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 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.
Granite’s Favourable Characteristics
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 so 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.
Manual Labour As Predicate
Granite inspection is carried out with a digital inclinometer.
Based on the favourable characteristics above, the base plate, rails, beams and sleeve of Wenzel measuring machines are also made of granite. And because they are made of the same material, a homogeneous thermal behaviour is provided.
In order for the qualities of granite to apply fully when operating a coordinate measuring machine, precision is imperative when processing of the granite components are carried out. The final processing step is the hand lapping of the granite. The evenness of the lapped granite is inspected minutely with a digital inclinometer.
The flatness of the surface can be determined sub-µm-precisely and be displayed as a tilt model graphic. Only when the defined limit values are followed and the smooth, wear-free operation can be guaranteed, the granite component can be installed.
Robust Measuring Systems
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 and heavy component or a small part.
Surface flatness can be determined sub-µm-precisely
and be displayed as a tilt model graphic.
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 one metre long aluminium component expands by 23 µm when temperature changes by 1 deg C. A granite component with the same mass however expands itself for only six 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 stable and precise in the long term.
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.
With granite being the foundation for high accuracy measurement, Wenzel purchased a granite processing business in Germany in 2006.
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.
Run by the same family since 1885, Wenzel Steintechnik stands on the same ground today, making the granite bases and components for 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. The division is responsible for bringing the raw material in from the quarry, and processing it to rough size for the 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.
Reducing Processing Time
A new 5-axis vertical mill was recently purchased to streamline the manufacturing process, and reduce the handling of the stone. This 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 percent reduction in overall processing time.
With a working volume that can handle up to six large machine bases, the facility now has the capability for lights out production of granite 24 hours a day, seven days a week.
Improvements like these allow reduced delivery times to the end customer, and also increase the flexibility of their production schedule to react faster to changing demands.
Traceability In Granite
Another more subtle change in the manufacturing process happened some years ago, when the Steintechnik team began serialising all the components they manufacture. This allows traceability from 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.
While a commonly seen in high volume production like automotive and aerospace sectors, this traceability is new in granite manufacturing.
APMEN Metrology & Design, Feb 2017