Mastering For CMM Precision

A Coordinate Measuring Machines (CMM) is a flexible measuring device and has developed a number of roles with the manufacturing environment, including use in the traditional quality laboratory, and the more recent role of directly supporting production on the manufacturing floor in harsher environments. The thermal behavior of CMM encoder scales becomes an important consideration between its roles and application.

In a recently published article,by Renishaw, the subject of floating and mastered encoder scale mounting techniques are discussed.

Encoder scales are effectively either thermally independent of their mounting substrate (floating) or thermally dependent on the substrate (mastered). A floating scale expands and contracts according to the thermal characteristics of the scale material, whereas a mastered scale expands and contracts at the same rate as the underlying substrate. The measuring scale mounting techniques offer a variety of benefits for the various measurement applications: the article from Renishaw presents the case where a mastered scale might be preferred solution for laboratory machines.

CMMs are used to capture three-dimensional measurement data on high precision, machined components, such as engine blocks and jet engine blades, as part of a quality control process. There are four basic types of coordinate measuring machine: bridge, cantilever, gantry and horizontal arm. Bridge-type CMMs are the most commonplace. In a CMM bridge design, a Z-axis quill is mounted on a carriage that moves along the bridge. The bridge is driven along two guide-ways in the Y-axis direction. A motor drives one shoulder of the bridge, while the opposite shoulder is traditionally undriven: the bridge structure is typically guided / supported on aerostatic bearings. The carriage (X-axis) and quill (Z-axis) may be driven by a belt, screw or linear motor. CMMs are designed to minimize non repeatable errors as these are difficult to compensate in the controller.

High-performance CMMs comprise a high thermal mass granite bed and a stiff gantry / bridge structure, with a low inertia quill to which is attached a sensor to measure work-piece features. The generated data used to ensure that parts meet predetermined tolerances. High precision linear encoders are installed on the separate X, Y and Z axes which can be many meters long on larger machines.

A typical granite bridge-type CMM operated in an air-conditioned room, with an average temperature of 20 ±2 °C, where the room temperature cycles three times every hour, permits the high-thermal mass granite to maintain a constant average temperature of 20 °C. A floating linear stainless steel encoder installed on each CMM axis would be largely independent of the granite substrate and respond rapidly to changes in air temperature due to its high thermal conductivity and low thermal mass, which is significantly lower than the thermal mass of the granite table. This would leads to a maximum expansion or contraction of the scale over a typical 3m axis of approximately 60 µm. This expansion can produce a substantial measurement error which is difficult to compensate due to it time-varying nature.

Temperature change of CMM granite bed (3) and encoder scale (2) compared with room air temperature (1)

A substrate mastered scale is the preferred choice in this case: a mastered scale would only expand with the coefficient of thermal expansion (CTE) of the granite substrate and would, therefore, exhibit little change in response to small oscillations in air temperature. Longer term changes in temperature must still be considered and these will affect the average temperature of a high-thermal mass substrate. Temperature compensation is straightforward as the controller only needs to compensate for the thermal behavior of the machine without also considering the encoder scale thermal behavior.

In summary, encoder systems with substrate mastered scales are an excellent solution for precision CMMs with low CTE / high thermal mass substrates and other applications requiring high levels of metrology performance. The advantages of mastered scales include simplification of thermal compensation regimes and potential for reduction of non-repeatable measurement errors due to, for instance, air temperature variations in the local machine environment.

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