Renishaw RCU10, Installation And User Manual, Page: 18 / 202

System overview 1-7
Expansion coefficients
The RCU10 recognises expansion coefficients as parts per million per degree Celsius or
degree Fahrenheit (the unit of temperature used depends on how the user configures the
system). The reference temperature for material expansion is 20 °C (68 °F).
Table 1.1 below shows example expansion coefficients for aluminium and steel:
Table 1.1 – Expansion coefficients
Material ppm/°C ppm/°F
Aluminium 20 11.11
Steel 10 5.56
Use the following formula to convert from ppm/°C to ppm/°F:
[ppm/°C] multiplied by 5/9 = [ppm/°F]
e.g. 20 ppm/°C x 5/9 = 11.11 ppm/°F
1.3.5 Structure thermal compensation
An additional source of positioning error may be the thermal distortion of the machine
structure. This could manifest in a number of ways including:

expansion of the spindle

expansion of the machine structure
As long as the thermal effect is linear and not related to axis position, the RCU10 can be
used to reduce the error.
To utilise this compensation mode, a material temperature sensor must be placed on the
applicable part of the machine structure and the RCU10 configured with the number of
micrometres of correction required per degree C.