Guest Post By Bryn Edwards, Vice President, Wenzel America
Before CMMs the only way to dimensionally inspect geometric parts to any kind of accuracy was either a surface plate with height gage and slip blocks or the shop jig borer with a dial indicator.
Having lived through the whole CMM evolution from then until today, my perspective on the industry might be of interest.
A brief, eyewitness history of CMMs
The CMM industry was actually started by a defense company in Scotland called Ferranti.
In about 1961 they invented the moiré fringe measuring scale that made low-friction, high-accuracy measuring scales available for the first time. Engineers at Ferranti actually devised the first CMM as a vehicle to sell the measuring scales themselves.
The device was very simple and effective. It was a small cantilever machine with two low-friction axes and a two axis digital readout. The concept was to use mechanical probes, ball, cylinder, or taper to manually contact the surfaces to be measured and record the positions from the digital readout. This same concept is still used today.
For the first decade after their invention, all CMMs were manually operated and very limited in scope. The only large machine made around that time was by Franco Satorio in Italy, the chief inspector at Fiat. He devised a large gantry machine to measure car body dies using a one-directional electronic indicator. His machines were also manually operated. Satorio left Fiat to form the DEA CMM Company.
Renishaw is born - The 70s brings a whole new player to metrology
In 1972, the whole metrology game changed. While working at Rolls Royce in Bristol UK, David McMurtry (now Sir David), applied his very innovative mind to the limitations of the rigid styli at the time that required manual control and a sensitive operator touch.
The resulting invention was a simple touch probe. While it may have been simple in concept, it was genius in application and changed the entire metrology industry forever.
Shortly after patenting his new touch probe styli, McMurty founded Renishaw Plc. in 1973.
This first Renishaw probe design was basically a stylus assembly mounted kinematically in a housing and spring loaded using three bars in the stylus and six balls embedded in the housing. A small voltage was applied to the contacts to allow the resistance to be measured very sensitively and therefore identify precisely when the contact was broken freezing the related scale positions.
The spring loading allowed the stylus to “over travel” into the part without damage and return to its exact location after contact was removed.
At the time Sir David invented his first touch probe there were many other people looking at probing systems and electronic indicators trying to figure out a solution to the limitations of the CMM styli then being used.
The problem, as they saw it, was to identify the exact contact point with the surface. The Renishaw touch probe just simply ignored this issue and left it to be computed separately. A possible advantage that allowed this out-of-the-box thinking was the computing power that was readily available to Sir David working within Rolls Royce.
So now we had a device that would automatically freeze and record all axes of the CMM on contact. The three axis data could then be used to compute just about any geometry. This also allowed the axes to be motorized and the size and concept of the CMM to be reinvented and ended this first era of manually controlled CMMs.
Motorization changed everything - machines could now be made to any size and controlled by remote joysticks. Contact speeds could be controlled allowing accuracy and repeatability to be much higher independent of the operator, and a full DCC programmable control could be applied.
PH9 touch probe - a true game changer for metrology and CMMs
With the first Renishaw probe the potential for the modern CMM was set on a new path.
Many refinements were to follow including: Driving at vectors normal to compound surfaces, allowing angular faces to be measured with a simple ball stylus radius correction. Following quickly was the utility of an indexing head, Renishaw’s PH9, along with probe changers and thus the foundation of today’s CMMs.
While CMMs may not have doubled in power every two years like was predicted for integrated circuits at the time, they certainly benefited from Moore’s law proving out, with advances in computer systems and resultant software. This was the time that computers went from huge main frames to “mini” computers and finally to PCs, allowing for suitable computing power to be added to the CMM and allowing ever more complex measuring computations.
CMM applications during those years were much more computer intensive than those used in NC machine tools.
In the case of CMMs, computers were used even before the machines were motorized, so it was logical to use the same computer to also drive the machine axes.
From the beginning, CMMs used this method for driving the machine as well as collecting and analyzing data. Doing both simultaneously was a very demanding application and required a robust multi-tasking environment.
After these initial uses and innovations for high speed computing of geometries and machine drivers, additional software was developed for error mapping enabling simplification of mechanical design. Real time interaction with CAD files greatly improved the utility of the CMM.
It would be many years before the machine tool industry went in this direction as well.
Touch Probes - still the most universal metrology sensors
A lot of refinements have been added to metrology probes and sensors over the years including scanning and optical sensors, but even today the most popular CMM is still the touch probe indexing head DCC machine conceived some 35 years ago and enabled by Renishaw.
The reason for this is that the CMM has become the industry workhorse for universal inspection of precision parts. The application refinements have been effective for specific applications, but the universality of the touch probe still meets most requirements.
After 30 years, Renishaw changes the game...again
The CMM design remained conceptually the same for some 30 years until Sir David again invented a new game changer - the Renishaw Revo.
The 5-axis REVO took the basic concept of the touch probe CMM and reinvented it. The concept of CMMs is to freeze axis data on contact for touch probes, or to coordinate the axis data with a scanning probe. This has the effect of limiting the speed at which the axes can move without distorting the CMM frame and the resultant data.
The Revo collects data by rotating the head without having to move the axes of the CMM. This allows it to scan surfaces up to 500mm per second, collecting thousands of data points per second at very high accuracy. In many applications this system can be 10 times faster than a regular touch or scanning probe because its speed is not axis dependent.
What further innovations this newest Renishaw game changer will usher in over the coming years are still to be seen. But, if the past is any indicator, Renishaw will have a hand in whatever comes next.
Applying innovation - the past, present and future of Renishaw
It is this dedication to innovation that has been the foundation for what is now a very large and successful UK company. Today, Renishaw’s products range from CMM probes and sensors, machine tool probes to transducers and laser systems and medical applications. The underlying theme of all their products still focuses on applying innovation to high-precision components for three-dimensional applications. For more information about Renishaw sensors, aftermarket products or fixtures please call us at 248.295.4300. We also guarantee we'll give you the absolute best prices on Renishaw Probes.