Understanding Coordinate Measuring Machines (CMMs)

The world on the micron level

What is a CMM

Historical development and evolution of CMM technology

How CMMs work and the main components

The granite table

The air bearings

The measuring head

Towards a connected future

The world on the micron level

When you get down to the micron level, the world becomes a very strange place indeed. A micron is one 1000^th of a millimetre. Split a human hair 90 times, and you’ll be approaching the micron level.

Objects that appear smooth to the naked eye, reveal themselves as jagged, mountainous landscapes. Tiny changes in temperature cause objects to shrink and grow. Liquids behave strangely. Water becomes sticky because of surface tension. The air itself is thick with particles. Dangerous creatures live on every surface. The e-coli bacteria measures 1 micron across.

Modern metrology devices provide a window into this bizarre world. A general purpose, small to medium sized CMM is reliable down to 2 or 3 microns per metre. A high specification CMM will go many times smaller, down to tenths of a micron. At that level, the world becomes an unrecognisable wilderness.

What is a CMM?

A CMM is a Coordinate Measuring Machine and they have been a key tool in metrology and measurement for many years. We use them for measuring shapes, sizes, positions and orientations of an object. CMMs use a system of coordinates on an X, Y and Z axis to locate an object in space. Mounted on each axis of the CMM are microscopic graduations. As the measuring head moves, a reader mounted on the CMM counts the lines which allows for precise location tracking.

The most common type of CMM is a bridge CMM where you place the object you want to measure on a granite base with a bridge gliding over the top carrying a measuring head.

Other types of CMM include the very large gantry CMM. These still have a moving bridge with a measuring head underneath but are scaled up for measuring much bigger objects, up to 20 metres or more. These very large CMMs are often used in the aerospace or shipbuilding industries. Another variety of CMM is the shopfloor CMM, specifically designed for use in harsh environments while maintaining a good level of accuracy.

Historical development and evolution of CMM technology

The first CMM is generally attributed to the industrial machine tool manufacturer Ferranti in 1959. The first of these CMMs was sold a year later in 1960 to the Western Electric plant in Winston-Salem, North Carolina, USA. The company reported a reduction in its measurement times from 20 minutes down to just one minute.

Five years later in 1965, an Italian company called Digital Electronic Automation (DEA), which is now part of Hexagon, sells the first gantry style CMM.

The first computer interface for a CMM came in 1967 developed by Data Corporation on behalf of Sheffield Corp. The computer in question had just 6 kilobytes of memory and no storage capability. Three years later in 1970, Sheffield Corp goes on to provide the first commercially viable computer controlled CMM with automated inspection and servo driven X, Y, and Z axis.

In 1972, the dynamic touch trigger probe was invented, and the following year was the first continuous contact scanning probe.

In 2001, Hexagon acquires Brown and Sharpe, PC-DMIS, Leica Geosystems, Leitz, Optiv, Quindos, Romer, and TESA Technology (and others) to become the world’s largest supplier of metrology solutions.

How CMMs work and the main components

A coordinate is a location in space, expressed in a set of numbers. To be more specific, we’re talking about the “Cartesian” coordinate system – named after the French philosopher René Descartes.

In the Cartesian system, the space is divided into a grid. We can locate any point on that grid by giving two numbers, the X axis (horizontal) and the Y axis (vertical). Add in a third axis (Z -vertical) and we have a 3-dimensional coordinate system. Each of these axes relates to a movement of the CMM which allows for accurate and precise measurements. If you’re looking to purchase a CMM, there’s a lot to consider.

When you look at a standard bridge type CMM, one of the first things you notice is the large granite slab. This is a vital part of a CMM and the reason why is to do with temperature.

The granite table

When you’re measuring objects on the micron level small changes in temperature can have a large effect on the size of an object. Different materials shrink and grow at different rates. This is the Coefficient of Thermal Expansion (CTE). The ambient temperature will effect the object you want to measure, but it will also affect the components of the CMM itself.

A CMM that’s changing temperature is just random number generator. That’s why it’s critical to maintain a CMM at a constant temperature and to make sure to calibrate the machine at the same temperature.

We typically use granite for the CMM table because it is the material that changes the least with changing temperatures. It has a Coefficient of Thermal Expansion (CTE) of 7.9. For comparison, steel is at 13, aluminium 23, and most plastics have a CTE into the 100s.

The air bearings

The next thing people often comment on is the smoothness of the movements, the way the bridge and measuring head seemingly glide into various positions.

Part of the magic of a CMM comes from the fact that there is no direct contact between most of the moving parts of the machine. Instead, air bearings provide frictionless supports in the form of a thin layer of compressed air between those moving parts and their guides. It creates an air cushion which allows for the characteristically smooth motion.

The air bearing technology makes a CMM quiet, fast, and free of vibrations. With oil based systems, for example, there is still a residual friction. Surfaces would still come into contact and wear would occur over time. Friction generates heat which causes thermal expansion.

While there are many benefits, there are significant challenges too. The design requires a constant supply of very clean compressed air. If any oil were to get into a CMM, it would quickly cause serious problems. It would clog and degrade the airlines, causing loss of accuracy and eventually immobilising the CMM entirely. If oil were to get inside the probe head itself, it would result in a very expensive failure.

The measuring head

The versatility of CMM comes in a large part from the technology incorporated into the measuring head itself. This is the component that holds and manoeuvres the probe or sensor, to measure the dimensions of the workpiece (the object you want to measured).

A flexible indexing wrist allows engineers to tilt probes at various angles required for reaching and inspecting unusual shapes and difficult to reach features. It enables the machine to access and accurately measure complex shapes without having to reposition the workpiece.

Sometimes a CMM operator will need to measure an object where it’s preferable not to make physical contact, for example, if the work piece consists of very delicate, or easily deformed features. In this case, we can use an optical probe with a high-resolution camera or a laser scanner. Laser scanners send a laser line or point onto the workpiece. From there, sensors capture detailed 3D data by detecting the reflected light. They are perfect for capturing complex or freeform surfaces, performing high-speed measurements, and creating detailed digital models of objects.

Towards a connected future

CMMs play a crucial role in modern manufacturing and quality assurance, providing essential data that supports innovation and maintains high standards across industries. As technology advances, so the variety and quantity of data that we collect also increases.

One of the key trends we’re seeing is the increasing level and requirement for connectivity, data sharing and cloud systems. Connected processes where automation is increasingly integrated with real-time data sharing across the entire manufacturing value chain are bringing in new forms of manufacturing and with it, new ways of performing inspection tasks.

We are looking towards a connected future and the CMM is very much part of that connected future. The MAESTRO system is Hexagon’s answer to the need for smart metrology systems, and it all begins with a next generation CMM designed to be infinitely upgradeable. Because it’s impossible to predict the future, the next best thing is to design with that in mind and keep systems open and flexible. The MAESTRO system is built to adapt, scaling up with the needs of the organisation.

With smart manufacturing technologies, the future of the CMM is very much part of industry 4.0, and MAESTRO is a case in point, the first fully digital CMM providing manufacturers with more than a snapshot of the world at the micron level, but a real-time window assessable from anywhere at anytime.