Several Methods of Scanning a Coordinate Measuring Machine

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  Key takeaways
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  Several methods of scanning and where each one fits
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  Contact scanning: touch-trigger and continuous
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  Non-contact scanning: laser and optical methods
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  Hybrid scanning: contact and laser in one program
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  Step by step: setting up a scan in PC-DMIS
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  Contact vs non-contact vs hybrid scanning

A coordinate measuring machine collects points along a surface, then software fits those points to a line, circle, plane or freeform surface. The method you choose decides how those points are gathered. Contact scanning drags a stylus across the part. Non-contact scanning projects a laser or light stripe. Hybrid scanning switches between the two inside one program.

The choice is not about which method is better. It is about part material, feature size, surface finish and tolerance. A hardened steel bracket at ±0.005 mm is a contact job. A 0.8 mm thick plastic housing that deflects under 0.05 N of stylus force is a laser job.

Most shops running a CMM keep one method as default and reach for the others only when geometry demands it. Knowing the boundaries of each method prevents the two most expensive mistakes: scrapping a good part because the scan lied, and accepting a bad part because the scan smoothed over a defect.

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  ContactStylus touches the surface. Stable on metal, slow on large freeform areas.
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  Non-contactLaser or optical. Fast on soft or complex surfaces, sensitive to reflectivity.
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  HybridBoth in one setup. Best for parts with tight holes plus sculpted surfaces.

Touch-trigger scanning takes discrete points. The probe hits the surface, the machine records the position and retracts. It is the slowest method but the most forgiving. Repeatability on a good machine with a Ø2 mm ruby stylus is around 0.5–1 μm. Use it for datums, hole centers and any feature smaller than 3 mm.

Continuous scanning keeps the stylus in contact while the machine moves along a defined path. A scanning probe such as a Renishaw SP25 or a TP200 with scanning module records thousands of points per second. Point spacing of 0.5–2 mm is typical for a 50 mm circle. Tighter spacing improves form error detection but adds cycle time.

Stylus selection matters more than most operators expect. A long stylus bends. Every 10 mm of extra length adds roughly 1 μm of uncertainty on a Ø2 mm tip. Keep the stylus as short as the feature allows, and use a star configuration only when you must reach under a flange.

Watch contact force. A standard scanning probe applies 0.05–0.3 N. On thin aluminium sheet below 1 mm, that force deflects the part before the probe triggers. The scan reads the deflection, not the part.

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  Best forMetal parts, holes, datums, features under 3 mm.
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  Avoid whenPart walls are thin, soft or flexible.
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  Typical spacing0.5–2 mm along the scan path.
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  Common errorStylus too long, tip too small for the feature.

Laser line scanning projects a stripe onto the surface and a camera reads the deformation. A single pass captures a dense point cloud, often 0.05–0.2 mm between points. It is fast and applies no force, which makes it the default for plastic, rubber, sheet metal and any part that would move under a stylus.

The limit is surface interaction. Shiny aluminium reflects the laser away from the camera, creating holes in the data. Matte black absorbs it and returns a weak signal. The fix is a thin removable coating, typically a spray that adds 5–15 μm. That coating changes the measured size, so you must correct for it or scan before coating.

Optical methods such as structured light and photogrammetry work on the same principle with a different light source. They suit large panels and full-part capture where you need the overall shape, not a 0.01 mm hole diameter. Accuracy on a 1 m panel is typically ±0.05 mm, not ±0.005 mm.

Edge detection is the weak point. A laser sees a rounded corner where the CAD model has a sharp one. If your drawing calls out a sharp edge or a small chamfer, verify it with a contact probe, not a laser.

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  Best forSoft parts, freeform surfaces, full-part capture.
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  Avoid whenThe surface is mirror-finish or matte black.
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  Typical spacing0.05–0.2 mm in the point cloud.
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  Common errorTrusting laser data on sharp edges and small holes.

Hybrid scanning uses both sensor types in a single measurement routine. The contact probe establishes the datum reference frame and measures tight holes. The laser then sweeps the freeform surfaces. Because both sensors are calibrated to the same machine coordinate system, the data aligns without manual stitching.

This is the practical answer for parts that have both. A machined housing with a Ø8 mm H7 bore and a sculpted outer skin is a classic case. Measure the bore with a contact probe at 8 points, then scan the skin at 0.1 mm spacing. One setup, one report.

The cost is calibration discipline. The laser and the probe must be qualified against the same reference sphere before every run. Skip that step and the two data sets drift apart by 10–30 μm, which is enough to fail a tolerance check on a good part.

Hybrid setups also take longer to program. Budget an extra 30–60 minutes for the first article. After that the program runs unattended.

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  Best forParts with tight holes plus freeform surfaces.
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  Avoid whenThe part is simple and one sensor covers it.
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  Key stepQualify both sensors to the same reference sphere.
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  Common errorMixing data sets without a shared datum frame.