Borescope Or Laser Rotary Probe? Inside-Hole Inspection At Ø1.5 mm

Overview

Inside-hole inspection is the part of dimensional and defect inspection that most often remains manual. The reason is access: a bore is a surface that faces inward, and the only ways to image it are to look along the axis with an optical system small enough to enter, or to rotate a sensor inside it and build the image line by line. Both approaches are in production use, and they fail in different ways.

A borescope-style optical probe delivers a real image with colour, which makes it intuitive for an operator and effective for visual defect classes such as scratches, dents and casting porosity. A laser rotary probe measures profile instead of colour, which makes it immune to the lighting problems that plague small-bore optics and gives it a collision-detection capability that an optical probe lacks.

Big Bird Industrial (formerly Shimada Big Bird Industrial) has been building cleaning and deburring systems for automotive powertrain components since 2001, and the inspection figures below come from production stations rather than laboratory rigs.

1. What An Optical Bore Probe Can And Cannot Do

The optical approach places a small camera or a lens-and-relay system inside the bore. Light is delivered to the far end and the image is returned to the sensor, either directly or through a fibre bundle. The result is a colour image that a human or a trained model can interpret for defect classes.

Three Optical Configurations

A rigid armoured lens is the most common arrangement for straight bores: a stainless tube with optics at the tip, giving a forward view or a side view at a fixed angle. A 360-degree panoramic lens images the full circumference in one exposure, which is efficient for large bores but has limited resolution per unit area. A fisheye configuration gives a wide field of view at short working distance, which suits shallow pockets and countersinks.

The three are not interchangeable. A panoramic view tells you that a defect exists somewhere on the circumference but not precisely where; a side-view rigid lens gives a precise azimuthal position but has to be rotated to cover the full circumference, which costs cycle time.

The Resolution Limit

Optical resolution inside a bore is limited by the aperture you can physically insert and by the working distance. A system designed around a minimum bore of 1.5 mm can resolve detail at 0.01 mm on the surface when the optics are correctly focused, but that figure is achieved at a specific working distance and deteriorates as the probe is withdrawn. Depth capability in the standard range covers 50 to 200 mm, with custom configurations reaching 310 mm.

2. What A Laser Rotary Probe Changes

A laser rotary probe projects a laser line onto the bore wall and rotates inside the bore, capturing a profile at each angular position. The result is a point cloud of the internal surface rather than a photograph of it.

Immunity To Ambient Light

Because the measurement is based on triangulating a laser line rather than on imaging reflected white light, the probe does not care about ambient illumination or about the colour of the surface. That removes two of the main sources of false calls in optical bore inspection: stray light entering the bore opening, and reflective or dark surfaces that change the apparent contrast of a defect.

Collision Detection And Foreign Matter

A rotating probe can detect an obstruction before it hits it, because the profile it is measuring changes abruptly. In practice this allows the probe to identify foreign matter in a bore and to stop, which is a requirement on parts where a chip or a casting nodule could damage a subsequent machining operation. On valve bodies, a foreign body with a profile above 0.25 mm is a detectable event, and cycle times of 14 seconds for a cylinder bore and 18 seconds for a booster bore are achievable with this method.

3. Selecting By Aperture And Depth

The first selection criterion is aperture, because it eliminates options before any other consideration applies. Bores in the 5 to 40 mm range and the 40 to 120 mm range are both routinely inspected, with a depth-to-diameter ratio of about one to one. Beyond that ratio the probe has to be longer relative to its diameter, which increases deflection and reduces positional accuracy at the tip.

Depth Versus Diameter Trade-Off

A long, thin probe bends. Even a rigid armoured lens deflects measurably over 300 mm, and the deflection changes the stand-off distance and therefore the focus. Systems that work at that depth either use a larger diameter housing or accept a lower resolution at the far end, and the specification should state the performance at the specified depth rather than the best-case figure.

4. Detection Limits And What They Hide

A stated detection limit is usually a best-case figure on a reference defect. The number that matters in production is the limit at the worst position in the bore, which is typically the far end and the region where the probe is moving fastest.

Lighting As The Dominant Variable

In optical systems, defect contrast depends on the illumination angle relative to the surface. A scratch on a honed bore is visible at one angle and invisible at another, which is why bore inspection stations often use more than one illumination direction and combine the results. A station that uses a single ring light will report a stable but incomplete picture.

Calibration And Drift

Both methods need a reference artefact. For an optical probe it is a target with known feature sizes at the working distance; for a laser probe it is a calibration cylinder with a known diameter and step. Checking the artefact at the start of each shift is the cheapest way to detect the slow drift that otherwise appears as a change in the false-call rate rather than as an error message. A probe that is out of focus still returns an image, and a probe that has lost its calibration still returns a profile; neither reports an error until someone compares the result against a known part.

5. Integrating Into A Production Line

A bore probe is a slow sensor compared with an area camera, so it is usually moved to the features rather than the features moved to it. That means a robot or a two-axis mechanism carrying the probe between bores, and it means the inspection station has to know where each bore is on the part.

The practical consequence is that cycle time scales with the number of bores inspected, not with the part size. A part with four critical bores takes roughly four times the inspection time of a part with one, and that arithmetic has to be in the takt budget before the station is specified. Where the count is high, sampling a subset per part rather than inspecting every bore is often the only way to fit the line.

Frequently Asked Questions

Can A Borescope Be Used For 100 Percent Inspection?

Yes, if the mechanism and cycle time allow it. The common compromise is to inspect every part for the two or three bores that carry the highest risk and to audit the rest on a sampling basis.

What Is The Smallest Bore That Can Be Inspected?

Down to 1.5 mm with dedicated optics. Below that the probe diameter limits the illumination and the resolution, and alternative methods such as air gauge measurement are usually more practical.

Which Method Handles A Dark Or Reflective Bore Better?

The laser rotary probe, because it does not depend on reflected white light. An optical probe on a dark surface needs more illumination and produces lower contrast, which raises both the miss rate and the false-call rate.

How Often Should The Probe Be Calibrated?

A reference check at each shift start, and a full recalibration on a schedule set by the manufacturer. The shift check catches mechanical damage and focus drift, which are the failures that occur between scheduled calibrations.

Conclusion

The choice between an optical bore probe and a laser rotary probe is decided by the defect class and the surface, not by resolution figures. If the criterion is a visual defect on an accessible bore with controlled lighting, optics give a richer image. If the criterion is geometry, position or the presence of foreign matter in a bore that may be dark or reflective, a laser profile is more robust.

Both are limited by aperture and depth before anything else, so the specification should start there. A detection limit quoted without the bore diameter, the depth and the surface condition it was achieved on is not a specification.