Why a FARO Arm Measures One Outside Diameter Incorrectly

1. Confirm the feature type

Make sure PolyWorks defines the measured feature as an external cylinder. An internal-cylinder definition can apply probe compensation in the wrong direction.

A FARO arm reports the position of the probe tip's center, not the point where it actually touches the part. For a spherical probe on a convex surface, the ball's center sits one probe radius away from the true contact point along the surface normal. Compensation is the correction that moves the reported point back to that contact point, and the direction it moves depends entirely on whether the feature is defined as external or internal. Get it backwards and the software doesn't just skip the correction — it applies it the wrong way, which can push the reported diameter off by roughly two probe diameters instead of a fraction of one.

2. Check probe compensation

Verify the active probe diameter and confirm that compensation is being applied to the correct side of the surface.

Feature type controls direction; probe diameter controls magnitude. Even with the correct external/internal setting, compensation only works if the diameter loaded in the software matches the stylus that's physically calibrated on the arm. A mismatched probe diameter is a separate failure mode from a wrong-direction compensation error — it shifts the diameter by a fixed amount rather than flipping its sign, so it's worth checking independently rather than assuming one fix covers both.

3. Review point distribution

Take points around the full circumference and at more than one height. Avoid collecting most points from one side.

A cylinder fit solves for two things at once: a radius and an axis — a line in space, not just a circle. Points taken from a narrow arc leave that axis direction weakly constrained, so several noticeably different cylinders can fit the same limited data almost equally well by the software's own residual score. Spreading points across the full circumference and across more than one height constrains both the radius and the tilt of the axis, which is what makes a fit trustworthy rather than merely low-residual.

4. Inspect the surface

Paint, burrs, surface roughness, chamfers, and edge contact can shift the fitted diameter.

Least-squares fitting treats every collected point as if it sits exactly on the nominal cylindrical surface. A point that lands on a burr, a paint buildup, or a chamfer isn't measuring that surface at all — it's measuring whatever local feature happens to be there — and the algorithm has no way to tell the difference unless the point is excluded. One bad point in an otherwise clean set can be enough to shift a diameter measurably.

5. Test with individual points

Create a new cylinder from manually selected individual points. This helps identify whether the issue comes from the measurement method or feature settings.

This step is a diagnostic, not a fix. If a small set of hand-picked points produces roughly the same diameter as the full automatic scan, the fitting math is doing its job and the error is upstream — feature type, compensation, or probe calibration. If the manual fit disagrees noticeably with the scan, the problem is more likely in the point cloud itself or in how the automatic feature was defined.