Bridge-Type Machines with Air Bearings

For measuring ranges exceeding roughly 400 mm and where higher accuracy requirements exist, it generally proves economical to use bridge-type machines equipped with air bearings (Fig. 31).

Fig. 31: Bridge-type machine with moving table and fixed bridge (Werth VideoCheck® 400×400×400).

Through appropriate design measures, a constant prestress of the entire guideway system can be attained similar to the machines described above.

A fixed-bridge construction is optimally suited to machines with transmitted-light illumination. The rigidity of the entire system enables excellent measuring uncertainty values. This is supported by manufacturing the bridge in a single piece with close tolerances. No subsequent adjustment of the guideway components is required.

During measuring operations, the workpiece is placed on the glass plate or in a device fastened to a measuring table which can be moved in the Y axis. The probe travels in the other two coordinate axes. A Werth Zoom lens system is standard equipment. In conjunction with the MultiRing® reflected-light technology, this enables great flexibility. The optical image distortion can be further reduced by using a telecentric lens system instead of the zoom lens system (see p. 18). Other sensors (for example, lasers, fiber probes and touch trigger or measuring mechanical probing systems) can also be added as options. If a coordinate measuring machine is equipped with multiple sensors, it should be noted when performing complex applications that this reduces the useful measuring range by the distance between the sensors. Under certain circumstances, it is therefore advisable to select a larger basic machine frame.

If larger objects are measured with different sensors, collision problems may occur – especially in the Z axis. The sensors not being used for the current measurement obstruct free positioning on and near the workpiece. This problem can be solved by using a second ram (see illustration on page 1). While one axis is used for tactile sensors, the second one remains reserved for optical sensors. The two sensor classes can thus be used to probe workpieces independently without any unnecessary risk of collision. This arrangement is also suitable for probe changer racks and motorized, articulating probe mounting devices like the ones used to measure tools and complex part shapes with the fiber probe. Measuring uncertainties of less than 1 µm can be achieved with this machine class. However, when performing measurements it should be noted that not all sensors have the same probing uncertainty. Higher measuring uncertainties may therefore occur.

The largest standard measuring range is 2 m × 2 m × 0.8 m. Machines featuring a larger measuring range are usually designed with a moving bridge (Fig. 32).

Fig. 32: Bridge-type machine with moving bridge (Werth ScopeCheck® MB).