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Adding a fourth axis CNC to your existing FANUC-controlled machine can greatly expand its capabilities—allowing for more complex machining, rotary motion, and multi-side processing in one setup.
If you’re still in the early stages of planning, we recommend checking out our Comprehensive Guide to Upgrading to a Four Axis CNC Machine: Harnessing four axis cnc for Efficient Multi‑Face Machining, where we cover hardware compatibility, application considerations, and system requirements.
This article focuses on the complete process of activating and configuring a Fanuc fourth axis, including servo motor selection, wiring, parameter settings, ladder logic setup, and how to temporarily disable or re-enable the axis.
When adding a Fanuc fourth axis, the most common mechanical form is a rotary table. The choice of servo motor and drive largely depends on the rotary table size and load requirement.
Common Rotary Table Sizes
The most typical rotary table diameters are 170mm, 210mm, 250mm, and 320mm, and these sizes directly affect the selection of motor and amplifier models.
Each size corresponds to specific FANUC servo motor and amplifier models, as shown below:
Rotary Table Diameter | Servo Motor Specifications | Amplifier Specifications |
φ170 | αi4F / β8iS | αiSV 40 / βiSV 20 |
φ210 | αi4F / β8iS | αiSV 40 / βiSV 20 |
φ250 | αi4F / β8iS | αiSV 40 / βiSV 20 |
φ320 | αi8F / β12iS | αiSV 40 / βiSV 20 |
Note: The numbers like 170, 250, etc., refer to the rotary table disc diameter in millimeters.
Choosing the correct FANUC motor-amplifier combination ensures optimal performance, torque, and long-term reliability.
After completing the hardware installation, power on the machining center and follow the steps below to activate and configure the Fanuc fourth axis.
Step 1: Enable Parameter Write Mode
Before modifying parameters, ensure that Parameter Write Enable (PARAMETER WRITE) is set to 1 to allow changes.
Step 2: Enable the Fourth Axis Function
Depending on your FANUC system model, set the following parameters to activate and define the fourth axis:
#9900 = 4 (Fourth axis mode)
#1010 = 4 (Total number of controlled axes)
#9943.3 = 1 (Axis expansion enabled)
For FANUC 0i-MD / Mate-MD:
#8130 = 4 (Total controlled axes)
#1010 = 4 (CNC-controlled axes)
#987 = 4 (Total controlled axes)
Additional related parameters are listed in Table 1 and Table 2 for your specific system version.
Step 3: Configure the FSSB (FANUC Serial Servo Bus)
FANUC’s FSSB (Fiber-optic Serial Servo Bus) enables high-speed communication between the CNC and servo amplifiers. Follow the steps below to configure it properly:
FSSB Initial Settings:
Step 4: FSSB Amplifier & Axis Mapping
After the above parameters are set, map the amplifiers and axes via the system interface:
Servo motor number (parameter 2020)
Motor Model | Motor Number | Motor Model | Motor Number |
α iF 2/5000 | 255 | β iS 2/4000 | 253/254 |
α iF 4/4000 | 273 | β iS 4/4000 | 256/257 |
α iF 8/3000 | 277 | β iS 8/3000 | 258/259 |
α iF 12/3000 | 293 | β iS 12/2000 | 272 |
α iF 22/3000 | 297 | β iS 22/2000 | 274 |
Common parameter table
Fourth Axis Model | AR-170/210/250 | ||
Motor Model | FANUC | ||
Parameters | Content | Set value | |
0i | Gear Ratio：1/90 | ||
N1020 | Fourth axis name (65→A, 66→B). | 65 | |
N1022 | Auxiliary axis control address setting. | 4 | |
N1023 | Servo axis control address setting. | 4 | |
N1005#1 | Deceleration during homing is controlled by a bumper. | 0 | |
N1006#0 | Rotary axis setting. (1: Rotary axis, 0: Linear axis) | 1 | |
N1008#0 | Rotary axis over/roll function enabled. | 1 | |
N1008#1 | Moves with the shortest distance. | 0 | |
N1008#2 | Relative coordinate value per revolution. | 1 | |
N1260 | Maximum rotary axis travel (one revolution = 0 to 360°). | 360.000 | |
N1320 | Maximum positive travel. | 99999.999 | |
N1321 | Maximum negative travel. | -99999.999 | |
N1420 | Rapid feedrate setting value (G00). | 8000 | |
N1421 | Rapid feedrate setting value for F0. | 500 | |
N1423 | Jog feedrate setting value. | 4000 | |
N1424 | Manual rapid feedrate setting value. | 8000 | |
N1425 | Deceleration speed during homing. | 200 | |
N1428 | Homing speed | 4000 | |
N1430 | Maximum cutting feedrate limit. | 8000 | |
N1620 | Rapid acceleration and deceleration time (T1). | 50 | |
N1621 | Rapid acceleration and deceleration time (T2). | 100 | |
N1622 | Acceleration and deceleration time of cutting speed state. | 10 | |
N1624 | Acceleration and deceleration time of inching speed. | 20 | |
N1815#4 | Incremental motor. (*Absolute motor, please set [1]) | 0 | |
N1815#5 | Incremental motor. (*Absolute motor, please set [1]) | 0 | |
N1816 | Reference counter capacity and detection magnification setting value. | 01110000 | |
N1820 | Command magnification setting value. | 2 | |
N1821 | Reference counter setting value. | 4000 | |
N1825 | Position gain value. | 3000 | |
N1826 | Rapid motion mode positioning width setting value. | 20 | |
N1827 | Cutting mode positioning width setting value. | 20 | |
N1828 | Position deviation limit value in moving state. | 8000 | |
N1829 | Position deviation limit value in stationary state. | 500 | |
N1850 | Home position compensation value. |
| |
1800#4 | G00/G01 backlash compensation value setting separately. | 1 | 0 |
N1851 | G00 backlash compensation value. |
|
|
N1851 | G01 backlash compensation value. |
|
|
N1852 | G00 backlash compensation value. |
|
|
N2020 | Fourth-axis servo motor specification code. |
| |
N2021 | Fourth-axis servo motor inertia ratio (load inertia). | 128 | |
N2022 | Fourth-axis servo motor rotation direction setting (111: forward, -111: reverse) | 111 | |
N2023 | Fourth-axis servo motor speed detection pulse number | 8192 | |
N2024 | Fourth-axis servo motor position detection pulse number | 12500 | |
N2084 | Fourth-axis servo motor transmission ratio setting value. | 1 | |
N2085 | Fourth-axis servo motor transmission ratio setting value. | 250 | |
Additional Notes:
The number of controlled axes is defined by parameter N8130 on the 0i-MD system, and N987 on the 0i-MF system.
Parameters 1620 to 1627 define the acceleration and deceleration time constants, which should match the machine’s existing configuration.
Servo-related parameters may require fine-tuning depending on the actual machine and mechanical setup. If the fourth axis does not behave correctly, it does not necessarily mean the parameter values are wrong—adjustments may be needed to suit your specific equipment and motion characteristics.
To ensure complete control, your PMC ladder must support the fourth axis. This includes:
Serial Number | Signal | Description |
1 | G100.3 | Fourth axis positive movement signal |
2 | G102.3 | Fourth axis negative movement signal |
3 | Y3.0 | Fourth axis release solenoid valve output |
4 | X9.3 | Release signal |
5 | X9.4 | Clamp signal |
6 | G18.2 | Axis select signal |
In some scenarios, you may need to disable the fourth axis—for example, if the rotary table is temporarily removed or not in use. This process is also referred to as axis shielding, which involves disconnecting the servo motor and its cables without causing system alarms.
There are two methods to properly disable (and later re-enable) the fourth axis on a FANUC CNC system:
Method 1: Virtual Feedback (Axis Shielding without Hardware Removal)
This method simulates the presence of the fourth axis motor and feedback signals, without physically connecting a servo motor.
Steps:
Note:
To restore the fourth axis later:
This method can be applied to any axis. No need to modify amplifier wiring—only a jumper plug is required at the JFX connector.
Method 2: Axis Removal Function (Logical Detach from Control)
This method removes the fourth axis from CNC control logic, making it fully ignored by the system.
Step 1: Enable Axis Detach Feature
Step 2: Choose a Detach Control Method
You can disable the axis in either of the following two ways:
1) Parameter-Based Detach
If this parameter is not set and the motor is missing, the system will trigger a 368 alarm.
2) PMC Signal-Based Detach
This method also requires shorting the JFX 11–12 pins to avoid 401 alarm.
In the self-diagnosis screen, the detached axis will appear as servo locked and cannot be moved.
By using either of these methods, you can safely disconnect or bypass a fourth axis without removing the amplifier or changing the wiring layout—ideal for temporary machine configurations or rotary table swaps.
Adding a Fanuc fourth axis is a great way to enhance your machine’s flexibility and productivity. From selecting the right servo motor, completing FSSB setup, configuring system parameters, and designing PMC logic, to safely disabling or enabling the axis—this guide covers it all. Be sure to follow safety precautions and FANUC documentation for your specific model.
If you’re looking for fourth axis hardware, cables, or configuration support, feel free to contact us— we offer expert advice, genuine parts, and technical help tailored to your CNC system.
A Fanuc fourth axis is an additional rotary axis added to a CNC machine controlled by FANUC. It enables the machine to rotate the workpiece around an axis (commonly labeled as the A axis) for multi-side machining, indexing, or continuous rotary cutting.
Adding a fourth axis CNC greatly expands machining flexibility:
The most common rotary tables are φ170, φ210, φ250, and φ320 mm. Each corresponds to specific servo motor and drive combinations (e.g., αiF4/β8iS motors with αiSV40 or βiSV20 drives).
To activate the fourth axis:
Enable Parameter Write = 1
Set the axis-related parameters (e.g., #9900, #1010, #8130, or #987 depending on the control model)
Configure FSSB (Fanuc Serial Servo Bus) for amplifier and axis mapping
Power cycle the machine
Refer to your specific FANUC system manual for exact parameter values.
Yes. There are two methods:
Both methods prevent system alarms while keeping the amplifier wiring unchanged.
These can be avoided by correct parameter settings and jumper installation.
How we process your repair requirements
Get in touch with us and provide as much detail as possible about the issue you are experiencing.
Our professional repair team will help assess the severity of the problem and the possibility of repair.
The customer sends or delivers the servo drive to our company. We register it in our system and prepare it for further inspection.
Our engineers diagnose the fault points of the servo drive and provide a detailed inspection report, including the final repair cost and estimated repair time.
We provide the repair quote to the customer and wait for their confirmation. If the customer agrees to the quote, we proceed with the repair.
Upon customer approval, we carry out the necessary repairs. After repairs are completed, we perform tests with the motor and conduct aging tests to ensure the servo drive operates reliably under various conditions. Once the testing is successful, we register the drive for delivery.
The customer completes the payment, and we deliver the repaired servo drive to the customer.
We provide attentive follow-up service to ensure customer satisfaction and address any further needs.
