Specify metrology fixtures when selecting the production CMM

By Grace Kim

Verus Metrology Partners builds 24-, 32- and 64-station metrology fixtures and says the coordinate measuring machine (CMM), not the fixture, should perform about 95% of part alignment. The company presented those figures as engineering practice, not published benchmarks, and supplied no independent repeatability, accuracy or cycle-time protocol.

Verus designs and builds fixtures, CMM and vision programs, and automated inspection systems. It focuses on moulded plastics, flexibles, thin-walled parts and polymer components for medical-device, pharmaceutical, personal-care, electronics and aerospace customers.

Production-floor CMMs remain the exception in Verus's sectors; most high-volume moulding and medical-device measurement stays in controlled laboratories. Where machined metal implants move from machining to inspection, plants often carry the heavily invested machining fixture to the CMM, despite spending far less on measurement holding.

What changes when a fixture touches the part?

Michael Nugent, chief operating officer, said: “A Verus fixture holds the part. It does not locate it, and it does not influence its geometry.” A manufacturing fixture holds a component rigidly during machining; a metrology fixture must avoid distorting the geometry being measured.

Andrew Hodson, chief executive officer, said moulded parts warp, respond to temperature and humidity, and vary legitimately within production tolerances. Verus tunes each fixture to let a conforming part float inside its validated process window, without hard clamping, full-form nests or friction fits.

Hard clamping, full-form nests and friction fits apply constraints that can alter measured geometry. Temperature, humidity and hand heat add variation, so fixture-induced shifts can resemble process effects. Letting a conforming part float within its validated process window keeps genuine moulding changes visible.

Can fixturing keep pace with production?

The fixture must match sampling and production rates. One or two loading positions cannot sustain a high-volume moulding line, Hodson said, so Verus builds 24-, 32- and 64-station fixtures, fills a large CMM stage and automates inspection to maximize throughput. Every station must hold identically or repeatability suffers.

Verus contrasts its multi-station work on flexible, plastic and thin-walled medical, pharmaceutical and personal-care parts with the engine blocks and turbine blades common at trade shows.

Design starts with the operator, then removes complexity from loading, unloading and alignment. Nugent places alignment in the measurement routine using Verus's method, not the fixture, consistent with the company's 95% machine-alignment target. Rotating stations give full access for load-once inspection on a CMM or vision system without moving the part to another fixture set; repeated handling can bend thin walls or warm parts.

Hodson set a limit: optimizing routine speed until repeatability deteriorates trades measurement integrity for throughput and pushes errors downstream.

Which strategy fits the part and sample plan?

The selection begins with three numbers:

• Feature tolerances.
• Required gauge repeatability as a percentage of tolerance.
• Sampling rate: 5%, 10% or a fixed number of parts per shift.

Modular fixtures suit R&D, early development and one-off inspection when the design still changes. Low-volume work can still carry tight tolerances and repeatability demands. Once the drawing, GD&T and gauge R&R targets are fixed, Hodson said, use a dedicated fixture with 1, 2, 4, 8 or more stations, selected by required volume and measurement frequency.

“Fixturing isn’t an afterthought. It has to be scoped at the start of a project,” Hodson said. CMM selection must run in parallel, covering size, type, software, probing and optics; Verus also uses 8-, 16- and 32-station fixtures.

Nugent's constraint was a fist-sized part: 32 stations need a large enough machine. He said budgets often arrive too late. Selection should follow production method, volume, mould cavitation and cycle time, which determine inspection load and the choice among vision, CMM and multisensor systems.

At 10 million parts per year, a sampling plan might require 20,000 measurements. The fixture should accompany the mould order; an undersized machine can force a second purchase or production concessions.

What changes on the shop floor?

Temperature, vibration, contamination and operator interaction matter more beside production. One orthopaedic implant CMM environment was hot and cold, dirty and gritty. The same fixture design ran in that client's laboratory and on the floor, but the production version wore faster through grit, debris and constant handling.

Verus therefore designs shop-floor fixtures for wear and contamination from the start and checks them regularly, preventing fixture wear from appearing as measurement variation.

How does closed-loop control change the fixture?

Closed-loop control triggers correction as results trend toward a specification limit. Metal removal supports it more readily; injection-moulded parts can change for 24, 48 or 72 hours after demoulding. Measuring at the press would apply offsets and bias to predict a later result.

Verus's in-house automation programme automates fixture movements and actuation, integrates robots for loading and unloading, and links fixtures to metrology machines. Results enter a central database that informs production while it runs and feed machining offsets, process control, statistical software and cloud storage. A 16-station hand-loaded fixture might become one robot-fed station handling a 100-part tray.

As volume and sampling demand rise, Hodson expects dedicated inspection systems with measurement technology, software and AI built into the fixture. The fixture would take measurements and supply data, replacing a universal platform adapted to whichever machine an OEM sells with a system tailored to a specific part and production environment.

What does ISO/IEC 17025 change for buyers?

Verus operates an ISO/IEC 17025-accredited inspection laboratory whose scope covers program creation and metrology, not calibration of one gauge type. The company says this combination, plus a laboratory with multiple machines and software versions, supports validation and subcontract metrology across client sites.

Multisite buyers need, in Verus's view, a traceable fixture with a validated process and program that works across platforms, sites and continents. Hodson contrasted this with machine shops and machine vendors, which can build fixtures but cannot supply the same metrology chain. Holding flexible, thin-walled and polymer parts is specialist work.

Hodson tied fixture specification to profitability, time through validation and scrap rate. Poor fixturing can scrap acceptable parts and admit defective ones, while later corrections cost more as assembly advances. Can the plant document a validated fixture, alignment routine and gauge R&R before ordering the mould, committing to a CMM and accepting production concessions?