Why Is an Interpolator in a CNC Machine Essential for Accurate Motion?

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  Interpolation faults: symptom, cause, action
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  What the interpolator in a CNC machine actually does
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  Chord error and tolerance: the trade you cannot avoid
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  How to tell an interpolator problem from a mechanical one
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  Why five-axis work stresses the interpolator most
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  What we check before blaming the control
• 7
  Step by step: isolate an interpolation fault

A CNC program never tells a machine to move a single axis to a single point. It hands the controller a sequence of blocks, each describing an endpoint, a feed rate, and a shape such as G01, G02, or G03. The interpolator is the part of the control that takes those endpoints and decides, thousands of times per second, where every axis should be at each moment along the path. Without it, you would get point-to-point motion and stepped contours.

The interpolator works in two stages. First it plans the geometry, breaking a commanded arc or spline into small segments and checking the chord error against the tolerance in the control. Then it plans the velocity, blending segment speeds so the machine does not stop at every block boundary. Both stages run ahead of the actual motion, which is why look-ahead depth matters on complex 3D surfaces.

The output is a position command stream to the servo drives. Each drive closes its own position loop, but the shape of the path is decided upstream. This is the key point for troubleshooting: a bad contour is not always a bad servo. It can be a geometry decision made by the interpolator before any axis moved.

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  Geometry stageConverts arcs and splines into segments within a tolerance band.
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  Velocity stageBlends feed between segments so axes never fully stop.
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  Look-aheadReads blocks ahead to plan deceleration before corners.
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  Servo outputSends synchronized position commands to each drive.

Every curved path is approximated by straight segments. The distance between the true arc and the straight chord is the chord error. If you allow 0.01 mm chord error, the interpolator can use longer segments and run faster. If you demand 0.002 mm, segments get shorter, block rate goes up, and the control may run out of look-ahead. That is the trade.

On a part held to ±0.005 mm, a chord error around 0.002–0.003 mm is usually safe for finishing passes. Roughing can sit at 0.01–0.02 mm with no harm to the final size. The mistake is leaving a finishing tolerance in the control while roughing, or the reverse: finishing with a coarse tolerance and blaming the machine for facets.

Surface finish follows the same logic. For Ra 0.8–1.6 μm you usually need tighter chord error and a stable feed, not just a finer tool. If the control is fighting to keep up with block rate, feed drops in the corners and you get visible marks even though the toolpath looks clean on screen.

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  Roughing0.01–0.02 mm chord error is typically fine.
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  Finishing at ±0.005 mmTarget 0.002–0.003 mm chord error.
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  Fine finish Ra 0.2–0.8 μmTighten further and slow the feed slightly.

Start with a simple test. Run the same program in dry run, first with the part removed, and watch the axis load and following error on the control. If the error spikes at the same block every time, the interpolator or the program is the suspect. If the error spikes at the same physical position, look at the machine: backlash, loose preload, or a sticky way.

Second, compare a short arc cut at two different feed rates. A mechanical fault usually gets worse as feed rises. An interpolation or look-ahead fault often appears as a sudden feed drop, not a gradual degradation. Plot commanded feed against actual feed if the control supports it.

Third, check the servo tuning after any change to acceleration limits. Interpolators and drives share parameters. Raising acceleration to improve cycle time without re-tuning the position loop will show up as overshoot at the end of fast moves. That looks like a contour error but it starts in the drive.

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  Repeatable at one blockProgram or interpolator setting.
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  Repeatable at one positionMechanical or drive issue.
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  Worse at high feedUsually mechanical, sometimes look-ahead.

In three-axis milling, the interpolator only coordinates X, Y, and Z. Add two rotary axes and the problem changes shape. A small move of the C axis near the part center can swing the tool tip by a large distance. The interpolator must synchronize linear and rotary motion so the tool tip follows the intended path, not the machine axes.

This is why five-axis surface finish often depends more on the control than on the spindle. If the rotary axes are not blended with the linear axes at the same look-ahead depth, you get witness marks where the rotary axis reverses. On a Ø400 mm rotary table, even a small reversal can leave a visible step.

For parts with compound angles and undercuts, the interpolator also manages tool orientation. The post processor defines the intended contact angle; the control has to hold it while the path curves. When that synchronization drifts, the tool rubs instead of cutting, and you see burnishing or rapid tool wear on one edge.

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  Rotary reversalCommon source of witness marks on five-axis parts.
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  Tool tip vs axis motionThe control must compensate for rotary offset.
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  Orientation driftShows up as rubbing, not cutting.

In our Dongguan and Singapore plants, a contour problem goes through the same checklist before anyone touches the interpolator parameters. We verify the CAM output first: arc tolerance, point spacing, and whether the post processor is writing G02 and G03 or just tiny G01 moves. A program made of thousands of 0.05 mm line segments will starve any look-ahead buffer.

Next we check the actual feed against the commanded feed, block by block, on a dry run. If the actual feed collapses on short segments, the problem is data rate, not servo gain. We then confirm the chord error setting against the drawing tolerance. Many facets disappear when the finishing tolerance is tightened to 0.002 mm and the program is re-posted with true arcs.

Only after that do we look at drive tuning, and we change one parameter at a time. On parts held to ±0.005 mm, a small acceleration change can move the contour by more than the tolerance if the position loop is not re-tuned. We inspect 100% of parts before shipment, so we would rather spend twenty minutes on the checklist than scrap a batch.

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  CAM output firstCheck arc tolerance and segment length.
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  Feed comparisonCommanded vs actual, block by block.
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  One parameter at a timeNever change accel and gain together.