Mach3 G3 Arc Error: Troubleshooting Missing I and J

The failing block is incomplete for a conventional clockwise/counterclockwise arc command: G3 X-75.987 Y-28.056 P10.212 defines an endpoint but supplies neither center offsets I/J nor a radius R. Correct the Fusion 360 post processor so it emits the arc format and I/J mode expected by Mach3, or configure the CAM output to linearize arcs. Changing Mach3's I/J setting can correct a center-mode mismatch, but it cannot create center coordinates that are absent from the block.

Interpret the failing G3 block

The preceding motion ends at approximately X-76.3 Y-30.234 Z-10. After G17 selects the XY plane, the next command requests counterclockwise circular interpolation:

G17
G3 X-75.987 Y-28.056 P10.212

The commanded XY endpoint is X-75.987 Y-28.056. The straight-line chord from the starting point to that endpoint is about 2.200 units:

delta X = -75.987 - (-76.300) = 0.313
delta Y = -28.056 - (-30.234) = 2.178
chord = sqrt(0.313^2 + 2.178^2) = 2.200

Those values identify the two ends of the arc, but two endpoints alone do not define a unique circle. The controller still needs a center or radius. The meaning of P varies among G-code dialects, so P10.212 must not be treated as an interchangeable replacement for I, J, or R. Check the selected post processor's output rules before assigning any geometric meaning to that word.

How G17, G3, I, J, and R define the motion

G17 selects the XY interpolation plane. In that plane, G3 commands counterclockwise motion from the current XY position to the programmed X/Y endpoint. A valid arc normally uses one of two geometric formats:

In incremental-center mode, I is the X offset from the arc start to the center and J is the Y offset. In absolute-center mode, they represent center coordinates in the active coordinate system. Interpreting the same numbers in the other mode moves the calculated center, so the controller may report a radius mismatch or reject the block.

The sample block contains neither format. Therefore, first repair the missing geometry. Only then investigate an I/J-mode mismatch if the newly generated blocks contain I and J but Mach3 still rejects them.

Run the diagnostic sequence

1. Confirm the exact highlighted block. Verify that the reported failure occurs on G3 X-75.987 Y-28.056 P10.212, not on a preceding modal or coordinate command.
2. Read the current position immediately before the arc. For this program fragment, use approximately X-76.3 Y-30.234 Z-10 as the arc start.
3. Confirm the active plane. G17 is present and selects XY, so an I/J center-format arc would require both applicable XY-center values.
4. Inspect nearby arc blocks from the same file. Determine whether the post emits I/J, R, or the same endpoint-plus-P construction repeatedly.
5. Inspect the Fusion 360 post selected for the machine. A post intended for another controller dialect can generate syntactically valid output for that controller while producing unusable Mach3 code.
6. If regenerated arcs include I/J values, open Mach3's Config > General Config page and inspect the I/J setting. Match it to the post processor's center-coordinate convention.
7. Backplot or dry-run the corrected file before cutting. Confirm the arc direction, endpoint, center, and continuity with the following G1 move.

Correct the CAM output or Mach3 configuration

The preferred repair is to select or modify a Mach3-compatible Fusion 360 post processor. Configure it to output XY arcs using a center or radius representation accepted by the control. Regenerate the entire program; do not patch only the first failing line because the same post behavior can affect every arc.

When the post emits center-format arcs, make its I/J convention match Mach3. In Mach3, locate the I/J setting under Config > General Config. Change that setting only after identifying whether the post outputs incremental center offsets or absolute center coordinates. An arbitrary toggle may make one block load while corrupting other arcs.

If a suitable arc post is not available, configure CAM to replace circular interpolation with short G1 segments. Set the CAM chordal tolerance from the drawing tolerance, surface-finish requirement, machine resolution, and acceptable program size. Linearization avoids dialect-specific arc-center interpretation, but tighter tolerances create more blocks and can reduce motion smoothness on controllers with limited block-processing capacity.

Verify the repair safely

1. Reload the regenerated program and confirm that Mach3 parses beyond the original failing line.
2. Review every G2/G3 block. Each should contain an endpoint and either the required center words or a radius word.
3. Use the toolpath display to check for full circles, long sweeping arcs, loops, or arcs displaced far from the part. These patterns commonly identify the wrong I/J mode.
4. Run above the work with the spindle and process outputs controlled according to the machine's commissioning procedure. Use reduced feed and remain ready to stop motion.
5. Confirm that the corrected arc ends at X-75.987 Y-28.056 and joins the following motion without a jump or reversal.
6. After the air cut passes, make a controlled test cut and measure the affected feature against the CAM geometry.

Avoid recurring arc-post problems

Do not delete G3 while leaving only its endpoint unless a straight chord is acceptable; that changes the toolpath geometry. Do not invent I, J, or R from the endpoint and P10.212. The missing center cannot be uniquely calculated from the two endpoints.

Keep the controller profile, CAM post, units mode, plane selection, and center-coordinate convention as one validated machine configuration. After changing a post, compare a small test program containing clockwise and counterclockwise arcs, different quadrants, and a known endpoint. Archive the validated post with the machine configuration so later CAM upgrades do not silently restore an incompatible dialect.

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