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. 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.
Good damping properties – Granite has optimal damping properties and thus can keep vibrations to a minimum.
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 aluminum.
Wear-free – Granite can be prepared that a nearly level, pore-free surface arises. This is the perfect base for air bearing guides. A technology which guarantees the wearfree operation of the measuring system.
Manual labor as predicate
So that the qualities of granite fully applies when operating a 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. Illustration 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 (Illustration 3). Only when the defined limit values are followed and the smooth, wear-free operation can be guaranteed, the granite component can be installed.
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 behavior shows clear benefits to the use of molding, steel and aluminum. A 1 meter long aluminum 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. Furthermore, granite is corrosion-free and shows no magnetism.
Some final thoughts about granite in metrology
On moving parts, excess material is removed, also granite specific weight is low equal to aluminum. A CMM structure that is precise before error map tuning with low pitch roll and yaw errors the helps eliminate temperature effect complications. Highly error mapped machines with high coefficient materials are more difficult to compensate. Aluminum has a 15% variability in coefficient of expansion between material batches.