Coordinate Measuring Machines (CMMs)
Bridge, gantry and horizontal-arm CMMs remain the reference standard for dimensional inspection of machined and fabricated parts. Independent, vendor-neutral guidance for UK manufacturers - free for the end user.
How it works
A CMM measures a part by moving a probe through a calibrated Cartesian volume (X, Y, Z) and recording the coordinate of each point of contact or optical hit. Points are combined into features (planes, cylinders, cones) which are compared against a nominal CAD model or drawing using GD&T. Probing is typically tactile - either touch-trigger (TTP) or scanning (analogue) - with optional laser-line or vision sensors on multi-sensor machines.
Typical accuracy
Length-measurement error (MPE_E) per ISO 10360-2 for a modern bridge CMM is typically 1.5 to 3.0 microns + L/300 to L/400. Shop-floor CMMs run in the 3-5 micron + L/250 range. Horizontal-arm CMMs used for body-in-white sit in the 30-60 micron range over multi-metre volumes.
Applications
- First-article inspection (FAIR / PPAP / AS9102)
- In-process and final dimensional control
- GD&T verification of machined and moulded parts
- Tool, fixture and gauge proving
- Reverse engineering with scanning heads
- Batch capability studies (Cp/Cpk)
Strengths
- • Highest tactile accuracy of any general-purpose technology
- • Fully traceable to ISO 10360 with UKAS calibration
- • Mature CAD-based programming and GD&T evaluation
- • Scales from shop-floor to CMM room to gantry
Limitations
- • Sensitive to temperature - typically specified at 20 degrees C
- • Tactile probing is slower than optical for high-density capture
- • Fixturing and stylus configuration drive real-world uncertainty
- • Capital cost and lead times higher than portable alternatives
Bridge, gantry, horizontal-arm and shop-floor CMMs
Bridge CMMs dominate general-purpose inspection up to roughly 2 x 3 x 1.5 metres. Gantry CMMs extend that envelope to 4-6 metres in each axis for large castings, moulds and aerospace tooling. Horizontal-arm CMMs are the traditional body-in-white choice, with dual-arm configurations measuring both sides of a car body simultaneously. Shop-floor CMMs (Zeiss DuraMax, Aberlink Xtreme, LK Altera S) trade a small amount of accuracy for thermal and vibration robustness so they can live next to machining centres.
Tactile vs scanning vs multi-sensor
Touch-trigger probes (TTP) collect discrete points quickly and remain the workhorse for prismatic features. Analogue scanning heads (Zeiss VAST, Renishaw REVO, LK Innovator) collect thousands of points along a controlled path, making them the right choice for form, freeform surfaces and gears. Multi-sensor CMMs add vision, laser-line or chromatic white-light sensors on the same coordinate system - useful for parts that mix tight machined features with delicate or hard-to-touch geometry.
GD&T, alignment and reporting
The value of a CMM is only as good as the alignment and GD&T interpretation behind it. Datum reference frames must reflect function, not just probing convenience, and reports should support conformance decisions rather than dump raw numbers. Modern software supports ISO 1101, ASME Y14.5 and customer-specific ballooned reports (PPAP, AS9102, VDA) directly from CAD.
Buying considerations
- Measuring volume matched to part envelope plus fixturing
- MPE_E and MPE_P specified against ISO 10360-2/-5
- Probing strategy: touch-trigger vs scanning vs multi-sensor
- Environment: temperature control, vibration and access
- Software: PC-DMIS, Calypso, MODUS, Metrolog or CAM2 - trained operators cost more than the software licence
- Service, calibration and application support in the UK
FAQs
Bridge, gantry or shop-floor - which do we need?+
It depends on part size, tolerance, throughput and environment. A 1.5 micron bridge CMM is wasted next to a hot machining cell; a shop-floor CMM is wasted in a temperature-controlled lab. We'll match the class to the parts and workload before recommending a supplier.
Do we need a temperature-controlled room?+
For sub-5 micron work, yes. For general 10-25 micron tolerances, a shop-floor CMM with structural thermal compensation is often sufficient and much cheaper to house.
How long does a CMM last?+
Mechanically, 15-25 years is normal with routine service. Controllers and PCs typically need refresh sooner. Retrofits and controller upgrades can extend life significantly.