How to Measure Deep Holes Bores Datums and Other Hidden Features in a 3D Inspection

Measure a deep hole or hidden feature by first defining the result required from it. A scanner may be suitable when the opening, wall, edge, and relevant depth remain visible from enough angles. Use a dedicated deep-hole scanning mode when standard laser patterns create excessive shadowing but the feature still has a clear optical path. Use tactile probing when a bore, recess, datum target, or hidden point is physically reachable but cannot be captured with enough optical coverage.

Many industrial parts need both methods. Scan the accessible exterior and visible features to obtain full-surface geometry, then probe the deep or hidden characteristics that require direct contact. The two datasets should share a controlled coordinate system when a bore position, hidden datum, or assembly point must be related to the scanned surface.

Do not decide from hole depth alone. Diameter, opening shape, viewing angle, required output, surface response, tolerance, and datum role all affect the method. A scan of the rim does not prove the diameter or axis of the full bore, and a few contact points do not describe an unknown defect across an internal surface. The measurement plan must match the actual characteristic.

Why Hidden Features Need a Separate Strategy

Optical 3D scanning depends on a usable light path. The laser must illuminate the surface, and the cameras must observe the reflected pattern. As a hole becomes deeper or narrower, the rim blocks more projected light and more of the camera view. Tilting the scanner or using a narrower laser pattern can extend coverage, but the captured data must still represent the section of the feature used for the result.

Tactile measurement has a different access condition. The stylus must reach the measurement location without the probe body, extension, or surrounding part colliding with the workpiece. The operator must also control the approach direction, contact force, probe qualification, and distribution of points.

Hidden-feature inspection therefore begins with geometry. High point density cannot replace an unseen surface, and a long stylus cannot define a cylinder if the touches cover only one side. The acquired points must support the required diameter, axis, depth, position, or datum.

Classify the Feature Before Measuring It

Open holes and shallow bores

A shallow feature with a clear opening may be captured with standard scanning. Inspect the point cloud or mesh to confirm that the edge and enough of the wall are present. Hole location derived only from an incomplete edge can shift as filtering and feature extraction change.

Deep bores with a clear opening

A deep bore may remain optically measurable when its diameter permits several viewing angles and the required wall region is visible. Dedicated single-line scanning reduces the optical width of the projected pattern and can reach farther into the bore than a multi-line pass. The operator should rotate the viewing direction around the opening rather than rely on one narrow arc.

Blind bores

Define whether the inspection needs diameter, axis, depth, bottom condition, or position relative to a datum. A scanner may capture the upper wall but not the floor. A probe may reach the floor or measure defined sections along the bore. One method can also establish the axis while the other supplies surrounding surface context.

Internal grooves and recessed channels

Grooves, slots, and channels often require a cross-section rather than a complete internal mesh. If the laser and cameras can see both walls and the floor, a scan can support width, depth, and profile analysis. Where a wall or bottom remains hidden, use a probe for the defined contact locations.

Hidden datums and rear-facing points

A hidden datum hole, tooling point, or assembly locator may serve mainly to establish coordinates. Dense surface coverage is unnecessary when the requirement is a defined center, plane, or point. Contact probing can measure those references and place the visible scan in the functionally correct frame.

Fully enclosed internal geometry

An enclosed void that neither light nor a stylus can reach requires an internal inspection method such as computed tomography or a sectioning procedure appropriate to the part. CAD reconstruction can represent nominal intent, but it cannot convert unmeasured internal geometry into inspection evidence.

Determine Whether the Feature Is Scannable

Start by checking whether the projected laser reaches the target surface and a camera returns stable data. Change the scanner angle and verify that additional views provide complementary coverage.

Next, examine the relationship between bore depth and opening size. There is no universal depth-to-diameter cutoff for every scanner and feature. As that ratio increases, shadowing usually becomes more severe. Counterbores, chamfers, shoulders, nearby walls, and physical clearance around the opening can matter as much as the nominal ratio.

Then define the output. Measuring the entrance diameter differs from establishing the axis over the full depth. The scan must contain data in the region that defines the required edge, diameter, form, depth, or position.

Review coverage before fitting geometry. Inspect the wall angle represented by the points, their distribution along the depth, gaps, reflections, and outliers. A software fit may return a diameter from a small arc, but a numerical result alone does not establish that the feature was adequately sampled.

Scan Deep Holes and Bores Methodically

Clean the opening and accessible wall without changing the surface. Select the scan mode, point spacing, and exposure for the material. If a temporary scanning coating is required, control its application and keep it away from contact features when its thickness could affect the later probing result.

Capture the surrounding exterior first so the hole remains connected to stable part geometry. Record the rim, nearby planes, bosses, and any visible datum features. Switch to the deep-hole mode, approach the opening from several directions, and monitor how far the data extend into the feature. Do not move the part unless the registration method supports the change.

After capture, isolate the intended feature region with a documented rule. Fit the circle or cylinder from an adequate angular and axial distribution of data, review residuals, and compare the extracted feature with the raw point cloud. Do not fill a mesh gap and treat the interpolated patch as measured surface. Hole filling is useful for visualization or downstream modeling only when the report distinguishes reconstructed geometry from captured data.

Current Scanners for Restricted Geometry

SIMSCAN-S Gen2 is designed for detailed capture on compact and medium parts and in restricted spaces. Its current product page describes a short-baseline camera design for gaps, deep holes, slots, and channels approached at a steep viewing angle. The Gen2 specification table also lists one additional blue laser line for deep-hole scanning. This combination suits compact housings, engine components, brackets, and parts with closely spaced geometry.

SIMSCAN-S Gen2 capturing geometry around restricted areas of an engine component

KSCAN-E supports mixed-size industrial tasks with multiple scanning modes. Its deep-hole mode uses one additional blue laser line for hard-to-measure regions such as engine cylinders and internal grooves in cast parts. The same system also provides hole and edge inspection, fine scanning, large-area scanning, and photogrammetry functions. It is useful when a part combines broad exterior surfaces with bores, edges, grooves, and detailed local areas.

KSCAN-E capturing the surface of a large industrial component during on-site inspection

Deep-hole modes extend optical access; they do not remove the need to review coverage. Select the scanner according to the complete part, required measurement volume, access around the opening, and the characteristics in the inspection plan.

When Tactile Probing Fits the Feature

Use tactile probing when the scanner cannot observe enough of the feature, when a specific hidden coordinate is required, or when the established control plan calls for a contact strategy. Typical tasks include deep blind bores, reference holes behind surrounding structures, recess depth, locator points, tooling targets, and the position of internal assembly features.

Plan the stylus as carefully as the scanner view. The tip diameter must suit the feature, the extension must reach without excessive deflection, and the probe body must clear the opening. Qualify the probe configuration that will be used. If the stylus or extension changes, the measurement procedure should require the corresponding qualification or verification.

Distribute touches around the circumference and along the relevant depth. A cylinder based on points clustered on one side may fit mathematically but represent the feature poorly. The drawing determines the number of sections, datum relationship, and geometric evaluation. Probe force, surface roughness, burrs, contamination, and operator technique also contribute to variation.

Select the Probing Architecture

AccuArm is a portable coordinate measuring machine, or PCMM. It is not a fixed CMM. Its articulated structure and full joint rotation support portable measurement around fixtures, assemblies, large components, and restricted geometry. In a hidden-feature workflow, it can measure defined bore sections, recess depths, locator points, and datum features, and it can be combined with the company’s scanning systems through the Arm+ concept.

NimProbe is a compact optical probing system for contact measurement within a tracked reference. The current product page describes a 500 mm i-Probe for reference holes, hidden points, and hard-to-reach areas, together with stylus and corner-extension options. It suits medium and large parts that require flexible multi-angle probing. When both marker-free scanning and probing are needed, NimbleTrack Gen2 can be paired with an i-Probe.

NimProbe measuring a bore on a large industrial component

Choose between an articulated arm and tracked probing by considering part size, access direction, measurement volume, available references, portability, and the inspection team’s established procedures.

Establish Datums from the Drawing

A broad visible datum surface can be constructed from scan data when the selection area and data-cleaning rules are controlled. Exclude edges, damaged areas, fastener holes, and irrelevant geometry according to the inspection plan rather than changing the selection until the alignment looks favorable.

A datum hole or cylinder may also come from scan data if the wall coverage is sufficient for the required characteristic. Probe the feature when depth, occlusion, or sampling makes the optical construction unreliable. Discrete datum targets should be measured at the specified locations; replacing them with a best fit over the surrounding surface changes the reference system.

Follow the datum precedence on the drawing. If probing establishes the primary coordinate frame and scanning supplies the surface, preserve that relationship through a verified transformation or common tracking reference. Check the coordinate relationship after moving a tracker, portable arm, target, fixture, or part.

Build a Hybrid Inspection Routine

Review the drawing and list the required output for each hidden feature. Assign visible surfaces and adequately captured holes to scanning. Assign inaccessible bore sections, hidden points, and discrete targets to probing. Define the datum sequence before measurement begins.

Keep the part in one support condition whenever movement could change its shape or reference. Capture the exterior and visible features, then use the deep-hole mode where optical access remains possible. Review coverage while the setup is intact. Probe the selected internal features and datums using the qualified configuration.

Process the results under one coordinate strategy. DefinSight, the company’s own 3D digitization and metrology software platform, supports data capture, processing, analysis, and reporting. A unified environment reduces manual transfers, but the procedure must still identify the source of every result and preserve the validated alignment.

Report measured surface maps, sections, hole dimensions, axes, positions, depths, and GD&T results as applicable. Mark reconstructed CAD or filled mesh regions separately from measured data. Save the software settings, feature definitions, probe configuration, reference revision, and environmental records required by the quality system.

Feature Selection Matrix

Validate the Measurement Result

Validate the routine with representative features or suitable traceable artifacts. A calibrated ring, bore, step, or check length may support different parts of the study. Compare the result with a reference method when the feature or tolerance justifies it.

Repeat the complete sequence across operators, setups, and relevant shop-floor conditions. For scanned bores, evaluate how viewing angle, coverage, surface preparation, filtering, and feature selection affect diameter and axis. For probing, vary the operator, approach direction, point distribution, and requalification events included in normal work. For a hybrid routine, include a check feature that tests the relationship between the scanned and probed datasets.

ISO 10360-13 addresses specified optical 3D coordinate measuring systems, and applicable VDI/VDE 2634 documents provide further optical performance evaluation. ISO 10360-12 addresses articulated-arm coordinate measuring machines. These standards define different test quantities, so their maximum permissible errors should not be compared as interchangeable accuracy figures.

Common Measurement Mistakes

· Reporting a full bore from data captured only at the entrance

· Fitting a cylinder from a short arc or one depth without reviewing coverage

· Treating mesh hole filling or CAD reconstruction as measured geometry

· Ignoring the relationship between the bore axis and the drawing datums

· Placing all tactile points on one side of the feature

· Using an unqualified stylus or changing an extension without verification

· Moving the part or reference hardware without recovering the coordinate system

· Substituting an unconstrained best fit for specified datum targets

· Validating the instruments separately but not the complete routine

· Failing to identify whether a reported value came from scanning or probing

Conclusion

Reliable hidden-feature inspection starts by defining the characteristic, then checking optical visibility and physical access. Scan holes, grooves, and recesses when the required surface is captured with adequate coverage. Probe features that remain optically incomplete but can be reached with a qualified stylus. Combine both methods when internal coordinates or datums must be related to a complete exterior surface.

Whichever method is used, control the datum structure, feature construction, coordinate relationship, and validation procedure. That evidence determines whether the result supports the manufacturing decision.

Frequently Asked Questions

Can a 3D scanner measure the full diameter of a deep bore

Yes, when enough of the wall is captured over the section and depth that define the result. Review angular coverage, axial distribution, and fit residuals. Data at the rim alone do not establish the full bore.

When should I use deep-hole scanning mode

Use it when the feature retains an optical path but a standard multi-line pattern produces shadowing or incomplete wall data. Single-line capture can extend access, though coverage must still be verified.

Can SIMSCAN-S Gen2 scan narrow gaps and channels

Its current product information describes a short-baseline camera design and a dedicated deep-hole line for gaps, deep holes, slots, and channels approached at a steep angle.

When is KSCAN-E suitable for bore inspection

KSCAN-E fits parts that combine bores or internal grooves with broad surfaces, edges, fine details, or larger measurement dimensions. Its multiple scanning modes allow the operator to address those regions with one system.

How many probe points are needed for a bore

There is no single number for every characteristic. The points must support the specified diameter, axis, form, or position and be distributed around the circumference and along the relevant depth. Define the pattern in the validated inspection plan.

Can a probe establish a datum the scanner cannot see

Yes. A probe can measure an accessible hidden hole, target, or point and establish the coordinate frame used to evaluate visible scan data.

Should CAD hole filling be used for acceptance

No. Filled or reconstructed geometry can support visualization and modeling, but acceptance results should come from measured data unless the approved procedure explicitly defines another evidence source.

Is AccuArm a fixed CMM

No. AccuArm is a PCMM, an articulated portable coordinate measuring machine. A fixed CMM is a different equipment category.