CMM machines are a vital part of the modern manufacturing process across many industries; from aerospace, automotive and motorsport through to energy and nuclear. However, one important component is often forgotten, a component that can significantly increase the CMMs capability and accuracy while reducing inspection times – the rotary table.
Bath-based RPI UK has been designing and manufacturing precision rotary tables since the 1950s and estimates that 80% of all CMMs in the UK using a rotary table rely on one of their devices. This includes machines manufactured by leading CMM manufacturers such as Hexagon, Nikon and Mitutoyo; and are used by leading aerospace companies such as Rolls Royce and GE.
The RPI rotary tables have precision geometry with axial performance well below 0.0005mm and positional accuracies to +/- 0.5 arc-seconds (+/-0.00014°). These features complement the CMM machine enabling it to achieve even greater precision.
RPI produce a range of rotary tables for various applications, including: full positioning and indexing tables; manual or NC controlled with mechanical and air bearings or a combination of both. They range in diameter from 100mm to 1200mm, though much larger bespoke tables are available.
Their high geometric performance is ideally suited for assembly, test, inspection and metrology applications. RPI’s tables can handle loads up to 14 tonnes which enables fine machining and precision testing of heavy components such as aircraft engine casings.
Rotary tables enable very precise measurement of many geometrical characteristics, including angle, roundness, concentricity, parallelism, flatness and runout. They are used with: probes such as plunger, transducer, and capacitance; optical such as autocollimator, laser interferometer, and diffractometer; and electrical – oscilloscope, volt meter.
RPI’s air bearing spin tables are becoming more popular and rely on air lubricated hydrostatic bearings which provide very high radial and axial stiffness. This type of bearing offers significant advantages over conventional rolling element bearings permitting operation with minimal drag, vibration and mechanical noise.
The design principle of these bearings ensure axial laminar flow between shaft and housing compared with conventional orifice entry arrangements thereby ensuring exceptionally good motion geometry of less than 0.0001mm for axial and radial runout.
These tables can be combined with direct drive motors and high accuracy angular encoders. The encoder couples directly to the motor control system, which has its own high-resolution interpolator ensuring precise positioning and repeatability.
Some CMM applications require the component being measured to be held with its axis horizontal. RPI has a specific table for these applications which combines preloaded axial-radial rolling bearings for high rigidity in both planes with self-locking worm gearing and a high accuracy angular encoder ensuring sensitivity and fine positioning.
It is possible to fit a table with a high accuracy Hirth Coupling, which provides location and angular positioning of the table-top. This delivers fast, accurate indexing between pre-set positions.
Rotary tables are so accurate that they can be forgotten, when using a CMM. And that’s no bad thing for manufacturers who need precision time and time again.