Why CMM selection matters
Buying the wrong CMM is an expensive mistake. Too small and you can't measure your largest parts. Too large and thermal drift and vibration become problems. Too fast and accuracy suffers. Too slow and it becomes a bottleneck.
The right CMM is the one that fits your part envelope, your tolerance band, your throughput requirement and your environment — not the one with the most impressive brochure.
Step 1: Define your part envelope
Start with the largest part you will ever need to measure — not your typical part, your worst case. Add 20% clearance on each axis for probe access and fixturing. That gives you your minimum working volume.
Most bridge CMMs are available in a range of sizes. A 700 × 1000 × 600 mm machine is a common starting point for automotive and aerospace components. If you're measuring large structural assemblies, you may need a gantry or horizontal arm configuration.
Step 2: Understand your tolerance requirements
The general rule of thumb is that your CMM should be capable of measuring to one-tenth of your tightest tolerance. If your tightest feature is ±0.05 mm, you need a machine with a volumetric accuracy of ±0.005 mm or better.
Check the manufacturer's specification carefully. Volumetric accuracy (MPE_E) is the figure that matters for most applications — not the individual axis repeatability figure, which is always more flattering.
Step 3: Consider your environment
CMMs are sensitive to temperature. Most are specified at 20°C ±2°C. If your shop floor fluctuates more than that, you have three options:
- Install the CMM in a temperature-controlled room
- Choose a machine with a wider temperature compensation range
- Accept reduced accuracy and factor that into your measurement uncertainty budget
Vibration is also a concern near presses, grinders or heavy machining centres. Anti-vibration mounts are available but add cost and height.
Step 4: Choose the right configuration
Bridge CMMs are the most common configuration. They offer a good balance of accuracy, speed and cost for parts up to around 2 metres. Most are available in manual, motorised and CNC variants.
Gantry CMMs are used for large parts — aerospace structures, automotive body panels, large castings. The part sits on the floor or a low table and the bridge travels over it.
Horizontal arm CMMs are common in automotive body shops. Two arms on a common rail can measure both sides of a body simultaneously. Less accurate than bridge CMMs but fast and flexible for sheet metal work.
Portable arms are not CMMs in the traditional sense but are worth considering if you need to measure parts that can't come to the CMM, or if you need to measure on the machine tool.
Step 5: Probe system selection
The probe system is often more important than the machine itself. A Renishaw REVO or PH20 system can dramatically increase throughput on complex parts. A simple TP20 touch-trigger probe is fine for straightforward prismatic parts.
Consider:
- Number of probe positions required
- Whether you need scanning (analogue) or touch-trigger capability
- Stylus reach and access requirements
- Automatic stylus changing
Step 6: Software
Most CMMs are supplied with the manufacturer's own software (PC-DMIS, Calypso, Rational DMIS). Make sure the software can output to your required format — whether that's a PDF report, SPC data to your MES, or a specific customer format.
If you're measuring to GD&T, check that the software handles your specific callouts correctly. Composite tolerances and simultaneous requirements can catch out less capable packages.
Summary
The best CMM for your application is the one that comfortably fits your largest part, achieves one-tenth of your tightest tolerance, suits your environment, matches your throughput requirement, and integrates with your quality system.
If you're unsure, speak to an engineer who has specified and used CMMs in production — not just someone who sells them.