do not carry out disassembly/assem-
bly work with wet hands in order to prevent electrical-shock accident.
4. Be sure to carry out replacement work after having installed the machine in a stable state in order to
protect against accident that can result in personal injury.
In addition, be sure to select appropriate tools.
5. In prevention of accident that can result in personal injury, be sure to tighten screws and nuts in assem-
bly work with a specied torque, if specied, or with an appropriate torque, if not specied.
After the completion of assembly work, be sure to check that screws and nuts are not loosened before
starting test run.
6. In prevention of accident that can result in personal injury, make sure, after the completion of adjust-
ment work, that neither screws nor nuts are loosened or come in contact with other parts.
7. Make sure, after the completion of replacement work, that neither connectors nor cables are damaged,
slipped off or loosened in order to protect against accident that can result in personal injury.
It should be remembered that some parts have been factory-insulated with tubes or tapes, or oated
above the PWB for safety's sake. In addition, internal wiring has been factory-routed or -clamped in such
a way that it does not come close to high-voltage parts. Be sure to re-place those parts as they are at the
time of delivery.
8. In prevention of accident that can result in personal injury, be sure to check whether the direction of
rotation is correct at the time of test run.
9. In prevention of accident that can result in personal injury or entanglement accident, be sure to ensure
safety at the time of test run. In addition, be sure to take care not to allow hair or cloths to come in con-
tact with the machine belt.

viii
Precautions to be taken so as to use the MF−7900 Series more safely
DANGER
1. To prevent accidents caused by electric shock, never open the motor control box cov-
er or touch the components inside the control box while the power switch is ON.
CAUTION
1. Never bring your ngers under the needle when the power switch is turned ON or the
machine is in operation..
2. Never bring your ngers, hair or clothes close to the pulley and needle or place any-
thing on the pulley and under the needle while the machine is in operation.
3. Never operate the machine with the safety devices such as belt cover, needle bar
thread take-up cover, nger guard, eye guard cover, etc. removed.
4. Be sure to turn OFF the power and perform the work after ascertaining that the sewing
machine does not run even when the starting pedal is depressed in case of checking,
adjusting, cleaning, threading or replacing the needle of the sewing machine.
5. Never operate the sewing machine with the ground wire for the power supply removed
so as to ensure safety.
6. Be sure to turn OFF the power switch in advance in case of inserting/removing the
power plug.
7. In time of thunder and lightning, stop your work and disconnect the power plug from
the receptacle so as to ensure safety.
8. When you move the sewing machine from a cold place directly to a warm place, dew
condensation may result. Turn ON the power to the machine after you have conrmed
that there is no fear of dew condensation.
9. In case of maintenance, inspection, or repair, be sure to turn OFF the power switch
and conrm that the sewing machine and the motor have completely stopped before
starting the work. (In case of the clutch motor, it continues rotating for a while by the
inertia even after turning OFF the power switch. So, be careful.)
10. Be careful of handling this product so as not to pour water or oil, shock by dropping,
and the like since this product is a precision instrument.

ix
CAUTION BEFORE OPERATION
• Be sure to ll the oil hole with oil designated by JUKI before use.
• Clean the sewing machine thoroughly before using it for the rst time.
• Remove all dust collected on the sewing machine during the transportation.
• Conrm that the voltage and phase are correct.
• Conrm that the power plug is properly connected.
• Never use the sewing machine in the state where the voltage type is different from the designated
one.
• The direction of rotation of the sewing machine is clockwise as observed from the pulley side. Be
careful not to rotate it in reverse direction.
• When operating the sewing machine, turn ON the power switch after properly setting the head on
the table.
• For the rst month after set-up, use the machine at a reduced sewing speed of 5,000 sti/min or less.
(4,000 sti/min or less for the MF-7900D, 4,500 sti/min or less for the MF-7900-E22/23)
• Operate the pulley after the sewing machine has totally stopped.
WARNING :
To avoid malfunction and damage of the machine, conrm the following.

x

xi
CONTENTS
1. Specications ........................................................................................................................ 1
2. Conguration of the machine components ........................................................................ 2
3. Motor pulley and belt ............................................................................................................ 3
4. Standard adjustment............................................................................................................. 4
(1) Threading the machine head ........................................................................................................ 4
(2) Adjusting the rocking thread take-up ..........................................................................................6
(3) Adjusting the needle thread loop by the eccentric cam............................................................. 8
(4) Adjusting the position of the needle thread guide rod............................................................. 10
(5) Adjusting the position of the thread receiver............................................................................ 12
(6) Adjusting the position of the spreader thread guide and spreader thread take-up .............. 14
(7) Adjusting the spreader ................................................................................................................ 16
(8) Adjusting the timing relation between the looper and needle bar .......................................... 20
(9) Returning amount of the looper ................................................................................................. 22
(10) Height of the needle..................................................................................................................... 24
(11) Locus and longitudinal motion of the looper ............................................................................26
(12) Adjusting the needle guard......................................................................................................... 28
(13) Adjusting the feed dog ................................................................................................................ 32
(14) Adjusting the feed relation.......................................................................................................... 38
(15) Adjusting the presser foot .......................................................................................................... 42
(16) Adjusting the micro-lifter ............................................................................................................ 44
(17) Adjusting the looper thread cam ................................................................................................ 46
(18) Adjustment value of balloon ....................................................................................................... 48
(19) With regard to lubrication ........................................................................................................... 50
(20) How to change needle bar stroke............................................................................................... 54
(21) How to change the spreader ....................................................................................................... 58
(22) MF-7500 Series/MF-7900 Series How to use the timing gauge................................................ 60
5. UT51, 52 (Solenoid type thread trimmer) .......................................................................... 62
(1) Installing the SC-921.................................................................................................................... 62
(2) Adjusting and setting SC-921 ..................................................................................................... 63
(3) Adjusting the looper thread trimmer mechanism ..................................................................... 68
(4) Adjusting the top covering thread trimmer mechanism ..........................................................80
(5) Adjusting the needle stop position at thread trimming............................................................ 82
6. UT55, 56, 57 (Pneumatic type thread trimmer) ................................................................. 84
(1) Installing the SC-921.................................................................................................................... 84
(2) Piping of the thread trimmer device...........................................................................................86
(3) Adjusting the looper thread trimmer mechanism ..................................................................... 88
(4) Adjusting the top covering thread trimmer mechanism ..........................................................90
(5) Adjusting the air-blow wiper (UT56)...........................................................................................92
(6) Adjusting the needle thread wiper (UT55) ................................................................................. 94
(7) Adjusting the needle stop position at thread trimming............................................................ 96
7. H22, 23 (Hemming type (with left hand fabric trimmer)).................................................. 99
(1) Specications...............................................................................................................................99
(2) Adjustment .................................................................................................................................100
8. MC37, MC40 (Pneumatic cloth chip suction device)...................................................... 107
(1) Specications and features ...................................................................................................... 107
(2) Installation and adjustment....................................................................................................... 108
9. PL12, 13 (Cloth puller) ...................................................................................................... 119
(1) Specications............................................................................................................................. 119
(2) Installation and adjustment....................................................................................................... 120
10. MF-7900-E11....................................................................................................................... 128
(1) Specications.............................................................................................................................128
(2) Adjustment procedure of engagement amount of knives...................................................... 128
(3) Adjustment procedure of the knife pressure ..........................................................................129
(4) Adjustment procedure of the knife cutting position............................................................... 129
(5) Adjusting the engagement angle of the knife .........................................................................130

xii
(6) Replacement procedure of the moving knife and the counter knife..................................... 131
(7) Adjustment procedure of the cloth guide................................................................................131
(8) Adjustment procedure of front-up amount of the presser foot ............................................. 132
(9) Adjustment procedure of the position of the tape guide ....................................................... 132
(10) Installing the accessories (tape guide base, tension roller and suction pipe)
supplied with the machine ........................................................................................................ 133
(11) Adjusting procedure of the position of the tension roller......................................................135
(12) Adjusting procedure of the position of the tape guide base ................................................. 137
(13) How to use the knife cancelling mechanism...........................................................................141
11. MF-7900-E22/23 ................................................................................................................. 142
(1) Specications.............................................................................................................................142
(2) In case of using E22................................................................................................................... 142
1) Installing procedure...........................................................................................................................142
2) Adjustment procedure.......................................................................................................................145
(3) In case of using E23................................................................................................................... 146
1) Installing procedure...........................................................................................................................146
2) Adjustment procedure.......................................................................................................................148
(4) With regard to the accessories (tension roller)....................................................................... 149
1) Installing procedure...........................................................................................................................149
2) Adjustment procedure.......................................................................................................................150
12. MF-7900/UT59 .................................................................................................................... 152
(1) Specications.............................................................................................................................152
(2) At the beginning.........................................................................................................................152
(3) Installing the accessories ......................................................................................................... 153
1) Installing the air regulator .................................................................................................................153
2) Installing the manifold .......................................................................................................................153
3) Connecting the cords ........................................................................................................................154
4) Setting procedure of the machine head...........................................................................................155
(4) Air piping drawing...................................................................................................................... 156
1) Piping of the thread trimmer device.................................................................................................156
2) Adjusting the air regulator ................................................................................................................156
(5) Threading the machine head .................................................................................................... 157
(6) Adjusting the looper thread trimmer mechanism ................................................................... 158
1) Adjusting the looper thread trimmer air cylinder............................................................................158
2) Adjusting the engagement of the lower knife .................................................................................158
3) Initial position of the looper thread trimmer mechanism...............................................................159
4) Longitudinal position of the blade point of lower knife .................................................................160
5) Adjusting the height of the lower knife............................................................................................160
6) Adjusting the stopper ........................................................................................................................161
7) Adjusting the thread trimmer sensor ...............................................................................................162
(7) SS lever unit ...............................................................................................................................163
1) Installing the SS lever unit ................................................................................................................163
2) Adjusting the SS lever unit ...............................................................................................................163
(8) Setting the upper stop position of the needle bar .................................................................. 164
(9) Setting the raveling prevention parameter .............................................................................. 166
(10) Adjusting the thread hook......................................................................................................... 167
13. Other cautions................................................................................................................... 170
(1) MF-7900D dry section ................................................................................................................ 170
(2) Application of appropriate grease............................................................................................170
(3) Adhesives application spots..................................................................................................... 172
14. Troubles and corrective measures .................................................................................. 173
15. Drawing of table ................................................................................................................ 189
(1) Table-mount type for the V-belt type ........................................................................................ 189
(2) Semi-submerged type for the V-belt type ................................................................................190
(3) Table-mount type for the direct-drive type ..............................................................................191
(4) Semi-submerged type for the direct-drive type ......................................................................192
(5) H22, 23 table-mount type for the V-belt type ...........................................................................193
(6) H22, 23 semi-submerged type for the V-belt type ................................................................... 194
(7) H22, 23 table-mount type for the direct-drive type .................................................................195
(8) H22, 23 semi-submerged type for the direct-drive type ......................................................... 196
(9) E11/E22/E23 for the direct-drive type.......................................................................................197
(10) E11/E22/E23 for the V-belt type................................................................................................. 198
(11) H22/H23-UT59 semi-submerged type for the direct-drive type..............................................199
(12) U11-UT59 semi-submerged type for the direct-drive type ..................................................... 200

– 1 –
1. Specications
No. Item Application
1 Model name High-speed, Cylinder-bed
Top and Bottom Coverstitch Machine
High-speed, Semi-dry-head, Cylinder-bed
Top and Bottom Coverstitch Machine
2 Model MF-7900 series MF-7900D series
3 Stitch type ISO standard 406, 407, 602, and 605
4 Example of
application
Hemming and covering for knits and general knitted fabrics
5 Sewing speed Max. 6,500 sti/min
(at the time of intermittent operation)
V-belt type
Max. 6,000 sti/min
(at the time of intermittent operation)
Direct-drive type
Speed of stitch at the delivery:
4,500 sti/min
(at the time of intermittent operation)
Max. 5,000 sti/min
(at the time of intermittent operation)
V-belt type
Max. 5,000 sti/min
(at the time of intermittent operation)
Direct-drive type
Speed of stitch at the delivery:
4,000 sti/min
(at the time of intermittent operation)
6 Needle gauge 3-needle..... 5.6 mm and 6.4 mm
2-needle..... 3.2 mm, 4.0 mm and 4.8 mm
7Differential feed
ratio
1 : 0.9 to 1 : 1.8 (stitch length : less than 2.5 mm)
(1 : 0.6 to 1 : 1.1, when the differential link hinge screw is changed)
Micro-differential feed adjustment mechanism is provided. (Micro-adjustment)
8 Stitch length 0.9 mm to 3.6 mm (can be adjusted up to 4.5 mm)
9 Needle UY128GAS #9S to #12S (standard #10S)
(Equivalent to Nm65 to Nm80, standard Nm70)
10 Needle bar
stroke
31 mm (33 mm when the eccentric pin is changed over)
11 Dimensions (Height) 450 mm x (Width) 468 mm x (Length) 264 mm
12 Weight 42 kg
13 Lift of presser
foot
8 mm (needle gauge : 5.6 mm without top covering), and 5 mm (with top covering)
Micro-lifter mechanism is provided.
14 Feed adjust-
ment method
Main feed............. dial type stitch pitch adjustment method
Differential feed ... lever adjustment method (micro-adjustment mechanism is provided.)
15 Looper mecha-
nism
Spherical rod drive method
16 Lubricating sys-
tem
Forced lubrication method by gear pump
17 Lubricating oil JUKI CORPORATION GENUINE OIL 18 (Equivalent to ISO VG18)
18 Oil reservoir
capacity
Oil gauge lower line : 600 cc to upper line : 900 cc
19 Installation Table-mount type, Semi-submerged type
20 Noise - Equivalent continuous emission sound
pressure level (LpA) at the workstation :
A-weighted value of 79.5 dB; (Includes
KpA = 2.5 dB); according to ISO 10821-
C.6.2 -ISO 11204 GR2 at 4,500 sti/min.
- Equivalent continuous emission sound
pressure level (LpA) at the workstation :
A-weighted value of 76.5 dB; (Includes
KpA = 2.5 dB); according to ISO 10821-
C.6.2 -ISO 11204 GR2 at 4,000 sti/min.

– 2 –
2. Conguration of the machine components
❶
❷
❸❹❻
❽
❾
󱢴
󱢶
󱢸
󱢹
󱢺
󱢻
󱢿
󱢼
󱢽
󱣀
󱢾
❺
󱢵
❼
󱢷
󱢳
❶Presser spring regulator 󱢶Throat plate
❷Needle bar thread take-up cover 󱢷Needle tip silicon oil lubricating unit
❸Oil circulation inspection windows 󱢸Differential lock nut
❹Oil hole cap 󱢹Micro-adjustment knob
❺Micro-lifter 󱢺Finger guard
❻Needle thread silicon oil lubricating unit 󱢻Rocking thread take-up
❼Thread guide No. 1 󱢼Front cover
❽Upper pulley 󱢽Needle bar thread take-up thread receiver
❾Thread tension nut 󱢾Silicon container thread guide
󱢳Oil gauge 󱢿Belt cover
󱢴Feed regulating knob 󱣀Differential feed regulating lever
󱢵Eye guard cover

– 3 –
3. Motor pulley and belt
Sewing speed of
the machine
(sti/min)
50Hz 60Hz
Pulley size
(mm)
Belt size
(inch)
Pulley size
(mm)
Belt size
(inch)
4,500 ø 100 M-39 ø 85 M-38
4,800 ø 110 M-40 ø 90 M-38
5,000 ø 115 M-40 ø 95 M-39
5,500 ø 125 M-41 ø 105 M-39
5,800 ø 135 M-42 ø 110 M-40
6,000 ø 140 M-42 ø 115 M-40
6,200 ø 145 M-43 ø 120 M-41
6,500 ø 150 M-43 ø 125 M-41
MF-7900 Series
Sewing speed of
the machine
(sti/min)
50Hz 60Hz
Pulley size
(mm)
Belt size
(inch)
Pulley size
(mm)
Belt size
(inch)
3,500 ø 80 M-38 ø 65 M-37
4,000 ø 90 M-38 ø 75 M-37
4,500 ø 100 M-39 ø 85 M-38
4,800 ø 110 M-40 ø 90 M-38
5,000 ø 115 M-40 ø 95 M-39
MF-7900D Series
1. The table shows the numbers when a 3-phase 2-pole 400 W clutch motor (1/2 HP) is used.
2. The commercially-available motor pulley near to the counted value is designated since the outside diam-
eter of the commercially-available motor pulley counts by 5 mm.
3. When you use a new sewing machine, use the following sewing speed for the rst 200 hours (approxi-
mately one month). A good result can be obtained in terms of the durability.
• MF-7900 Series : 4,500 sti/min or less
• MF-7900D Series : 4,000 sti/min or less
(Caution) 1. Use a motor pulley which is adaptable to this sewing machine.
2. This sewing machine will exceed its max. revolutions and max. sewing speed, nally
leading to the occurrence of malfunction, unless the adaptable motor pulley is used.

– 4 –
4. Standard adjustment
(1) Threading the machine head
WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the
sewing machine.
Standard adjustment
When using more stretchable
thread
❷
When covering thread is excessively
loosened = B
When covering thread is excessively
loosened even after passing B= C
When covering thread is
excessively tense = D
Broken lines Awhen stretcher thread
is used
When using less stretchable
thread
Broken line Ewhen using more stretchable thread
❸
A
❶
E
❹
D
B
C
❶
❷
❹
❸
F

– 5 –
Adjustment procedures Results of Improper adjustment
1. When motion of thread is very rough, and stable seams can
not be obtained, there are cases where the condition can be
improved by the following threading.
(1) Wind thread around 1st thread guide ❺.
❺
2. When using more stretchable thread (wooly thread or the like),
the threading as given below does not make thread stretch and
neat seams can be obtained.
(1) Do not pass thread through spreader auxiliary thread ten-
sion ❹.
Refer to section E.
(2) Do not pass thread through the looper thread tension disk.
(F)
(3) Do not pass thread through the looper thread small tension
disk ❻.
❻
◦ When threading is not proper-
ly performed, not only sewing
trouble occurs, but also needle
breakage is caused. So, be
careful.

– 6 –
(2) Adjusting the rocking thread take-up
WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the
sewing machine.
Standard adjustment
1. Length of the rocking thread take-up
(1) It is the standard that the length of rocking thread take-up ❷from the center of the shaft of rocking
thread take-up support arm ❹to the thread hole face of rocking thread take-up ❷is 83 mm.
❶
❷
❹
83mm
2. Position of the rocking thread take-up
(1) Install so that rocking thread take-up ❷ is ush with rocking thread take-up ❷when the needle bar is
in its lowest position.
Flush
3. Timing of the rocking thread take-up
(1) It is the standard timing that rocking thread take-up ❷is in its lowest position as well when the needle
bar is in its lowest position.
Timing of rocking thread take-up ❷is performed by adjusting the position of rocking thread take-up
ball arm ❽.
4. Relation between the timing of rocking thread take-up and the needle thread loop
(1) When using the excessively stretchable thread or the hard-to-stretch thread, the size of needle thread
loop can be changed by changing the timing of rocking thread take-up ❷.
(2) Relation between the timing of the rocking thread take-up ❷and the size of needle thread loop is as
shown in the table on the next page.
↑
Anti-operator's side
Operator's side
↓
8 mm