Two circular features can have the correct centre distance and still leave too little material between their edges. This matters when an inspection report is used to judge a connecting web, a sealing land or the separation between adjacent holes. We start by asking exactly which distance the drawing controls. A report labelled only “distance” can conceal a perfectly repeatable calculation of the wrong geometric quantity.

The company video demonstrates optical dimensional inspection. The numerical examples below are ideal geometric examples for explaining measurement definitions; they are not customer results or guaranteed machine performance.

Name the distance before constructing it

Centre distance describes the separation of two centres. A distance along the X axis describes only the corresponding coordinate difference. The shortest distance between ideal circles describes the nearest separation of their circumferences. These quantities can be related, but they are not interchangeable. On an automatic video measuring machine, we give each constructed result an explicit name and associate it with the relevant drawing feature.

For two separate circles in the same plane, let their centre distance be d and their radii be r1 and r2. If they are externally separated, their ideal nearest edge gap is d − r1 − r2. The qualification matters: the same expression is not a general formula for intersecting circles, nested circles, arbitrary curves or three-dimensional surfaces. Software may also distinguish a signed separation from an unsigned distance. We confirm the definition rather than inferring it from a familiar icon.

A simple example exposes the missing information

Take two ideal holes with a centre distance of 10.00 mm. If both diameters are 4.00 mm, the ideal gap between their nearest circumferences is 6.00 mm. If both diameters increase to 4.10 mm while the centres remain unchanged, the ideal gap becomes 5.90 mm. The centre-distance result has not changed, but the remaining web has become narrower. These arithmetic examples deliberately exclude form error and measurement uncertainty so that the distinction is clear.

A desktop instant vision measuring machine can capture several features in an image, but collecting them together does not decide which construction answers the requirement. If the drawing specifies centre distance, report centre distance. If it specifies a local minimum web, investigate that web directly using the prescribed definition and suitable sampling. Do not add a gap requirement merely because the software can calculate one, and do not remove a drawing requirement because another result looks reassuring.

Fitted circles and real edges answer different questions

A fitted circle summarises the selected edge points using an association method. Its radius and centre depend on those points and that method. A local protrusion, an irregular hole boundary or an excluded edge segment can make the nearest physical separation differ from the gap calculated between two fitted circles. Reporting the ideal construction as the smallest material web without qualification can therefore conceal the feature that matters.

When using a vertical and horizontal instant vision measuring machine, we also identify the optical view and measurement plane. A top-view gap does not automatically describe separation at every depth. A chamfer, taper or hidden internal surface may require another method. If the inspection cannot see the limiting surfaces, the report should state the scope of what was measured rather than presenting an inferred three-dimensional clearance as an observed value.

Do not confuse a coordinate component with the full distance

Suppose two centres differ by 6 mm in X and 8 mm in Y in an orthogonal planar coordinate system. Their straight-line centre distance is 10 mm. An X-only result would be 6 mm. Rotating the coordinate axes changes the components even though the straight-line distance between the same centres remains unchanged. We use a small, easily checked construction like this when reviewing a new report template or an unfamiliar distance command.

In a video measuring system, retain the source element identities and the construction type with the reported result. A clear label such as centre-to-centre planar distance is more useful than an unnamed number. Where a directional web or spacing is specified, retain the direction or datum reference too. A minimum-distance command should not silently replace a distance required along a specified direction.

Validate the report with deliberately different geometry

Before releasing a routine, we test whether its outputs respond to changes in the quantities they are intended to describe. Ideal CAD geometry or a verified reference dataset can help check the construction logic without pretending to validate the complete physical measurement process. Change a radius while keeping the centres fixed; then change a centre while keeping the radii fixed. Review the expected calculations separately from the image acquisition and calibration checks.

For batch work on an instant optical measuring machine, verify that each constructed distance remains linked to the intended pair of features on the intended part. For work involving several optical views, the integrated instant vision measurement system should be evaluated with the actual viewing arrangement and report definitions. We can review the drawing, accessible surfaces and required distance together at handing3d@163.com. The useful outcome is a result whose geometric meaning is clear enough for the next person to act on.