Troubleshooting Rotary Axis Shortest Path: M126 vs M94

Problem Details

A rotary axis (typical 4th/5th axis, A/B/C) was commanded to index between angular positions and took the long way around instead of the shortest angular route. The operator attempted to force shortest-path behaviour by inserting M126 into the program. The M-code had no useful effect on the observed motion. The behaviour was ultimately corrected with M94, which was the M-code that actually addressed the underlying condition on that control.

This is a classic symptom pattern on rotary axes:

• Axis position display accumulates past 360 degrees (e.g. 720.000, 1080.000) after repeated indexing in one direction.
• A subsequent absolute command such as G90 C90. produces a very long move (several full revolutions) instead of a short index.
• Cable-carrier, rotary union or coolant line wrap-up warnings appear after a production run.
• Cycle time grows over the run as accumulated angle increases.

Root Cause

The failure was a mis-selected M-code, not a servo, backlash or parameter fault. The engineering root cause has two layers:

1. Accumulated absolute rotary coordinate. The rotary axis was configured (or left in a state) where the absolute coordinate keeps counting past one revolution. In that condition an absolute positioning command is interpreted as a linear-style move in degrees, so the control legitimately drives through every intermediate degree between the current large value and the target. That is not an error — the control is executing exactly what was asked.
2. Wrong M-code family applied. M126 was assumed to be the shortest-path enable. On the control in question it did not produce the expected behaviour. M94 did. M-code numbering above the mandatory range is not standardised across controls or across OEM machine builders; the same number can be an unassigned code, an OEM auxiliary function, or a completely unrelated macro call.

Why M-code assumptions fail

Solution

Step 1 — Identify the correct code from the machine's own documentation

1. Open the machine builder's programming manual, not a generic control manual. The builder's M-code list overrides the control vendor's default list where they conflict.
2. Locate the rotary/index axis section and note the exact codes for: rollover / coordinate reduction, shortest-path selection, direction-forced positioning (CW only / CCW only), and their cancel codes.
3. Cross-check against the actual machine: on the control, review the assigned M-codes screen or the PMC ladder M-code decode block if the manual is unavailable or outdated.

Step 2 — Verify the rotary axis parameter configuration

Before blaming the program, confirm the axis is set up as a rotary axis with rollover, not as a linear axis with degrees as the unit. Check with the machine builder or control documentation:

Step 3 — Apply the correct M-code in the program

In the case documented here, M94 produced the required behaviour and M126 did not. Structure the program so the mode is set explicitly and cancelled explicitly rather than relying on a modal state left over from a previous job:

O1000 (ROTARY INDEX EXAMPLE)
G90 G17 G21 G40 G80       (safe modal reset)
M94                        (rotary handling code verified on THIS machine)
G00 C0.                    (reference the table at a known angle)
...
G00 C90.                   (index - short move expected)
...
G00 C270.                  (index - direction chosen by active mode)
...
M30

If the builder's manual defines a separate cancel code for the mode, place it in the program tail together with the other modal resets so the next operator does not inherit an unexpected state.

Step 4 — Alternatives if no shortest-path M-code exists

Not every machine offers the function. Workarounds that require no parameter changes:

• Incremental commands. Command the index with G91 and a signed increment, e.g. G91 C-90., so the direction and travel are fully explicit. Return to G90 immediately after.
• Programmed unwind. After a group of indexes, command a return to the base angle explicitly in the direction that unwinds the utilities, or reset the coordinate with a work-offset/preset command permitted by the control.
• Macro-computed shortest move. Compute the delta in a macro variable, normalise it to the range −180 to +180, and output an incremental move. Example logic pattern:

  #101 = [ #TARGET - #CURRENT ]
  WHILE [ #101 GT 180. ] DO1
    #101 = #101 - 360.
  END1
  WHILE [ #101 LE -180. ] DO2
    #101 = #101 + 360.
  END2
  G91 C#101
  G90

  Confirm the available macro variables for current axis position on your control before using this pattern.

Verification

1. Dry run with the axis clear. With the tool retracted and no workpiece contact possible, single-block through the index sequence at reduced rapid override.
2. Watch the distance-to-go display. On the block that indexes to the new angle, the remaining distance should be less than or equal to 180.000 degrees if shortest path is active. A value above 180 or a multi-revolution value means the mode is not active or the coordinate is still accumulating.
3. Check absolute position after a full cycle. Run the program ten times without homing. If rollover is working, the displayed C position stays within one revolution instead of climbing by 360 per cycle.
4. Confirm direction on the 180-degree case. A move of exactly 180 degrees is ambiguous. Note which direction the control chooses and, if the choice matters for cable wrap or a probe cable, replace that move with an explicit incremental command.
5. Cycle-time check. Compare the measured cycle time of a full part before and after the change. Elimination of multi-revolution moves should show up as a repeatable reduction, and cycle time should no longer drift upward across consecutive parts.
6. Utility wrap check. Inspect the rotary union, air/coolant lines and any encoder cable after a production run to confirm no net accumulated wind-up.

Preventive Practice

• Keep a machine-specific M-code sheet at the control. Do not copy auxiliary M-codes between machines, even from the same builder, without checking the list.
• Treat any M-code that "does nothing" as unassigned until proven otherwise — a silently ignored code is the normal outcome for a number with no PMC decode.
• Start every rotary program with an explicit safe-modal block and, where supported, an explicit rotary-mode selection so behaviour does not depend on the previous job.
• Where absolute rotary positioning is not strictly needed, prefer incremental indexing. It is unambiguous, portable between machines, and immune to accumulated-coordinate problems.
• Document the verified code in the program header as a comment so the next programmer does not repeat the M126 substitution.

FAQ