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Bypassing a linear scale on a FANUC 0i‑T /0i‑M control can be done by switching the axis position feedback source from the linear scale to the motor encoder, then re-tuning backlash/pitch error and revalidating reference/home logic and servo gains. This is a builder-level change that requires controlled conditions, data backup, and a clear risk plan to avoid positional accuracy regressions and soft-limit crashes during the transition. In full closed-loop control mode, the position feedback signal comes from the linear scale, while in semi closed loop mode, it relies on the motor encoder to feedback position.

When to bypass

• Linear scale failure or intermittent noise causing encoder communication or position deviation alarms and machine downtime pressure.
• Temporary production continuity while waiting for scale/cable replacement, acknowledging loss of volumetric accuracy and thermal drift compensation the scale provided.

Safety and preparation

• Full CNC/PMC backup: parameters, diagnostics, pitch error tables, grid shift, and tool/work offsets before any edits. Use the parameter I/O procedure and enable Parameter Write only during the change.
• Lockout/tagout, move axes to mid‑stroke, remove tool, increase stroke margins, and reduce rapid/feed overrides for the first motion checks.
• Plan for metrology verification: ball bar or laser later; at minimum, run dial test indicator checks on key points.

Concept: switching feedback source

FANUC 0i controls support multiple feedback sources per axis; with scale fitted, the axis usually runs in “dual/separate detection” using the linear scale for position and the motor encoder for velocity/commutation. To bypass, select motor encoder only, disable scale-related options for that axis, and adjust related in-position, grid, and backlash parameters. The exact bit names vary by 0i‑C/D/F/Plus generation and option set but are consistently documented in the Parameter Manual sections for Axis Control, Servo, FSSB, and “Linear scale with absolute address reference position.”

Step-by-step procedure

1) Enable parameter write

• Setting screen: set PARAMETER WRITE = 1, acknowledge alarm; change only with E‑STOP on. Reset to 0 and power cycle if a “POWER MUST BE OFF” prompt appears.

2) Identify axis and backups

• Record current axis-related parameters: increment system, grid shift, reference return method, in-position widths, backlash tables (e.g., 1851 family on 0i‑F), pitch error compensation activation/data, and any linear-scale specific items under “Linear scale with absolute address reference position.”
• Output parameters via I/O channel as per manual Sections “Input/Output formats” and channel parameters 0020/0101–0123.

3) Select motor encoder feedback

• Linear scale disable/switch: On 0i‑F/0i‑F Plus, use the Axis Control/Servo and FSSB parameter groups to deselect the secondary feedback for the target axis and run with the motor encoder as sole position source; this is the core “bypass.” The official parameter numbers are listed per-axis in the Servo and FSSB sections; confirm in your specific 0i‑TF/MF manual build since numbering can differ with options.
• If the machine used “linear scale with absolute address reference,” disable that function in the corresponding parameter section so the CNC no longer expects absolute scale references on power-up for that axis.

4) Reference return logic

• If dogs/home switches exist, set the reference return method to dog-based and re-execute reference point setting per Parameter Manual “Butt-type reference position setting/Reference point with mechanical stopper” sections.
• If previously using absolute scale homing without dogs, implement dog-based or marker-based home using motor encoder Z and decel dogs, then set new grid shift for accurate machine zero.

5) In-position and following error

• With motor encoder only, increase in-position width to avoid nuisance in-position alarms; adjust the “in-position width for rapid” and “cutting” groups as per Accel/Decel and Feedrate sections. These are standard parameters under in-position width tables identified in the manual’s feed/accel chapters.
• Verify error detect bits are appropriate; excessive tight error windows configured for scale-level accuracy may cause SV alarms; relax to typical motor‑encoder values cautiously.

6) Backlash and pitch error

• Expect 15–20 µm or more repeatability loss if the ballscrew/backlash was being “seen” by the scale and now is uncompensated; measure and input backlash compensation per axis (e.g., the 1851 family on 0i‑F for backlash value per direction/axis; verify exact table for your release).
• Re-enable and review pitch error compensation if it was disabled earlier due to scale; pitch comp is recommended when running on motor encoder only. Use the Pitch Error Compensation section to input lead error at defined grid intervals.

7) Servo tuning and gains

• Without scale, servo loop stiffness must be balanced for encoder quantization and screw compliance; start from standard gain set, then fine-tune following error during rapid and cut. Use Servo parameter chapter guidance for proportional/integral filter parameters common to 0i‑F.
• If high-frequency oscillation occurs, reduce acceleration time constants or apply the recommended gain/acceleration parameters in Acceleration/Deceleration and Servo sections.

8) Stroke limits and soft limit margins

• Because absolute position trust shifts, increase safety margin temporarily on stored stroke limits; verify positive and negative limits against physical travel using handwheel first, then jogging. Stored stroke parameters are in the Stroke Check section.

9) FSSB/Encoder communications

• If bypassing was driven by linear scale FSSB errors, confirm no active FSSB scale channels remain mapped for that axis; the FSSB parameter groups for feedback channel assignment must reflect motor encoder only.
• After changes, power cycle if required where the manual states “power must be off” for communication parameter changes to take effect.

10) Verification checklist

• Re-home all axes; check machine coordinate repeatability to home within target microns on multiple power cycles.
• Perform grid and offset checks, then cut a simple square and circle; measure squareness/size drift with temperature to document expected loss vs scale mode.

Parameter groups to review and typical items

Note: Exact numbers and bit labels depend on 0i generation and options; use Fanuc manual chapters to locate the precise parameters in your control’s build.

Before doing any change in parameters you have to note down old values of parameters and take the SRAM back up at reference position & Change parameters at machine at Reference Position.

Here we are giving an example of X axis linear scale bypass in Milling machine in which we have changed following parameter. In your case these parameters may be changed so you need to check with your machine’s manual.

Milling machine and turning machine’s linear scale parameter will be different.For turning machine’s linear scale parameter you may contact me by contact form. I will share the detail.

You will found detection unit in Servo setting X parameter. Now we will discuss about the important parameter details mentioned above in the table to understand better.

Parameter

• 1815#1 OPTx Position detector

1: A separate pulse coder is used.

0: A separate pulse coder is not used

NOTE
If you are using full – closed system then set this parameter to 1 when using a linear scale with reference marks or a linear scale with an absolute address zero point.

• CMR 1820

You can also set parameter 2084 & 2085 with the help of our following blog for fanuc controller

Practical tuning tips

• Mechanical first: check ballscrew end float and thrust bearing preload; 15–20 µm repeatability errors after bypass usually indicate mechanical backlash that the scale previously masked; correct mechanically and then fine-tune backlash parameters.
• EMC/noise: if the motive to bypass was scale noise, investigate cable routing and shielding; plan for proper long-term scale repair rather than indefinite bypass in precision machines.
• Documentation: archive “Scale Mode” and “Encoder Mode” parameter sets separately so service can revert in minutes when the scale is replaced.

Test and acceptance

• Jog tests: verify smooth motion, no SV410/servo in-position alarms after in-position window changes; SV410 commonly relates to excessive following error/in-position issues after bypass unless tuned.
• Home repeatability: 10 cycles, record machine coordinate drift; target within your process tolerance acknowledging encoder‑only baseline.
• Geometry: run a quick circularity test in XY and straightness tests along X and Y; use pitch comp to trim long-axis size drift.

Troubleshooting after bypass

• Intermittent servo alarms at stop: slightly widen in-position range and verify decel/jerk parameters; check for stiction on axes.
• Dimensional drift with temperature: expected without scale; recommend warm-up cycles and temperature-stable offsets, and keep pitch comp active.
• Poor finish or chatter: retune gains conservatively and review acceleration tables; consider feedforward tweaks available in Servo parameters.

Compliance and reversibility

• Clearly label the machine status on the nameplate and in the maintenance log: “Linear scale bypassed on Axis … ; date; parameter set archived.”
• Keep the “Linear scale with absolute address reference position” parameters documented to restore full function immediately after hardware repair.

References for parameter locations

• Series 0i‑MODEL F/Plus Parameter Manual: see “Parameters of Axis Control/Increment System,” “Parameters of Servo,” “Parameters of FSSB,” and “Parameters of Linear scale with absolute address reference position,” plus I/O and parameter write procedures.
• Series 0i Parameter Manual (Model C/D style) for general navigation: reference return, stroke checks, pitch error comp, in-position, and I/O procedures; screen flows and bit conventions are consistent across generations.
• Operator/Maintenance handbooks caution that incorrect settings can cause unexpected motion; observe power-cycle requirements when communication/encoder mapping parameters change.
• Field note on backlash impact after bypass and use of backlash parameter 1851 family to compensate repeatability loss.

By following this structured approach—disabling the linear scale feedback, re-establishing reference logic, tuning in-position and gains for motor encoder only, and applying backlash/pitch compensations—you can keep a 0i‑TF/0i‑MF machine productive until the scale system is repaired, while maintaining safe, predictable motion under shop conditions.

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These blogs should not be considered as official repair or service manuals. For detailed instructions, critical repairs, or advanced troubleshooting, it is always necessary to contact and work under the guidance of the respective *machine manufacturer* or *CNC controller support team*.  

The content provided is focused only on *diagnosis and awareness*. We do not take responsibility for any kind of damage, error, or malfunction that may occur if someone directly applies the information shared here without proper technical supervision.#