What is a CMM?

A CMM, or Coordinate Measuring Machine, describes a range of mechanical precision measurement solutions. First invented in the late 1950s, CMMS measure parts using XYZ-coordinates, more officially known as the Cartesian coordinate system. 

Key highlights: 

• XYZ coordinates for precision measurements (Cartesian coordinate system)
• Touch probe system for accuracy 
• Robotic or manual, computer-aided for speedy calculations 
• Invented in 1958; touch trigger probe invented in the 1970s; perfected over the decades 

CMMs tend to work with what's called a touch-probe: a mechanical probe that physically touches the part it needs to measure. The machine knows the exact location of the probe at all times and by calculating its coordinates at the time of measurement, it can accurately measure components. The probe looks like a metal stylus with a little colourful ball on the end. Think of it like a robot is using his finger to feel the shape of an object. 

CMMs come in many different shapes and sizes. A popular smaller type is that mounted on the end of a manually-operated robotic arm. This means the actual measuring does not require a pre-programmed scanning routine: it is instead operated by a worker moving the probe manually to the relevant points on the component. Pressing a button on the handle then records measurements when the probe is in touch with the part. This type of CMM is ideal for smaller components and can be easily installed on factory floors. It requires relatively little training and can measure as fast as the worker can move the machine's arm.

Modern versions of these machines may also come with a 3D scanner attached to the probe, which allows the part to be roughly scanned first, allowing the machine to get automatically produce a measurement routine for the operator to follow. This works best by having the arm paired with an electronic turntable, which the computer tracks to always know the position of both the arm and the component. 

It is also possible to have a CMM as a completely automated machine, carrying out all of its measurements without an operator controlling the individual actions. This usually requires in-depth measurement routines to be programmed, although it is possible to automate this part of the process if CAD files for the component are available. The most advanced and modern systems have ways of automatically measuring components, even if there is no CAD file present or the component is completely unknown to the machine. This often involves a machine using a range of techniques beyond just the touch probe, such as 3D scanning, profilometry or photogrammetry. 

A drawback of a conventional CMM machine is that hollow spaces are difficult to reach. In some cases, a solution for this may be a custom made stylus for the probe, allowing to measure beyond the standard range of the machine, or even around curves. Some examples of companies who supply CMMs are Renishaw, who have the credit of inventing the touch-trigger probe in the 1970s, Mitutoyo, Hexagon, FARO and Keyence. However, the technology is fairly ubiquitous now, meaning the list of companies that produce CMMs is practically inexhaustible.