Design and accuracy analysis of pneumatic gauging for form error of spool valve inner hole

The inner hole of a spool valve is a slender hole required to have high precision form, and the form error is generally not allowed to exceed 1 μm in servo valve production. Aiming at ultra-precision measurement for the spool valve inner hole in situ, this paper proposes a novel non-contact measurement method based on the differential pressure pneumatic measuring principle and develops a form error pneumatic gauge system for the spool valve inner hole. The pneumatic measuring circuit, gauge head and precise driving mechanism have been designed. The factors that influence the accuracy of the pneumatic gauge system have been thoroughly analyzed and tested to evaluate the measuring uncertainty. The system calibration and measurement experiments have been carried out. Results show that the measuring error does not exceed 0.3 μm, which can be further improved by the error separation and compensation technique.

► A differential pressure pneumatic measuring method for the form error of an inner hole is presented.
► The pneumatic gauge system is developed to realize the non-contact inspection for the form error in situ.
► The characteristics of the pneumatic gauge and system error source are analyzed.
► Measuring uncertainty is evaluated as 0.3 μm.
► Potential methods to enhance the measuring accuracy of the pneumatic gauge are proposed and validated.