A CNC Dimensional Error That Grows With Distance Is Telling You Something

Error that grows with commanded distance suggests a different branch from a nearly constant offset. Measure several lengths in both directions and compare the slope, reversal difference, tool effect, and measurement setup before editing scale.

Plot Error Against Commanded Distance

Measure at several commanded distances and preserve direction. Plot actual minus commanded travel rather than reducing everything to one percentage. A straight slope supports a scale investigation, a near-constant offset after reversal suggests lost motion, and irregular jumps point toward slip, missed steps, binding, or measurement instability.

Validate the Drawing and Export Scale

Use a reference whose uncertainty is small relative to the travel. Short caliper checks across soft or poorly defined points can create a false slope. Repeat from the same datum, approach consistently, and include enough range to see whether error really grows linearly.

Separate Axis Travel From Cut Size

Test X and Y separately before using a diagonal or circular part. If one axis scales incorrectly, rotated features will combine both errors and become harder to interpret. Keep temperature, machine state, and travel location noted when long components or warm drive systems could affect measurement.

Check for Local Mechanics and Load Effects

Separate motion from the cut. After non-cutting travel is credible, machine a simple coupon with known dimensions in both axes. Cutter diameter, runout, tool deflection, stock movement, and material recovery can add a nearly constant or direction-dependent size error that is not corrected by steps per millimeter. A straight residual-error slope is the case addressed by Calibrate CNC Steps per Millimeter Without Hiding Backlash; irregular jumps need another branch.

Calculate Scale Only From Stable Measurements

Avoid scaling production artwork to hide machine error. Artwork compensation transfers the fault into every file and can produce different errors at other sizes. Correct a proven axis-scale problem through documented configuration, then return the source geometry to nominal and verify several distances again.

Confirm With Independent Geometry

Archive raw distances, residual errors, plot or table, axis and direction, instrument, original and revised settings, repeat spread, and cutting-coupon results. A later reviewer should be able to distinguish scale from backlash and process offset without reverse-engineering one final calibration number.