Granite as Foundation for High Accuracy Measurement

The use of granite in 3D coordinate metrology has already proven itself for many years. No other material fits with its´ natural properties as well as granite to the requirements of metrology. The requirements of measuring systems regarding temperature stability and durability are high. They have to be used in a production-related environment and be robust. Long-term downtimes caused by maintenance and repair would significantly impair production. For that reason, WENZEL uses granite for all important components of measuring machines. (Image 1)

Image 1: The WENZEL LH Gantry CMM showcases a monolithic design with an integral granite table negating the need for a special foundation. (Source: WENZEL)

For many years now, manufacturers of coordinate measuring machines trust in the quality of granite.  It is the ideal material for all components of industrial metrology which demand high precision. The following properties demonstrate the advantages of granite:

 

• High long-term stability – Thanks to the development process which lasts many thousand years, granite is free of internal material tensions and thus extremely durable.

• High temperature stability – Granite has a low thermal expansion coefficient. This describes the thermal expansion at the temperature changing and is only half that of steel and only a quarter of aluminium.

• Good damping properties – Granite has optimal damping properties and thus can keep vibrations to a minimum.

• Wear-free – Granite can be prepared that a nearly level, pore-free surface arises. This is the perfect base for air bearing guides and a technology which guarantees the wear-free operation of the measuring system.

 

Based on the above, the base plate, rails, beams and sleeve of the WENZEL measuring machines are also made of granite. Because they are made of the same material a homogeneous thermal behaviour is provided.

 

Manual labour as predicate

So that the qualities of granite fully applies when operating a coordinate measuring machine, the processing of the granite components must be carried out with the highest precision. Accuracy, diligence and particularly experience are imperative for the ideal processing of the single components. WENZEL carries out all processing steps itself. The final processing step is the hand lapping of the granite. The evenness of the lapped granite is checked minutely. Image 2 shows the inspection of the granite with a digital inclinometer. The flatness of the surface can be determined sub-µm-precisely and be displayed as tilt model graphic (Image 3). Only when the defined limit values are followed and the smooth, wear-free operation can be guaranteed, the granite component can be installed.

Image 2: Using a digital inclinometer the flatness measurement of the granite elements is performed to micron accuracy.

Image 3: An inclination report generated with a digital inclinometer.

Measuring systems have to be robust

In today’s production processes the measuring objects have to be brought as fast and uncomplicated as possible to the measuring systems, irrespective of whether the measuring object is a large/heavy component or a small part. It is therefore of great importance that the measuring machine can be installed close to production. The usage of granite components supports this installation site as its uniform thermal behaviour shows clear benefits to the use of moulding, steel and aluminium. A 1 meter long aluminium component expands by 23 µm, when temperature changes by 1°C. A granite component with the same mass however expands itself for only 6 µm. For additional safety in the operational process bellow covers protect machine components from oil and dust.

 

Precision and durability

Reliability is a decisive criterion for metrological systems. The usage of granite in the machine construction guarantees that the measuring system is long-term stable and precise. As granite is a material which has to grow for thousands of years, it does not have any internal tensions and thus the long-term stability of the machine base and its geometry can be ensured. So granite is the foundation for high accuracy measurement and WENZEL takes this statement very seriously and purchased a granite processing business in Germany in 2006.

 

WENZEL’s Granite Processing Plant

In 1885 in the German village of Groß-Bieberau, a stone cutter began a business making tools, utensils, and decorative pieces from the stone of local quarries. He likely never imagined that 130 years later his family would still be cutting stone on the same spot, though for drastically different purposes. Today, WENZEL Steintechnik stands on the same ground, run by the same family as it has since 1885, making the granite bases and components for WENZEL CMMs.

 

A division of Wenzel since 2006, Steintechnik evolved over the years from simple hand carved items, to designing and fabricating tombstones, evolving to the production of granite for industrial applications, and along the way became a critical partner for WENZEL. WENZEL Steintechnik is responsible for bringing the raw material in from the quarry, and processing it to rough size for our factory.

 

Work begins typically with a 35 ton block of raw material which is sawn into workable sizes for either machine tables, or components such as X beams. These smaller blocks are then moved to other machines for finishing to their final sizes. Working with such massive pieces, while also trying to maintain high precision and quality, is a balance of brute force and a delicate touch that requires a level of skill and passion to master.

 

Recently WENZEL has invested in a new 5 Axis vertical mill, to streamline the manufacturing process, and reduce the handling of the stone (Image 4). This massive gantry mill allows combining cutting, grinding, and milling operations into one, while also improving the cycle time of the processes themselves, which led to a 60% reduction in overall processing time.

Image 4: 5 Axis vertical mill, to streamline the manufacturing process, and reduce the handling of the granite.

With a working volume that can handle up to 6 large machine bases, WENZEL Steintechnik now has the capability for lights out production of granite, 24/7. Improvements like these allow reduced delivery times to the end customer, and also increase the flexibility of our production schedule to react faster to changing demands.

 

Another more subtle change in the manufacturing process happened some years ago, when the Steintechnik team began serializing all the components they manufacture. This allows them complete traceability form the time a block of raw granite leaves the quarry, until the pieces are assembled into a CMM.

 

Should problems arise with a certain component, all other components which could be affected can be easily contained and verified for their quality, ensuring that no quality defects escape the facility. This may be something for granted in high volume production like Automotive and Aerospace, but it is unprecedented in the world of granite manufacturing.

 

About WENZEL Group

The WENZEL Group is a leader in Innovative Metrology. The comprehensive product portfolio from WENZEL includes unique solutions in 3D Metrology, Computed Tomography, Optical High Speed Scanning. WENZEL technology solutions are installed throughout many industries such as Automotive, Aerospace, Power Generation and Medical. WENZEL has an installed base of more than 10,000 machines worldwide. Subsidiaries and agents in more than 50 countries take care of sales and provide after sales service support to our customers.