A novel vibrating touch probe is proposed for on-machine measurement of small optical elements machined on ultraprecision machine tools. Advantage of the proposed probe is its low measuring force that can be less than 50μN (5mgf). Maximum measurable contact angle of the probe is larger than 60 degrees. Therefore, the probe is suitable for measuring a hemispherical lens and its die. The probe is excited by a piezoelectric actuator in a sinusoidal manner. When the probe touches an object, the probe vibration becomes distorted, and harmonics appear in the vibration spectrum. It is judged that the probe touches the object at the point where the power of the second harmonic exceeds a threshold level. The performance of the probe is evaluated by measuring the form of a spherical lens die of 4.2mm in diameter. The result of the measurements is examined and the on-machine measurement using the proposed probe is discussed.
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