Practical Inspection Tips

Fixturing that survives production

Cheap fixturing is expensive. On many part families, repeatable holding is the single biggest lever for reducing inspection variation - larger than any probe upgrade or software feature. Best-practice fixturing constrains six degrees of freedom deterministically (3-2-1), locates on functional datums rather than convenient flat faces, and is designed to be cleaned quickly between parts.

• 3-2-1 locating on functional datums, not the nearest flat surface
• Kinematic mounts (Renishaw, Erowa) for repeatable setup across shifts
• Design for cleaning - swarf that gets under a locator ruins repeatability
• Weld and machine fixtures on the same datums as the CMM fixture where possible
• Fixture GR&R separately from the machine - it hides in the total number otherwise

Probing strategy - the underrated lever

Stylus length, tip diameter, approach direction and touch speed change results more than most teams realise. Longer styli bend under contact force; smaller tips filter surface roughness differently; approach normal to the surface reduces cosine error. On tight-tolerance features, a two-minute probing review often delivers more than a new machine.

• Shortest stylus that reaches the feature - bending scales with length cubed
• Tip diameter matched to smallest form feature you need to resolve
• Approach normal to the surface where possible; document offsets otherwise
• Qualify the stylus at the start of every shift; check drift daily
• Scanning: control speed, point density and filter carefully - defaults are rarely right

Alignment discipline

The datum reference frame is the single most consequential decision in a measurement program. Function-based alignment (mounting features, mating faces, functional axes) produces reports that customers and auditors accept first time. Convenience-based alignment (nearest flat, best-fit plane, RPS with arbitrary weights) produces reports that survive nobody's scrutiny.

• Align to function first, then to fixturing, only then to convenience
• Keep the same alignment across drawing, fixture, CMM and gauge
• Document iterative or best-fit alignments with rationale - never in silence
• Re-check alignment discipline whenever a customer disputes a result

Scanning technique - dense doesn't mean better

Point-cloud density is not the same as measurement quality. Over-dense scans amplify surface noise, blow out storage and slow reporting. Under-dense scans miss form features. The right density is driven by the smallest form feature you need to resolve - typically 5-10 points across the feature, matched to sensor resolution and stand-off.

• Density based on feature size, not sensor maximum
• Filter and decimate consciously, with documented settings
• Watch stand-off - accuracy degrades at the edge of the working range
• Use reference targets or dynamic referencing on parts larger than a single field

Environment and thermal control

The best CMM in the country will disappoint in the wrong environment. Best-practice inspection labs log temperature at the machine (not the wall), soak parts before measurement, and treat draughts, direct sunlight and neighbouring machinery as first-class engineering problems. Shop-floor CMMs with structural thermal compensation lower the bar - but do not remove it.

• 20 C +/- 1 C for sub-5 micron work; +/- 2 C acceptable for general precision
• Soak parts for 30-60 minutes per 10 mm of thickness before critical measurement
• Log temperature at the point of measurement, on the calibration certificate
• Watch for hidden heat sources: PCs, lights, operators, adjacent machines

Reporting that means something

A report should answer a specific question: does this part ship, is this process capable, is this supplier improving. Best-practice reports lead with the conformance decision, include the uncertainty and decision rule (ISO 14253), show trend where it matters, and keep the raw data traceable for audit. Report templates should be standardised across the business so results from different machines are comparable.

• Lead with conformance, not raw data
• State uncertainty and decision rule (guardbanding per ISO 14253)
• Standardise templates across CMMs, arms and scanners
• Keep raw data retrievable for at least the customer's retention requirement

How we help

We run practical, on-site training on your equipment and your parts, audit existing inspection processes and help teams standardise fixturing, probing and reporting across sites. Most engagements start with a walk-round of the inspection area and a review of the parts causing the most pain.