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# Several Basic Principles Of Length Measurement (II)

May 08, 2017

2. Elastic Deformation

Overview:

During the measurement, the measured work-piece will produce elastic deformation because of measuring force, contact form, weight, which will cause measurement error.

Generally, elastic deformation are mainly include instrument stents deformation or support deformation; Contact deformation between probe, workbench and the measured parts, etc. Why there is a measuring force of contact type measuring instruments; Why to stipulate contact form; Why to properly select supporting points when measuring elongated piece, etc., These are trying to reduce the elastic deformation, in order to reduce the measuring error caused by the elastic deformation.

（1）Support deformation

When tested pieces horizontal placed, the bending deformation state, supporting way and the fulcrum position has a close relationship with the choices of supporting position. For long and thin pieces, the influence of gravity is more outstanding .

（2）Contact Deformation

If we fail to correctly choose the measure force, the contact body material and contact way, it will cause larger surface contact deformation.

① Contact Deformation Caused by Force

During contact measurement, there must be enough force of measurement instrument to ensure the probe and measured work-piece reliably contact. But the existence of measuring force will produce sag deformation in the contact position, which will cause the error. General we decide the measuring force according to the tested pieces tolerance, when the tolerance is less than 2um, measuring force should not be higher than 2.5 N; when tolerance is 2 ~ 10um, the force should be 2.5 ~ 4N; when tolerance greater than 10um, the force should be greater than 4N, but less than 10N.

② Contact Deformation Caused by Contacting Way

Common contacting way divided into point contact, line contact and surface contact. Tell from the relationship between the deformation and contact surface, the smaller contact area, the higher pressure, and the greater deformation. Clearly in the case of the same measuring force, point contact deformation is the largest and surface contact deformation is minimal.

Contact way will be stipulated in the measurement requirements. Such as in the gauge block measurement, using the ball touch the plane; When measuring diameter by using outside micrometer valve, using flat probe touch cylinder plane, etc. We generally should follow these principles in the process of measurement.