FANUC CNC parameters: what each group controls and when to change it

• 1
  What FANUC CNC parameters actually store
• 2
  Reference position and pitch error parameters
• 3
  Backlash parameters and reversal error
• 4
  Servo gain and filter parameters
• 5
  Spindle and tool change parameters
• 6
  Stroke limits and option bits
• 7
  Which parameter group fixes which symptom

A FANUC control holds thousands of parameter numbers, but they fall into a small number of jobs. Some describe the machine to the control: how many pulses equal 1 mm of travel, where the reference point sits, how much backlash the ball screw has. Others describe how the servo should behave: gain, filter time, torque limit. A third group is pure safety and option configuration, and touching it can lock a function you paid for.

The control does not know your machine. It only knows the numbers you feed it. That is why two identical-looking VMCs from the same builder can behave differently after a board swap: the new board arrived with default parameters, and the geometry stored in the old board was never written down. The machine still moves, but the pitches, backlash compensation and reference offsets no longer match the iron.

This is the practical reason a parameter backup belongs next to the program library. It is not bureaucracy. A single lost file can turn a machine that holds ±0.005 mm into one that drifts 0.05 mm over a 300 mm cut, and no amount of tool offset tweaking will recover it.

In the sections below we walk through the groups that show up most often on the shop floor: reference position, pitch error, backlash, servo gain, spindle, stroke limits and option bits. For each one we cover what it changes, the symptoms of a wrong value, and the boundary where you should stop and call the builder.

• 1
  Machine geometryPulses per unit, reference point, pitch error, backlash.
• 2
  Servo behaviourLoop gain, feed-forward, filter time, torque limits.
• 3
  Safety and optionsStroke limits, alarm thresholds, function enable bits.

The reference position parameter tells the control where machine zero physically is. On a typical FANUC mill this is stored per axis as a grid shift value relative to the one-rotation signal from the encoder. Change it by even a few counts and every tool offset in the library becomes wrong by that amount. The symptom is clean, repeatable, wrong parts: bores come out consistently oversize by the same value on all features.

Pitch error compensation is the other half of the geometry story. Ball screws are not perfectly linear. Over 500 mm of travel the actual distance moved can deviate from the commanded distance by tens of microns. Pitch error parameters store a table of correction points along the axis, and the control adds a small offset at each point. A machine with a healthy compensation table holds size across the whole stroke. A machine with a blanked or wrong table holds size only near the point where it was last calibrated.

You can spot a missing pitch table without a laser interferometer. Cut a test bar with steps at 50 mm intervals along the axis and measure each step with a micrometer. If the error grows in a smooth ramp instead of wandering randomly, the compensation table is either absent or belongs to a different machine.

Do not rebuild a pitch table by trial and error. It needs a laser or a ball bar under controlled temperature, and the measurement should be repeated in both directions. On a machine used for ±0.005 mm work this is a scheduled task, not a breakdown task.

• 1
  Reference positionGrid shift per axis. Wrong value shifts every feature equally.
• 2
  Pitch errorCorrection table along the stroke. Wrong table gives ramped size error.

Backlash parameters tell the control how much lost motion exists when an axis reverses direction. The control adds that amount as a pulse burst during reversal so the tool reaches the commanded position. Set it too low and climb-milled walls show a visible step at every direction change. Set it too high and the axis overshoots on reversal, which shows up as a bump on the opposite side of the feature.

The right value comes from measurement, not from a datasheet. Mount a dial indicator on the table, touch the spindle nose against it, and jog the axis in one direction, then the other. The difference between commanded and indicated movement is the reversal error. Do this at several points along the travel, because backlash is rarely constant.

Backlash compensation is a patch, not a cure. It works well on a healthy ball screw with a few microns of lost motion. On a worn screw with 40 μm of backlash the compensation becomes unstable: the axis chatters at reversal and surface finish collapses. At that point the screw needs replacement or preload adjustment, and the parameter should be reset after the mechanical repair.

A useful rule: if the measured backlash grows between two quarterly checks, schedule the mechanical inspection. Do not keep raising the parameter value to chase it.

• 1
  Too lowVisible step on walls at each reversal.
• 2
  Too highOvershoot bump, chatter, poor finish.
• 3
  Mechanical limitBacklash above roughly 20–30 μm needs repair, not more compensation.

Servo parameters decide how hard the motor pushes to close the position error. Raise the gain and the axis follows the commanded path more tightly, which improves contour accuracy on corners and arcs. Raise it too far and the axis starts to vibrate, then trips an overload or position error alarm. Lower the gain and the machine sounds calm, but corners round off and the tool lags behind on fast moves.

The first gain to check when a machine is retuned is the position loop gain, usually expressed as a velocity loop gain and a position loop gain pair. Most builders publish a starting range, and on a typical VMC the position loop gain sits in the region of 30–60 per second. The exact number depends on the machine mass, the screw pitch and the rigidity of the structure.

Filter parameters matter as much as gain. A low-pass filter in the velocity loop smooths the current command and suppresses resonance from the mechanical structure. Too much filtering and the axis feels sluggish and lags on reversal. Too little and a high-pitched whine appears at certain feed rates. The goal is the smallest filter that removes the resonance peak.

A practical tuning sequence is: set gain to the builder's baseline, run a circular test with a ball bar, raise gain in small steps until the roundness error stops improving, then add filter only if a resonance peak appears. Never tune gain by ear alone. The number that sounds good may not be the number that cuts round.

• 1
  Higher gainBetter contour accuracy, higher risk of vibration.
• 2
  Lower gainStable but sluggish, corners round off.
• 3
  FilterSuppresses resonance. Use the minimum that works.

Spindle parameters cover orientation position, acceleration and deceleration ramps, and the speed limits the control will accept. Orientation position matters on machines that use a two-pin drive or a specific keyway alignment for tool holders. If it drifts, tool holders stop seating cleanly and runout increases.

Acceleration ramps decide how fast the spindle reaches commanded speed. A very aggressive ramp shortens cycle time but loads the drive and the belt. On older machines a softened ramp can remove an intermittent spindle alarm that only appears on rapid speed changes. This is a legitimate trade: a small cycle time loss in exchange for a stable spindle.

Tool change parameters are usually stored in the PMC side rather than the CNC parameter page, but they behave the same way: they define positions, timings and interlocks. A wrong tool change position parameter is one of the fastest ways to crash a machine. Never adjust these without the builder's documentation and a dry run at reduced rapid override.

Spindle and tool change parameters are also where the machine's safety interlocks live. If someone has disabled an interlock to make a cycle run, the parameter file will show it. Backups are worth reading, not just storing.

• 1
  OrientationSets spindle stop angle for tool holder seating.
• 2
  RampsTrade cycle time against drive load and stability.
• 3
  InterlocksSafety logic. Never disable to save a few seconds.

Stroke limit parameters define the soft limits the control enforces before the hard limit switch is reached. They protect the machine, the fixture and the part. When a fixture is taller than usual, the right move is to verify the soft limit value against the actual travel, not to widen it casually. A soft limit set wider than the physical travel offers no protection at all.

Option bits enable functions such as high-speed machining modes, additional axes or specific canned cycles. These are licensed features. Changing an option bit to enable a function the machine was not sold with is a licensing violation and can also destabilise the control, because the function may depend on hardware that is not installed.

The safe rule for option bits is simple: read them, record them, do not edit them. If you need a function that is not enabled, talk to the machine builder or the control supplier about the proper license path.

One last point that ties the whole page together. Parameter changes are invisible in the finished part. That is what makes them dangerous. A wrong tool offset shows up immediately as an obvious size error. A wrong pitch table shows up as a slow drift across a batch, and by the time the trend is clear you may have shipped parts. Keep the backup current, log every change with a date and a reason, and re-verify with a test cut after any edit.

• 1
  Soft limitsMust stay inside the physical travel.
• 2
  Option bitsLicensed features. Record, do not edit.
• 3
  Change logDate, parameter number, old value, new value, reason.