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The effect of temperature for Coordinate Measuring Machine

Physical phenomena of thermal expansion and contraction illustrate the effects of temperature on the object. However, the coefficients of thermal expansion (CTE) are different with various materials and the amount of length variations per degree C in temperature are also various with materials. For example, the CTE of steel is about 11.5 x 10-6/℃ and the length variation is 0.0115mm per degree C in temperature. However, the CTE of aluminum is about 23 x 10-6/℃ and the length variation is 0.023mm for 1 meter.

The other effect is from the coefficient of thermal conductivity. The definition is that the amount of energy per second (W) can be transferred in units of W / mK. Where "W" refers to the thermal power units, "m" is the length in meters, and the "K" is the absolute temperature units. The greater the value, the better the thermal conductivity; thermal conductivity of steel is about 80, while aluminum is about 237. With the same shape and volume, the overall change in the steel smaller than aluminum where these two materials increased from 20 to 21 degree in temperature.

 

The Principle of Temperature effects to CMM

The Coordinate Measuring Machine (CMM) is constructed from a variety of different materials. Generally the normal operating temperature is 20 ℃ ± 2 ℃. The reference length(optical scale) is corrected and compensated at 20 ℃, while the CMM is also adjusted and assembled under relatively stable condition in temperature. When the temperature is far away from this, it will affect the precision and operation of the measurement.

For reducing the effects of measuring instrument with environmental variations in temperature, stepped temperature is introduced. 0.5 ℃ / hr is representative of the temperature variation per hour less than 0.5 ℃, 1 ℃ / 8hrs every 8 hours representatives of temperature variation less than 1 ℃, and 0.5 ℃ / m variation representative of per meter height less than 0.5 ℃ temperature variation.

 

Use CMM under Constant Temperature

Since the CMM requires almost a constant temperature in the environment, the measuring room must be equipped with the air conditioner. With the scientific and technological progress, computational power of the commercial frequency conversion air-conditioning in the room is chose and the most appropriate condition that the temperature is able to controlled at 20 ℃ ± 1 ℃.

  • Since the cold air flow is downward and warmer air is upward, therefore, the installing of the fan circulating makes the temperature uniformly distributed throughout the room if necessary.
  • The air conditioning vent is not 20 ℃ exactly in temperature, so definitely avoid blowing directly into the CMM.
  • The measuring room has better with insulation measures, such as the transition zone of entrances and exits, the use of double-glazed windows, avoiding direct sunlight (if necessary, install the curtains).

The rule of some users to measure the room air conditioning is: open the air conditioner when using CMM, and turn off it when you have finished it. Such a behavior would make the indoor air temperature to reach operating temperature in a shorter time. But the CMM is manufactured from various materials; the temperature variation of each part is not the same. The operating temperature reached and the time line is not the same, the result is a temperature CMM changing continuously and so that CMM is under an unstable state. Generally small CMM takes about 8-12 hours, medium-sized CMM takes about 12 to 16 hours, and a large CMM requires 24 hours to stabilize in precision. Such usage would seriously affect the efficiency of the CMM, particularly it is difficult to ensure precision in summer or winter, for the stability of the machine can also cause adverse effects.