ASME B89.4.21.1:2020 pdf free download

ASME B89.4.21.1:2020 pdf free download.Environmental Effects on Coordinate Measuring Machine Measurements.
lf a sinusoidal environmental temperature variation is applied to a simple body, its length variation is also sinusoidal.For materials with relatively high diffusivities,e.g., metals, at frequencies well below the reciprocal of the time constant,the length will track the temperature according to eq.(3-1).However, as the frequency increases, the length response isattenuated as the body’s temperature is unable to keep up with the environmental variation.This also leads to anincreasing phase shift between the environmental temperature and the body’s length.A plot of a simple body’s responseto an external sinusoidal temperature variation is shown in Figure 3.5.2-2.The time constant is 1 h (3600 s).
NOTE: 1 cycle/min = 0.0167 Hz (16.7 mHz); 1 cycle/h = 0.00028 Hz. (0.28 mHz).
Asimple body’s thermal response does not have a resonant response like a spring-mass system.That is, the change inlength will never exceed the steady-state change predicted by eq. (3-1) for the amplitude of temperature change.However, systems with more than a single thermal element and systems with significant gradients do show afrequency-dependent maximum in their thermal responses that may exceed their steady-state responses. Byanalogy with the spring-mass resonance,this phenomenon is called “thermal resonance”whether or not the responseexceeds 1.0.
Figure 3.2-1 illustrates differential expansion between the workpiece and the measuring scale.In a steady temperatureenvironment at 20C.+AT, the differential expansion can be calculated as described in para.3.2.However, when both aresubjected to fluctuating air temperature, the workpiece and scale will respond according to their individual thermal timeconstants and their heat transfer situations. In general, they will have different length changes and different phase lags inresponse to the fluctuating temperature.
Consider a simple example with a scale attached to an axis beam having a time constant of 1 h (3600 s)being comparedto a thin workpiece with a time constant of 2 min (120 s).The workpiece will have a fast response relative to the scale/beam assembly. The differential response even when they have the same CTE is shown in Figure 3.5.2-3.
The case where thermal resonance may occur deals with a multielement structure such as a CMM and the effects ofthermal gradients on the clements.Each mechanical component of a CMM in the structural loop from workpiece to sensorhas its own unique response to temperature change. Because heat transfer boundary conditions are often not symmetricsurrounding these components, temperature gradients occur within and among the structural components, causing thestructure to bend.Therefore,displacements occur that are tangential, or normal to the direction of the elements’ simple,linear thermal expansion.The magnitudes of these displacements are complicated to predict as they depend on themagnitude and frequency of the temperature fluctuations and the thermal response characteristics of each component.When multiple components are connected.