Improving the Accuracy of CMMs by 20%

25.04.17 - When you have been building accurate measurement devices and CMMs (Coordinate Measuring Machines) as long as WENZEL (almost half a century), it gets harder and harder to find accuracy improvements.

But not only are customers demanding lower specifications and uncertainties of measurement, but WENZEL also push themselves to stay in front of the competition.

Earlier this year, WENZEL set themselves a target of a 20% improvement in accuracy, but also decided to report CMM specifications to the ISO10360-2:2010 standard. The latest generation of LH CMMs would therefore have to have improvements of greater than 20% as the latest ISO 10360 standard requires that the stated accuracy takes account of and includes the uncertainty of equipment used to determine the CMM’s uncertainty.

Let us imagine a CMM with an accuracy of 1.5 microns and a targeted new accuracy of 1.2 microns. How on earth would you go about making that large structure 0.3 microns more accurate? Make 10 improvements that each in themselves contribute a 0.03 micron improvement, that’s how!

It’s very easy to throw around these numbers but what do we really mean when we talk about microns and fractions of microns.

A micron is millionth of a metre or one thousandth of one millimetre. Here are some things considered to be ‘small’ and their approximate size in microns;

• The diameter of human hair – 40 to 80 microns
• The diameter of a water droplet in fog – 10 microns
• The diameter of a human red blood cell – 5 microns
• The diameter of a particle of wood smoke – 1 micron

So, the engineers at WENZEL are trying to find accuracy improvements 1/50th of the size of a particle of smoke!

Of course, WENZEL don’t give all the details of the improvements made – that would be selling the family silver, but they include improvements and changes to components, materials, assembly processes and calibration methods.

Here’s a few of the improvements to the LH CMM to whet your appetite;

• Larger air bearings to improve stiffness and reduce microscopic vibrations
• A slightly less porous granite for the X and Z axes
• Flatness and Parallelism manufacturing tolerances halved on some components
• Moving the X scale under a cover to reduce temperature effects
• Increasing the bend radius in the cable drag chains to reduce ‘pull’ on the axis
There are others too, and each improvement was rigorously tested to make sure there were repeatable gains in accuracy.

WENZEL has always been a company that made attention to detail their daily work, but nowadays these details are getting smaller and smaller. It’s this kind of focus that makes a WENZEL CMM have the finest mechanical construction in the world. A bit of software compensation can be added to this structure to make it even a little more accurate, but doesn’t it give you a large comfort feeling that underneath the covers the WENZEL mechanical design and construction is by far the best in its class?